TP Total Phosphorus

Phosphorus is a constituent of human wastewater, averaging around 10 mg/liter in most cases. The principal forms are organically bound phosphorus, polyphosphates, and orthophosphates. Organically bound phosphorus originates from body and food waste and, upon biological decomposition of these solids, is converted to orthophosphates. Polyphosphates are used in synthetic detergents, and used to contribute as much as one-half of the total phosphates in wastewater. Massachusetts has banned the sale of phosphate-containing clothes washing detergent, so phosphorus levels in household wastewater have been reduced significantly from previous levels. Most household phosphate inputs now come from human waste and automatic dishwasher detergent. Polyphosphates can be hydrolyzed to orthophosphates. Thus, the principal form of phosphorus in wastewater is assumed to be orthophosphates, although the other forms may exist. Orthophosphates consist of the negative ions PO₄^3-, HPO₄^2-, and H₂PO^4–. These may form chemical combinations with cations (positively charged ions).

It is unknown how much phosphorus is removed in a conventional septic system. Some phosphorus may be taken up by the microorganisms in the septic system and converted to biomass (of course, when these microorganisms die the phosphorus is re-released, so there really is no net loss of phosphorus by this mechanism). Any phosphorus which is removed in the septic system probably is removed under the leaching facility by chemical precipitation.

At slightly acidic pH (as is found in the soils of Cape Cod and most of New England), orthophosphates combine with tri-valent iron or aluminum cations to form the insoluble precipitates FePO₄ and AlPO₄.

Domestic wastewater usually contains only trace amounts of iron and aluminum. However, the sandy soil of Cape Cod frequently contains significant amounts of iron bound to the surface of sand particles. It is likely that this iron binds with phosphorus and causes some removal of total phosphorus below the leaching facility.

One caveat must need be added here. If the soil below the leaching facility becomes anaerobic, iron may become chemically reduced (changed to the Fe²⁺ form), which is soluble and able to travel in groundwater. In this case, the iron phosphate compounds may breakdown and phosphorus may also become soluble. Anaerobic conditions under the leaching facility can occur when the leaching facility is not well aerated, when there is a small vertical separation to groundwater, or when BOD in the sewage is so high that all oxygen present is depleted to oxidize BOD. In the conditions found on Cape Cod, the best method for maximizing phosphorus removal is probably to locate the leaching facility well above groundwater (>5 feet vertical separation) thereby providing a well-aerated area under the leaching field. To date, no alternative on-site technologies are capable of significant phosphorus removal. However, many are trying to achieve this goal and it is likely that within the next few years we may begin to see some technologies that are successful at phosphorus removal.