(1) Step-by-step culture method: raw sludge is gradually put into anaerobic digester, so that raw sludge is gradually transformed into anaerobic digested sludge. This method makes the activated sludge undergo a transformation process from aerobic to anaerobic, and the growth rate of anaerobic microorganisms is much lower than that of aerobic microorganisms, so the step-by-step culture process takes a long time, generally it takes about 6 months to 10 months to complete.
(2) Inoculation culture method: that is, the sludge with the total volume of 10% ~ 30% is anaerobically inoculated into the sludge anaerobic digester. Inoculated sludge is generally taken from the sludge anaerobic digester of the running municipal sewage treatment plant. When liquid digested sludge is inconvenient to transport, dry sludge after mechanical dehydration can be used. In places lacking anaerobic digestion sludge, rotten organic sediments can be taken out of pits and ponds, and human manure, pig manure, cow dung, distiller's grains or sludge from primary sedimentation tanks can also be used as strains. Firstly, the sludge is dissolved in water, then filtered with a 2mm×2mm filter screen to remove large impurities, and then the sludge with a solid concentration of 3% ~ 5% is allowed to stand and settle to remove part of the supernatant, and then it is put into a digestion tank.