Nutrient Removal Techniques | Activated Sludge Process | Wastewater Treatment Plant

NUTRIENT REMOVAL TECHNIQUES

Nutrient removal generally implies the removal of nitrogen or phosphorous. Nitrogen and phosphorus in wastewater effluent can have a fertilizing impact on the receiving water. This can lead to excessive growth of algae in the receiving water. In fresh water impoundments, this can result in green “pea soup” conditions. Also, as algae die off and sink to the bottom of the impoundment, cell decomposition can lead to hypoxia (lack of dissolved oxygen) in the lower levels of the water, which is extremely deleterious to the aquatic environment. As a general rule, it has been found that phosphorus is the limiting nutrient leading to eutrophication in freshwater aquatic systems and nitrogen is the limiting nutrient in marine (salt water) environments. Therefore, depending on the receiving water, there may be a need to meet low effluent requirements for N, P, or both (e.g., discharge to freshwater river impoundments that ultimately discharge to tidal marine waters).

Phosphorous has become more of an issue in recent years due to nonpoint discharges to rivers and streams causing significant algal blooms. Phosphorous control initially came about as a regulatory concern because nitrogen discharge control did not reduce the incidence of algal blooms in all receiving waters to the degree expected. Further investigations indicated a phosphorous limited growth phenomenon rather than, or in addition to, a nitrogen limited growth phenomenon. Consequently, phosphorous has been subjected to much greater scrutiny of late, and processes to reduce phosphorous to previously unattainable concentrations are being developed, and improved regularly.

Phosphorus is removed either chemically or biologically. Both processes involve changing a solubilized or liquid form of phosphorus (generally an ortho-phosphate) to a solid form. Chemically this is most often accomplished using either an iron salt (such as ferric chloride or ferrous sulfate) or an aluminum salt (such as alum, sodium aluminate, or poly-aluminum chloride). Lime can also be used to precipitate phosphorous through an increase in the pH in the water. This does not generally reduce the concentrations low enough to meet regulatory standards, however, and then requires an additional recarbonation step to lower the pH. Any chemical process also requires chemical storage facilities, chemical feed pumps, and extra equipment maintenance, as well as larger or additional sludge removal and processing equipment.

Biologically, phosphorus is removed from solution and concentrated inside bacterial cells. Treatment involves two different processes. The first is a fermentation zone where short chain organic compounds are created, notably volatile fatty acids (VFAs) that are attractive energy sources for phosphate accumulating organisms (PAOs). This zone also provides the opportunity for the uptake of those volatile fatty acids by the single celled PAOs. The second step requires an oxygen-rich zone where the PAOs oxidize the volatile fatty acids that they ingested in the fermentation zone as an energy source, thereby removing the soluble phosphorus from the solution.

TSS control is important during phosphorous removal because both chemical and biological phosphorus removal results in TSS containing up to 5 percent total-phosphorus. Achieving a total-P limit of 0.2 mg/L, which assumes a residual, post treatment, soluble phosphorus concentration of 0.05 mg/L, requires an effluent TSS concentration of no more than 3 mg/L.

Nitrogen is almost universally removed biologically. The process results in the conversion of solubilized (liquid) organic nitrogen (such as urea, which is the most common form of organic nitrogen) to nitrogen gas. The nitrogen escapes harmlessly to the atmosphere; ambient air is 75 to 80 percent nitrogen gas.

In some unusual cases, the organic nitrogen readily converts to ammonium during normal treatment at the treatment plant. Those wastewater treatment plants convert ammonia to nitrate (with nitrite as an intermediate product) in highly aerobic conditions such as those created by two stage trickling filters or extended aeration treatment plants. The nitrate is then converted to nitrogen gas under anoxic (low oxygen) conditions. Unlike phosphorus removal, nitrogen removal does not result in a sludge that must be removed since the nitrogen escapes into the atmosphere as a gas.