Effect of ultrasound, low-temperature thermal and alkali pre-treatments on waste activated sludge rheology, hygienization and methane potential

Abstract

Waste activated sludge is slower to biodegrade under anaerobic conditions than is primary sludge due to the glycan strands present in microbial cell walls. The use of pre-treatments may help to disrupt cell membranes and improve waste activated sludge biodegradability. In the present study, the effect of ultrasound, low-temperature thermal and alkali pre-treatments on the rheology, hygienization and biodegradability of waste activated sludge was evaluated. The optimum condition of each pre-treatment was selected based on rheological criteria (reduction of steady state viscosity) and hygienization levels (reduction of Escherichia coli, somatic coliphages and spores of sulfite-reducing clostridia). The three pre-treatments were able to reduce the viscosity of the sludge, and this reduction was greater with increasing treatment intensity. However, only the alkali and thermal conditioning allowed the hygienization of the sludge, whereas the ultrasonication did not exhibit any notorious effect on microbial indicators populations. The selected optimum conditions were as follows: 27,000 kJ/kg TS for the ultrasound, 80 °C during 15 min for the thermal and 157 g NaOH/kg TS for the alkali. Afterward, the specific methane production was evaluated through biomethane potential tests at the specified optimum conditions. The alkali pre-treatment exhibited the greatest methane production increase (34%) followed by the ultrasonication (13%), whereas the thermal pre-treatment presented a methane potential similar to the untreated sludge. Finally, an assessment of the different treatment scenarios was conducted considering the results together with an energy balance, which revealed that the ultrasound and alkali treatments entailed higher costs.

Ultrasound, thermal and alkali treatments affect extracellular polymeric substances (EPSs) and improve waste activated sludge dewatering

“…Although ultrasonication was expected to disrupt the flocs and cellular membranes, the LB-EPS content did not increase as much as it did during the alkali or thermal treatments. This is consistent with the results obtained in a previous work, where the ultrasound treatment was effective enough to dissipate sludge flocs but not for killing bacteria and spores or for inactivating bacteriophages [26]. The TB-EPS fraction was slightly decreased after the ultrasound and alkali treatments and noticeably decreased after the thermal treatment.…”

The viscoelastic characterisation of thermally-treated waste activated sludge

“…With regard to this, the major concern is waste activated sludge (WAS) since common sludge minimization methods such as anaerobic digestion and dewatering process are far from being optimized for WAS [1,2]. The complex structure of WAS flocs which primarily consists of extra polymeric substances (EPS), makes the process of breaking down the cell walls and releasing the bound water in the anaerobic digester and dewatering process so hard [3].…”

“…inhibitory impact of some of chemical pretreatments on microorganisms in anaerobic digestion). Ruiz-Hernando et al (2014) [2] compared ultrasound, low-temperature thermal and alkali pre-treatment processes and reported ultrasound as the most expensive method. Appels et al (2010) [4] reported high temperature thermal-pretreatment (>180°C) produced toxic or inhibitory products which resulted in reducing the biodegradability of sludge.…”

“…Furthermore, they mentioned high energy consumption as the major drawback of high-temperature thermal pre-treatment. Although among the pretreatment processes, low-temperature thermal pretreatment (<100°C) does not significantly advance the disintegration of organic matter in sludge but , reportedly, low temperature thermal pretreatment improve the bio-gas production in anaerobic digestion [5], dewaterability [1] and hygienization of sludge [2].…”

Effect of Ultrasound Pretreatment on Sludge Digestion and Dewatering Characteristics: Application of Particle Size Analysis

“…Different pretreatment methods have been studied to improve the anaerobic degradability of sewage sludge including thermal hydrolysis, ozone oxidation, alkaline hydrolysis, and ultrasound [6][7][8][9]. Among these, ultrasonication is a well-known method to break-up microbial cells and to extract intracellular material [10].…”