Anaerobic digestion of thickened waste activated sludge under calcium hypochlorite stress: Performance stability and microbial communities.

Hypochlorite pretreatment has been proven effective in enhancing waste activated sludge (WAS) anaerobic digestion performances recently. In this study, two semi-continuous anaerobic sequencing batch reactors (ASBRs), one fed with Ca(ClO)2 pretreated thickened WAS (TWAS) and one with raw TWAS, were operated at mesophilic conditions (35 °C) for 145 days. Three loading shocks were introduced to each reactor to compare the performance stability and resilience between the digestion of Ca(ClO)2 pretreated TWAS and untreated TWAS. Microbial community shifts were quantified to reveal the microbiome responses to disturbances. The results suggested that 1% Ca(ClO)2 enhanced the digestion of TWAS by inactivating and transforming the biomass to more easily digested substrates. Co-occurrence network analysis revealed that the strongest interactions in the microbial community occurred in the steady state of TWAS anaerobic digestion.

Impacts of granular activated carbon addition on anaerobic granulation in blackwater treatment.

Upflow anaerobic sludge blanket (UASB) reactors, with or without granular activated carbon (GAC) amendment, were applied for blackwater treatment. The impact of GAC on the formation of granules and biomethane recovery was assessed. High organic loading rates (OLRs) up to 15.7 ± 2.1 kg COD/(m3d) were achieved with both reactors. Similar chemical oxygen demand (COD) removal and methane production rate were observed with OLRs ranging from 5.1 ± 1.0 to 9.3 ± 1.5 kg COD/(m3d). Under higher OLR conditions (13.6 ± 1.1 to 15.7 ± 2.1 kg COD/(m3d)), the GAC-amended UASB achieved a higher COD reduction than the UASB without GAC addition. Interestingly, volatile suspended solids (VSS) concentrations, granule size, and extracellular polymeric substance concentrations were lower in the GAC-amended UASB reactor as compared to the UASB without GAC. The methanogenesis activity of the granules in the GAC-amended UASB reactor was significantly higher than the methanogenesis activity of the UASB granules. The microbes o_Bacteroidales and Syntrophus were predominant in both reactors. The acetoclastic methanogens dominated in the UASB reactor without GAC addition; while hydrogenotrophic methanogens dominated in the GAC-UASB reactor. A phylogenetic investigation of communities by reconstruction of unobserved states (PICRUSt) indicated that syntrophic acetate oxidation improved with GAC addition. The co-occurrence network indicated that interactions between dominant bacteria and archaea were higher in the GAC-amended UASB reactor than in the UASB reactor without GAC addition. This study demonstrated the improved blackwater treatment performance as a result of granulation in UASB with the addition of GAC.

Blackwater biomethane recovery using a thermophilic upflow anaerobic sludge blanket reactor: Impacts of effluent recirculation on reactor performance.

Thermophilic anaerobic digestion is a promising process for high-solid blackwater (BW) treatment due to improved hydrolysis rates, high methanogenesis efficiency, and pathogen removal, when compared with mesophilic treatment. In the present work, the effects of effluent recirculation (i.e., mixing) on thermophilic blackwater treatment were studied. A laboratory-scale thermophilic upflow anaerobic sludge blanket reactor was operated with and without effluent recirculation. The methanogenesis efficiency of the BW treatment increased from 45.0 ± 2.9% when effluent recirculation was applied to 56.7 ± 5.5% without effluent recirculation. Without effluent recirculation, the COD accumulation in the bioreactor was reduced from 17.2 to 3.8% and the effluent volatile fatty acids (VFA) concentration was reduced from 0.64 ± 0.18 to 0.15 ± 0.10 g/L. Further, both acetoclastic and hydrogenotrophic methanogenic activity increased from 101.3 ± 10.8 and 93.9 ± 6.1 to 120.4 ± 9.4 and 118.2 ± 13.2 mg CH4-COD/(gVSS⋅d), respectively, after effluent recirculation was discontinued. The predominant methanogens changed from Methanothermobacter (67%) with effluent recirculation to Methanosarcina (62%) without effluent recirculation. As compared to the effluent recirculation conditions, the enhanced biomethane recovery and treatment performance without effluent recirculation can be attributed to the close proximity of bacteria and archaea groups and the reduced VFA accumulation. Predicted functional gene comparison showed higher prevalence of function for intermediate metabolite transportation (transporters, ATP-binding cassette (ABC) transporters, and two-component system) after discontinuing effluent circulation, which contributed to improved syntrophic propionate oxidation and syntrophic acetate oxidization and enhanced hydrogenotrophic methanogenesis.

Spatial distributions of granular activated carbon in up-flow anaerobic sludge blanket reactors influence methane production treating low and high solid-content wastewater.

The impacts of granular activated carbon (GAC) spatial distributions in up-flow anaerobic sludge blanket (UASB) reactors treating different solid-content wastewater were evaluated in the present study. When treating high solid-content wastewater, the highest methane yield was observed for UASB supplemented with self-floating GAC (74.2 ± 3.7 %), which was followed by settled + self-floating GAC reactor (65.1 ± 3.8 %), then settled GAC reactor (58.3 ± 1.4 %). When treating low solid-content wastewater, all UASBs achieved improved methane yield, and settled + self-floating GAC reactor achieved the highest methane yield (83.4 ± 3.3 %). Self-floating GAC amended reactor showed the best performance for treating high solid-content wastewater, while settled + self-floating GAC amended reactor was optimal for treating medium and low solid-content wastewater. The spatial distributions of microbial communities differed in the reactors with settled GAC and floating GAC. This study underlines the importance of considering feedwater characteristics when adopting GAC-based UASB processes.

Metagenomic insights into direct interspecies electron transfer and quorum sensing in blackwater anaerobic digestion reactors supplemented with granular activated carbon.

The addition of granular activated carbon (GAC) enhanced the performance of up-flow anaerobic sludge blanket (UASB) reactor treating blackwater at 35 °C. DNA were extracted from the sludge and biofilms attached to GAC and submitted for shotgun sequencing. In addition, the acyl-homoserine lactones (AHLs) were quantified. Diverse partners for direct interspecies electron transfer (DIET) were enriched in the sludge or biofilm (GAC-biofilm) of GAC amended UASB. Pedosphaera parvula, Syntrophus aciditrophicus and Syntrophorhabus aromaticivorans were dominant syntrophs. The analysis for type IV pilus assembly genes further suggested DIET may be functioned through GAC for GAC-biofilm, while through conductive pili for sludge aggregates. AHLs quantification and the analysis for quorum sensing (QS) related genes indicated higher QS activity at the population level was induced by GAC. Overall, the work illustrated the different DIET patterns, and suggested that QS played an important role in controlling the performance in GAC amended USAB.

Microbial co-occurrence network topological properties link with reactor parameters and reveal importance of low-abundance genera.

Operational factors and microbial interactions affect the ecology in anaerobic digestion systems. From 12 lab-scale reactors operated under distinct engineering conditions, bacterial communities were found driven by temperature, while archaeal communities by both temperature and substrate properties. Combining the bacterial and archaeal community clustering patterns led to five sample groups (ambient, mesophilic low-solid-substrate, mesophilic, mesophilic co-digestion and thermophilic) for co-occurrence network analysis. Network topological properties were associated with substrate characteristics and hydrolysis-methanogenesis balance. The hydrolysis efficiency correlated (p < 0.05) with clustering coefficient positively and with normalized betweenness negatively. The influent particulate COD ratio and the relative differential hydrolysis-methanogenesis efficiency (Defficiency) correlated negatively with the average path length (p < 0.05). Individual genera's topological properties showed more connector genera in thermophilic network, representing stronger inter-module communication. Individual genera's normalized degree and betweenness revealed that lower-abundance genera (as low as 0.1%) could perform central hub roles and communication roles, maintaining the stability and functionality of the microbial community.

Calcium hypochlorite enhances the digestibility of and the phosphorus recovery from waste activated sludge.

Waste activated sludge (WAS) can be treated using anaerobic digestion (AD) for biogas recovery and volume reduction. However, the poor digestibility and hydrolysis of WAS limit AD applications. The current study investigated the feasibility of applying calcium hypochlorite as a WAS pretreatment strategy to improve AD treatment efficiency using laboratory reactors. The results showed that pretreatment with 5 - 20% Ca(ClO)2 (total suspended solids basis) significantly enhanced WAS anaerobic digestibility, and led to significantly enhanced methane production rate and biomethane yield comparing to the AD of raw WAS (P < 0.05). Low Ca(ClO)2 pretreatment (5 - 10%) significantly enhanced digestion efficiency, which can be attributed to the development of fermentative and syntrophic bacteria. However, high Ca(ClO)2 doses (>20%) reduced microbial activities, leading to slow release of dissolved organic compounds and prolonged methane production lag phase. In addition, high Ca(ClO)2 removed 82.7% of the initial phosphate by calcium-phosphate binding, reducing the phosphorus in liquid digestate.

Calcium phosphate granules formation: Key to high rate of mesophilic UASB treatment of toilet wastewater.

Toilet wastewater, a rich source of organic matter and nutrients, can be treated anaerobically to recover energy and resources at mesophilic conditions (35 °C) using an upflow anaerobic sludge blanket (UASB) digester. However, low organic loading rates (OLR) have often been reported, which may be attributed to the flocs biomass applied in previous studies. In the present study, CaP granules were developed in the UASB reactor during the reactor operation of 250 days, which accounted for 89.2% of the UASB sludge, and had high VSS (25.9 ± 0.3 g/L) and high methanogenesis rates (0.34 ± 0.04 g CH4-COD/(gVSS·d)). An OLR of 16.0 g/(L·d) and a hydraulic retention time (HRT) of 0.25 days, were achieved, with a total chemical oxygen demand (COD) removal rate of 75.6 ± 6.0%, and a methane production rate of 8.4 ± 0.9 g CH4-COD/(L·d). The efficiency of the hydrolysis of organic substrates ranged from 32.6 ± 2.8% to 43.4 ± 1.4%. Microbial community analysis revealed that syntrophic bacteria Syntrophus, together with diverse H2-utilizing methanogens, proliferated; and eventually resulted in a hydrogenotrophic dominant pathway in the UASB reactor. The performance, mechanism of CaP granule formation, and the application of the process were discussed in detail. The present paper provided insight of high rate biomethane production from anaerobic toilet wastewater treatment.