An Optimization in the Design of Vermifilter for Synchronous Municipal Sewage-Sludge Stabilization

The sewage purification and simultaneous sludge stabilization effect of both filter beds of a double-layer filter were compared to optimize the design of vermifilter and improve the economics of vermifilter systems. The upper filter bed exhibited much higher COD and NH₄ ⁺-N removal efficiency than the lower bed (43.28-62.86%, 14.31-40.38% for the upper and 1.70-17.77%, 2.34-12.40% for the lower). The average of TP removal rate of upper bed was 17.66%, and a markedly reduction was observed in SS in the upper bed (50.38%), whereas there was slight removal effect of TP and SS in the lower bed. VSS/SS for effluent from the upper filter bed (34.52-56.01 %) was apparently lower than influent (57.71-70.00 %). Moreover, for excess sludge, the upper bed significantly contributed to the nitrogen and phosphorus availability improvement, organics stabilization, organic nitrogen and phosphorus form transformation for better uptake by plant. The arrangement of the lower filter bed has little significance for sewage purification and stabilization, and after the lower bed treatment, slight increase of SS and VSS/SS even could be observed, thus, in view of economical respect, the design of vermifilter could be simplified by doing away with the lower filter bed.