During advanced biological wastewater treatment, a huge amount of sludge is produced as a by-product of the treatment process. Hence, reuse and recovery of resources and energy from the sludge is a big technological challenge. The processing of sludge produced by Wastewater Treatment Plants (WWTPs) is massive, which takes up a big part of the overall operational costs. In this regard, anaerobic digestion (AD) of sewage sludge continues to be an attractive option to produce biogas that could contribute to the wastewater management cost reduction and foster the sustainability of those WWTPs. At the same time, AD reduces sludge amounts and that again contributes to the reduction of the sludge disposal costs. However, sludge volume minimization remains, a challenge thus improvement of dewatering efficiency is an inevitable part of WWTP operation. As a result, AD parameters could have significant impact on sludge properties. One of the most important operational parameters influencing the AD process is temperature. Consequently, the thermophilic and the mesophilic modes of sludge AD are compared for their pros and cons by many researchers. However, most comparisons are more focused on biogas yield, process speed and stability. Regarding the biogas yield, thermophilic sludge AD is preferred over the mesophilic one because of its faster biochemical reaction rate. Equally important but not studied sufficiently until now was the influence of temperature on the digestate quality, which is expressed mainly by the sludge dewateringability, and the reject water quality (chemical oxygen demand, ammonia nitrogen, and pH). In the field of comparison of thermophilic and mesophilic digestion process, few and often inconclusive research, unfortunately, has been published so far. Hence, recommendations for optimized technologies have not yet been done. The review presented provides a comparison of existing sludge AD technologies and the gaps that need to be filled so as to optimize the connection between the two systems. In addition, many other relevant AD process parameters, including sludge rheology, which need to be addressed, are also reviewed and presented.

Urban Environmental Management Kotebe University College 31248 Addis Ababa Ethiopia

Water Technology and Environmental Engineering University of Chemistry and Technology Technická 5 166 28 Prague 6 Dejvice Czech Republic

See more in PubMed

Jiang J., Wu J., Poncin S., Li H.Z. Rheological characteristics of highly concentrated anaerobic digested sludge. Biochem. Eng. J. 2014;86:57–61. doi: 10.1016/j.bej.2014.03.007. DOI

Appels L., Baeyens J., Degrève J., Dewil R. Principles and potential of the anaerobic digestion of waste-activated sludge. Prog. Energy Combust. Sci. 2008;34:755–781. doi: 10.1016/j.pecs.2008.06.002. DOI

Nges I.A., Liu J. Effects of solid retention time on anaerobic digestion of dewatered-sewage sludge in mesophilic and thermophilic conditions. Renew. Energy. 2010;35:2200–2206. doi: 10.1016/j.renene.2010.02.022. DOI

Verstraete W., Vlaeminck S.E. ZeroWasteWater: Short-cycling of wastewater resources for sustainable cities of the future. Int. J. Sustain. Dev. World Ecol. 2011;18:253–264. doi: 10.1080/13504509.2011.570804. DOI

Zábranská J., Dohányos M., Jenícek P., Kutil J. Thermophilic process and enhancement of excess activated sludge degradability—Two ways of intensification of sludge treatment in the Prague central wastewater treatment plant. Water Sci. Technol. 2000;41:265–272.

Ge H., Jensen P.D., Batstone D.J. Relative kinetics of anaerobic digestion under thermophilic and mesophilic conditions. Water Sci. Technol. 2011;64:848–853. doi: 10.2166/wst.2011.571. PubMed DOI

Gavala H.N., Yenal U., Skiadas I.V., Westermann P., Ahring B.K. Mesophilic and thermophilic anaerobic digestion of primary and secondary sludge. Effect of pre-treatment at elevated temperature. Water Res. 2003;37:4561–4572. doi: 10.1016/S0043-1354(03)00401-9. PubMed DOI

McCarty P.L., Smith D.P. Anaerobic wastewater treatment. Environ. Sci. Technol. 1986;20:1200–1206. doi: 10.1021/es00154a002. DOI

De la Rubia M.A., Riau V., Raposo F., Borja R. Thermophilic anaerobic digestion of sewage sludge: Focus on the influence of the start-up. A review. Crit. Rev. Biotechnol. 2013;33:448–460. doi: 10.3109/07388551.2012.726962. PubMed DOI

Chi Y.Z., Li Y.Y., Ji M., Qiang H., Deng H.W., Wu Y.P. Mesophilic and Thermophilic Digestion of Thickened Waste Activated Sludge: A Comparative Study. Adv. Mater. Res. 2010;113–116:450–458. doi: 10.4028/www.scientific.net/AMR.113-116.450. DOI

Amani T., Sreekrishnan T.R. Experimental Study on Key Dissimilarities between Mesophilic and Thermophilic Anaerobic Digestion of Waste Activated Sludge. Int. J. Environ. Res. 2011;5:10.

Cavinato C.B.D., Pavan P., Fatone F., Cecchi F. Mesophilic and thermophilic anaerobic co-digestion of waste activated sludge and source sorted biowaste in pilot- and full-scale reactors. Renew. Energy. 2013;55:260–265. doi: 10.1016/j.renene.2012.12.044. DOI

Bitton G. Wastewater Microbiology. John Wiley & Sons, Inc.; Hoboken, NJ, USA: 2005. Anaerobic Digestion of Wastewater and Biosolids; pp. 345–369.

Ho D.P., Jensen P.D., Batstone D.J. Methanosarcinaceae and acetate-oxidizing pathways dominate in high-rate thermophilic anaerobic digestion of waste-activated sludge. Appl. Environ. Microbiol. 2013;79:491–500. doi: 10.1128/AEM.01730-13. PubMed DOI PMC

Weedermann M., Seo G., Wolkowicz G.S.K. Mathematical model of anaerobic digestion in a chemostat: Effects of syntrophy and inhibition. J. Biol. Dyn. 2013;7:59–85. doi: 10.1080/17513758.2012.755573. PubMed DOI PMC

Lee M.J., Zinder S.H. Hydrogen Partial Pressures in a Thermophilic Acetate-Oxidizing Methanogenic Coculture. Appl. Environ. Microbiol. 1988;54:1457–1461. PubMed PMC

Kaspar H., Wuhrmann K. Product inhibition in sludge digestion. Microb. Ecol. 1977;4:241–248. doi: 10.1007/BF02015080. PubMed DOI

Sung S., Liu T. Ammonia inhibition on thermophilic anaerobic digestion. Chemosphere. 2003;53:43–52. doi: 10.1016/S0045-6535(03)00434-X. PubMed DOI

Rajagopal R., Massé D.I., Singh G. A critical review on inhibition of anaerobic digestion process by excess ammonia. Bioresour. Technol. 2013;143:632–641. doi: 10.1016/j.biortech.2013.06.030. PubMed DOI

Franke-Whittle I.H., Walter A., Ebner C., Insam H. Investigation into the effect of high concentrations of volatile fatty acids in anaerobic digestion on methanogenic communities. Waste Manag. 2014;34:2080–2089. doi: 10.1016/j.wasman.2014.07.020. PubMed DOI PMC

Kim M., Ahn Y.H., Speece R.E. Comparative process stability and efficiency of anaerobic digestion; mesophilic vs. thermophilic. Water Res. 2002;36:4369–4385. doi: 10.1016/S0043-1354(02)00147-1. PubMed DOI

Li Q., Qiao W., Wang X., Takayanagi K., Shofie M., Li Y.-Y. Kinetic characterization of thermophilic and mesophilic anaerobic digestion for coffee grounds and waste activated sludge. Waste Manag. 2015;36:77–85. doi: 10.1016/j.wasman.2014.11.016. PubMed DOI

Kardos L., Juhász Á., Palkó G., Oláh J., Barkács K., Záray G. Comparing of thermophilic and mesophilic anaerobic fermented sewage sludge based on chemical and biochemical tests. Appl. Ecol. Environ. Res. 2011;9:293–302. doi: 10.15666/aeer/0903_293302. DOI

Ruffino B., Campo G., Genon G., Lorenzi E., Novarino D., Scibilia G., Zanetti M. Improvement of anaerobic digestion of sewage sludge in a wastewater treatment plant by means of mechanical and thermal pre-treatments: Performance, energy and economical assessment. Bioresour. Technol. 2015;175:298–308. doi: 10.1016/j.biortech.2014.10.071. PubMed DOI

Khemkhao M., Nuntakumjorn B., Techkarnjanaruk S., Phalakornkule C. Comparative mesophilic and thermophilic anaerobic digestion of palm oil mill effluent using upflow anaerobic sludge blanket. Water Environ. Res. 2012;84:577–587. doi: 10.2175/106143012X13378023685637. PubMed DOI

Song Y.C., Kwon S.J., Woo J.H. Mesophilic and thermophilic temperature co-phase anaerobic digestion compared with single-stage mesophilic- and thermophilic digestion of sewage sludge. Water Res. 2004;38:1653–1662. doi: 10.1016/j.watres.2003.12.019. PubMed DOI

Suhartini S., Heaven S., Banks C.J. Comparison of mesophilic and thermophilic anaerobic digestion of sugar beet pulp: Performance, dewaterability and foam control. Bioresour. Technol. 2014;152:202–211. doi: 10.1016/j.biortech.2013.11.010. PubMed DOI

Moen G., Stensel H.D., Lepistö R., Ferguson J.F. Effect of Solids Retention Time on the Performance of Thermophilic and Mesophilic Digestion of Combined Municipal Wastewater Sludges. Water Environ. Res. 2003;75:539–548. doi: 10.2175/106143003X141330. PubMed DOI

Ocansey F.N. Master’s Thesis. Lundto University; Lund, Sweden: 2005. New Trends In Treatment of Reject Water from Dewatering of Sludge.

Zhou J., Zheng G., Zhang X., Zhou L. Influences of extracellular polymeric substances on the dewaterability of sewage sludge during bioleaching. PLoS ONE. 2014;9:e102688. doi: 10.1371/journal.pone.0102688. PubMed DOI PMC

Zhou J., Mavinic D.S., Kelly H.G., Ramey W.D. Effects of temperatures and extracellular proteins on dewaterability of thermophilically digested biosolids. J. Environ. Eng. Sci. 2002;1:409–415. doi: 10.1139/s02-029. DOI

Braguglia C.M., Gianico A., Gallipoli A., Mininni G. The impact of sludge pre-treatments on mesophilic and thermophilic anaerobic digestion efficiency: Role of the organic load. Chem. Eng. J. 2015;270:362–371. doi: 10.1016/j.cej.2015.02.037. DOI

Liu J.B., Ni X.T., Wei Y.S., Tong J., Wang Y.W. Enhancement for anaerobic digestion of sewage sludge pretreated by microwave and its combined processes. Environ. Sci. 2014;35:3455–3460. PubMed

Braguglia C.M., Gianico A., Mininni G. Effect of ultrasound on particle surface charge andfilterability during sludge anaerobic digestion. Water Sci. Technol. 2009;60:2053–2033. doi: 10.2166/wst.2009.505. PubMed DOI

Braguglia C.M., Gianico A., Mininni G. Comparison between ozone and ultrasound disintegration on sludge anaerobic digestion. J. Environ. Manag. 2012;95:S139–S143. doi: 10.1016/j.jenvman.2010.07.030. PubMed DOI

Houghton J.I., Stephenson T. Effect of influent organic content on digested sludge extracellular polymer content and dewaterability. Water Res. 2002;36:3620–3628. doi: 10.1016/S0043-1354(02)00055-6. PubMed DOI

Lau S.W., Chong S.H., Ang H.M., Sen T.K., Chua H.B. Dewaterability of Anaerobic Digested Sludge with Cations and Chitosan as Dual Conditioners. In: Pogaku R., Bono A., Chu C., editors. Developments in Sustainable Chemical and Bioprocess Technology. Springer US; New York, NY, USA: 2013. pp. 11–17.

Baudez J.C.M.F., Eshtiaghi N., Slatter P. The rheological behaviour of anaerobic digested sludge. Water Res. 2011;45:5675–5680. doi: 10.1016/j.watres.2011.08.035. PubMed DOI

Aranowski R., Hupka J., Jungnickel C. Changes in Rheological Properties during Anaerobic Digestion of Activated Sludge. Physicochem. Probl. Miner. Process. 2010;44:13–22.

Farno E., Baudez J.C., Parthasarathy R., Eshtiaghi N. Rheological characterisation of thermally-treated anaerobic digested sludge: Impact of temperature and thermal history. Water Res. 2014;56:156–161. doi: 10.1016/j.watres.2014.02.048. PubMed DOI

Feng G., Tan W., Zhong N., Liu L. Effects of thermal treatment on physical and expression dewatering characteristics of municipal sludge. Chem. Eng. J. 2014;247:223–230. doi: 10.1016/j.cej.2014.03.005. DOI

Wang L., Zhang L., Li A. Hydrothermal treatment coupled with mechanical expression at increased temperature for excess sludge dewatering: Influence of operating conditions and the process energetics. Water Res. 2014;65:85–97. doi: 10.1016/j.watres.2014.07.020. PubMed DOI

Urrea J.L., Collado S., Laca A., Díaz M. Rheological behaviour of activated sludge treated by thermal hydrolysis. J. Water Process Eng. 2015;5:153–159. doi: 10.1016/j.jwpe.2014.06.009. DOI

Disposal and Recycling Routes for Sewage Sludge, part-3 Scientific and Technical Report. [(accessed on 18 February 2015)]. Available online: http://ec.europa.eu/environment/archives/waste/sludge/pdf/sludge_disposal3.pdf.

Waste-activated sludge disruption by dry ice: bench scale study and evaluation of heat phase transformations