Extent of bioleaching and bioavailability reduction of potentially toxic heavy metals from sewage sludge through pH-controlled fermentation.

Utilization of anaerobically stabilized sewage sludge on arable lands serve as a renewable alternative to chemical fertilizers as it enables recycling of valuable nutrients to food chain. However, probable presence of heavy metals in sewage sludge restricts the use of stabilized sludge on lands. In this study, a novel approach based on pH-controlled fermentation and anaerobic metal bioleaching was developed to reduce ecotoxicity potential of fermented sludge prior to its land application. Sewage sludge was subjected to pH-controlled fermentation process at acidic, neutral, and alkaline pH levels with the aim of increasing metal solubilization and decreasing bioavailable metal fractions through anaerobic bioleaching. Alkaline reactor performed the best among all reactors and resulted in 3-fold higher hydrolysis (34%) and 6-fold higher acidification (19%) efficiencies along with 43-fold (in average) higher metal solubilization than that of neutral pH reactor. As a result of alkaline fermentation, 32-57% of the metals remained as bioavailable and 34-59% of the metals were encapsulated as non-bioavailable within solid fraction of fermented sludge (biosolid), whereas 8-12% of total metal was solubilized into fermentation liquor. Our results reveal that anaerobic bioleaching through alkaline fermentation enables biosolid production with less metal content and low bioavailability, facilitating its utilization for agricultural purposes.

Removal and recovery of heavy metals from sewage sludge via three-stage integrated process.

Heavy metal contamination of sewage sludge is one of the concerns preventing its land application. Traditional processes applied for stabilization of sewage sludge are still inadequate to serve sustainable solutions to heavy metal problem. In this study, fermentation and bioleaching potentials of sewage sludge were investigated in anaerobic reactors for either non-pretreated or ultrasonicated sludge at three different pH regimes (free of pH regulation, acidic, and alkaline). The results of the study revealed that combination of ultrasonication pretreatment and alkaline fermentation performed the best among the other cases, resulting in 33.7% hydrolysis, 10.5% acidification, 11-33% metal leaching, and up to 25% reduction in bioavailability of potentially toxic heavy metals. Bioleaching effluent obtained from the best performing reactor was subjected to membrane-based metal recovery. A supported liquid membrane impregnated with a basic carrier successfully recovered soluble metals from the bioleaching effluent with an efficiency of 39-68%. This study reveals that the proposed three-stage process, ultrasonication pretreatment-alkaline fermentation-supported liquid membrane, effectively produces stable sludge with reduced heavy metal toxicity and recovers metals from organic waste streams.

Enhanced heavy metal leaching from sewage sludge through anaerobic fermentation and air-assisted ultrasonication.

Interest in using stabilized sewage sludge in agriculture is mainly to benefit from its nutrient content, soil conditioning properties, and water holding capacity. Therefore, sludge management practice needs to be directed from treatment liability towards the recovery of chemical assets embedded in sludge. In this study, anaerobic fermentation process integrated with a new treatment method; i.e., air-assisted ultrasonication, was used to assess the leaching of heavy metals (HM) from waste activated sludge (WAS). Fermentation processes resulted in 9390 mg COD/L of volatile fatty acids (VFAs) production, 26% Ni solubilization and up to 3.4% solubilization of other target metals (Cu and Zn). Application of the air-assisted ultrasonication as a post-treatment to fermentation process stimulated the migration and transformation of HMs to the liquid fraction of the digestate. Applying specific energy input greater than 9 kJ/g total solids (TS) through ultrasonication and supplying air with constant flow rate of 0.875 L of air/(L of digestate.min) resulted in leaching of more than 83% of Ni, 82% of Cu and 80% of Zn.

Influence of volatile fatty acids in anaerobic bioleaching of potentially toxic metals.

Potentially toxic metals are common contaminants associated with sewage sludge, and limited information is available on migration and transformation behavior of potentially toxic metals during anaerobic digestion (AD) process. The aim of this study was to reveal the influence of volatile fatty acids (VFAs) on the solubilization of metals through VFAs-metal complexation. Addition of readily biodegradable extra carbon source at organic loading rate (OLR) of 17.65 gVS/L.d resulted in accumulation of 67,255 mg chemical oxygen demand (COD)/L as VFAs. Low pH values due to VFAs accumulation enhanced the solubilization of Ni and more than 22% of its total concentration became soluble. Subsequent to consumption of VFAs and increase of pH to neutral levels (~7.5), solubility of Ni decreased below 10% of its total concentration. Contrarily, the solubility of Cr reached to 25% of its total concentration at neutral pH values. Presumably the complexation of Cr with dissolved organic matter (DOM) have increased its concentration in the liquid fraction at neutral pH values. Fractionation analysis of metals revealed that AD process altered Cu and Zn speciation between organically-bound and residual fractions, and hence solubility of Zn and Cu remained consistently low over the entire period of the AD process.

Anaerobic digestion of chicken manure: Influence of trace element supplementation.

In this study, anaerobic digestion of nitrogen-rich chicken (egg-laying hen) manure at different trace element (TE) mix doses and different total ammonia nitrogen (TAN) concentrations was investigated in batch digestion experiments. With respect to nonsupplemented TE sets, addition of TE mixture containing 1 mg/L Ni, 1 mg/L Co, 0.2 mg/L Mo, 0.2 mg/L Se, 0.2 mg/L W, and 5 mg/L Fe at TAN concentrations of 3000 mg/L and 4000 mg/L, cumulative CH4 production and CH4 production rate improved by 7-8% and 5-6%, respectively. The results revealed that at a very high TAN concentration of 6000 mg/L, the effect of TE addition was significantly high and the cumulative CH4 production and production rate were increased by 20 and 39.5%, respectively. Therefore, it is concluded that at elevated TAN concentrations the CH4 production that was stimulated by TE supplementation was presumably occurred through syntrophic acetate oxidation.

Long-term influence of trace element deficiency on anaerobic mono-digestion of chicken manure.

Recent findings showed that some trace elements essential for anaerobic digestion might be deficient in chicken (laying hens) manure. In this study, the long-term influence of trace element deficiency on anaerobic mono-digestion of chicken manure was investigated. Three bench-scale anaerobic reactors were operated with or without trace element supplementation. As trace element, only Se or a mix containing Co, Mo, Ni, Se, and W was added to the reactors. The results revealed that in anaerobic digestion of chicken manure at total ammonium nitrogen concentrations over 6000 mg L-1, Se supplementation was critical but not sufficient alone for long-term stable CH4 production. Addition of a mix consisting of Co, Mo, Ni, Se and W resulted in a more stable digestion performance. Daily trace element mix supplementation promoted the hydrogenotrophic Methanoculleus bourgensis, which is an ammonia tolerant methanogen. The decrease in the relative abundance of Methanoculleus detected after termination of trace element addition and resulted in accumulation of acetate and propionate that followed by a significant decrease in CH4 production.