Around one third of Minnesotans rely on septic systems to treat their wastewater. Also referred to as subsurface sewage treatments systems (SSTS), septic systems utilize the power of the soil beneath our feet to remove pathogens and other contaminants from sewage so it can be safely returned to the water cycle. Explore this page to grow your knowledge of how SSTS help keep Minnesota's water clean and abundant.
What are the different types of onsite systems?
Onsite system options overview
In Minnesota, there are different methods to deal with on-site wastewater treatment. Standard systems are the simplest way to effectively treat wastewater, but occasionally site characteristics such as lot size, soil type, site disturbance, flow and wastewater characteristics often limit the options available. This document will define the different options.
According to Chapter 7080 a standard system is a trench, at-grade or mound system which is designed and constructed according to applicable requirements. The Minnesota Pollution Control Agency administrates these rules from a state wide level. Local governmental units such as a township or county administer the problem locally.
Standard systems set the acceptable treatment standards for system performance and management. These systems are designed with at least 3 feet of vertical separation. This assures that appropriate treatment levels can be met. An example would be a standard septic tank followed by gravity fed trench.
Alternatives to standard systems
There are two types of alternative to standard systems - 'other' or 'performance'. All of these systems need reviewal as a treatment system and different methods in achieving sewage treatment are what make them alternative. These systems have some similar characteristics. These systems do not have the same design criteria as standard systems. The permitting authority must approve the design and have reasonable assurance of performance. There must be a flow meter to monitor the flow. The estimated cost for construction, operation, monitoring, service, component replacement and management must be submitted to the local governmental unit. In addition, the anticipated system life and hydraulic and organic load must be determined. These systems are required to have a monitoring plan, which evaluates how is it working. The performance of these systems is measured. It includes sampling and testing to assure the waste is treated. The determination of what is to be sampled and how often is determined by the site characteristics, choice in system and local governmental unit (LGU). A mitigation plan, which lays out what will happen if the system fails to adequately treat needs to be submitted to the LGU. The differences between these systems are explained below.
These systems must be designed with a vertical separation of three feet or greater and have a soil texture of medium sand or finer. This soil treatment assures performance. At least 1-foot of the three feet must be original soil. They can not be loaded more than 1.2 gpd/ft2. An example would be a mound that was slightly downsized to accommodate a small lot.
Septic Tank ----> Downsized Mound
The permitting authority must approve performance system's design, monitoring plan and mitigation plan. Certain measurements must be taken to assure that the environment and public health are still protected. There must be at least 1 foot of vertical separation and an unsaturated zone between the seasonably saturated zone or bedrock. Twenty-five feet from the system the concentration of fecal coliform must not exceed the background concentration. If the system is close to a lake, fifty feet from the system the concentration of phosphorous must not be greater than 1 mg/L above background levels. An example would be an aerobic tank followed by a trench system with less than 3-feet of vertical separation.
Septic Tank ----> Aerobic tank ----> Trench with less than 3 feet
An in-ground system is one where the drainfield is entirely underground. They include both trenches and seepage beds, and can use gravity or pressure distribution.
The trench is the most common of the soil treatment systems in Minnesota. A trench is defined as a soil treatment and dispersal system, the absorption width of which is 36 inches or less. Trenches are narrower than they are wide, no wider than three feet, and are laid out along the contours of the soil. The method of distributing the septic tank effluent can be either pressure or gravity.
There are a number of different configurations by which the trenches can be connected with each other and with the septic tank.
The trench soil treatment system consists of distribution media, the bottom of which must not be deeper than 48 inches, covered with a minimum of 12 inches of soil and a close-growing and vigorous vegetation. Many trench systems utilize a pipe and gravel distribution system where effluent passes through the pipe and is stored within the media until it can be absorbed into the soil. Other distribution media are allowed including chambers and expanded polystyrene to distribute the effluent to the soil.
Seepage bed systems
A seepage bed system is a wide area (wider than three feet) prepared to accept septic tank effluent. It is created below the surface of the soil and built the same way as a trench system. Seepage beds treat the effluent effectively as long as they are located in appropriate soils. A seepage bed is defined as a soil treatment and dispersal system, the absorption width of which is greater than three feet but no greater than 25 feet. The system uses either gravity or pressure distribution.
The construction of a seepage bed is essentially the same as that for a trench, except the bed is wider. In Minnesota, the bottom area of seepage beds with gravity distribution must be 50% greater than that of trenches to allow for the fact that there is very little sidewall with a seepage bed and low oxygen transfer. The bottom of the bed must be within 48 inches of the final grade and be covered with at least 12 inches of soil and a close-growing and vigorous vegetation.
Gravity distribution of septic tank effluent has been the most common design over the history of treatment in Minnesota. Less expensive to install and maintain than systems in which effluent is pumped, gravity distribution systems take advantage of natural elevation differences. Effluent flows down from its sources to the septic tank, then on to the soil treatment system.
Today, gravity distribution systems for conveying septic tank effluent are used in two types of systems: either trench or seepage bed systems.
There are several types of gravity distribution devices that receive and transfer effluent from the supply pipes to distribution pipes or down slope components : drop boxes, distribution boxes, valve boxes, and manifolds. The primary purpose of flow distribution devices is to control the flow of the effluent into the drainfield.
In an above-ground system, the drainfield is located above ground. These include ag-grades, and mounds. Above-ground systems always use pressure distribution.
An at-grade, as its name implies, is a system installed with the distribution media placed at the original soil surface. It is designed to solve similar issues as the mound, but where the soil conditions are somewhat more favorable. The operation of the at-grade component is a two-stage process involving both effluent treatment and dispersal into the underlying soil. Treatment is accomplished predominately by physical and biochemical processes within the soil. These processes are affected by the physical characteristics of the effluent wastewater, influent application rate, temperature, and the nature of the receiving soil.
The at-grade component contains a septic tank(s), pump tank, distribution system that consists of distribution media and a pressure distribution system, which is installed directly on top of the plowed natural soil and covered by loamy or sandy cover material and topsoil. Effluent flows into the soil, where it undergoes biological, chemical, and physical treatment and dispersal into the environment. The natural soil serves as the treatment medium and disperses the effluent into the environment.
Mound systems were developed in the early 1970s to overcome soil and site conditions, which limit the use of trenches and beds. Limiting conditions include high water tables, shallow soil depth to bedrock, slowly permeable soil, or soil too coarse for treatment.
A sewage treatment mound is nothing more than a seepage bed elevated by clean sand fill to provide adequate separation between where sewage effluent is applied and a limiting soil layer.
The mound system consists of septic tank(s) followed by a pump tank. Effluent is dispersed evenly via the pump into the absorption bed, where it flows through the clean sand material and undergoes biological, chemical, and physical treatment. It then passes into the underlying soil for further treatment and dispersal to the environment. A minimum of twelve inches of cover material which is crowned to shed surface water is placed over the top of the mound with the upper six inches being topsoil.
Aerobic treatment units
An aerobic treatment unit (ATU) pretreats wastewater by adding air to break down organic matter, reduce pathogens, and transform nutrients. Compared to conventional septic tanks, ATUs break down organic matter more efficiently, achieve quicker decomposition of organic solids, and reduce the concentration of pathogens in the wastewater.
More than 20 brands of ATUs are available, but efficiency varies widely. A properly operating ATU should produce high-quality effluent with less than 30 mg/liter BOD (biochemical oxygen demand, a measure of the organic matter), 25 mg/liter TSS (total suspended solids), and 10,000 cfu/100mL fecal coliform bacteria, an indicator of pathogens and viruses.
Aerobic Treatment Application
Since wastewater leaves an ATU as high-quality effluent, the soil in the trench or mound soil treatment system may be better able to accept it, and the system should last longer. Because ATUs produce cleaner wastewater, they are useful in sites with "disturbed" (compacted, cut, or filled) soil, and in environmentally sensitive areas such as those near lakes in shallow bedrock areas, aquifer recharge areas, and wellhead protection areas. Pretreatment may allow a reduction in the three-foot separation requirement between the soil treatment system and the limiting soil layer. Researchers are testing this hypothesis.
ATU systems may also be successfully retrofitted into drainfields that have failed because of excessive organic loading from lack of maintenance.
How Do Aerobic Treatment Units Work?
By bubbling compressed air through liquid effluent in a tank, ATUs create a highly oxygenated (aerobic) environment for bacteria, which uses the organic matter as an energy source. In another stage bacteria and solids settle out of the wastewater and the cleaner effluent is distributed to a soil treatment system.
ATUs are more complicated than septic tanks. In a septic tank, solids are constantly separating from liquid. As individual bacterial cells grow, they sink to the bottom, along with less decomposed solids, to form a layer of sludge. Floating materials, such as fats and toilet paper, form a scum layer at the top of the tank.
In an ATU, the bubbler agitates the water so solids cannot settle out, and floating materials stay mixed in the liquid. Well designed ATUs allow time and space for settling, while providing oxygen to the bacteria and mixing the bacteria and its food source (sewage). Any settled bacteria must be returned to the aerobic portion of the tank for mixing and treatment.
There are three basic ATU operation styles: suspended growth, fixed-film reactor, and sequencing batch reactor. All three types usually have a septic tank (sometimes called a trash tank) ahead of them that removes the large solids and provides some protection to the ATU.
- A suspended-growth tank has a main treatment chamber where bacteria are free-floating and air is bubbled through the liquid. The second chamber where the solids settle out is separated from the main tank by a wall or baffle. The two chambers are connected at the bottom or by a pump, and settled bacteria from the second chamber are brought back into the main treatment chamber. This return and mixing is critical for proper operation. Treated effluent from the second chamber is piped to the soil treatment system (Figure 2). Though simple, the system is likely to have problems with bulking (the formation of chains or colonies of bacteria that don't settle or sink to the bottom as they should). Bulking is caused by changes in wastewater strength or quantity. When too much water/wastewater is added to the system, the bacteria can run out of food or become overloaded. Bulked bacteria remain suspended in the liquid and can clog the outflow.
- A fixed-film reactor has bacteria growing on a specific surface medium and air is provided to that part of the tank. The bacteria can grow on any surface including fabric, plastic, styrofoam, and gravel. Decomposition is limited to this area, and settling occurs in a second chamber. This design is expensive, but the effluent is of consistently high quality, and bulking is uncommon. There is no need for a return mechanism because the bacteria stay on the film (Figure 3).
- In a sequencing batch reactor, aerobic decomposition, settling, and return occur in the same chamber. Air is bubbled through the liquid during the decomposition cycle. The bubbler shuts off, and the wastewater goes through a settling cycle (Figure 4). Once the bubbler turns back on, the tank reenters the decomposition cycle, and settled bacteria mixes back into the aerobic environment. After settling of bacteria and solids, the treated effluent is discharged to the soil treatment system. Bacteria settle out more consistently in this kind of tank, but since it has more moving parts and requires a controller, it has more potential for mechanical and electrical failure.
ATUs require very little installation space, which allows placement flexibility. A typical ATU space requirement is 25 square feet for a 3-bedroom home. Most ATUs are located after a septic tank. However, because of strict permitting requirements this is not practical for onsite systems.
Designing Aerobic Treatment Units
When sizing an ATU, consider the amount of water generated by the home, the addition of oxygen, the concentration of organic matter in the wastewater, and the settling characteristics of the chosen system. Proper design of these systems depends on the waste strength. Siting and construction considerations are the same as those for typical onsite treatment systems.
Final Disposal of Wastewater
Some ATUs produce the same high-quality effluent as large wastewater treatment plants that discharge to surface water. Therefore, in an onsite system, effluent is sent to a soil treatment system for final treatment. Options for a soil treatment system include trenches, mounds, and drip distribution.
Pressure distribution (rather than gravity) to the soil system is necessary, since the effluent from ATUs contains very little organic matter. The effluent is so "clean," a biomat layer does not form the way it does in a soil treatment system receiving effluent from septic tanks. A pressure distribution network is needed to apply effluent evenly throughout the system.
The soil treatment system may last longer when receiving effluent from an aerobic tank than it would receiving effluent from a septic tank, since ATUs remove more solid matter from the wastewater. It may be possible to make the soil treatment systems somewhat smaller than those used for conventionally pretreated effluent. It also may be possible to reduce the vertical separation to the seasonally high water table or bedrock. Changes to size or vertical separation would put these systems into the "performance" classification, requiring local approval and an operating permit. An operating permit requires a monitoring and mitigation plan and the installation of a flow meter.
Operation and Maintenance
All routine operation and maintenance practices suggested for any onsite treatment system apply to ATUs. ATUs are more maintenance intensive than septic tanks. A maintenance contract is strongly recommended and required for standard use according to Minnesota Rules Chapter 7080. Depending on the local governmental unit requirements and the recommendations of the manufacturer, the system may require quarterly to yearly maintenance. Maintenance includes inspecting all components and cleaning and repairing the system when needed. A visual inspection of the effluent is required and often a lab analysis is necessary.
If problems arise with the supply of air to the bacteria, the tank loses all effectiveness. If there are problems with settling, which are more likely in these designs than in conventional tanks, there will be problems in the soil treatment system. These tanks must be monitored frequently and repaired as needed.
The costs to operate an ATU are based on the run-time of the compressor, and average less than ten dollars per month for an individual home. Overall operational costs of $200–$500 per year include pumping, repairs, maintenance, and electricity. Some homeowners notice the constant hum of the blower.
With proper design and a good maintenance program, the aerobic system should perform well and treat wastewater for a long time. ATUs have a small land requirement, but are highly mechanical and require more management than most onsite systems.
A constructed wetland system (CWS) pretreats wastewater by filtration, settling, and bacterial decomposition in a natural-looking lined marsh . Constructed wetland systems have been used nationally and internationally with good results, but performance levels decrease in cold climates during winter.
A properly operating constructed wetland should produce an effluent with less than 30 mg/liter BOD (biochemical oxygen demand, a measure of organic material), less than 25 mg/liter TSS (total suspended solids), and less than 10,000 cfu/100mL fecal coliform bacteria, an indicator of viruses and pathogens.
The SF system is usually a basin or channel surrounded by a barrier of ponded wastewater and soil to support the growth of rooted emergent vegetation. The two types of SF wetlands are shown in Figures 2 and 3, (page 1). The open-water wetland in Figure 2 has a small layer of sand to root the plants while the hydroponic wetland in Figure 3 does not. SF wetlands are better suited for large community systems in milder climates for several reasons: the system can be fenced to prevent public contact, mosquito habitat is not a major issue, freezing is unlikely, and the amount of gravel is minimal, therefore lowering cost. In the SSF system, the water level is maintained below the surface of the gravel substrate by a stand-pipe structure at the discharge end of the cell which minimizes the risk of exposure to people and animals and greatly reduces mosquito breeding. The SSF is the most common constructed wetland system used for small flows (less than 10,000 gallons per day in Minnesota) and is often used for individual homes, small clusters of houses, or resorts.
How do wetlands work?
Solids are removed by physical filtration and settling within the gravel/root hair matrix. Organic matter may also be removed by these physical processes, but is ultimately removed through biodegradation. Biological treatment may be anaerobic (as in a septic tank where very little or no oxygen is present in the wastewater); or aerobic, with oxygen supplied by both diffusion from the atmosphere at the surface of the beds (much lower in SSF than SF systems) and by "leaking" of oxygen from the roots of cattails, bulrushes, reeds, and other emergent aquatic plants.
Aerobic treatment processes are faster than anaerobic processes, but oxygen is limited within the wetland. Some designers include an active aeration component to fully break down the BOD and nitrify the ammonium present in the septic tank effluent to nitrate-nitrogen.
Constructed wetlands have four parts: the liner, distribution media, plants, and underdrain system.
The liner keeps the wastewater in and groundwater out of the system. Although the liner can be made from a number of materials, 30 mil polyvinyl chloride (PVC) is the most common and the most reliable. Clay liners are not recommended because they can crack if too thin, allowing untreated wastewater to move into the soil and contaminate groundwater.
The distribution medium at the inlet is usually coarse drainfield rock that is 3/4 to 21/2 inches in diameter. This first part of the distribution system spreads the wastewater across the width of the wetland. Both gravity and pressure distribution can be used to spread the wastewater evenly over the system. The media in the filter is pea gravel that is 3/8 to 3/4 inch in diameter. The depth of the pea gravel varies from 18 to 24 inches.
Plants growing in the cell are often cattails, but other species include bulrushes, reeds, and sedges. Flora must grow and
flourish in the system for it to operate at maximum efficiency. The underdrain system at the end of the wetland is a slotted 4-inch pipe covered with drainfield rock. The underdrain moves the treated effluent out of the wetland and keeps the effluent level below the surface of the gravel. This prevents the effluent from coming into contact with people and keeps mosquitoes from breeding in the wetland. It also keeps the water level high enough to sustain plant growth.
Designing wetland systems
Subsurface flow systems, as the name implies, are constructed so the effluent moves through the medium below the surface. These systems require more space than surface flow systems, but can add treatment area. They also have fewer odor problems and are less prone to freezing (although they need freeze prevention management, particularly during winters with subnormal snowfall). SSF systems are generally more expensive to build than SF systems because of the cost of transporting materials, but are often recommended for Minnesota because of their relative simplicity and established performance record throughout the world.
Freezing may be minimized by deepening the bed, which increases the cost and decreases performance if much of the wastewater can then move through the system without contacting the root zone. Difficulties at research sites in northern Minnesota during mild, low-snow winters suggest that the cells should be covered with a layer of insulating material. This may reduce their treatment performance by decreasing oxygen transfer.
The size of the system depends on the level of treatment desired balanced against the wastewater strength. Wetlands should be sized in cold climates for a minimum detention time of 10–13 days to ensure high quality effluent. For SSF wetlands, the rock medium must be included in calculations for the system size. A 30% porosity ratio takes the rock volume into account, increasing the system volume necessary for adequate retention time. The length should be two to three times larger than the width to ensure that the wastewater is not flowing too quickly through the system.
The system should be located on the contour with drainage directed away from the system. Surface water inflow can cause overloading problems. A barrier to sediment, such as rock landscaping or sod, is needed to minimize erosion and wetland cell problems.
Final disposal of wastewater
Wastewater from the CWS must be routed to the subsoil for final treatment. This route may take the form of an additional unlined seepage cell or a standard set of drainfield trenches or a drip distribution system.
The size and design of a soil treatment system for wetland effluent has not yet been established, although it could be smaller than conventional soil treatment systems for similar lows using septic tank effluent. Two types of final treatment systems are shown in.
In Minnesota, a CWS is considered a "performance" system unless followed with a soil treatment system with three feet of unsaturated soil. The local unit of government must allow performance systems and issue an operating permit. Operating permits have a monitoring and mitigation plan, requiring the installation of a flow meter.
Operation and maintenance
All the routine operation and maintenance practices suggested for any onsite treatment system apply to wetlands.
Constructed wetlands require more maintenance than conventional septic-tank-drainfield systems. A maintenance contract is strongly recommended. Depending on the local governmental unit requirements and the recommendations of the manufacturer, the system may require quarterly to yearly maintenance. Maintenance includes inspecting all components and cleaning and repairing the system when needed. Visual inspection of the effluent is required and often a lab analysis is necessary. The pump and electrical dosing unit must be inspected annually. Plants should be inspected and, if a good stand does not exist, replanted. Consider introducing a different species mix.
The flow meter and timer should be checked to ensure the right amount of effluent is being applied to the system. Water must be present at all times or the system will dry out, killing the plants and bacteria that treat the waste. Large flows (caused by excessive wastewater flows or by natural events such as torrential rains) can impair treatment by washing pathogens and nutrients through the media. They can lead to short-or long-term reduction in the ability of the system to provide treatment.
The quality of the wastewater that goes out of the septic tank and into the wetland (influent) affects system operation. Toxic chemicals can harm or kill plants and bacteria in the wetland, with serious consequences. In commercial applications, plugging the media with solids, organic matter, or grease may be a problem. Higher strength influents to the CWS may also decrease its performance and the influent should be carefully monitored before design stage.
Daily running costs for a wetland are based on the operation of a small submersible pump and average less than a dollar per month for an individual home. Overall operational costs of $200–$500 per year includes pumping, repairs, maintenance, and electricity.
Years with below-normal snowfall may require extra insulation, such as extra bed depth (adding cost and possibly reducing efficiency) or extra surface insulation. The water level may also be lowered in winter, allowing an ice cap to form an insulating layer of air in the system.
Constructed wetlands are an effective option for on-site wastewater treatment when properly designed, installed, and maintained. They do require more maintenance (such as insulating, plant replacement, and weeding) than other systems.
A peat filter pretreats septic tank effluent by filtering it through a two-foot-thick layer of sphagnum peat before sending it to the soil treatment system. Peat is partially decomposed organic material with a high water holding capacity, large surface area, and chemical properties that make it very effective in treating wastewater. Unsterilized peat is also home to a number of different microorganisms, including bacteria, fungi, and tiny plants. All of these characteristics make peat a reactive and effective filter.
In research conducted in Minnesota, peat filters removed high concentrations of nutrients (nitrogen and phosphorus) and produced a high-quality effluent with less than 30 mg/liter BOD (biological oxygen demand, a measure of organic material), less than 25 mg/liter TSS (total suspended solids), and less than 1,000 cfu/100ml fecal coliform bacteria, an indicator of pathogens and viruses.
The two main types of peat filters are modules and lined filters. Modules are manufactured plastic peat treatment cells. Lined peat filters are built on site and usually lined with 30 mil polyvinyl chloride (PVC).
Peat filter application
Since wastewater leaving a peat filter system is a high-quality effluent, the soil in the trench or mound soil treatment system may be better able to accept it, and the system should last longer. Because peat filters produce cleaner wastewater, they are useful for sites with "disturbed" (compacted, cut, or filled) soil and for environmentally sensitive areas such as shoreland areas in shallow bedrock areas, aquifer recharge areas, and wellhead protection areas. Pretreatment may allow a reduction in the three-foot separation required between the bottom of the seepage trench of the soil treatment system and the limiting soil layer. In locations with difficult access, such as small lots on lakeshores or in heavily wooded areas, modular peat filters may be easier to install than other systems.
How do peat filters work?
Wastewater flows from the home into a septic tank where the large solids settle out and the liquid flows into a pump tank. An effluent screen or filter is often installed to restrict smaller solids and grease from flowing out of the septic tank. The liquid effluent is then pumped to the peat filter, where it is pretreated and delivered to the soil treatment system for final treatment.
A peat filter has three components: the peat, a pressure distribution system, and a drain. Wastewater must move through the peat under unsaturated conditions. The peat layer should be from 2 to 2.5 feet deep. Most of the peat used in Minnesota comes from the northern regions of the state. It is harvested from large natural beds and screened for consistency. Bord na Mona brand modular filters use a coarser peat from Ireland. Systemsusing this coarser medium also provide excellent treatment.
With a gravity distribution system, wastewater may pond on top of the peat and compress it, reducing the flow of wastewater through the filter. With a pressure distribution system, wastewater is applied evenly over the peat surface, allowing rapid infiltration. Filters using pressure distribution are long lasting and provide good treatment of wastewater.
The drain is a liner or module that holds the effluent inside the filter. The drain collects the effluent and delivers it to the soil treatment system. In a lined filter, the drain is a four-inch slotted PVC pipe surrounded by twelve inches of drainfield rock. The bottom of the filter slopes slightly (one inch in eight feet) to keep effluent from ponding. With a module peat filter, the drainage system is built into the module.
Designing a peat filter
In a modular peat filter system, the recommended design is one module per bedroom. For a constructed peat filter, the recommended size is 1 gallon/sq. ft./day.
To determine the design size of the filter, the volume of wastewater flow from the residence is divided by the loading rate. The length-to-width ratio is not as important as a distribution system that applies wastewater evenly to the filter surface at regular intervals. The use of a timer to spread the application out is recommended.
Final disposal of wastewater
Effluent leaving a peat filter is sent to a soil treatment system. Options for soil treatment systems include trenches, mounds, drip distribution systems, and linerless peat filters. A linerless or "bottomless" drain system, in which the effluent from the peat is allowed to drain directly into the soil.
The effluent is so "clean," a biomat layer does not form the way it does with effluent from septic tanks. A pressure distribution network is needed to apply effluent evenly throughout the system.
Most soil treatment systems will last longer when treating effluent from a peat filter than when treating effluent from a conventional septic tank. These systems can be smaller than those designed to receive conventionally pretreated effluent. It also may be possible to reduce the vertical separation distance to the seasonal high water table or bedrock. Systems with these modifications are called "performance systems," and require local approval and an operating permit. The operating permit requires a monitoring and mitigation plan and the installation of a flow meter. Researchers in Minnesota, Wisconsin, and Pennsylvania are currently testing size reductions for soil systems using pretreated effluent, so the sizing requirements may be subject to change.
Operation and maintenance
All routine operation and maintenance practices suggested for onsite treatment systems apply to peat filters.
Peat filters require more maintenance than conventional septic-tank-drainfield systems. A maintenance contract is strongly recommended. Depending on the local governmental unit requirements and the recommendations of the manufacturer, the system may require quarterly to yearly maintenance. Maintenance includes inspecting all components and cleaning and repairing when needed. The flow meter and timer should be checked to ensure that the right amount of effluent is being applied to the system. A visual inspection of the effluent is required, and a lab analysis of effluent is often necessary.
Because of the high organic content of peat, the filter media must be periodically replaced. This means physically removing the layer of peat when it has begun to decompose. Life expectancy of the peat media in a filter is estimated to be ten to fifteen years. The system should be designed to make it easy to remove and replace the peat. Module peat filters are easier to maintain than lined peat filters because they are open to the surface.
Daily running costs for a peat filter are based on the operation of a small submersible pump, and average less than one dollar per month for an individual home. Overall operational costs of $200–$500 per year include pumping, repairs, maintenance, and electricity.
Because of the unique treatment abilities of peat and its availability in the state, the peat filter appears to be a very promising method of treatment for Minnesota's wastewater.
Recirculating media filters
A recirculating media filter (RMF) pretreats septic tank effluent by filtering it through a medium of coarse sand gravel, peat, or textile before sending it to the soil treatment system. Sand is the most reliable and widely used medium. RMFs have been used since the 1970s in communities with flows of more than 5,000 gpd (gallons per day), but use for small flow application (less than 1200 gpd) has been growing. RMFs are an attractive alternative because they require less land and can handle higher strength waste.
A recirculation system has an advantage in areas where nitrogen contamination is a problem. As wastewater moves through the filter and becomes oxygenated, ammonia is transformed into nitrate. In the recirculation tank, conditions are anoxic (low in dissolved oxygen) and bacteria break down nitrates and releases N back to the atmosphere, a process called denitrification. Because of the large media size, RMFs do not remove fecal coliform as effectively as single-pass sand and peat filters.
Recirculating media filters application
Since wastewater leaving an RMF is a high-quality effluent, the soil in the trench or mound soil treatment system may be better able to accept it, and the system should last longer. Researchers are looking into whether soil treatment systems receiving pretreated wastewater could be downsized to reduce the total area required.
Because the wastewater flowing into the soil treatment system from an RMF is much cleaner than wastewater from a septic tank would be, RMFs are also useful for sites that have been compacted, cut or filled, and may be the best alternative in environmentally sensitive areas with shallow separation to bedrock or seasonally high water tables near lakes and rivers, groundwater recharge areas, and wellhead protection areas. RMFs can also help recover drainfields that have failed due to excessive organic loading. This is especially attractive for small lots with little room for replacement. Recently RMFs with gravel filters have been used to treat high strength waste from restaurants and other establishments.
How do recirculating media filters work?
Recirculation means cycling wastewater through the filter a number of times, allowing for continued filtering and increased bacterial decomposition. Wastewater moves from the house into a septic tank where solids settle out and some organic matter is decomposed (Figure 2). Liquid effluent moves, usually by gravity, to the recirculation tank. Here effluent that has been recirculated through the filter is mixed with septic tank effluent. Effluent is pumped repeatedly through a lined filter and then back (by gravity or pump) to the recirculation tank. In the filter, biological treatment occurs as the effluent passes the surfaces of the filter media. Treated effluent is collected at the bottom and returned to the recirculating tank where the cycle begins again. After the effluent has gone through the filter several times a controlling mechanism sends the effluent to the soil for final treatment. Depending on the site, final treatment could be trenches, a mound, at-grade, or drip distribution.
The filter is in a watertight liner or container. Although the liner can be made from a number of materials, 30 mil polyvinyl chloride (PVC) is the most common and the most reliable material. The filter is composed of twelve inches of drainage media. Outflow from the filter is provided by a four-inch pipe surrounded by drainfield rock. Depth of the outflow should be from one foot to eighteen inches below the bottom of the treatment media. Effluent must drain freely out of the media since saturation reduces the filter's effectiveness. The layer above the outflow drain is the treatment media (usually two feet of coarse sand, 0.05-2.0 mm in diameter). The top layer is the distribution media (usually drainfield rock), where the pressure distribution system is located.
Designing recirculating media filter
A properly designed, installed, and managed RMF can achieve the treatment levels shown in Table 1. These results are typical for RMF using sand as the treatment media. The major considerations when designing an RMF are loading rate and recirculation rate.
The loading rate describes how much wastewater is applied per square foot. In an RMF, the loading rate can range from 1–20 gallons per day per square foot. The most widely used and researched loading rate is 4–5 gallons per day per square foot.
The recirculation rate describes how many times the effluent goes through the filter before being released to the soil treatment system. This rate is generally in the range of 2–10 times. To achieve acceptable treatment levels a minimum recirculation rate is 4 times. Coarse media is needed for the higher loading rates to accomplish the recirculation without plugging problems.
A vertical-flow system (popular in northern Europe and now used in Minnesota) is simply small, planted gravel beds that are dosed intermittently with wastewater across their entire surface. The intermittent application of wastewater allows oxygen from the atmosphere to be more efficiently drawn through the gravel medium and its attached microbial community. The difference between this system and other RMFs is the addition of wetland plants on the surface.
Site flexibility is a major advantage of filter systems. Because the filter is watertight and uses a medium for treatment, the soil where it is constructed is not as important as the ability of the system to transfer oxygen. Without adequate oxygen, bacterial action will be compromised. Landscape rock is recommended for the filter surface to maximize gas exchange. The location is mounded in the center to avoid excessive surface runoff into the system. Landscaping should divert surface water around the filter to minimize surface water additions. RMFs are much smaller than single-pass sand filters; therefore, they are a better option for small lots.
Final disposal of wastewater
Effluent from the RMF will be very clean, but must still be applied to the soil for final treatment. Because effluent is so clean, a biomat layer does not form the way it does with effluent from septic tanks. A pressure distribution network is needed to apply effluent evenly throughout the system.
There are many options for the soil treatment system including trenches, mounds, and drip distribution.
Operation and maintenance
All the routine operation and maintenance practices suggested for any onsite treatment system apply to RMFs.
RMFs require more maintenance than conventional septic tank-drainfield systems. A maintenance contract is strongly recommended. Depending on the local governmental unit and the recommendations of the manufacturer, the system may require quarterly to annual maintenance. Maintenance includes inspecting flow meters, pump, recirculation tank, recirculation pump, distribution systems, media and effluent quality; and cleaning and repairing when needed. In particular, the flow meter should be read to make sure the application rates are within the design limits. Spray heights on the pressure distribution system should be inspected to ensure even distribution over time. A visual inspection of the effluent is required and often a lab analysis is necessary.
Over time the upper layer of the RMF may become plugged with solids or a build-up of organic matter. If this happens, the upper layer should be removed and replaced with new media. Adding air to the system may minimize this problem.
The running costs for an RMF are based on the operation of a small submersible pump, and average less than five dollars per month for an individual home. Overall operational costs of $200–$500 per year includes pumping, repairs, maintenance, and electricity.
Recirculating media filter systems are a viable onsite sewage treatment option. The small land requirement makes them attractive for small lots. The increased ability to remove nitrogen is an advantage in areas with nitrogen contamination. Because of the large media size, RMFs do not remove fecal coliform as effectively as single-pass sand and peat filters.
Design and performance
A recent development in the on-site sewage treatment industry is the textile filter. There are several different types of textile filters, but they all share common characteristics. The media is high in surface area, void space, and water holding capacity and is fibrous, not solid. They generally have a small land use requirement, some as small as 10-20 square feet. They are usually loaded at high rates, such as 20-45 gallons per day per square foot. Many times they are recirculating meaning the effluent passes through the filter several times before going to a soil treatment component. To date, several textile filters have been installed in Minnesota. Preliminary data has shown these filters to effectively pre-treat wastewater. The effluent from a textile filter then goes to a soil treatment system.
At some point the textile media may need to be removed and replaced with new media, but because this technology has only been around for a few years it is uncertain when this will be needed. The media itself is a synthetic fiber made of durable and biodegradation-resistant polymers, so it should last forever. Overtime the media may fill up with solids, grease and oil. Proper maintenance of the septic tank will facilitate the media lasting longer.
Single-pass sand filters
A single-pass sand filter system pretreats septic tank effluent by filtering it through sand before sending it to a soil treatment system. Various sand filter types and designs have been extensively tested and used in the United States. Other wastewater treatment filters use peat, pea gravel, crushed glass, or other experimental media, but sand is the best understood and the most predictable.
Treatment mechanisms in a sand filter include physical filtering of solids, ion exchange (alteration of compounds by binding and releasing their components), and decomposition of organic waste by soil-dwelling bacteria. A properly operating sand filter should produce high-quality effluent with less than 10 mg/liter BOD (biological oxygen demand, a measure of organic material), less than 10 mg/liter TSS (total suspended solids), and less than 200 cfu/100 ml fecal coliform bacteria, an indicator of viruses and pathogens.
Sand filters application
Since wastewater leaves a sand filter system as high-quality effluent, the soil in the trench or mound soil treatment system may be better able to accept it, and the system should last longer. Because sand filters produce cleaner wastewater, they are useful for sites that have been compacted, cut, or filled; and for environmentally sensitive areas like those near lakes, in shallow bedrock areas, aquifer recharge areas, and wellhead protection areas. Pretreatment may allow a reduction in the three-foot separation required between the soil treatment system and the limiting soil layer. Researchers in several states, including Minnesota, are testing reduced separation distances in soil treatment systems receiving wastewater pretreated in a sand filter.
Sand filter systems may also be successfully retrofitted into drainfields that have failed because of excessive organic loading from lack of maintenance.
How do sand filters work?
Sewage flows from the house into one or several septic tanks, depending upon the size of the house and local requirements. Effluent from the septic tank(s) flows into a pump or lift tank. A pump introduces the effluent at the top of the watertight sand filter, using pressure distribution to apply the wastewater evenly to the filter surface to maximize treatment. A timer is used to dose the entire surface of the filter intermittently with wastewater. This draws oxygen from the atmosphere through the sand medium and its attached microbial community. The effluent is treated by physical, chemical, and biological processes. Suspended solids are removed by mechanical straining due to enhanced contact and sedimentation. Treatment occurs through the bacteria that colonize in the sand grains. Microorganisms use the organic matter and nutrients in the effluent for growth and reproduction.
Designing sand filter systems
The two main types of sand filters differ in the rate at which wastewater is introduced into the system. Loading rates determine the amount of maintenance needed and how long the system will last. A single-pass filter with a high loading rate needs regular cleaning (every two to three months) of the sand surface to prevent clogging.
In high-rate sand filters, effluent is applied at rates of 1.6 to 5 gallons per day per square foot. This application rate means the surface of the filter must be easy to access. That is why high-rate sand filters are more common in warmer climates where they can be left open or have a lid that is easily removed.
Low-rate sand filters are the most common designs in Minnesota. Effluent from the pump tank is applied at rates of 0.8–1.5 gallons per day per square foot. Sizing criteria used for low-rate sand filters are similar to those for rock beds in mound soil treatment systems. These systems are covered with 6 inches of loamy topsoil and vegetation to provide insulation during the winter.
To determine the design size of the filter, the volume of wastewater flow from the residence is divided by the loading rate. The length to width ratio is not as critical as providing a system that distributes wastewater evenly across the filter surface at regular intervals. Timed dosing and a two-foot spacing of inlet pipes are recommended. In Minnesota, to be considered a standard pressure distribution system, anywhere from 2-to 5-foot spacing is allowed. Perforations in the laterals can be 3/16 inch to 1/4 inch in diameter. While laterals with 1/4 inch perforations require a larger pump, smaller perforations are subject to plugging.
Site flexibility is probably the biggest advantage of a sand filter system. Because the filter is watertight and uses media for treatment, the soil where it is constructed is not as important as the ability of the media in the filter to transfer oxygen. Without enough oxygen, bacterial action will be compromised. The system should be constructed to keep surface water from entering the filter.
Outflow drainage from the filter is provided by a four-inch pipe surrounded by pea rock. Depth of outflow should be one foot to 18 inches below the bottom of the sand. The effluent must drain freely out of the sand, since filter saturation reduces treatment effectiveness.
Final disposal of wastewater
Effluent discharged from this system will be very clean, but must still be applied to the soil for final treatment. The design of this part of the system is still being tested and sizing requirements are being developed. Effluent leaving the sand filter is sent to a soil treatment system. The effluent is so "clean," a biomat layer does not form the way it does in soil treatment systems receiving effluent from septic tanks. A pressure distribution network is needed to apply effluent evenly throughout the system. Options for the soil treatment system include trenches, mounds, and drip distribution. Figure 5 shows a sand filter system with pressurized trenches for even effluent application throughout the soil treatment system.
Operation and maintenance
All the routine operation and maintenance practices suggested for any onsite treatment system apply to sand filters (See Septic System Owner's Guide, PC-06583, for details.) Sand filters require more maintenance than a conventional septictank- drainfield system. A maintenance contract is strongly recommended.
At high loading rates (2 to 6 gal./sq. ft.), the sand must be replaced every 2–5 years. At lower loading rates, the system will operate properly for a longer time. If higher loading rates are necessary, recirculating the waste is an attractive alternative to the single-pass design.
Maintenance includes inspecting all components and cleaning and repairing when needed. Visual inspection of the effluent is required and often a laboratory analysis is necessary. A flow meter and timer should be installed and periodically checked to ensure the right amount of effluent is being applied to the system.
Daily running costs for a sand filter are based on the operation of a small submersible pump, and average less than a dollar per month for an individual home. Overall operational costs of $200–$500 per year includes cleaning tanks, repairs, maintenance, and electricity.
In an intermittent sand filter treatment system, wastewater receives primary treatment in the anaerobic environment of a septic tank, secondary treatment in the aerobic environment of a sand filter and is disposed of in soil trenches where additional biodegradation occurs. Most of the decomposition takes place in the sand filter. Naturally occurring microorganisms reside on the surfaces of the sand particles and thrive on the regular doses of nutrients contained in the wastewater. Following sand filter treatment and prior to drainfield discharge, effluent can be expected to produce:
- BOD <15 mg/L
- TSS <15 mg/L
- Fecal 1000 cts/100mL
- NH3-N <1.0 mg/L
- NO3-N <30 mg/L
Operationally, it works like this: The septic tank is equipped with a screen vault and a pump or followed by a lift station with a pump to discharge effluent under pressure to the sand filter. Piping on the surface of the filter is the vehicle for uniformly distributing effluent over the surface of the sand bed or filter media. Solids and other contaminants in the effluent are mechanically, biologically, and chemically reduced as the effluent passes through the filter media. The treated effluent is collected at the bottom of the sand filter by the underdrain and either drains out by gravity into a lift station or is pumped out for final treatment and disposal in a shallow drainfield.
Typical design for low-rate intermittent sand filter with pump basin
Sand filters are usually loaded at 0.5 to 1.2 gpd/ft2. Here are the main component parts of sand filters normally installed:
- A 19’ x 21’ x 35" deep hole, flat on the bottom, with a depression where the pump basin will be located. Any over-excavation should be filled and well compacted. A two-inch cushion of sand in the bottom of the hole is to protect the liner from penetration by sharp objects. The depth of hole can actually vary from 0" to 41" depending on specific site conditions.
- A perimeter support frame to hold the liner in place during construction may be needed. This box protects the liner and provides support to prevent the whole from collapsing. If the whole will maintain its shape and no sharp objects are present in the soil this may not be necessary. If a box is needed plywood with 2" x 4" framing support is appropriate. Treated wood is unnecessary as once the system is backfilled and complete, it is supported by the earth and sand and the wood can decompose without harm. During construction of the sand filter (placement of the media), it is important that sand be placed between the excavated soil and the plywood framework. This keeps the framework and liner vertical during the course of construction and results in a sand cushion around the outside perimeter of the liner. All nails or staples used must have their sharp ends pointed away from the liner.
- The 30-mil PVC liner is unfolded from the center of the excavation and draped over the top edges of the perimeter support frame (b). Care must be taken to ensure that the liner is in full contact with the bottom and sides and that no bridging occurs. Pleats or wrinkles in the liner are not a concern.
- There are two methods for constructing the removal of the effluent after it has based through the sand filter:
- The first is a PVC pump basin is installed in the depression located in the center of the sand filter. The pump basin must have a PVC or fiberglass bottom to prevent damage to the liner. It is important to verify all dimensions to ensure the pump basin is the correct height. The pump basin should have 4" grommets installed opposite one another to accommodate the 4" slotted PVC underdrain pipe.
- The other option is to have the bottom of the filter sloped slightly and the effluent flowing out by gravity through the underdrain out to a tank. A boot will then be needed to be used. The PVC boot permits a watertight penetration of the liner. When installing boots, the manufacturer’s installation instructions must be followed exactly, orienting the boot so the clamp is outside the sand filter. In the event high ground water was to reach that elevation, the boot would prevent infiltration. The 4" slotted PVC underdrain pipe should be Class 125 (or higher) pressure rated. Slots are cut half way through the pipe _ " wide, 4" on center. If using a pump basin the pipe is laid flat with the slots pointed upward and capped with 4" end caps and the 4" underdrain pipe should penetrate the pump basin 1"-2".
- Drainfield rock is then mounded at least 2" over the 4" slotted PVC drainpipe to prevent finer material from entering the pipe.
- A level course of _" to 3/8" pea gravel, 6" deep, is placed in the bottom of the sand filter. Water will pond 2" to 3" deep in the bottom of the sand filter and the pea gravel will prevent unwanted capillary action from occurring and will allow the treated effluent to move freely toward the 4" drain pipe.
- Filter sand must be placed and compacted while it is damp. If the sand is not damp, it will not compact well and settlement may cause dislocation and breakage of the distribution laterals; wet the sand when necessary. The sand surface must be flat. It is critical that water never come in contact with the sand to saturate it. This saturation will greatly impact performance of the system.
- 3" of _" to 3/8" pea gravel is placed on top of the compacted sand, disturbing the sand as little as possible. The pea gravel serves to support the distribution system and to keep the sand from eroding under the action of the effluent being applied. After the laterals are installed and a pressure test performed, more pea gravel (enough to cover the pipe) will be added.
- A valve box located at the end of each lateral is to permit annual flushing of the laterals. Turning on the pump in the dosing septic tank for one or two minutes and opening a flushing valve allows water to flow in the _" diameter laterals at a velocity of more than three times the normal dosing velocity. The effluent may be dosed directly in the valve box. The higher flow rate scours biological growth that occurs on the inside walls of the lateral pipes. If these growths are not removed periodically, they may slough off and plug the orifices. When too many orifices are plugged, the effluent is not spread over the entire sand surface and a small portion of the filter becomes overloaded, turns anaerobic and can eventually cause the entire sand surface to plug. Following annual flushing of the laterals, the PVC 90° elbows are removed and replaced temporarily with a PVC capped adapter having a 1/8" orifice drilled in its top. The dosing septic tank pump is turned on and the proper squirt height verified–normally five feet for systems without distribution valves and twenty feet for systems with distribution valves. If the squirt increases from five to eight feet, or from twenty to thirty feet or more, it is likely that too many orifices are plugged and each lateral should be cleaned with a bottlebrush or a pressure washer.
- Assembly of the 2" PVC manifold (Class 200 minimum), 1". The 1/8" or _" diameter orifices should be drilled with a drill press or drill guide using a new 1/8" drill bit and should not have any visible burrs. All PVC joins should be glued according to the manufacturer’s instructions, e.g. primers shall be used if required.
- Orifice shields are required to prevent the orifices from being blocked by rocks resting against the outside of the PVC pipe. In locations where there is a possibility the laterals could freeze solid, orifice shields for cold climates should be used. These are an optional feature of sand filters.
- Filter fabric is placed over the final course of pea gravel. The purpose of the filter fabric is to keep soil, silts, and fine-grained material from moving down into the sand filter, at the same time allowing air and water to pass freely. It is important not to use a filter fabric so dense that air and water movement are impaired since that could cause the sand filter to clog and turn anaerobic.
- When attached to a PVC stem, the floats should have a maximum tether length of 2". The floats should be either mercury or mechanical and must be UL or CSA listed unless otherwise approved. The on, off, and alarm settings depend on the type of drainfield to be dosed. The designer should provide a narrative, describing what is to be accomplished. The high-water alarm float must be connected to the pump control panel in such a manner that a high-water alarm in the sand filter will disable the pump in the dosing septic tank until the high-water alarm is canceled.
- The electrical splice box must be UL or CSA listed and corrosion-proof, with the proper number of cord grips installed. Heat shrink must be used on the individual wire splices within the box. Sufficient length of wires must be provided in the box to allow for future repairs.
- The conduit seal must be UL or CSA listed and must be installed using the proper conduit sealant as recommended by the manufacturer. Bubble gum or silicone is not allowed.
- The electrical conduits must be UL or CSA listed for the purpose. _" diameter is most common. There are electrical code rules restricting the number of bends between panels and junction boxes. Refer to NEC 1993 section 347-14.
- The pump must be UL or CSA listed and specifically selected for its intended use by a knowledgeable person (not necessarily a pump salesperson). Pump capacity (gpm) and total dynamic head (Tdh~ft) should be considered.
- The hose and valve discharge assembly should be easily removable and have the flexibility to be easily installed. It should be constructed of moisture- and corrosion-resistant materials.
- The pump basin lid should be durable and should have durable, tamper-resistant mechanical fasteners.
- The soil cover must be loamy-sand. Its purpose is to provide insulation against cold winter temperatures, to allow the free movement of air into the sand filter below, and to prevent odors from escaping the sand filter. Establishing a grass cover over the sand filter is very beneficial.
Single-pass sand filters are an effective way to treat wastewater in an onsite application. The sand filter system has been used for more than 30 years across the United States and there is significant design, treatment, and maintenance experience with these systems. Sand filter systems are very reliable in treatment of BOD, TSS, and fecal coliform. The system protects the final soil treatment area because failure will occur in the sand filter before the soil treatment system is significantly affected. Single-pass sand filters require more area that recirculating filters and are not a good choice for small lots.
Drip distribution pumps pretreated wastewater to a soil treatment site where the wastewater provides nutrients and moisture for plants. The pretreated effluent "drips" out of tubing at regular intervals, allowing a small amount of wastewater to irrigate a large vegetative area. Topsoil with vegetation is an excellent environment because it maximizes the treatment of sewage and minimizes the risk of untreated water flowing quickly through the soil.
Drip distribution application
Drip distribution is often used in places where standard trenches are hard to install, such as steep slopes and forested areas. It is also used in places that only operate during the warmer months of the year, such as resorts and golf courses. Drip distribution systems are often used after a pretreatment system, such as an aerobic treatment unit or sand filter, is used.
How does drip distribution work?
The basic principles for drip distribution are the same as for other soil-based treatment systems: filtering and bacterial decomposition of waste. The difference is that a drip distribution system distributes the effluent evenly over a large area.
A drip distribution system has four main parts: a pretreatment device, a pump tank, a filtering/flushing device, and the distribution system.
The pretreatment device depends on the drip tubing and the manufacturer's recommendations. Some drip systems require advanced pretreatment, but others work with only a septic tank. All systems can plug without a good filtering device.
The pump tank stores the water until the drip field is ready for a dose of effluent. A high head pump is needed for even application of the wastewater. Pump selection and installation follows typical onsite treatment system design practices. The filters remove all particles larger than 100 microns from the effluent. Some filters have automatic cleaning systems. Flushing capacity and the total dynamic head are important design features that assure effluent passes through the emitters in the tubing. Even with excellent filtration, growth in the tubing can cause plugging. Flushing the system removes the growth and prevents plugging.
The most common types of filters are sand, disc, and spin. All three filters will function adequately with proper pretreatment so the designer and owner must weigh the advantages and disadvantages of each type.
- Sand filters are similar to swimming pool filters and are inexpensive and easy to maintain. On the down side, if sand particles make their way into the pump tank they can wear out the pump or escape the filter and plug the emitters.
- Disc filters were recently developed in Israel and use serrated discs as the filtering medium (Figure 3). They are more expensive than sand filters, but have fewer problems. The manufactured medium allows flexibility in waste strength and the filter maintenance can be completely automated with pressure gauges and control panels.
- Spin filters have a stainless steel screen that filters the effluent. The wastewater is forced through a directional nozzle plate onto the inside of the screen, creating a centrifugal action that rotates debris down the screen wall to a large debris holding basin. Most are self-cleaning.
The distribution system includes the components that carry wastewater from the pump to the soil treatment area. It is the most complex part and has seen the most design changes in the last twenty years. At one end, the tubing is connected to the pump. Along its length, tiny orifices or emitters allow the wastewater to drip out into the soil. The tubing is generally 1/2 inch in diameter with an emitter in the tubing wall (Figure 4). The pressure inside operates at 15–20 pounds per square inch. The collection manifold for the drip system connects back to the tank for flushing solids in the drip tubing.
Most problems occur in the tubing, including plugging of the tubes or the emitters by dissolved or suspended solids or roots. This results in uneven distribution of wastewater. Each brand of tubing is unique and tubing suppliers approach problems differently. System designers or operators should research tubing choices thoroughly before making a selection. Pressure compensating tubing is the easiest choice because it automatically increases the flow if an emitter starts to plug (Figure 5). It is also designed for systems installed on non-level sites, so even distribution is provided with no additional design requirements.
Designing drip systems
The sizing of drip distribution systems is based on the wastewater flow and the soil texture and structure. Manufacturers give a range of system sizes based on long-term soil acceptance rates. These values are similar to the design rates for soil treatment systems, but the area used is larger (since the goal is to maximize the surface area in contact with the effluent).
Location of the system depends on soil conditions, including depth of soil to bedrock or zone of saturation, texture, and temperature. The tubing type dictates siting requirements. These may include equal length runs, a level distribution field, equal distance from the pump, and equal manifold heights. Pressure compensating tubing has the fewest siting restrictions; in particular, a level field is not required and the system-dosing controller allows for different lengths of tubing runs.
Since most product development occurs in warm climates, freezing can be a problem in cold climates. In the initial operation of a drip system near Duluth, Minnesota, parts of the system (most notably the filters) worked well in the winter, while other parts froze. The filter portion must be both well insulated and heated. Depth of placement is an important consideration. Systems used only during the summer (at camps) often have tubing placed six inches deep. A minimum depth of twelve inches is recommended for all other systems in Minnesota. A research site near Hastings had freezing problems attributed to compaction of snow cover by foot traffic over the area. Repeated walking or driving over the system reduces the insulating ability of the snow.
There is debate about the necessary depth of separation from bedrock or saturated soil. Current Minnesota standards require three feet of separation for a standard soil treatment system. Use of drip distribution automatically makes this system a nonstandard system. If three feet of separation to the limiting layer are present below the tubing, the system is classified as "another system" and required to have a monitoring and mitigation plan and a flow meter installed. If three feet of separation does not exist, the system is classified as "performance" and only allowed by local governmental permitting agencies that have adopted performance standards. These systems must have a monitoring and mitigation plan, a flow meter, and an operating permit.
Maintenance and operation
All routine operation and maintenance practices suggested for any onsite treatment system apply to drip distribution systems. (See Septic System Owner's Guide, PC-06583, for details.) Maintenance should be done annually (or ideally, quarterly) and the entire system must be examined. First, pressure gauges should be checked for changes in pressure, which would signal clogging or leaks. Second, walking around the application area while the effluent is applied can uncover obvious leaks. Third, the air relief valve must be checked to make sure the unit is not wearing down or operating improperly. If the air relief valve malfunctions, soil particles can be sucked into the emitters and cause a problem that is very difficult to fix. The flow meter should be checked to measure actual wastewater application. This tells homeowners if the amount of effluent is above the design level. Finally, the tubing and filters may have to be flushed.
Daily running costs for drip distribution are based on the operation of a small submersible pump and average less than a dollar per month for an individual home. Maintenance costs range from $200–$500 per year, which includes a periodic pump-out of the septic tank, electricity, and maintenance visits.
Drip distribution can be an effective option in areas with forests or steep slopes, but must be properly designed, installed, and maintained. Particular care must be taken to prevent freezing.
Systems in floodplain areas are usually associated with existing structures as new construction would be prohibited in the floodplain. An existing structure in a floodplain with a failing system may need a replacement system. The concerns about floodplain areas during flood events include surface water contamination and system damage by fine soil particles contained in the flood waters. A system cannot be located in the floodway but may be sited in the "flood fringe".
Privies and outhouses
A privy or outhouse is a self-contained waterless toilet used for disposal of non-water carried human waste consisting of a shelter built above a pit in the ground into which human waste falls. If the pit has an earth bottom, this point should be at least three feet above saturated soil conditions. If this separation distance cannot be achieved in the location of the privy, then the pit should be liquid-tight, with the wastes periodically removed by someone who services septic tanks. The privy should be securely attached to the ground or to the tank used for the pit.
While holding tanks are not recommended for installation on newly developed lots, there are some developed lots which do not have adequate area for a sewage treatment system. In some instances, a holding tank may be the only alternative. The figure below shows a schematic diagram of a holding tank, together with the tank capacity as recommended by Chapter 7080. Holding tanks are constructed of the same materials and by the same procedures as septic tanks. For a single family dwelling, not located in a flood plain, holding tank capacity should be 1,000 gallons or 400 gallons times the number of bedrooms, whichever is greater. In floodplain areas, the capacity is 100 times the number of bedrooms, times the number of days the site is flooded during a ten-year flood, or 1,000 gallons, whichever is greater. Information regarding the number of days of flooding is available from the 100-year hydrograph or by contacting the local planning and zoning agency. For other establishments, the capacity should be based on measured flow rates or estimated flow rates. The tank capacity should be at least five times the average design flow.
Holding tanks may be allowed by the local unit of government as replacements for existing failing systems which pose an imminent threat to public health and safety, or on existing lots. Holding tanks are prohibited for new construction unless approved by the local unit of government. A monitoring and disposal plan must be submitted, signed by the owner and a licensed pumper. A contract for disposal and treatment of the sewage wastes should be maintained by the homeowner or pumper with a municipality, agency or firm established for that purpose. Holding tanks can only be installed:
- In an area readily accessible to the pump truck under all weather conditions.
- At least 10 feet from property lines, buried pipe distributing water under pressure, and occupied buildings at least 50 feet from any source of domestic water supply or buried water suction line.
- Where accidental spillage during pumping will not create a nuisance.
The tank should be protected against flotation under high water table conditions by weight of tank, earth anchors or shallow bury depth. A cleanout pipe of at least six inches diameter shall extend to the ground surface and be provided with seals to prevent odor and to exclude insects and vermin. A cleaning access of at least 20 inches least dimension shall extend through the cover to a point within 12 inches, but no closer than six inches below finished grade. The cleaning access cover shall be covered with at least six inches of earth. Holding tanks must be monitored to minimize the chance of accidental sewage overflows. A mechanical or electrical alarm must be activated when the tank has reached 75 percent capacity.
The cost of hauling the sewage can be excessive. Costs of pumping septic tanks are $50.00 to $120.00 for approximately 1,000 gallons. Costs may differ somewhat for holding tanks since they are usually readily accessible. A family of four is likely to generate at least 200 gallons of sewage per day. At a cost of $50 per 1,000 gallons, the annual cost to remove the sewage would be $3,650. Cost will vary with amount of sewage and hauling fees. Water conservation will reduce sewage flow and hauling costs. The liquid level in the holding tank will need to be continuously monitored in order to prevent an overflow. A water meter should be used to measure all flow except to the outside sillcocks. Water meter readings can be used to determine the amount of sewage pumped and hauled. Weather conditions or road restrictions may prevent hauling when necessary and require that the plumbing systems not be used until the holding tank has been pumped. A continuous contract must be maintained to be sure that pumping service is available and that the sewage can be treated and disposed of.
Separation technology separates the toilet waste (blackwater) from the rest of the sewage (graywater). There are two part of a system that uses separation technology. The first part is the toilet, which instead of a flush toilet is a composting, incinerating or chemical toilet. By removing the toilet waste from the system the soil treatment system can be downsized by 40%. The separation removes a lot of the organic material and nitrogen. The second part is the soil treatment system for the graywater.
Incinerator toilets can completely eliminate liquid and solid toilet wastes from the sewage treatment system. The initial cost may vary from $800 to $1,500, including electric wiring and a fireproof vent for the waste gases. In addition to the initial expense, there may be some replacement costs of component parts for an incinerator toilet. Average energy use is 1.5 pounds of gas or 1.0 kwh for each toilet cycle (flush). Current energy costs can be used to determine the actual use cost. Because an incinerator toilet requires a cool-down period after each incineration cycle, it may not be a particularly desirable device for a large family where demands on the toilet may come in short spans of time. An incinerator toilet is not particularly effective for situations where there is a considerable amount of liquid waste. Liquid is difficult to burn. The waste gases from an incinerator toilet have some odor and, under certain atmospheric conditions, may settle to the ground and be objectionable to occupants or neighbors. There have been reports along lakeshore areas, where temperature inversions are common, of incinerator toilets causing serious odor problems. The firepot requires regular cleaning to remove ashes and other residue and will need to be periodically replaced, depending upon the amount of use.
In most chemical toilets, a charge of chemical is added to a small amount of water. After use, the liquid is recirculated by an electric- or hand-operated pump to flush the wastes into the holding chamber. The initial charge of chemical is adequate for 40 to 160 uses, depending upon the model. When the holding chamber is full, a valve can be opened to discharge wastes into the septic tank. On some chemical toilets, the holding chamber can be removed for disposal of wastes. Wastes are reduced to about two percent of those from a conventional flush toilet. The initial cost of chemical toilets varies greatly depending on model and size, but it will likely range from $200 to $700 plus installation. The cost of the chemical may be from two to three cents per toilet use. Because most chemical toilets are plastic, they should not corrode. Maintenance costs should be minor.
Composting toilets are more appropriately called biological toilets and have two basic principles of operation: liquid is evaporated, and solid wastes are biologically decomposed into compost. The biological toilet uses no water and requires no connection to house plumbing. Every biological toilet has a capacity limit, which depends on its ability to evaporate moisture. To increase the capacity, most room-sized biological toilets use heating elements and fans, together with mixers for the organic material. All biological toilets designed for year-round use must have electricity to run the fan and the heating element. Large-volume biological toilets may be used in seasonal residences without having electricity available, but care must be taken that excess liquid is not discharged into them. All biological toilets must have the compost removed periodically. The frequency, which depends upon the type of toilet and the number of people using it, might vary from every three weeks to once per year. A biological toilet requires frequent examination and care so that it will continue to function in a satisfactory manner. Care and maintenance requirements vary with the different brands of toilets. It is advisable to obtain an accurate cost estimate from the supplier of the model you are interested in, as well as information about energy consumption, installation, maintenance and replacement. Energy costs may be appreciable for the year-round use of a composting toilet. The prices of composting toilets may range from $750 to $3,000, plus installation.
The second part is the graywater treatment system. Graywater is sewage that does not contain toilet wastes. Graywater systems receive, treat, and dispose of only graywater. Toilet wastes from the residence or other establishment have to be treated in some other system, or the residence has to have a privy. Graywater systems cannot accept garbage disposal waste. Graywater tanks are constructed to the same specifications as other sewage tanks , except that the liquid capacity of the tank is 40% smaller than if the tank was receiving toilet waste too. Aerobic treatment units can be used as part of a graywater. system; they have to meet the same requirements as for aerobic units serving any other systems. A picture of a grays water aerobic unit is shown below.
If a dwelling has a graywater system, it is a Class IV residence, and average daily flow is estimated as 60 percent of a similar house. Effluent from a graywater tank has to enter a soil treatment system for final treatment. This can either be a trench, bed or mound system. It cannot be discharged to the surface. Proper sizing of the soil absorption system is based on Class IV flows and the appropriate soil sizing factor.
Cesspools and seepage pits
The presence of improperly treated sewage is a threat to public health and the environment. Human exposure to sewage has resulted in disease outbreaks, severe illnesses, and in some instances death from the bacteria, viruses and parasites contained in the waste. Wastewater disposal systems that do not adequately treat wastewater also negatively affect our lakes, rivers, and groundwater by potentially introducing sediment, nutrients, and chemicals that result in contamination.
There is also a safety concern with many of these systems because the tank lids may collapse resulting in an unsafe environment for people, animals and infrastructure.
What is proper wastewater treatment?
In Minnesota, we are concerned about all of our water resources for both beneficial use and recreation. One way to minimize damaging our waters is to ensure effective wastewater treatment is achieved across the State.
Effective wastewater treatment is simply the removal of solids, nutrients, bacteria and viruses from the wastewater and the predictable acceptance of the treated waste into the natural environment. In the case of individual sewage treatment systems (ISTSs), this level of treatment and acceptance is fundamental to our ISTS design requirements.
Specifically what is required for this level of treatment has been researched for over 100 years and remains true today – three vertical feet of dry, well-aerated soil with a wastewater distribution network sized based on use (e.g. single family home, day care facility, etc.) and soil properties. Often times our older ISTSs placed too much importance on wastewater going away (i.e. disposal), without adequate understanding or concern for treatment.
There are two primary reasons why seepage pits and cesspools do not provide adequate wastewater treatment; size and depth. Sewage is discharged into a small diameter pit and causes the wastewater to disperse under saturated, anaerobic conditions, limiting soil treatment. The small size of these systems also increases the likelihood of sewage back-up into the dwelling and surfacing sewage. Many cesspools and seepage pits were intentionally sited with the bottom of the pit in groundwater, as the natural water movement carried the sewage away. Raw or partially treated sewage should never reach groundwater, as the impacts to an aquifer are similar to the damages in a ditch, stream, or lake. There have been numerous studies documenting contamination of ground and surface water from wastewater systems in contact with groundwater (Allen and Morrison, 1973; Anan'ev and Demin, 1971; Crane and Moore, 1983; Kligler, 1921; Vaisman, 1964).
Do I have system with a cesspool or a system with a seepage pit, drywell or leaching pit?
A cesspool is an underground tank with holes in the side and/or bottom through which wastewater is discharged. The wastewater seeps into the surrounding soil through the bottom and openings in the side of the pit. Some designs may have a septic tank prior to the leaky tank and if so, it is considered a seepage pit, drywell or leaching pit.
When and how do we fix noncompliant seepage pits and cesspools?
In Minnesota, noncompliant seepage pits and cesspools must be replaced with a compliant system. The time period for upgrade is based on local public health and environmental priorities and varies from location to location. Be sure to check with your local governmental unit (LGU). A list of MN LGUs can be found under "SSTS Local Units of Government" at: www.pca.state.mn.us
The noncompliant cesspool or seepage pit must be properly abandoned to eliminate the safety hazard and impact to public health and the environment. A licensed designer must be hired to evaluate site and soil conditions to determine the proper replacement ISTS to treat and disperse the wastewater at the site. This design is reviewed by a LGU to ensure it meets Minnesota Rules, Chapter 7080 and any additional LGU requirements. A list of licensed septic professionals can be found by contacting your LGU or under "SSTS Business Licensing, Individual Certification, and Enforcement" at: www.pca.state.mn.us
Allen, M.J. and S.M. Morrison. 1973. "Bacterial movement through fractured bedrock." Ground Water 11 (2): 6-10.
Anan'ev, N.I. and N.D. Demin. 1971. "On the spread of pollutants in subsurface waters." Hygiene and Sanitation 36 (8):292-294.
Crane, S.R. and J.A. Moore. 1984. "Bacterial pollution of groundwater: A review." Water, Air, and Soil Pollution. 22 (1): 67-83.
Kligler, I.J .1921. "Investigation on soil pollution and the relation of the various types of privies to the spread of intestinal infections." p. 1- 75 in ( ed.) Monograph of the Rockefeller Institute for Medical Research. Vol. No.15 .The Rockefeller Institute for Medical Research, New York.
Vaisman, Y .I. 1964. Hygiene and Sanitation. 29:21.
Systems for small lots and poor soils
Septic systems are a safe and effective soil based system to treat household wastewater provided there's enough soil area and soil conditions are conducive to treatment. Septic systems treat sewage as well as or better than municipal treatment facilities when they are properly designed, installed and maintained.
There are many situations in wet soils or on small lots where traditional septic system designs must be modified or they won't work. Space problems are common in small rural towns, around lakes, and in existing or new small-lot suburban developments. Poor or wet soil conditions may occur in any of these same areas, as well as in wide-open spaces.
The soil treatment portion of a septic system, often referred to as a "drainfield", is the most important part of the treatment. It kills the disease-causing pathogens and filters out most of the nutrients in the sewage. All of this depends on having two to three feet of unsaturated soil and separation from bedrock. Treatment will not occur if untreated wastewater is allowed to leak into bedrock or enter soil that is filled with water at any time during the year. This separation determination is made at the time the system is designed by doing a soil boring.
If you don't have the two to three feet of separation necessary for a traditional trench drainfield, the treatment area must be raised (mound), relocated on the property, or the raw sewage stored and hauled off of the property (holding tank). If the individual property is not large enough or does not have the right soil conditions, additional property may be needed. Two or more property owners may join together to locate an appropriate treatment site nearby.
Several treatment alternatives are available. Individual or multiple-household septic systems may utilize an in-ground trench, an at-grade trench, a mound, a constructed (lined) wetland, or drip or spray irrigation system to disperse and treat septic tank effluent. Special enhancement devises may be added to improve the performance or allow for the modification of the system. These include peat or sand filters, aerobic septic tanks, or the separation of solid wastes into a composting system. Current research and trial systems are evaluating the effectiveness of these units. Some of them may result in reduced size requirements and/or smaller soil separation distances.
The importance of proper operation and maintenance is increased as multiple-household systems and special enhancements are introduced. Private interests such as homeowner associations, private joint ventures, or water quality cooperatives can do this management. Public management options include municipal utilities, sanitary sewer districts or Environmental Subordinate Service Districts.
In solving sewage treatment problems on small lots or in difficult soil conditions, it is very important to remember that the ultimate goal is to "achieve proper sewage treatment for the protection of human health and the environment". The most cost- effective solution to the problem may not be "one system for everyone" but rather a combination of treatment and management options.
More details about treatment and management options are available from your local planning and zoning or environmental services office.
Alternative sewage treatment systems
Traditional septic systems are typically thought of as an on- site sewage treatment system serving one household with a drainfield or mound. These are probably still the best way to treat sewage when space and good soil conditions exist. When properly designed, installed, operated and maintained, they treat sewage as well as or better than municipal treatment systems.
When space is limited or soil conditions are poor (wet or close to the water table), homeowners may need to consider a modified treatment system. Because a typical septic system does not remove all of the nitrates from sewage, additional treatment steps may be used to reduce or eliminate them. These options may be considered for individual homes or multiple household units.
Typically, alternative treatments provide "pre-treatment" of septic tank effluent before it enters the soil of a drainfield or mound. These pre-treatment systems include containers using sand, peat or gravel as a medium where filtration and biological degradation of fine solids, pathogens and nutrients occur. The containers may be manufactured or assembled on the site.
The effluent in a pre-treatment system usually passes through the system one time, but some systems collect and re-circulate the effluent several times. These systems often require more space than a traditional drainfield or mound. Research has still to determine if the size or the separation distance of the drainfield receiving the pre-treated water will be able to be reduced. These same considerations are being made when aerobic septic tanks are used to pre-treat wastewater.
Alternative methods of dispersing septic tank effluent into the soil are also being tested. These include drip irrigation over a large soil area, particularly appropriate in shallow bedrock and high water table situations. Spray irrigation onto the soil surface is another option, but presents special health risks with potential human contact. Uses of the water for watering lawns and golf courses also offer opportunities to recycle the water.
Separation technology is another concept in alternative treatment for individual homes. The idea of separating the solid wastes from the toilet and delivering them to a composting unit reduces household water use up to 40 percent and removes many of the pathogens and nutrients from the system. Some systems use worms, while others use bacteria and aeration to accomplish the composting process. Homeowners are sometimes reluctant to have a bin of composting wastes and even worms in their basements. The composted materials must be removed periodically.
Alternative treatment systems will always require increased attention to operation and maintenance. As the treatment becomes more sophisticated and technical, the need for monitoring of its performance increases. Multi-household installations must have a functional management plan.
Most alternative treatment systems require special permits for their design, installation and operation. Any system discharging to the surface must have a State Disposal System permit and any system discharging 10,000 gallons per day must have a National Pollutant Discharge Elimination System permit. Both require on-going monitoring. Information about permits is available from local planning and zoning or environmental service offices.