By Jared Alder, MS
There has been a big focus in domestic wastewater on the removal of phosphorus and the potential for excess phosphorus to cause eutrophication in receiving water. Treatment facilities of all different shapes and sizes with inadequate phosphorus treatment technologies have the potential for excess phosphorus release.
The removal of phosphorus from wastewater can be performed using physico-chemical methods, biological treatment, and/or combinations of both. Physico-chemical processes of phosphorus removal have been widely used. Such physico-chemical processes are generally effective, reliable, and do need a lot of large capital equipment; however, they are not without limitations. For example, adding chemicals to treatment processes can impact the pH of the treatment process, thus resulting in the need for additional chemicals to adjust the pH before the treated water can be discharged. In some cases, because of the chemical usage, a chemical sludge can be created and there may need to be additional treatment steps for removing the sludge.
The most common chemical phosphorus removal options utilize dosing metal salts—such as ferric chloride as part of pre-treatment—into activated sludge reactors or as part of the secondary clarifier process. Ferric chloride (or similar metal salts) precipitates phosphorus in the wastewater and the resulting solids residuals are removed either by settling under gravity or by filtration. The subsequent precipitates may be rich in phosphorus but, since it is chemically bound, it can make recovering the phosphorus challenging—which presents a disadvantage over Enhanced Biological Phosphorus Removal (EBPR) systems because it reduces the economic benefits of the phosphorus-rich sludge.
Removal rates for phosphorus are typically proportional to the mass of the chemical added, which influences the amount of extra solids produced; therefore, a balance between the two is critical. Phosphorus effluent concentrations of 1 mg/L or greater can generally be achieved by gravity settling. Techniques including filtration and tertiary-ballasted flocculation are sometimes combined with metal-salt dosing to achieve lower phosphorus levels, even down to concentrations of < 0.50 mg/L.
Since chemical dosing is generally reliable and widely accepted, it is the most commonly used treatment option.