Thermophilic (55 °C) and hyper-thermophilic (70 °C) anaerobic digestion as novel treatment technologies for concentrated black water

Thermophilic and hyper-thermophilic anaerobic digestion (AD) are promising techniques for the treatment of concentrated black water (toilet fraction of domestic wastewater collected by low flush volume toilets; BW) and recovery of nutrients. High temperatures are applied in this study to ensure pathogen removal and thus safe reuse of recovered nutrients. This technique is applicable when ultra-low flush-volume collection (e.g. vacuum toilets) is used to obtain a highly concentrated waste stream (15-20 g COD/L) suitable for high rate anaerobic treatment and efficient nutrient recovery. This study showed that thermophilic AD (55 °C) of concentrated BW could achieve the same methanisation and COD removal as mesophilic anaerobic treatment of BW (collected with conventional vacuum toilets) and kitchen waste (KW) while applying a higher loading rate (2.5-4.0 kgCOD/m3⋅day). With a retention time of 8.7 days, and loading rates exceeding 3 kgCOD/m3⋅day a COD removal of 70% and a methanisation of 62% (based on CODt) was achieved during thermophilic anaerobic treatment, despite the high temperature (55 °C) and possible increased ammonia toxicity. Hyper-thermophilic (70 °C) reached lower levels of methanisation (38%). A UASB treating (highly) concentrated BW can be started-up in 12 days. Furthermore, microbiological analysis showed that the high temperature combined with high ammonia concentrations resulted in increased abundance of species closely related to Thermacetogenium phaeum and Syntrophaceticus schinkii, two syntrophic acetate oxidizers, along with an increased abundance of hydrogenotrophic methanogens (eg. species closely related to Methanothermobacter tenebrarum). This indicates that a shift from acetoclastic methanogenesis to hydrogenotrophic methanogenesis combined with syntrophic acetate oxidation can be observed as a consequence of the thermophilic conditions and possibly the related increased ammonia toxicity.