Performance of anaerobic treatment of blackwater collected from different toilet flushing systems: Can we achieve both energy recovery and water conservation?

Source-diverted blackwater (toilet wastewater) contains most of the organic energy in domestic wastewater and can be treated anaerobically to maximize energy recovery. Blackwater collected from toilets of different water saving options (e.g., conventional, dual and vacuum toilets) represents different characteristics, but their digestibility has not been discussed. In the present study, blackwater collected from different toilet flushing systems were characterized and compared in terms of chemical composition, digestibility and microbial population development during biochemical methane potential (BMP) tests. Interestingly, the highest BMP (48%) was achieved for conventional/dual flush toilet (5-9 L water/flush) blackwaters, whereas vacuum toilet (0.5-1.2 L water/flush) blackwater BMP was only 34%. Elevated free ammonia (FA) concentration (>205 mg L-1) appeared to contribute to the reduced digestibility of high-water saving toilet (< 1.5 L water/flush) blackwaters. Methanogenesis was the major FA inhibition step in anaerobic digestion as evident by batch kinetics studies; where Methanosarcina methanogens predominate in all blackwater, but ammonia-tolerance methanogens Methanoculleus and Methanomicrobiales were also predominant in blackwater collected from vacuum toilets. This work underlines that overall measures of sustainability also need to consider blackwater characteristics when designing resource recovery based source-diverted sanitary treatment systems.