Abstract: A multistage residential wastewater treatment system. The preferred system has three chambers. Wastewater flows into the first chamber via an inflow line. Primarily anaerobic treatment occurs in the first chamber. From the first chamber, wastewater flows into a second chamber. The second chamber is aerated, and primarily aerobic treatment occurs there. Substantially fully treated wastewater flows from the second chamber to the third chamber. The third chamber is substantially quiescent and anaerobic. Most remaining solids settle out in the third chamber. A bactericide pump is configured to introduce batch doses of bactericide in the desired volume to the third chamber at set intervals. After sufficient delay following introduction of the bactericide to allow for substantially complete toxicity to the bacteria in third chamber, effluent is pumped out of the tank by an effluent pump via an effluent line. The effluent pump preferably powers the bactericide pump.