Testing of sequential sludge washing in triplicate using typical biosurfactant saponin was conducted to remove heavy metals. The aim of the testing was to improve efficiency and reduce mobility of heavy metals. Different washing conditions such as concentration, contact time, liquid/solid ratio and pH were optimized. The heavy metal desorption performances were investigated strictly and the experimental data fitted the pseudo-second order equation very well. A sequential washing step and heavy metal mobilization was investigated, and the results demonstrated that the total content and fractions of the heavy metals reached remarkable extraction efficiencies, after three washings, Cr obtained the highest extraction efficiency (65.00%), and the Pb exchangeable fraction acquired the highest extraction efficiency (85.00%). Also, the heavy metal binding intensity and mobility were investigated, and the results indicated that the heavy metal binding intensity (I_R) increased after washing, conversely, the mobility (M_F) decreased, due to the weak fractions extracted by saponin. The toxicity characteristic leaching procedure (TCLP) indicated that heavy metals have more stable fractions in the after washing sludge. SEM, FTIR and XRD revealed that mechanism of saponin enhanced heavy metal extraction from the sludge, elucidated that carboxyl and hydroxyl were the main functional groups. This study indicated that the saponin may be a replacement for organic acids, inorganic acids and less environmentally friendly refractory chelators in the extraction of heavy metals from the sludge.

Quorum sensing (QS) signaling has been extensively studied in granules and single species populations. However, the knowledge regarding QS in biofilm formation on packing media has neither been experimentally explored nor theoretically addressed. We performed a long-term study to investigate the links between QS, organization and composition of complex microbial communities when biofilm developed on packing media. Three distinct phases were defined according to the weight of the biofilm and the removal rate of COD and ammonia nitrogen. Four kinds of N-acyl-homoserine-lactone (AHL) were found to be regularly and positively correlated with biofilm in growth stage (activation phase) (p < 0.01), while the correlation was not significant in other phases. Total AHL level was also closely related to the productions of extracellular polymeric substance (EPS), especially in polysaccharide. Sequence analysis results revealed that community structure changed with growth of biofilm, and top 34 most abundant community members had a significant correlation with AHLs production. Notably, biofilm and production of EPS, as well as community structure changed after dosing with nanomolar level of exogenous AHL in stable and mature biofilm, but this change was not a signal concentration-dependent manner. This study provides a foundation for investigation of QS effect on biofilm formation on packing media in many natural and engineered ecosystems, where it coordinates community behavior.