Occasional inspection of the sludge levels in tanks, or regularly scheduled pumping intervals, are thus important for maintaining the proper functioning of septic tanks. During the past five to ten years at least two different types of proprietary filters have been developed for use in septic tanks. These have become increasingly used to filter the effluent prior to discharging either to the disposal field, or further treatment processes. One type of filter uses a series of plastic trays, and another filters the effluent through a series of long tubes and screens as the effluent flows upward through the outlet pipe of the septic tank. These filters can provide for enhanced solids removal, with associated BOD reduction.
Manufacturers recommend that the filters be cleaned once every two to three years.
Technology Status Septic tanks are used by 25 to 30 percent of the residences in the United States. It is a well-developed technology. If site conditions are such that primary treatment is not sufficient prior to final onsite subsurface disposal, then additional treatment will be necessary if the effluent is to be disposed of onsite. Performance Performance of septic tanks is a function of proper sizing, design, and installation, as well as use which is in keeping with the design assumptions. Some organic nitrogen removal occurs through the solids removal process.