Abstract
Anaerobic digestion (AD) appears to be a popular unit operation in wastewater treatment plant to treat waste activated sludge (WAS) and the produced methane gas can be harvested as renewable energy. However, WAS could inhibit hydrolysis stage during AD and hence pre-treatment is required to overcome the issue. This paper aimed to study the effect of electrochemical pre-treatment (EP) towards efficiency of AD using titanium coated with ruthenium oxide (Ti/RuO2) electrodes. The investigation has been carried out using in-house laboratory batch-scale mesophilic anaerobic digester, mixed under manipulation of important operating parameters. Optimization was performed on EP using response surface methodology and central composite design to maximize sludge disintegration and dewaterability. By operating at optimal conditions (pH 11.65, total solids 22,000 mg/L, electrolysis time 35 min, current density 6 mA/cm2, and 1000 mg/L of sodium chloride), the pre-treated WAS in terms of mixed liquor volatile suspended solids (MLVSS) removal, soluble chemical oxygen demand (sCOD), capillary suction time (CST) reduction, and extracellular polymeric substance (EPS) were 38%, 4800 mg/L (increased from 935 mg/L), 33%, and 218 mg/L, respectively. Following AD, the volatile solids (VS) removal and chemical oxygen demand (COD) removal by EP were enhanced from 40.7% and 54.7% to 47.2% and 61.5%, respectively, at steady-state. The biogas produced from control and electrochemical pre-treated WAS were in the ranges of 0.12 to 0.17 and 0.2 to 0.24 m3/kg VSfed, respectively, and the volume of biogas produced was 44–67% over the control. Based on the results obtained, suitability of EP for WAS prior to AD was confirmed.
Highlights
The effect of four important process variables discussed in our previous study, such as pH, sludge concentration, electrolysis time, and current density, towards four important indicators of the disintegration and dewaterability properties, viz. mixed liquor volatile suspended solids (MLVSS) removal, soluble chemical oxygen demand (sCOD) concentration, capillary suction time (CST) reduction, and Extracellular polymeric substances (EPS) concentration, were provided quantitatively
Via operation at optimal operating condition, waste activated sludge (WAS) anaerobic digestion has been substantially improved via the electrochemical pre-treatment step, viz. 47.2% total volatile solids (VS) reduction, 61.5% chemical oxygen demand (COD) removal and
0.2–0.24 m3 /kg VS·d biogas production rate; compared to 40.7% total VS solids reduction, 54.7% COD removal and 0.12–0.17 m3 /kg VS·d biogas production rate for control. It has been demonstrated in this study that the integration of the optimally operated electrochemical-anaerobic digestion system as a WAS pre-treatment strategy is able to improve production rate and capacity of biogas by 44–67% in comparison to conventional process using mere anaerobic digestion
Summary
Biological treatment is a common method applied in most wastewater treatment plants (WWTP). During treatment of municipal wastewater, waste activated sludge (WAS). Is produced in huge amounts as a by-product, with the total solids concentration varying between 0.8 and 1.2% [1]. The transportation, treatment and disposal of WAS is an important issue as it adds to the overall cost of wastewater treatments, as well as for sludge management [2,3]. An efficient sludge treatment strategy is highly desired to minimize its volume and reduce the WWTP operating cost
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