The goal of nutrient reduction is to produce an effluent quality to meet effluent limitations for phosphorus, ammonia nitrogen and total kjeldahl nitrogen (TKN). All designs should be based on pilot plant studies or full scale operating data obtained at design loadings.
Nutrient control processes include:
a.) Natural Systems - Aquatic plant removal (APU) and proper plant management.
b.) Suspended growth systems with adequate sludge treatment and management.
c.) Attached growth system.
d.) Covered anaerobic ponds.
e.) Packed bed filters.
Aquatic Plant Systems are derived from natural treatment and typically involve three phases: aquatic plant growth, harvesting and management. Design should be based on seasonal climate and available sunlight in accordance with the provisions of this chapter. The basin or channel design should achieve the required removal rate at the minimum encountered liquid temperature and include sufficient capacity to achieve permit requirements during periods of low temperatures and little or no sunlight.
Biological nitrification is a process whereby autotrophic nitrifying bacteria convert ammonia nitrogen to nitrate nitrogen. This process is capable of removing most of the nitrogenous oxygen demand from domestic wastewater but does not remove the nitrogen itself. Should nitrogen removal be required, dentrification facilities must follow nitrification facilities. Although the nitrification phenomenon has been observed for some time, unit process design for optimum nitrification performance has only recently been employed. Optimum nitrification factors include wastewater temperature and pH, as-well-as maintaining nitrifying organisms (nitrobacter and nitrosomonas) at a sufficient level in the reactor biomass. The mean cell residence time required to develop a well nitrified effluent will exceed 10 days and may range up to 30 days at colder temperatures. The demand for dissolved oxygen will increase significantly during nitrification. Denitrification can be achieved through an activated sludge biomass subjected to anoxic conditions to promote the reduction of nitrate nitrogen to nitrogen gas which escapes to the ambient air. Anoxic conditions are defined as a dissolved oxygen level of 0.2 mg/l or less and a nitrate nitrogen level exceeding 0.2 mg/l.
Complete denitrification can recover fifteen (15) percent or more of the dissolved oxygen utilized for complete nitrification. In addition, denitrification can recover approximately one-half of the alkalinity utilized for nitrification. A sufficient level of carbonaceous energy in the form of a biodegradable organic substrate must be provided to the anoxic reactor to achieve the design denitrification potential. The degree of nitrogen removal will be a function of the ratio or the carbonaceous energy level available, to the level of TKN oxidized to nitrate nitrogen. The minimum ratio of influent total (5-day) BOD to TKN appears to be approximately ten (10) or more to achieve effluent levels of ten (10) mg/l or less of total nitrogen.
Phosphorus removal typically involves the use of activated sludge biomass exposed to varying levels of dissolved oxygen. Anaerobic conditions serve to select organisms that release phosphorus and store carbonaceous substrate. Biomass is processed through anaerobic conditions to a combination of anoxic and aerobic conditions. The subsequent exposure to dissolved oxygen results in biological metabolism of stored organics with subsequent uptake and storage of phosphorus by the biomass.
The efficiency of biological phosphorus removal is highly dependent on the influent levels of phosphorus and biodegradable substrate (BOD or COD). The optimum ratio of process influent total (5-day) BOD to phosphorus appears to be approximately twenty (20) to achieve final effluent levels of phosphorus of one (1) mg/l or less.