Assessment of microbial viability in municipal sludge following ultrasound and microwave pretreatments and resulting impacts on the efficiency of anaerobic sludge digestion
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.
KeywordsAnaerobic digestion Sludge pretreatment Microwave irradiation Ultrasonication Microbial viability Municipal waste
The authors would like to thank the City of Kelowna and the Government of British Columbia Ministry of Environment for supporting this project. This work has been carried out under the sponsorships of the Natural Science and Engineering Council of Canada (NSERC) Strategic Project Grant (#396519-10) and the financial support of the Scientific and Technological Research Council of Turkey (TUBITAK).
- American Public Health Association (APHA) (2005) Standard methods for the examination of water and wastewater. Washington DC USA.Google Scholar
- British Columbia Organic Matter Recycling Regulation (BC OMRR) 2008 Land application guidelines for the organic matter recycling regulation and the soil amendment code of practice, best management practice. BC OMRR and Soil Amendment Code of Practice (SACoP) Land Application Guidelines. Prepared for: BC Ministry of Environment. Prepared by: SYLVIS Environmental. New Westminster, British Columbia.Google Scholar
- Cai Y, Strømme M, Welch K (2014) Bacteria viability assessment after photocatalytic treatment. Biotechnology. 3 Biotech 4:149–157Google Scholar
- Dholiya K, Patel D, Kothari V (2012) Effect of low power microwave on microbial growth, enzyme activity, and aflatoxin production. Research in Biotechnology 3(4):28–34Google Scholar
- Droste RL (1997) Theory and practice of water and wastewater treatment. John Wiley & Sons, New Jersey, USAGoogle Scholar
- Fleming, J. (1961) Microwave irradiation in relation to biological systems and neural activity. In: Biological effects of microwave radiation. Plenum Press, New York, USA.Google Scholar
- Houtmeyers, S., Appels, L., Degrève, J., Van Impe, J., Dewil, R. (2013) Comparing the influence of ultrasonic and microwave pretreatment on the solubilization and semi-continuous digestion of waste activated sludge. Proceedings of the anaerobic digestion conference, Santiago de Compostela, Spain.Google Scholar
- Nielson PH (2002) In: Bitton G (ed) Activated sludge—the floc. Encyclopedia of Environmental Microbiology. Wiley-Interscience, Hoboken, New Jersey, USAGoogle Scholar
- Takatani S, Takayama S, Yamauchi T (1981) A study of anaerobic digestion for sewage sludge. Mitsubishi Juko Giho (Japan) 18:1–7Google Scholar