The impact of phosphorus discharged from septic systems can vary widely between different systems and different watersheds
Phosphorus is a chemical element and nutrient essential for all life-forms. Organic phosphorus is formed primarily by biological processes, and its sources in sewage include bodily wastes, food residues, and the conversion of orthophosphates in biological treatment processes. Phosphorus is found in body wastes, food residues, fertilizers and detergents.
Total phosphorus is the most commonly measured analysis you would request from a laboratory. It's the sum of all forms of phosphorus in effluent, and the results are provided in milligrams per liter. It is important to note that septic systems are only one of the sources contributing phosphorus into the environment; and depending on the watershed and quality of the septic systems, the impact may be minimal or significant.
Elemental phosphorus occurs in natural water and wastewater almost solely as phosphate (a group of phosphorus and oxygen atoms with an electric charge, in varying formations). Phosphorus is a nutrient essential to the growth of plants and microorganisms.
The concern with phosphorus in most freshwater is its impact on surface water. Most freshwater is phosphorus-limited, meaning that any additional phosphorus will result in the growth of more plant life. Growth of algae and weeds dramatically affects lake ecosystems, lowering their water clarity, oxygen levels, aesthetic appeal and recreational value. Algae blooms and heavy growth of emergent vegetation not only make surface water bodies unappealing for recreation, they also threaten the health of fish and other aquatic creatures. During algae blooms, there can also be the dispersal of toxic compounds (hepatotoxins and neurotoxins). Impaired waters for phosphorus are common across the U.S.
Phosphorus in septic systems
Under many septic systems, groundwater is discharged to surface water. Therefore, it is important that phosphorus is removed before reaching surface water. Phosphorus retardation in soil absorption areas is dependent upon sorption and precipitation reactions. Precipitation occurs as the phosphorus reacts with calcium, aluminum, magnesium or iron in the soil. It can also move in surface water or in groundwater during erosion episodes or under anaerobic soil conditions.
In a conventional septic system, phosphorus removal starts with pretreatment in the septic tank. The primary removal mechanism is the settling of solids containing phosphorus in the sludge or floating of soaps, although some precipitation may occur as well. This removal is typically 20 to 30 percent. Remaining phosphorus is removed from the septic tank effluent by two processes:
- Absorption involves the formation of a two-dimensional structure on the surface of a mineral (iron, calcium and aluminum) rather than within the mineral itself. Surface adsorption is usually limited by a fixed availability of sorption sites in a particular soil that eventually will be used up if sewage loading occurs over long periods. When the adsorption sites are filled, newly added phosphorus must travel deeper in the soil to find fresh sites. Soils that are higher in clay content have more surface area and binding sites on the soil particles than soils that are high in sand.
- Mineral precipitation involves the formation of a three-dimensional solid phase arrangement of molecules from the solution phase. Iron, calcium and aluminum are minerals that chemically bind with phosphates. Compared to absorption, precipitation is potentially sustainable, provided that the supply of minerals necessary to complete the reaction is sufficient.
The characteristics of the soil, wastewater and site influence the degree to which phosphate is retained beneath the soil treatment area. If the soil treatment system has adequate mineral content and a sufficient zone of separation before limiting conditions such as water tables, bedrock or coarse soils, and proper setbacks are maintained from surface waters, then problems from phosphorus movement to surface water or groundwater should be minimal.
In situations where these characteristics don't exist, additional steps in the pretreatment prior to the soil may be needed.
About the author
Sara Heger, Ph.D., is an engineer, researcher and instructor in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota. She presents at many local and national training events regarding the design, installation, and management of septic systems and related research. Heger is education chair of the Minnesota Onsite Wastewater Association and the National Onsite Wastewater Recycling Association, and she serves on the NSF International Committee on Wastewater Treatment Systems. Ask Heger questions about septic system maintenance and operation by sending an email to firstname.lastname@example.org.