Insights into the dewatering of excavated landfill sludge conditioned by polyferric silicate sulfate.

Large quantities of landfill sludge (LS) with higher water content (WC) were stored underground, and excavation and re-dewatering of LS is a sustainable and economic strategy to save landfill space and reduce the leaching of contaminants. In this study, polyferric silicate sulfate (PFSS) was first applied in the conditioning of excavated LS, and the effects of the Si/Fe mass ratio and PFSS dosage on physicochemical properties, dewaterability and rheological properties were investigated. At the best Si/Fe of 0.18, PFSS conditioning obtained compact aggregates with the strongest internal structure, thus achieving the lowest WC. Large sludge flocs were formed, and slime and loosely-bound extracellular polymeric substances were effectively removed with the PFSS dosage above 100 mg/g dried solids, which made the WC to be lower than 51.4%. The whole mechanical compression process of conditioned LS can be described by the modified Terzaghi-Voigt model, and increasing the PFSS dosage induced the release of bound water and migration of the consolidation stage from ternary to secondary. PFSS is an economically sustainable conditioner for LS, integrating multiple functions such as charge neutralization, particle aggregation, interparticle bridging and skeleton building in one chemical.

Applying organic polymer flocculants in conditioning and advanced dewatering of landfill sludge as a substitution of ferric trichloride and lime: Mechanism, optimization and pilot-scale study.

In this study, poly dimethyl diallyl ammonium chloride (PDADMAC) and polyacrylamide (PAM) were applied to substitute ferric trichloride (FeCl3) and lime conditioning for advanced dewatering of landfill sludge (LS). Four response surface methodology (RSM) models were constructed for FeCl3-lime, FeCl3-PAM, PDADMAC-lime and PDADMAC-PAM, and identical dosages, namely 29.86, 57.91, 5.73 and 2.99 mg/g dry solids (DS) for FeCl3, lime, PDADMAC and PAM, were obtained by solving the system of four RSM equations at water content of 60% to investigate conditioning mechanisms. Compared to FeCl3-lime, PDADMAC-PAM conditioning had strong charge neutralization and bridging performance, and obtained conditioned LS with large flocs size, strong network structure and rapid dewatering rate. By integrating RSM with nonlinear programming for optimization, the total cost of PDADMAC-PAM route was saved by 7.9% and close to FeCl3-lime, and the optimized condition with dosages of 1.93 and 3.47 kg/t DS was further confirmed by pilot-scale experiments. The results indicated that PDADMAC-PAM was a feasible substitute for FeCl3-lime in sludge conditioning, and showed more advantage if dewatered sludge was further treated by incineration.

A full-scale survey of sludge landfill: sludge properties, leachate characteristics and microbial community structure.

In this study, a full-scale survey was conducted of a sludge landfill that had been sealed for 10 years to investigate sludge properties, leachate characteristics and microbial community structure. Vertical distribution of sludge and leachate pollutants in the landfill site showed that the sludge and soluble pollutants in the leachate were both distributed almost evenly even after long-term anaerobic digestion, and higher concentrations of soluble pollutants and richness of microbial community were observed at the middle layer. Compared to dewatered excess sludge generated from the activated sludge process before landfill, landfill sludge had a much lower organic content (28.1%), smaller particle size and worse dewaterability. Compared to municipal waste landfill, sludge landfill generated leachate with a lower concentration of organic substances, and comparable concentrations of nitrogenous and phosphorus pollutants. Bacterial community analysis by Illumina MiSeq sequencing showed that Proteobacteria, Firmicutes, Chloroflexi, Actinobacteria and Bacteroidetes were the major phyla, and some new genera (Methylocystaceae, Mariniphaga and Aminicenantes) were enriched in the sludge landfill. Archaeal community analysis showed that aceticlastic methanogenesis by Methanosaeta and Methanosarcina was the main pathway for methane production in the sludge landfill, in contrast to waste landfill with hydrogenotrophic methanogenesis as the main pathway.

Insights into conditioning of landfill sludge by FeCl3 and lime.

In this study, landfill sludge (LS) was excavated from a 10 year old full-scale sludge landfill and used to investigate effects of dosage on sludge dewaterability, rheological properties and extracellular polymeric substances (EPS) variations by FeCl3-lime conditioning. LS had lower content of organic matters (0.28) and smaller particle size than excess sludge (ES), and greatly lower viscosity and high flowability. The suitable concentration of LS for conditioning (107.2-118.6 g/L) was much higher than that of ES (34 g/L) by rheological analysis. Both FeCl3 and lime improved dewaterability of LS and caused decline of slime and loosely bound EPS (LB-EPS). FeCl3 destroyed proteins in slime and LB-EPS owing to coagulation and acidification effects, weakened internal structure strength, and thus improved dewaterability. Lime addition caused alkaline hydrolysis of polysaccharides in slime and LB-EPS, reduced viscosity and flowability, and improved flowability and dewaterability for LS. The optimal dosage for dewatering using 57.6 mg lime/g dried solids (DS) and 53.6 mg FeCl3/g DS was obtained by using an integrative response surface methodology (RSM) coupled nonlinear programming approach under water content constraint of 55%. The integrative optimization achieved 26.0% cost saving in comparison to RSM optimized condition.