Techno-economic and environmental assessment of upgrading alternatives for sludge stabilization in municipal wastewater treatment plants

Feasibility and potential of separate anaerobic digestion of municipal sewage sludge fractions

“…A model of separate anaerobic digestion was studied and different degrees of biodegradability were obtained as 52(±1) and 40(±5)% VS removal for PS and SS, respectively. High removal and methane yield levels according to literature were correlated to high oil and grease content (Tchobanoglous et al, 2004) and a related toxic effect was observed as a consequence. The best operational parameters were determined to guide full-scale application, both in terms of energy gain and chemical savings.…”

“…The opposite behaviour of the two sewage sludge fractions in the thickening unit was the first result supporting the necessity of separate thickening or at least direct feeding of PS to the digesters. The design criteria range for thickening units is very large, at 2-8% dry solids (DS) (4% typical) (Tchobanoglous et al, 2004). Gravity thickening of PS does not usually produce a higher solid percentage (5-10% dry solids) than raw PS (5-9% dry solids), possibly due to its floating behaviour which is a natural outcome of anaerobic degradation of sewage solids, consisting of mostly raw organic compounds in the form of proteins, lipids, carbohydrates and bacteria, during their transport in long sewage pipes and this can be observed by the blackish colour, sulphurous odour and gas production of PS solids.…”

“…Each fraction possesses different contents and degree of biodegradability (Winter and Pierce, 2010). Settleable solids, mostly proteins and lipids, in the wastewater are separated into PS reaching a high dry and volatile solid (DS and VS) concentration (5-9% DS (typical 6%)) and transferred to the thickening unit where mixing with the SS is provided (Tchobanoglous et al, 2004). Conversion rate to methane is faster for PS due to raw organic matter content but SS has a lower DS and VS content (0.5-1.5% DS (typical 0.8% DS)) composed of viable microorganisms and extracellular polymeric substances (EPS) for which hydrolysis rate is low.…”

“…In the current situation in Turkey, biogas production is emphasized and ADs in municipal WWTPs are operated at a constant hydraulic retention time (HRT) of 18-20 d, and need improvement for more efficient VS removal, higher biogas yield and higher final sludge quality via potential alternative operational systems. Separate stabilization of PS and SS is an alternative method to optimize anaerobic digestion of sewage sludge fractions (Winter and Pierce, 2010;Tomei et al, 2016). Both fractions have a comparable VS/total solids (TS) percentage but different biodegradability and dewatering characteristics as well as VS and pathogen contents.…”

How far are we from closing the loop of sewage resource recovery? A real picture of municipal wastewater treatment plants in Italy

“…However, it is the authors' belief that any new process/technology or management strategy should be carefully evaluated before its implementation by considering as many factors as possible; examples of such an approach are reported in Gianico et al (2015), Bertanza et al (2016) and Tomei et al (2016).…”