Septic tank is primary treatment structures which remove the suspended matter in domestic sewage by adopting sediment and anaerobic fermentation principle, domestic sewage contain plenty of feces, Paper Scrap, parasite etc impurity, the suspended solid concentration is 100-350mg, organism concentration BOD5 is between 100 to 400mm, the suspended organism BOD is 50 to 200mg among of them. sewage enter into septic tank will remove 50% to 60% suspended matter after 12 to 24h sediment , the sediment sludge will anaerobic digestion above 3 month, which will make the organism decompose stable inorganics, corruptible raw sludge translate to ripe sludge , it changed sludge structure, reduce sludge water ratio, clear it and transport out regular. Landfill or reuse it as fertilizer.