Abstract
The production and management of sludge in wastewater treatment plants is a significant environmental issue. Sludge is a complex material, treated primarily by biological stabilization methods, i.e., anaerobic and aerobic digestion. However, the presence of complex organics, microbial flocs, extracellular polymeric substances, and various inhibitory compounds, considerably hinders the efficiency of these processes. In order to overcome the effect of these rate-limiting factors, the literature proposes a number of pretreatment technologies, which can be used either as single pretreatment methods, or in combination. The present review describes both the anaerobic and aerobic digestion of sludge, and highlights the issues that limit the efficiency of the process. Emphasis is placed on the potential use of pretreatment methods, including: thermal; ultrasonic; microwave; Fenton; wet oxidation; photocatalysis and some others. These pretreatment approaches demonstrate varying potential for sludge disintegration and solubilization under different circumstances (e.g., operating conditions and sludge composition). However, the ultimate goal is to improve the subsequent biological treatment of sludge. In short durations, thermal, ultrasonic and microwave processes can efficiently solubilize the components of sludge and disrupt the cell walls of microbial flocs. However, issues related to high levels of energy requirements render these processes uneconomical for field application. The Fenton process can be used in combination with either bioleaching or ultrasound. Visible-Photocatalysis pretreatment for sludge can improve the anaerobic treatment of sludge and biogas production, with low energy demand. (C) 2016 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.