Responses of microbial capacity and community on the performance of mesophilic co-digestion of food waste and waste activated sludge in a high-frequency feeding CSTR

Comparison of the short- and long-term effects of zinc ions on the anaerobic mesophilic co-digestion of food waste and waste activated sludge: Digester performance, antibiotic resistance gene reduction and the microbial community

Heavy metals contained in waste activated sludge (WAS), especially zinc ions, have an inhibitory effect on the anaerobic digestion. However, the effects of zinc ions on digester performance, antibiotic resistance genes (ARGs) reduction, and the microbial community involved in the anaerobic mesophilic co-digestion (AcoD) of WAS and food waste (FW) have not been fully characterized. Therefore, batch trials and continuous stirred tank reactors were used under different zinc-ion concentrations. Findings showed that the AcoD system can tolerate a maximum zinc ion of 540 mg/L in a short-term batch and 470 mg/L in a long-term AcoD system, promoting methane production of approximately 1.0-17.0 %. Metagenomic analysis revealed that syntrophic H2 transfer occurred between Syntrophomonas and Methanoculleus and the aceticlastic and hydrogenotrophic methanogenic pathways were both enhanced by 1.18- and 1.16 times, respectively. Moreover, the relative abundance of Methanosarcina increased from 58.4 % to 72.5 % at 470 mg/L to adapt to the high zinc ion concentration during long-term continuous operation. These results revealed that AcoD with a low zinc ion concentration can effectively increase the removal percentage of ARGs. The results provide guidance for biogas recovery and use of mesophilic AcoD with FW and WAS containing high zinc ion concentrations without pretreatment process.

Enhanced digestion of sludge via co-digestion with food waste in a high-solid anaerobic membrane bioreactor: Performance evaluation and microbial response

A 15 L high-solid mesophilic AnMBR was operated for the digestion of food waste, primary sludge and excess sludge. The digestion performance was evaluated from the perspective of methane generation, permeate quality and organic reduction. Furthermore, the change in the microbial community was investigated by 16S rRNA gene analysis. The results showed that the introduction of sludge decreased the H2S levels in biogas compared with the mono-digestion of food waste and the co-digestion with food waste increased biogas and methane production compared with the mono-digestion of sludge. A substitution ratio of 25 % became a turning point of permeate composition and reaction rates. The energy recovery ratios of the mesophilic AnMBR were over 75 % based on stoichiometric analysis. In reaction kinetics analysis, hydrolysis as the first step of anaerobic digestion was found to be most influenced by the composition of the substrate. Finally, the microbial community structures were stable under tested conditions while the evolutionary relationships within the dominant phyla were observed. In the archaea community, Methanosaeta was the dominant methanogen regardless sludge ratio in the substrate.

Long-term in-situ starvation and reactivation of co-digestion with food waste and corn straw in a continuous AnDMBR: Performance, sludge characteristics, and microorganism community

To explore the effects of in-situ starvation and reactivation in a continuous anaerobic dynamic membrane reactor (AnDMBR), the anaerobic co-digestion system of food waste and corn straw was firstly start-up and stability operated, and then stopped feeding substrate approximately 70 days. After long-term in-situ starvation, the continuous AnDMBR was reactivated using the same operation conditions and organic loading rate as the continuous AnDMBR used before in-situ starvation. Results shown that the anaerobic co-digestion of corn straw and food waste in the continuous AnDMBR can resume stable operation within 5 days, and the corresponding methane production of 1.38 ± 0.26 L/L/d was completely returned to the methane production before in-situ starvation (1.32 ± 0.10 L/L/d). Through analysis of the specific methanogenic activity and key enzyme activity of the digestate sludge, only the acetic acid degradation activity of methanogenic archaea can be partially recovered, however, the activities of lignocellulose enzyme (lignin peroxidase, laccase, and endoglucanase), hydrolase (α-glucosidase) and acidogenic enzyme (acetate kinas, butyrate kinase, and CoA-transferase) can be fully recovered. Analysis of microorganism community structure using metagenomic sequencing technology showed that starvation decreased the abundance of hydrolytic bacteria (Bacteroidetes and Firmicutes) and increased the abundance of small molecule-utilizing bacteria (Proteobacteria and Chloroflexi) due to lack of substrate during the long-term in-situ starvation stage. Furthermore, the microbial community structure and key functional microorganism still maintained and similar with that of starvation final stage even after long-term continuous reactivation. The reactor performance and sludge enzymes activity in the continuous AnDMBR co-digestion of food waste and corn straw can be well reactivated after long-term in-situ starvation, even though the microbial community structure can not be recovered to the initiating stage.

Strategies for enhancing the efficacy of anaerobic digestion of food industry wastewater: An insight into bioreactor types, challenges, and future scope

Food waste have become a growing concern worldwide with raising population and economic growth. Wastewater discharged from food industries contains many valuable and toxic components that have a negative impact on the ecological system. Large amounts of wastewater are discharged from the food industry, which necessitates the creation of effective technologies. Wastewater from the food industry can be seen as a rich source of energy and a primary source for generating valuable products. Waste disposal and resource recovery are sustainably valued by anaerobic digestion of wastewater from the food sector. The characteristics, composition, and nature of wastewater produced from various food sectors are elaborated upon in this review. An overview of the anaerobic digestion process for wastewater treatment in the food industry is included. Enhancement strategies for the anaerobic digestion process have been discussed in detail. In addition, various types of reactors utilized for performing anaerobic digestion is illustrated. Though anaerobic digestion process possesses advantages, the challenges and future scope are examined for improving the outcome.

Comparative study of the effect of loading increments on the mesophilic codigestion of waste activated sludge and food waste: Reactor performance, stability analysis, and microbial community

The performance and stability of mesophilic codigestion of waste activated sludge (WAS) and food waste (FW) were compared in two parallel, continuously stirred tank reactors using high- and low-magnitude loading increments for the loading regimes. The results indicated that a high methane (CH₄) production of 6.98 L L^-1·d^-1 was realized without volatile fatty acid accumulation via low-magnitude loading increments at a high loading of 26.5 g-COD·L^-1·d^-1, and this system was more stable and achieved a higher efficiency than the codigestion system that used high-magnitude loading increments at similar loading and operating conditions. Furthermore, higher CH₄ yields of 258-334 mL-CH₄·g-COD^-1, TCOD removal efficiencies of 64-79%, conversion ratios of 62-88%, and methanogenic activities of 0.37-0.40 g-CH₄-COD·g-VS^-1·d^-1 were consistently maintained via the low-magnitude loading increments during the high-rate period. High abundances of the phyla Firmicutes (63.3%) and genus Methanosarcina (94.5%) contributed to the high rates and stable operating conditions of the mesophilic system for WAS and FW codigestion using low-magnitude loading increments.

Effects of organic loading rate and pretreatments on digestion performance of corn stover and chicken manure in completely stirred tank reactor (CSTR)

The performance, system stability, and microbial community response in anaerobic co-digestion (AcoD) of corn stover (CS) and chicken manure (CM) were investigated by running completely stirred tank reactor (CSTR) under controlled organic loading rate (OLR). Prior to anaerobic digestion (AD), potassium hydroxide (KOH) or liquid fraction of digestate (LFD) was applied to pretreat CS, respectively. The results showed that the daily biogas production (DBP) in co-digestion showed a gradual increasing trend with an increase in the OLR from 65 g TS·L^-1 to 100 g TS·L^-1. The daily methane production per g volatile solids (DMP-VS) in co-digestion increased by 23.0%-27.1%, 18.7%-18.8%, and 17.5%-18.0% at the OLRs of 65, 80, and 100 g TS·L^-1, respectively, upon pretreatment with KOH or LFD, as compared to that in co-digestion CSTR without any pretreatment. In addition, all co-digestion CSTRs were operated in stable state. Approximately half of the total carbon in the substrates was recovered in the form of a biogas product, with the carbon mass balance being impacted by the OLR as well as pretreatment. The diversity as well as function of the microbial community varied in response to different OLRs and pretreatment methods. The majority of bacterial genera were strongly correlated with operational parameters. The study indicates that management of OLR and selection of proper pretreatment method could enhance the efficiency and productivity of CS and CM co-digestion in CSTR.

Sequencing Batch Integrated Fixed-Film Activated Sludge Membrane Process for Treatment of Tapioca Processing Wastewater

Tapioca processing industries are very popular in the rural community to produce a variety of foods as the end products. Due to their small scales and scattered locations, they require robust modular systems to operate at low capacity with minimum supervision. This study explores the application of a novel sequencing batch-integrated fixed-film activated sludge membrane (SB-IFASM) process to treat tapioca processing wastewater for reuse purposes. The SB-IFASM employed a gravity-driven system and utilizes biofilm to enhance biodegradation without requiring membrane cleaning. The SB-IFASM utilizes the biofilm as a secondary biodegradation stage to enhance the permeate quality applicable for reuse. A lab-scale SB-IFASM was developed, preliminarily assessed, and used to treat synthetic tapioca processing industry wastewater. The results of short-term filtration tests showed the significant impact of hydrostatic pressure on membrane compaction and instant cake layer formation. Increasing the pressure from 2.2 to 10 kPa lowered the permeability of clean water and activated sludge from 720 to 425 and from 110 to 50 L/m²·h bar, respectively. The unsteady-state operation of the SB-IFASM showed the prominent role of the bio-cake in removing the organics reaching the permeate quality suitable for reuse. High COD removals of 63-98% demonstrated the prominence contribution of the biofilm in enhancing biological performance and ultimate COD removals of >93% make it very attractive for application in small-scale tapioca processing industries. However, the biological ecosystem was unstable, as shown by foaming that deteriorated permeability and was detrimental to the organic removal. Further developments are still required, particularly to address the biological stability and low permeability.

Influence of initial pH on the production of volatile fatty acids and hydrogen during dark fermentation of kitchen waste

The purpose of this work was to determine the effect of initial pH on the production of volatile fatty acids (VFA) and hydrogen (H₂) in the dark fermentation processes of kitchen waste. The study was conducted in batch bioreactors of working volume 1 L for different initial pH in the range from 5.5 to 9.0. The dark fermentation processes were carried out for 4 days at 37°C. Initial organic load of the kitchen waste in all bioreactors amounted to 25.5 gVS/L. Buffering of pH during the fermentation process was carried out with the use of ammonia contained mainly in digested sludge. The optimal conditions for the production of VFA and H₂ were achieved at the initial pH of 8. Production of VFA and H₂ in these conditions was, respectively, 13.9 g/L and 72.4 mL/gVS. The main produced components of VFA were acetic and butyric acids. The production of ethanol and lactic acid was at very low levels due to the high ratio of the volatile fatty acids to total organic content of 0.86. With the optimal initial pH of 8 the yield of CO₂ production was 0.30 gC/gC. High initial pH value (above 8) extended the lag phase duration in the course of H₂ production. The dominant groups of micro-organisms at the most favourable initial pH of 8 for the production of VFA and H₂ were Bacteroidetes, Firmicutes, Spirochaetes and Waste Water of Evry 1 (WWE1) at the phylum level.

Production of Volatile Fatty Acids in a Semi-Continuous Dark Fermentation of Kitchen Waste: Impact of Organic Loading Rate and Hydraulic Retention Time

The aim of the study was to evaluate the possibility of using the process of dark fermentation to convert kitchen waste into valuable volatile fatty acids in a semi-continuous process at different values of the organic loading rate (2.5 and 5.0 gVS/(L × d)) and hydraulic retention time (5 and 10 d) using anaerobic mixed microbial consortia. The experiments were performed in a bioreactor of working volume 8L with pH control. The maximum volatile fatty acids yield in a steady state (22.3 g/L) was achieved at the organic loading rate of 5.0 gVS/(L × d) and HRT of 10 days. The main products of dark fermentation were acetic and butyric acids, constituting, respectively, 35.2–47.7% and 24.1–30.0% of all identified volatile fatty acids. Additionally, at the beginning of the fermentation and in a steady-state condition, the microbial population analysis (16S rDNA) of the fermentation mixture with the most effective volatile fatty acids generation has been performed to monitor the DF microflora development. The dominant microorganisms at a phylum level in a steady state were Firmicutes (44.9%) and Bacteroidetes (30.1%), which indicate the main role of those phyla in the volatile fatty acids synthesis.

Achieving single-stage partial nitritation and anammox (PN/A) using a submerged dynamic membrane sequencing batch reactor (DM-SBR)

Single-stage partial nitration and anammox (PN/A) process was achieved in a sequencing batch reactor (SBR) using a submerged dynamic membrane (DM) in this study. The reactor was stably operated for 200 days, and the nitrogen removal efficiency (NRE) was sustained at 70.3 ± 7.2% at a nitrogen loading rate (NLR) ranging from 0.1 to 0.3 kgNm^-3  day^-1 with a hydraulic retention time (HRT) of 24 hr. When the NLR was 0.2 kgN m^-3 day^-1 , the NRE achieved was high as 80% with a low concentration of dissolved oxygen (DO) of 0.13 mg/L. In addition, the specific activity of anammox bacteria and ammonia-oxidizing bacteria (AOB) reached was 2.72 and 16.80 gN gVSS^-1 day^-1 , respectively. The DM intercepted the biomass due to the lamellar, intact, dense biofilm self-generated on the surface of the supporting material, which had an effluent turbidity of 10 NTU. The enriched anammox functional bacteria were Candidatus Jettenia (11.06%) and the AOB-like functional bacteria consisted primarily of Nitrosomonas, with a relative abundance of 2.76%, which ensured the PN/A process proceeding. This study provides a novel reactor configuration of the single-stage PN/A process in the view of practical applications. PRACTITIONER POINTS: Single-stage partial nitration and anammox (PN/A) process was achieved using a submerged dynamic membrane (DM) in this study. The reactor was stably operated for 200 days, and the nitrogen removal efficiency was sustained at 70.3 ± 7.2%. The feasibility of the PN/A system with DM is evaluated. The main objective is to provide a control strategy of the DM-SBRs for practical applications.

Ammonia-induced inhibition of manure-based continuous biomethanation process under different organic loading rates and associated microbial community dynamics

Three Continuously Stirred Tank Reactors (CTSRs) were operating at steady state conditions with Organic Loading Rates (OLR) of 2.09, 3.024 and 4.0 g VS L^-1 d^-1. Glucose was used as the sole factor for increasing the OLR, linking the increase of the OLR with the C/N ratio increase. The reactors were stressed by increasing the ammonia concentration to 5 g L^-1 from 1.862 g L^-1. The results showed elevating inhibition of the anaerobic process by increasing the C/N ratio just by increasing the OLR, under the high ammonia concentration. A different response of the bacterial and archaeal community under ammonia stressed conditions was also observed. Under the high ammonia concentration, hydrogen-depended methylotrophic was the dominant methanogenesis route at OLR of 2.09 g VS L^-1d^-1, while the hydrogenotrophic route was the dominant at the high OLR of 4 g VS L^-1d^-1, which coincided with high acetate and propionate concentrations.

Effects of long-term acclimatization on the optimum substrate mixture ratio and substrate to inoculum ratio in anaerobic codigestion of food waste and cow manure

The effects of long-term acclimatization on the optimum food waste to cow manure ratio (FW/CM) and substrate to inoculum ratio (S/I) in anaerobic codigestion with FW and CM were investigated by batch trials. For the unacclimated sludge, the highest CH₄ yields of 646.6 and 653.4 mL/g VS were achieved under the optimum FW/CM (2.5 VS/VS) and S/I (0.07 VS/VS) ratios, respectively. After more than 550 days of acclimatization, the optimum FW/CM and S/I of the acclimated sludge were improved to 3.4 and 0.68 VS/VS with more anaerobic digestion enzymes and lignocellulose, respectively. Based on high-throughput sequencing analysis, the microbial community structures of bacteria, fungi, and archaea were changed, which was the main reason for the change in the optimum FW/CM and S/I. Therefore, the FW/CM and S/I should be periodically optimized during the long-term operation of codigestion to improve the codigestion efficiency for biogas production.

Survey of microbes in industrial-scale second-generation bioethanol production for better process knowledge and operation

The microbes present in bioethanol production processes have been previously studied in laboratory-scale experiments, but there is a lack of information on full-scale industrial processes. In this study, the microbial communities of three industrial bioethanol production processes were characterized using several methods. The samples originated from second-generation bioethanol plants that produce fuel ethanol from biowaste, food industry side streams, or sawdust. Amplicon sequencing targeting bacteria, archaea, and fungi was used to explore the microbes present in biofuel production and anaerobic digestion of wastewater and sludge. Biofilm-forming lactic acid bacteria and wild yeasts were identified in fermentation samples of a full-scale plant that uses biowaste as feedstock. During the 20-month monitoring period, the anaerobic digester adapted to the bioethanol process waste with a shift in methanogen profile indicating acclimatization to high concentrations of ammonia. Amplicon sequencing does not specifically target living microbes. The same is true for indirect parameters, such as low pH, metabolites, or genes of lactic acid bacteria. Since rapid identification of living microbes would be indispensable for process management, a commercial method was tested that detects them by measuring the rRNA of selected microbial groups. Small-scale testing indicated that the method gives results comparable with plate counts and microscopic counting, especially for bacterial quantification. The applicability of the method was verified in an industrial bioethanol plant, inspecting the clean-in-place process quality and detecting viability during yeast separation. The results supported it as a fast and promising tool for monitoring microbes throughout industrial bioethanol processes.

Stable and high-rate anaerobic co-digestion of food waste and cow manure: Optimisation of start-up conditions

Food waste (FW) and cow manure (CM) were co-digested to achieve a stable and high-rate of methane production. The start-up conditions (substrate mixing (FW/CM) ratio, substrate to inoculum ratio, and initial pH) were optimised, and the optimised parameters were experimentally confirmed by batch operation under mesophilic temperatures. To further verify the effects of start-up conditions on the long-term co-digestion process, a semi-continuous dynamic membrane bioreactor was operated for over 300 days with an FW/CM ratio of 2.5. Following the optimised operation scheme, the organic loading rate gradually increased to 11.9 g COD/L/d. Thus, stable anaerobic co-digestion was maintained at FW/CM ratio of 2.5, and a high CH₄ production of 2.71 L CH₄/L/d and CH₄ yield of 441 mL CH₄/g VS was achieved. In the long-term operation, the digestate pH was stable at approximately 8.4, which indicated a very favourable anaerobic reaction condition without volatile fatty acid accumulation.

Optimization ofS/Fe ratio for enhanced nitrobenzene biological removal in anaerobicSystem amended withSulfide-modified nanoscale zerovalent iron

Anaerobic reduction of nitrobenzene (NB) can be efficiently enhanced bySupplementing withSulfide-modified nanoscale zerovalent iron (S-nZVI). In thisStudy,S/Fe ratio ofS-nZVI was further optimized for enhancing biological NB removal in anaerobicSystem amended withS-nZVI and inoculated by anaerobicSludge. The results indicated that the performance andStability of the coupled anaerobicSystem for NB reduction and aniline formation were remarkably improved byS-nZVI atS/Fe molar ratio of 0.3 (0.3S-nZVI). TheSecretion of extracellular polymericSubstances (EPS), transformation of volatile fatty acids (VFAs), yield of methane and activity ofSeveral key enzymes could be efficiently improved by 0.3S-nZVI. Furthermore,Species related to NB reduction, fermentation, electroactivity and methanogenesis could be enriched in 0.3S-nZVI coupled anaerobicSystem, with remarkable improvement in the biodiversity observed. ThisStudy demonstrated thatSulfidation would be a promising method to improve the performance of nZVI in coupled anaerobicSystems for the removal of recalcitrant nitroaromatic compounds from wastewater.

Cow manure as additive to a DMBR for stable and high-rate digestion of food waste: Performance and microbial community

Cow manure (CM) was added to a dynamic membrane bioreactor (DMBR) operated under anaerobic condition for enhancing food waste (FW) digestion for over 300 days with stepwise increase of organic loading rates (OLRs) from 1.07 to 11.9 g COD/L/day. At a FW/CM ratio of 3.5:1 (based on volatile solids), the mixed liquor pH was always above 8.0 and no apparent volatile fatty acids (VFAs) accumulation occurred even at the highest OLR of 11.9 g COD/L/day (hydraulic retention time as 10 days and solid retention time as 15.5 days, correspondingly), indicating a very stable operation condition which resulted in an average CH₄ yield as high as 250 mL/g COD and CH₄ production as high as 2.71 L CH₄/L/day. The hardly biodegradable organic components, such as cellulose, hemicellulose, and lignin, were effectively degraded by 78.3%, 58.8%, and 47.5%, respectively. Significantly high anaerobic digestion reaction ratios, especially the hydrolysis ratio which is usually the limiting factor, were calculated based on experimental results. Furthermore, the high lignocellulase contents and coenzyme F₄₂₀ levels, along with the decrease of cellulose crystallinity from 72.6% to 16.4% in the feedstock, provided strong evidence of an enhanced biological activity by CM addition. By high-throughput sequencing analysis, more abundant and diverse bacterial, archaeal, and fungal genera were identified from the DMBR sludge. With CM addition, the biodegradation of lignocellulose might have produced sufficient H₂ and CO₂ for the hydrogenotrophic methanogens such as Methanoculleus, Methanomassiliicoccus, and Methanobacterium, which were highly tolerant to ammonium inhibition, and then the elevated ammonium level would have provided high buffering capacity in the DMBR thus ensuring a stable condition for high rate FW digestion and CH₄ production.

Effect of clarithromycin on the production of volatile fatty acids from waste activated sludge anaerobic fermentation

This study investigated the effect of Clarithromycin (CLA) on volatile fatty acids (VFAs) production during waste activated sludge (WAS) anaerobic fermentation for the first time. Experimental results showed that when CLA concentration in WAS increased from 0 to 1000 mg/kg TSS, the maximum yield of VFAs increased from 27.7 to 35.7 mg COD/g VSS (without pH pretreatment) and from 59.3 to 65.6 mg COD/g VSS (initial pH 9 pretreatment), respectively. Mechanism exploration revealed that CLA facilitated the disruption of extracellular polymeric substances, thus promoting WAS solubilization. CLA inhibited all the other anaerobic fermentation processes. However, its inhibition to acetogenesis and methanogenesis was severer than that to hydrolysis and acidogenesis, resulting in the decrease in VFAs consumption. Microbial analysis showed that CLA slightly increased the abundance of microorganisms responsible for hydrolysis and acidogenesis whereas decreased the proportion of VFAs-consuming microorganisms.

Performance and microbial community of an expanded granular sludge bed reactor in the treatment of cephalosporin wastewater

In this study, the anaerobic treatment and microbial characteristics of high-concentration cephalosporin wastewater were studied. A pilot-scale expanded granular sludge bed (EGSB) reactor was designed to treat cephalosporin wastewater, whose diameter, height and effective volume were 0.5 m, 4.9 m, 0.92 m³, respectively. With mixed high-concentration cephalosporin wastewater and municipal wastewater as a substrate, the anaerobic reactor was started and operated 414 days. An average COD removal efficiency of 72% was achieved at an organic loading rate (OLR) of 9.96 kg COD/(m³·d), with a hydraulic retention time (HRT) of 25 h. The average methane content reached 82%. Methanobacterium and Methanomassiliicoccus were predominant archaea in the granular sludge for each of the organic loading rates, and the predominant methane-producing pathway was hydrogenotroph and methylotroph. Those results demonstrated that the EGSB reactor could treat high-concentration cephalosporin wastewater with a unique methane-producing pathway.

Functional biodiversity and plasticity of methanogenic biomass from a full-scale mesophilic anaerobic digester treating nitrogen-rich agricultural wastes

The effect of ammonia on methanogenic biomass from a full-scale agricultural digester treating nitrogen-rich materials was characterized in batch activity assays subjected to increasing concentrations of total ammonia N. Acetotrophic and methanogenic profiles displayed prolonged lag phases and reduced specific activity rates at 6.0 gN-TAN L^-1, though identical methane yields were ultimately reached. These results agreed with the expression levels of selected genes from bacteria and methanogenic archaea (qPCR of 16S rRNA and mrcA cDNA transcripts). Compound-specific isotope analysis of biogas indicated that ammonia exposure was associated to a transition in methanogenic activity from acetotrophy at 1.0 gN-TAN L^-1 to intermediate and complete hydrogenotrophy at 3.5 and 6.0 gN-TAN L^-1. Such pattern matched the results of 16S-Illumina sequencing of genes and transcripts in that predominant methanogens shifted, along with increasing ammonia, from the obligate acetotroph Methanosaeta to the hydrogenotrophic Methanoculleus and the poorly understood methylotrophic Methanomassiliicoccus. The underlying bacterial community structure remained rather stable but, at 6.0 gN-TAN L^-1, the expression level increased considerably for a number of ribotypes that are related to potentially syntrophic genera (e.g. Clostridium, Bellilinea, Longilinea, and Bacteroides). The predominance of hydrogenotrophy at high ammonia levels clearly points to the occurrence of the syntrophic acetate oxidation (SAO), but known SAO bacteria were only found in very low numbers. The potential role of the identified bacterial and archaeal taxa with a view on SAO and on stability of the anaerobic digestion process under ammonia stress has been discussed.