RESUMO
Objective To evaluate non-hazardous treatment efficiency of night soil in the three-grille mode septic tanks so as to give the evidence to government for policy-making. Methods One hundred and thirty normally runningthree-grille mode septic tanks were surveyed in five cities of Jiangsu Province in 2009. The fecal samples from first chamber and wastewater samples from third chamber were collected and examined for fecal coliform (FC) , parasite eggs including Schistosoma japonicum eggs, As-caris eggs and hookworm eggs, chemical oxygen demand ( COD), five-day chemical oxygen demand (BOD_5) and ammonia nitrogen (NH,-N) in lab. The data were analyzed by statistical software SPSS 13.0. Results The mean qualified rates of FC in the first chamber and third chamber of septic tanks were 3.1% and 100% respectively, showing significant difference (P <0.001). Hookworm eggs and Ascaris eggs were found in the first chamber of one and four septic tanks respectively and the density was one or two eggs in one hundred milliliter. The mean removal rates of FC, COD, BOD_5 and NH_3 -N were (99.96 ± 0.03) % , (60.69 ± 21.77) % , (60.13 ± 23. 20 ) % and (44. 14 ± 24.61) % , respectively. For the value of FC, COD, BOD, and NH_3 -N, there were significant difference between the first chamber and the third chamber (P_(FC) =0.000 1, P_(COD) =0.000 1, P_(BOD5) =0.000 1, P_(NH3-N) = 0.000 1, P < 0.001). Conclusion The treatment efficiency of septic tank could meet the sanitary standard for non-hazardous treatment of night soil, and it could use as a useful primary facility for human feces and urine treatment in rural areas.