Assessment of microbial viability in municipal sludge following ultrasound and microwave pretreatments and resulting impacts on the efficiency of anaerobic sludge digestion

Abstract

A range of ultrasonication (US) and microwave irradiation (MW) sludge pretreatments were compared to determine the extent of cellular destruction in micro-organisms within secondary sludge and how this cellular destruction translated to anaerobic digestion (AD). Cellular lysis/inactivation was measured using two microbial viability assays, (1) Syto 16® Green and Sytox® Orange counter-assay to discern the integrity of cellular membranes and (2) a fluorescein diacetate assay to understand relative enzymatic activity. A range of MW intensities (2.17-6.48 kJ/g total solids or TS, coinciding temperatures of 60-160 °C) were selected for comparison via viability assays; a range of corresponding US intensities (2.37-27.71 kJ/g TS, coinciding sonication times of 10-60 min at different amplitudes) were also compared to this MW range. The MW pretreatment of thickened waste activated sludge (tWAS) caused fourfold to fivefold greater cell death than non-pretreated and US-pretreated tWAS. The greatest microbial destruction occurred at MW intensities greater than 2.62 kJ/g TS of sludge, after which increased energy input via MW did not appear to cause greater microbial death. In addition, the optimal MW pretreatment (80 °C, 2.62 kJ/g TS) and corresponding US pretreatment (10 min, 60 % amplitude, 2.37 kJ/g TS) were administered to the tWAS of a mixed sludge and fed to anaerobic digesters over sludge retention times (SRTs) of 20, 14, and 7 days to compare effects of feed pretreatment on AD efficiency. The digester utilizing MW-pretreated tWAS (80 °C, 2.62 kJ/g TS) had the greatest fecal coliform removal (73.4 and 69.8 % reduction, respectively), greatest solids removal (44.2 % TS reduction), and highest overall methane production (248.2 L CH4/kg volatile solids) at 14- and 7-day SRTs. However, despite the fourfold to fivefold increases in cell death upon pretreatment, improvements from the digester fed MW-pretreated sludge were marginal (i.e., increases in efficiency of less than 3-10 %) and likely due to a smaller proportion of cells (10-20 %) in the polymeric network and mixed sludge fed to digesters.

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

“…Unfortunately, no studies were found regarding the relationship between particle size distribution and temperature increase, at temperatures higher than 100°C, although a further decrease in particle size is expected at temperatures around 150°C, followed by an increase after this temperature, as has been reported for thermal hydrolysis. Cella et al (2015) found that the highest microbial destruction occurred at 2.62 kJ/g TS at a temperature of 80°C and 9 min of application time, which lowered the live/dead ratio from around 3 for the control to around 0.25 for the pre-treated sample. An additional input of energy did not cause significantly higher microbial death.…”

“…Regarding sludge biodegradation, results match with observations for low and high temperature pre-treatment. Most of the studies observed an increase in biodegradation (Table 5), although some papers reported no increase (Cella et al, 2015;Eskicioglu et al, 2008). The effect of the temperature increase rate during pre-treatment on sludge biodegradation should not be neglected.…”

“…For instance, Salsabil et al (2010) postulated that the degree of sludge solubilization depends upon the kind of treatment (mechanical, oxidative, thermal) rather than upon the specific energy input. Similarly, Cella et al (2015) concluded that the innate characteristics of the pre-treatment method are likely more important than energy input. The objectives of this review paper are a) to identify the effects of different pre-treatment methods on the proteins, carbohydrates, humic substances and cells contained in the WAS; b) to assess and compare the biological degradability (hydrolysis rate, biogas production and removal of organic matter), dewaterability of the digestate Table 1 Literature review of WAS composition.…”

Microwave as an emerging technology for the treatment of biohazardous waste: A mini-review

“…However, when subsequently fed to anaerobic digesters, the improvements of microwave pretreated sludge were relatively small (Cella et al, 2016). Other positive effects for the treatment of sludge were also observed Pino-Jelcic et al, 2006).…”

Optimization and Performance of Moderate Combined Alkali and Microwave Pretreatment for Anaerobic Digestion of Waste-Activated Sludge

“…Besides the thermal effect, another potential mechanism of the microwave is the "athermal effect," referring to an effect that is unrelated to temperature rise (i.e., bond breakage caused by dipolar rotation of molecules) [7,10]. The athermal effect is likely to dominate a low-intensity pretreatment, while the thermal effect plays a major role in a higher intensity pretreatment [11].…”