Effects of Bentonite Amendment on Detoxification, Heavy Metal Passivation and Estrone Elimination of Sewage Sludge Compost

Abstract

Sewage sludge was amended with calcium-based bentonite with addition of no more than 10% in dry weight during the 52-day aerobic composting process, the variations of temperature, pH, organic carbon, EC, total nitrogen, nitrate(NH4+-N) and ammonium(NO3--N) were investigated, as well as the compost detoxification(germination test), heavy metals(Zn, Cu, Pb, Cd) passivation and estrone(E1) elimination. The results showed that the amendment facilitated the thermophilic phase, promoted the compost heat inactivation and brought the organic carbon mineral up to more than 15.27% -19.71%. During the composting, the compost pH increased at the beginning and then gradually decreased before reaching values of 6.76-7.05, while the amendments alleviated the dramatic pH value fluctuation. The bentonite amendment reduced the salinity of the compost with final product EC remarkably lower than 1132 μS·cm-1 of the control treatment, and the effect was enhanced with the increase of addition amount. The total nitrogen content increased with time, and there was a remarkable ammonia loss in the beginning stage for the control treatment, while the bentonite addition could facilitate the total nitrogen content increase by reducing the ammonia loss. With the composting variation, the contents of NH4+-N increased and then decreased while the NO3--N content increased gradually. The bentonite addition had a slight inhibitory effect on the plant germination but did not influence the compost maturity and detoxification; meanwhile, the amendment improved the heavy metal passivation and reduced the E1 content, especially from 90.48 to 28.27 μg·kg-1 with 5% treatment during the composting. The study indicated that bentonite addition of lower than 5% was acceptable for the sludge compost amendment, which had great potential in sludge hygienization, detoxification, heavy metal passivation and E1 elimination.