via central valved outlet line 38 the precipitation sludge falls to the bottom of precipitation zone 37 from whence it is scraped by sweep 39 rotatably driven by rotor 40 into a sludge hopper 42 the sludge is propelled from the sludge holding hopper 42 by auger 41 driven by motor 43 into a waste sludge line 44 and into a recycle line 44 valves at 441 leading to main flow line 12 main flow line 12 is connected to outflow line 14 by pump recycle line 45 valved at 451 in use the basic flow pattern is as follows the raw sewage is drawn by pump 13 through lines 11 and 12 and is pumped to the mixing chamber 17 before the raw sewage goes to the mixing chamber 17 it is mixed with recycle sludge in line 4 and a recycle mixing flow both to the pump 13 occurs through recycle line 45 the sludge recycle rate is about 1 20 preferably about 5 7 of the total flow rate through the system which varies from about 5 40 gallon minute alum is added to the pump outflow line 14 in an amount of about 10 200 preferably about 20 50 ppm by weight in the mixing chambers 17 the chemicals are added to provide the chemically treated raw sewage the chemicals are added.