Parameters to Monitor Septic System Performance

By Sara Heger 

Monitoring Wastewater Characteristics

There are many characteristics to monitor an on-site wastewater treatment system’s performance. They vary from something as simple as checking for sewage on the surface, to complicated laboratory analysis. All cost and required amount of sample vary from lab to lab, but estimates are given. Be sure to contact a lab prior to dropping off samples.

Certified Labs

When choosing a lab to perform analysis of wastewater characteristics a certified lab is always the best choice. These labs use standard procedures. The Minnesota Department of Health maintains a list of labs across Minnesota that are certified. This can be found on their Web site (

Sampling Locations

There are many locations where samples can be taken. It is best if the sample locations are determined when the system is being designed. These sample locations must be built into the design. Effluent chambers, pump tanks and designed sampling ports are suggested locations to obtain samples.

Some obvious locations where the wastewater characteristics are of interest are:

  • Out of home
  • After septic tank.
  • Sampling at system’s "end-of-pipe"
  • Sampling groundwater (lysimeter, sampling wells)
  • Sampling soil (dry g/microgram fecal)

Piezometers can be used to determine the amount of separation. Lysimeter or soil access ports can be used to determine the amount of fecal under system.

Monitored Parameters

Biochemical Oxygen Demand (BOD5) is the most widely used parameter applied to wastewater. It is a measurement of the dissolved oxygen used by microorganisms in the oxidation of organic matter in sewage in five days. Because of the timeliness of these results the samples for a BOD5 test must be run within 24 hours of taking the sample. An average cost for a BOD5 test is $12. A minimum of 500 milliliters is required to run the test. A typical BOD5 value for septic tank effluent is 100- 250 milligram per liter (mg/l).

Color is an indication of how ‘clean’ the wastewater is. A black sample represents wastewater that is anaerobic and still need significant treatment. A clear sample represents a sample where the BOD5 and TSS have been minimized. The amount of fecal coliform cannot be estimated with a visual inspection.

Dissolved Oxygen (DO) is a measure to determine how much oxygen is in wastewater. Septic tanks usually have very low values of DO because the microorganisms in the septic tank use up all oxygen initially present. A typical value for DO in a septic tank is less than 1 mg/L. This can be measure with a probe or kits are available which evaluate the DO by comparing the color of sample after a chemical is added. The DO must be measured when the sample is taken or soon after because the level will decrease over time.

Fecal Coliform
 is an indicator organism. There are many pathogenic organisms present in wastewater. They are difficult to isolate and identify. The intestinal tract of man contains countless coliform bacteria. The presence of fecal coliform organisms, which are easily tested for, is an indication that pathogenic organisms may be present. The number of fecal coliform organisms will change over time; therefore fecal coliform test must be run within 6 hours of taking the sample. An average cost for a fecal coliform test is $16. A 500-milliliter sample is sufficient. An average value for septic tank effluent is 100,000- 100,000,00 cells/100 milliliter.

Fats, Oils and Grease (FOG)
 are added to wastewater through the use of butter, lard, margarine, vegetable oil, and meat. A typical value for FOG from a septic tank is 10 — 50 mg/L. A restaurant can produce very high values, often greater than 100 mg/L. A quart of more of the effluent is required to run this test. An average cost of this test is $42. The cost is so high because of the chemicals required for this test.

 is a nutrient essential to the growth of plants and microorganisms and in high levels can be toxic to humans. Wastewater naturally contains fairly high levels of nitrogen, typically in the range of 50-90 mgN/L. It is found in four different forms: organic nitrogen, ammonia, nitrite and nitrate. High nitrate (greater than 10 mg/L) and nitrite (greater than 1 mgN/L) in drinking water (greater than 10 mg/L) can cause blue baby syndrome in infants, which is potentially fatal blood disorder. At high concentrations of ammonium and if the pH gets about above 9, unionized ammonia may be formed which is toxic to fish and other aquatic animals. High ammonium also adds to the BOD load of the effluent when it is oxidized to nitrate. Typical ammonium levels in the septic tank effluent are 30-50 mgN/L. A 100-milliliter sample should be sufficient for a nitrogen test. An average cost for a nitrogen test is $10.

 is a nutrient essential to the growth of plants and microorganisms. This nutrient may cause increased growth of aquatic vegetation and algae in surface waters that can result in eutrophication impacts. A typical value for septic tank effluent is 7- 20 mg/L of phosphorus. A 100-millilter sample is needed at a minimum. An average cost for a total phosphorus test is $10.

Total Suspended Solids (TSS)
 is a measure of the organic and inorganic solids, which remain in wastewater after separation occurs in the septic tank. Typical suspended solids values of septic tank effluent range from 20-140 mg/L. A 100-millilter sample is needed. An average cost for a TSS test is $4.

 of wastewater is a very important parameter because of its effect on chemical reactions. Temperature of wastewater varies from 45-70 ° F depending on the season. Wastewater temperature is usually not a problem for individual residents, but very low or very high temperatures can be a problem from restaurants and infrequently used homes. The temperature of the wastewater must be done when the sample is being taken.

Turbidity is a measure of the light-transmitting properties of water. It is another test to easily measure the quality of waste with respect to suspended matter. Turbidity can be roughly measured in the field or a lab analysis can be performed. A 100-milliliter sample should be collected. An average cost of the lab test is $4.

Odor is often detected when a system is not performing properly. A properly functioning system will have little or no odor.

Flow Meters
 are used to measure the volume of wastewater going to an on-site system. They are usually located in the basement. This data, when collected on a regular basis can indicate how much water is being delivered to the on-site system. A typical flow for a 3-bedroom home is 450 gallons per day, but this is highly variable depending on the water conservation measures used by the residents.


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Abney, Jack, L. Selection of an Appropriate Wastewater Disposal System. Proceedings of the Seventh National NSF Conference 1980. Ann Arbor, MI. 1981

Canter, Larry, W. and Robert C. Knox. Septic Tank System Effects on Ground Water Quality. Lewis Publishers, Chelsea, MI. 1986.

Crites, R and G. Tchobanoglous. Small and Decentralized Wastewater Management Systems. McGraw-Hill. Boston, MA. 1998.