Progressive Anaerobic and Aerobic Treatment of Domestic Wastewater

Using raw sewage from a sewage treatment plant, this study was conducted to evaluate the performance of specially designed Anaerobic/Aerobic Treatment Units (AATU) developed for residential and commercial applications.The treatment train consisting of an anaerobic filter chamber and contact aeration chamber is common for small-scale wastewater treatment systems in Japan. Almost 30,000 onsite wastewater treatment systems of this type are installed monthly there and the stable function of this system is confirmed. In order to fit the life style in the U.S. and to enhance more stable removal of BOD, SS and nutrient, the structure of the traditional Japanese AATU was modified for this study.Raw sewage was delivered from the influent pump station of the sewage treatment plant to a 1000 gallon precast concrete septic tank. Then, specially designed air displacement pump installed in the septic tank transfers constant amounts of wastewater to the AATU with the treatment capacity of 500 gallons per day.The purposes of this paper are to:describe the air displacement pumps andcompare performance of several small scale wastewater treatment systems.The air displace pump is a device to dose uniform amounts of liquid by releasing air stored in an air tank into an airlift pipe intermittently. These pumps are called the Geyser™ pump and used for transporting wastewater from septic tanks to the AATU. Compared to ordinary airlift pumps, it creates higher lift with smaller volumes of air. The Geyser pump doesn't have any mechanical or moving parts, and it is operated by a small air pump, so that it is durable and energy efficient.An anaerobic filter chamber and two contact aeration chambers are cascaded in two AATU's. The anaerobic filter chamber is filled with plastic media. The influent flow is applied to the top of the chamber. The media provide surface area for biological growth and effectively prevent short circuit. The anaerobic biological action is similar to that of a septic tank, but greatly enhanced by the presence of the media. Filtration of suspended solids contained in the influent flow also occurs, and any entrapped biomass will also contribute to denitrification. Various kinds of packing are available in the market for trickling filter or stripping, and one of them was selected as anaerobic media. Optimum conditions should be selected for the size of void, structure and configuration of media, and flow pattern inside the anaerobic filter chamber to avoid clogging and to enhance solid separation. Headloss occurs as denitrifying bacteria grow on the media surface. Comparing to the ordinary effluent filters used in the U.S., as anaerobic filter media is used as 3-dimensional filter, it has more capacity to hold and store solids without clogging, and cumbersome cleaning is not necessary. Periodical pump-up and resuspension of sludge is enough to clean.The partially treated wastewater flows from the anaerobic filter chamber into the first contact aeration chamber, then the second contact aeration chamber, where the fixed media process and the activated sludge process are integrated. As an aerobic media, corrugated plastic sheets are used. Treated water in the aerobic treatment is returned to the anaerobic part for removal of excess sloughed biofilm and denitrification. Liquid is returned from the second contact aeration chamber to the anaerobic filter chamber in AATU 1, and from the clarifier to the septic tank in AATU 2. The Geyser pump installed in the chamber transfers the treated water 3 to 4 times the influent flow.Sewage has been fed to the AATU's at the flow rate of 0.347 gallon/min (500 gallon/day) since September 1999. Almost 99% of the raw sewage used is domestic wastewater and the monthly mean of influent BOD, SS and TN are 272, 239, and 47 mg/l. Water temperature varied from 14 to 21 degrees Celsius from September through April 2000. Effluent BOD and SS from AATU 1 were less than 8 and 3, respectively. AATU 2 had more stable performance and Effluent BOD and SS from AATU 2 were less than 6 and 4, respectively. Almost 80% of the data shows that ammonium-nitrogen was less than 0.3 mg/l, and nitrification is almost completed in the contact aeration chambers. The effluent nitrate-nitrogen was less than 5.5 mg/l, owing to the recirculation of the nitrified liquor to the anaerobic chamber.