Improving dewaterability of sewage sludge by inoculating acidified sludge and Fe2+: Performance and mechanisms

Abstract

Bioleaching is considered to be an economical and environmentally friendly strategy for sewage sludge (SS) dewatering and heavy metal removal. Acidified sludge (AS), exhibiting strong Fe2+ oxidation capacity, was obtained by repeatedly domesticating sewage sludge with Fe2+, and was treated as a novel inoculum to enhance SS dewaterability during the bioleaching process. AS addition accelerated Fe2+ oxidation and reduced the pH from 7.57 to 2.81 because of bio-acidification and bio-oxidation. After conditioning for 48 h, the SS dewaterability improved in Fe2+-treated and AS conditioned samples; the capillary suction time (CST) and specific resistance to filtration (SRF) decreased from 157.6 s and 12.5 × 1012 m/kg of raw sludge to 40.9 s and 0.53 × 1012 m/kg. Compared with the raw sludge, the total protein content in tightly bound extracellular polymeric substances (TB-EPS) decreased by 82.5% after AS and Fe2+ treatments, while it decreased by 9.69% after treatment with only Fe2+. Three-dimensional fluorescence spectroscopy showed that the decomposition of aromatic proteins, tryptophan-like proteins, and soluble microbial products in TB-EPS contributed in improving SS dewaterability. The results of microbial diversity revealed that in addition to bioleaching microorganism Acidithiobacillus playing an important role, other acid-tolerant microorganisms such as Acidocella, Thiomonas, and Metallibacterium also contribute in improving the SS dewatering performance. The symbiotic association of the above microorganisms reduced the environmental pH and protein concentration, changed the protein composition, thus promoting the release of the bound water contained in SS.