Anaerobic digestion is shifting from a single-purpose technology for renewable energy recovery from organic waste streams to a process for integrated resource recovery. The valorisation of high-rate energy-and phosphorus-rich sludge creates the opportunity for their combined recovery. This phosphate is present in a precipitated form in the sludge, and its release into the liquid phase is an important issue before recovery can be achieved. The objective of this research was to exploit the "unwanted" sulphate reduction process for the release of phosphate into the liquid phase during anaerobic digestion, thus, making it available for recovery. Two different treatments were considered, i.e., a control digester and a digester to which sulphate was added, each operated in triplicate for a period of 119 days. The control digester showed stable methane production at 628 ± 103 mL CH4 L -1 d -1 , with a feedstock COD (chemical oxygen demand) conversion efficiency of 89.5 ± 14.6 %. In contrast, the digester with sulphate addition showed a 29.9 ± 15.3 % decrease in methane production, reaching an "inhibited steady state", but phosphate release into the liquid phase increased with a factor 4.5, compared to the control digester. This inhibited steady state coincided with a clear shift from a Methanosaetaceae to a Methanosarcinaceae dominated methanogenic community. Overall, the sulphate reduction process allows phosphate release during the anaerobic digestion process, yet, at the cost of a reduced methane production rate.

22Anaerobic digestion is shifting from a single-purpose technology for renewable energy 23 recovery from organic waste streams to a process for integrated resource recovery. The 24 valorisation of high-rate energy-and phosphorus-rich sludge creates the opportunity for their 25 combined recovery. This phosphate is present in a precipitated form in the sludge, and its 26 release into the liquid phase is an important issue before recovery can be achieved. The 27 objective of this research was to exploit the "unwanted" sulphate reduction process for the 28 release of phosphate into the liquid phase during anaerobic digestion, thus, making it available 29 for recovery. Two different treatments were considered, i.e., a control digester and a digester to 30 which sulphate was added, each operated in triplicate for a period of 119 days. The control 31 digester showed stable methane production at 628 ± 103 mL CH4 L -1 d -1 , with a feedstock COD 32 (chemical oxygen demand) conversion efficiency of 89.5 ± 14.6 %. In contrast, the digester 33 with sulphate addition showed a 29.9 ± 15.3 % decrease in methane production, reaching an 34 "inhibited steady state", but phosphate release into the liquid phase increased with a factor 4.5, 35 compared to the control digester. This inhibited steady state coincided with a clear shift from a 36Methanosaetaceae to a Methanosarcinaceae dominated methanogenic community. Overall, the 37 sulphate reduction process allows phosphate release during the anaerobic digestion process, 38 yet, at the cost of a reduced methane production rate. 39 40 Keywords: Biogas, methanogenesis, resource recovery, sulphate reducing bacteria 41 42 449 Jo De Vrieze is supported as postdoctoral fellow by the Research Foundation Flanders (FWO-450 Vlaanderen). The authors would like to thank Tim Lacoere for his assistance with the molecular 451 analysis, and Cindy Law, Amanda Luther and Inka Vanwonterghem for carefully reading the 452 manuscript. The authors also kindly acknowledge Harry Vrins from Waterschap Brabantse 453 Delta for his assistance with the A-sludge collection. 454 455