Granular activated carbon (GAC) has been widely studied for its roles in improving anaerobic digestion (AD) through its conductive properties in stimulating direct interspecies electron transfer or its surface functional groups in altering redox conditions in AD. In the present study, the impacts of GAC on the biosynthesis of extracellular microbial secretions (e.g., amino acids and vitamins), and the roles of these secretions in AD were investigated. Four continuous laboratory reactors were operated under ambient temperature (20 ± 0.5 °C) conditions, including two up-flow anaerobic sludge blanket (UASB) bioreactors with GAC addition to one of the UASBs; and two anaerobic sequencing batch reactors (ASBRs) each fed with the centrifuged supernatants (to remove suspended biomass and GAC debris) of UASB effluents supplemented with additional carbon source (glucose). Our finding shows that GAC addition stimulated extracellular microbial secretions in UASB, and these secretions, which can work as essential nutrients, enhanced COD removal (from 48 ± 3 % to 60 ± 1 %) and methane production (31 ± 3 % to 47 ± 5 %) in its downstream ASBR. Interestingly, microbial kinetics test showed that extracellular microbial secretions from the GAC-amended UASB improved methane production rate from 72.5 ± 1.5 to 105.6 ± 1.2 mg CH4-COD/g VSS/d without dramatic microbial community shift. We conclude that the impact of GAC on AD does not only rely on the direct contact of microorganisms and GAC materials, and the GAC-stimulated microbial extracellular secretions represent an important mechanism in conductive material amended AD.
Granular activated carbon (GAC) has been shown to mediate direct interspecies electron transfer (DIET) in anaerobic digestion. Adding GAC to up-flow anaerobic sludge bed reactor increased the total biomass slightly from 20.0 to 26.6 gVSS/reactor, and maximum organic removal capacity remarkably from 285 to 1660 mgCOD/L/d. Since GAC occupied 7% of reactor volume (denser than suspended sludge, settled to the reactor bottom), we used a spatial sampling strategy (sludge bed top/mid/bottom layers, and tightly attached GAC-biofilm) and DNA- and RNA-based community analyses. RNA-based analysis demonstrated significant community differences between the non-GAC and GAC-amended reactors (p < 0.05) based on ANOSIM statistical analysis. In comparison, DNA-based analysis showed little community difference between these reactors (p > 0.05). RNA-based analysis revealed active enrichments in GAC-biofilm, including bacteria Geobacter, Syntrophus, Desulfovibrio and Blvii28, and archaea Methanosaeta and Methanospirillum. These are potential electro-active syntrophic microorganisms related with DIET, which expand the previously defined list of DIET microorganisms.
Abstract Anaerobic bioreactors for source‐separated blackwater are mostly operated at low organic loading rates (OLRs) due to low biodegradability and the potential of ammonia inhibition. In this study, an anaerobic biofilm reactor having conductive carbon fibers as the media was investigated for the high‐rate treatment of blackwater collected from vacuum toilets. The bioreactor was operated at different OLRs ranged from 0.77 to 3.01 g COD/L‐d in four stages for a total operating period of ~ 250 days. With the increase of OLRs, the specific methane production rate increased from 105.3 to 304.6 ml/L‐d with high methane content in biogas (75.5%–83%). The maximum methane yield was achieved at hydraulic retention time (HRT) of 15 days. Highest organics and suspended solids removal (80%–83%) were achieved at 20‐days HRT, while increased OLRs resulted in diminished removal efficiencies. The state variables, including pH, total ammonia nitrogen, short‐chain volatile fatty acids, and soluble chemical oxygen demand, indicated the system had a great capability to withstand the high OLRs. Microbial community analysis revealed that the high performance might be attributed to direct interspecies electron transfer (DIET) facilitated by potentially electroactive bacteria (e.g., Syntrophomonas , Clostridium ) and electrotrophic archaea (e.g., Methanosaeta and Methanosarcina species) enriched on the carbon fibers. Practitioner points An anaerobic biofilm reactor was investigated for biomethane recovery from source‐separated blackwater. Conductive carbon fibers were utilized as the media to stimulate enrichment of potentially electroactive methanogenic communities. The bioreactor was operated at ambient temperature for over 250 days. High methane production rate and high‐quality biogas were achieved at OLRs ranged from 0.77 to 3.01 g COD/L‐d. Microbial community analysis suggested direct interspecies electron transfer (DIET) between specific electroactive bacteria and electrotrophic archaea.
Decentralized wastewater treatment represents a promising sustainable option for future wastewater management. Blackwater collected from toilets contains high concentrations of organic matter, ideal for energy recovery using anaerobic digestion. Up-flow anaerobic sludge blanket (UASB) reactors treating conventional toilet (CT, 9 L water per flush) and vacuum toilet (VT, 1 L water per flush) blackwater with increments of loadings were successfully operated to steady state in three phases. The organic loading rates were maintained at comparable levels between the two reactors. The methanisation rates were 0.23–0.29 and 0.41–0.48 gCH4-COD/gfeedCOD in the CT and VT reactors, and the COD removal rates were 72% and 89%, respectively. The enriched microbial consortia and the community dynamics under different loading phases were compared. The rank abundance distributions and alpha-diversity showed that archaeal communities were predominated by mono-enrichments in both CT and VT reactors, while bacterial communities showed lower diversity in the VT reactor. Through principal coordinates analysis (beta-diversity), clear divergences of archaeal and bacterial communities between the CT and VT reactors were revealed, and the archaeal community developed at a slower rate than the bacterial community. The enriched archaea were hydrogenotrophic methanogens, Methanolinea in the CT reactor (56.6%), and Methanogenium in the VT reactor (62.3%). The enriched bacteria were Porphyromonadaceae in both CT (15.9%) and VT (13.4%) reactors, sulfate-reducing bacteria in the CT reactor, and Fibrobacteraceae in the VT reactor (13.8%). Links between enriched consortia and ammonia stress were discussed. Isotope fraction analysis of the biogas showed a slight shift from acetoclastic methanogenesis to hydrogenotrophic methanogenesis. A closer look into the predicted metagenomic functional profiles showed agreeing results, where hydrogenotrophic methanogenesis and fhs gene abundances were higher in the VT reactor. We demonstrated that different blackwater types enriched different microbial consortia, probably due to ammonia concentrations and sulfate loadings, which should be taken into consideration for practical applications.
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