By Sara Heger, Ph.D.
As referenced in a previous article, most septic systems remove phosphorus by absorption and precipitation in the soil treatment system. Septic professionals and property owners should consider first if additional phosphorus removal is needed.
Typical water body setbacks are established to limit the movement of phosphorus (P) to freshwater bodies. Often, when septic systems are impacting water bodies it’s because they are not currently meeting treatment standards. On sites that have limited appropriate soil and/or high water tables, what can be done to enhance phosphorus removal?
Phosphorus removal from wastewater can be achieved using multiple techniques. Included are precipitation, ion exchange and biological methods. Common to all techniques is the need to physically remove the phosphorus from the system. Unlike nitrification followed by denitrification where nitrogen gas is formed, phosphorus cannot practically be converted to an inert gas that passively exits the system. Unfortunately, most P-removal technologies have been developed for use at larger wastewater treatment plants and have yet to have mainstream application in the septic system industry.
Below is an overview of the options and status of these technologies.
- Precipitation can be achieved with a chemical addition to the wastewater or by adding enriched media into a filter bed or possibly even the soil.
- Chemical precipitation is used to remove phosphorus from wastewater. In this process, aluminum and iron salts are commonly used to convert the phosphorus into insoluble compounds, primarily through a precipitation reaction. The addition of treatment chemicals may increase the volume of waste sludge up to 50% and obviously create septage/biosolids with elevated levels of phosphorus. Typical chemical precipitation is difficult as a feed pump and the removal of precipitates would be routinely required. Alum injection in a septic tank could require tank pump-outs several times per year. To date this technology has had very limited use in larger scale septic systems.
- Iron rich media has been added to soil treatment systems, constructed wetlands and media filters to absorb and precipitate out the phosphorus. Media that has been researched to remove phosphorus typically contains iron oxides, zerovalent iron, and/or aluminum oxides. The media’s selectivity and effectiveness depends on other ions that are present, pH, dissolved oxygen levels, contact time, and the relative concentrations of the constituents. There are several commercial units on the market that can be added to septic systems, but installation has been limited.
- Ion exchange has been studied as a potential option to remove phosphorus but to date no septic system call applications have been developed. Research has shown good performance through pretreatment of ion exchange media with ferric oxide or aluminum hydroxide to increase selectivity for phosphate ions resulting in high P-removal rates, but more work is needed to bring this concept to market.
- Biological phosphorus reduction requires uptake by microorganisms and is achieved using an anaerobic/aerobic sequence in a biological reactor that allows for microorganisms to accumulate high levels of phosphorus. The phosphorus is then removed with the excess biosolids. If biosolids accumulate for an extended period, phosphorus may be released back into the water. A challenge with this type of system is the required routine maintenance requiring biosolids removal, making it impractical for single family homes — but it may be an option for community or commercial systems. The use of microalgae systems for the treatment of wastewater has also been tested for P removal but testing at the septic system scale is very preliminary at this time.
Although there are many ways to potentially remove phosphorus from wastewater, very few pretreatment systems have been designed to be added into septic systems. To date iron rich media or the addition of iron into systems are the only commercially available technology developed. When evaluating new options, the capital, operational and environmental costs associated with the application of new technologies should be assessed.