This invention relates to septic and anoxic tanks, and in particular, to a media added to said tanks to enhance denitrification of effluent.
A septic tank, also termed an anoxic tank, typically provides primary treatment for domestic wastewater where municipal treatment facilities are unavailable. In a conventionally operated septic tank, raw untreated sewage wastewater having a significant concentration of waste solids is introduced into the tank from an adjacent building. In the septic tank, solids separate from the liquid portion of the sewage. Solids having a lower density than the liquid move to the top of the liquid to form a scum layer, and solids having a higher density than the liquid sink to the bottom of the tank to form a sludge layer, resulting in a relatively clear liquid layer between the scum and the sludge. The liquid portion of the wastewater, which exits the discharge end of the tank by means of gravity, a pump, or a siphon, is the septic tank effluent. The quality of the septic tank effluent is generally measured by the biochemical oxygen demand (BOD), total suspended solids (TSS), and total nitrogen present in the effluent. The solids are periodically removed by having the tank pumped out and disposed in a facility specifically designed for solid disposal. Without secondary treatment, the septic tank effluent directly enters a drain field from which it is dispersed by percolation into the surrounding soil, with a certain amount of aerobic conversion taking place at this point. The effluent then migrates through the soil into the ground water, or sometimes into adjacent surface water, such as a stream or lake.
Nitrogen in raw untreated wastewater is primarily organic nitrogen combined in proteinaceous material and urea. Decomposition of the organic material by bacteria present in the anaerobic environment of the septic tank changes the organic nitrogen to ammonia nitrogen. Thus, in conventionally treated septic tank effluent, nitrogen is present primarily as ammonia nitrogen.
It has become more common now to provide secondary treatment of the septic tank effluent before disposing of the effluent in a drain field. Secondary treatment of septic tank effluent is typically an aerobic treatment. In addition to its reduction of BOD and TSS, the aerobic environment of secondary treatment causes bacteria to oxidize ammonia nitrogen to nitrate nitrogen, a process known as nitrification. Thus, in a conventional system, nitrogen in the secondary treatment effluent is primarily nitrate nitrogen. The secondary treatment effluent is recycled back to the septic tank. However, the nitrogen content of the secondary treatment effluent is still often unacceptable. It is desirable to minimize the nitrate concentration of the secondary effluent to about 10 mg-N/L, which is the nitrate concentration allowed by the U.S. EPA drinking water standards before disposing it into a drain field.
In order to remove nitrogen from the system, it is desirable to convert nitrate nitrogen to a readily removable gaseous form of nitrogen. This conversion can be accomplished biologically under anoxic (without oxygen) or anaerobic conditions by several genera of denitrifying bacteria. Denitrifying bacteria are capable of converting nitrate to nitrite, followed by production of nitric oxide, nitrous oxide and nitrogen gas. The latter three compounds are gaseous products that are released to the atmosphere and are thus removed from the system. The conversion is known as denitrification.