Several weeks ago in my post Nutrient Overload, I discussed the problems that wastewater facilities face when dealing with phosphorous in their water sources. Today, I will continue that discussion by writing about the biological removal of phosphorous from wastewater sources.
By definition, eutrophication is “the process by which a body of water acquires a high concentration of nutrients, especially phosphates and nitrates. These typically promote excessive growth of algae. As the algae die and decompose, high levels of organic matter and the decomposing organisms deplete the water of available oxygen, causing the death of other organisms, such as fish.” This may occur naturally, but it can also be the result of human activity, including cultural eutrophication from fertilizer runoff and sewage discharges.
Removing phosphorous from wastewater can be expensive and complex. To remove phosphorous using biology, you must alter how you treat the wastewater and how you set up the aeration in your facility. Biological phosphorous removal is less expensive because of reduced chemical costs, as well as less sludge production and disposal, as compared to chemical precipitation.
In the biological removal of phosphorous, the phosphorous in the influent wastewater is incorporated into cell biomass, which is subsequently removed from the process as a result of sludge wasting. The reactor configuration provides the phosphorus accumulating organisms (PAO) with a competitive advantage over other bacteria. Consequently, the PAO are encouraged to grow and consume phosphorous. In this circumstance, the reactor configuration is comprised of an anaerobic tank and an activated sludge tank.
The retention time in the anaerobic tank is about 0.50 to 1.00 hours, and its contents are mixed to provide contact with the return activated sludge and influent wastewater. Under anaerobic conditions, PAOs assimilate fermentation products (i.e. volatile fatty acids) into storage products within the cells with the simultaneous release of phosphorous from stored polyphosphates.
In the aerobic zone, energy is produced by the oxidation of storage products, and polyphosphate storage within the cell increases. The stored volatile fatty acids (VFA) are metabolized, providing energy from oxidation and carbon for new cell growth from the biochemical oxygen demand in the influent. The energy released from VFA oxidation is used to form polyphosphate bonds in the cell for polyphosphate storage.
The soluble orthophosphate is removed from the solution and incorporated into polyphosphates within the bacterial cell. VFA utilization also enhances cell growth, and this new biomass with high polyphosphate storage accounts for phosphorous removal. As a portion of the biomass/sludge is wasted, the stored phosphorous is removed from the treatment plant for ultimate disposal as waste sludge.
The process of phosphorus removal from municipal wastewater effluent has become a high priority due to the impact it has on our lakes, rivers and streams. Biological removal is a viable option available to municipalities across the Commonwealth. For more information, see our wastewater treatment training manual located on our test preparation materials page.