Influence analysis for the behavior of dewaterability of excess sludge in a two-stage vermifilter

To improve excess sludge dewaterability, a two-stage vermifilter was developed to qualitatively and quantitatively analyze sludge physico-chemical properties (fractal dimension, zeta potential, extracellular polymeric substances (EPS), particle size distribution, etc.) and to correlate them with sludge dewatering characteristics (specific resistance to filtration (SRF) and capillary suction time (CST)). Results demonstrated that sludge dewatering performance was significantly improved after the primary vermifilter VF₁ and the second-stage vermifilter VF₂. In addition, the further VF₂ treatment exhibited higher effects on sludge dewatering performance. The particle boundary of sludge after VF₂ treatment was clearer and smoother than VF₁ sludge (VF₁S), apart from the fact that sludge morphological structure got denser and more compact. Comparing with VF₁S, the fractal dimension D₁ calculated within 1D topological space was closer to 1 after VF₂ treatment, and the fractal dimension D₂ within 2D topological space closer to 2, indicating a better dewatering performance after VF₂ treatment. Additionally, the changes of sludge floc surface properties (such as zeta potential and EPS) resulted in small particles agglomerating into larger ones and then the increase of particle diameter. In summary, the two-stage vermifilter got a better sludge dewatering performance, and thus beneficial for subsequent processing of sludge.