Fresh feces enter the first pool from the fecal inlet. Feces in the pool begin to ferment and decompose. Due to different specific gravity, fecal liquid can be naturally divided into three layers. The upper layer is pasty fecal skin, the lower layer is massive or granular fecal residue, and the middle layer is relatively clear fecal liquid. The upper fecal skin and lower fecal residue contain the most bacterial and parasitic eggs, and the middle layer contains the least insect eggs. The initially fermented middle fecal liquid overflows to the second tank through the fecal pipe, while most of the fecal skin and fecal residue that have not been fully fermented are prevented from remaining in the first tank for further fermentation. The fecal fluid flowing into the second pool is further fermented and decomposed, the insect eggs continue to sink, the pathogens gradually die, the fecal fluid is further harmless, and the fecal skin and fecal thickness are significantly reduced compared with the first pool. The fecal liquid flowing into the third pool has generally been rotten, in which the bacteria and parasite eggs have been basically killed. The function of the third tank is mainly to store the basically harmless fecal liquid.