Pre-treatments to enhance the biodegradability of waste activated sludge: Elucidating the rate limiting step

Abstract

Pre-treatments for waste activated sludge (WAS) are, in most cases, an attempt to increase the biodegradation and/or improve hydrolysis rate of WAS after anaerobic digestion. This review presents an extensive analysis of WAS pre-treatments effectiveness focusing on increasing the biodegradability. In the first part of the review, WAS is considered as a cluster of organic components: proteins, carbohydrates, humic substances and cells. Based on this breakdown into components, the effect of different pre-treatments on each component (and in combination) is described. Also, possible reasons for the contradictory results frequently found among different studies dealing with the same pre-treatment are included. In the second part, the review describes the effects on volatile solids removal by digestion after pre-treatment and on the dewaterability of the final digestate. The energy balance and potential limiting factors for each pre-treatment are also taken into account. From the published works it is concluded that some pre-treatment techniques, such as thermal hydrolysis, thermal phased anaerobic digestion and low-temperature pre-treatment are effective ways to increase energy production and to improve other sludge properties, such as dewatering. However, these techniques are very energy intensive and require a large capital outlay, so research on milder pre-treatment techniques is valuable.

Approaches to energy and resource recovery from municipal wastewater

“…• Chemical pretreatment: Ozonation has been used to combine with AD as a pre-treatment step. After pretreatment of waste sludge with 0.15 g O 3 /g TS, biogas production was increased from 150 to 367 mL/g influent COD Gonzalez et al (2018) and González et al (2018). (Bougrier et al, 2007).…”

High-Rate Anaerobic Digestion of Waste Activated Sludge by Integration of Electro-Fenton Process

“…Cell lysis has been referred as a possible method for releasing intracellular organics and increasing the rate and efficiency of the digestion process [6]. Indeed, this is possible when extracellular polymeric substances (EPS) become more bioavailable [11].…”

“…During several studies performed in half-scale and lab-scale plants, these methods, including chemical and thermal methods [12][13][14], as well as mechanical [15] and biological hydrolysis with enzymes [16,17], were investigated for sludge disintegration purposes. By increasing the digestion rate, biogas yields were maximized in smaller digesters while the HRTs decreased [11]. Currently, advanced oxidation processes (AOPs) are considered as valuable sludge pretreatments that might reduce hydraulic retention times and increase methane production rates [18,19].…”

Evaluation of Feasibility of Using the Bacteriophage T4 Lysozyme to Improve the Hydrolysis and Biochemical Methane Potential of Secondary Sludge

“…Secondary sludge particles are essentially concentrated aerobic microbial cells, so anaerobic bacteria must release exo-enzymes that solubilize microbial cell walls, which are composed of peptidoglycan, protein, and lipid bilayers [12]. Degradation of these components is difficult, so the conventional AD process has difficulty digesting more than 35% of the organics in secondary sludge [13]. Therefore, the biological hydrolysis of cells is considered to be a rate-limiting step in most AD of secondary sludge [14].…”