Daniel Zitomer

Bioaugmentation for Improved Recovery of Anaerobic Digesters After Toxicant Exposure

Bioaugmentation was investigated as a method to decrease the recovery period of anaerobic digesters exposed to a transient toxic event. Two sets of laboratory-scale digesters (SRT = 10 days, OLR = 2 g COD/L-day), started with inoculum from a digester stabilizing synthetic municipal wastewater solids (MW) and synthetic industrial wastewater (WW), respectively, were transiently exposed to the model toxicant, oxygen. Bioaugmented digesters received 1.2 g VSS/L-day of an H2-utilizing culture for which the archaeal community was analyzed. Soon after oxygen exposure, the bioaugmented digesters produced 25-60% more methane than non-bioaugmented controls (p < 0.05). One set of digesters produced lingering high propionate concentrations, and bioaugmentation resulted in significantly shorter recovery periods. The second set of digesters did not display lingering propionate, and bioaugmented digesters recovered at the same time as non-bioaugmented controls. The difference in the effect of bioaugmentation on recovery may be due to differences between microbial communities of the digester inocula originally employed. In conclusion, bioaugmentation with an H(2)-utilizing culture is a potential tool to decrease the recovery period, decrease propionate concentration, and increase biogas production of some anaerobic digesters after a toxic event. Digesters already containing rapidly adaptable microbial communities may not benefit from bioaugmentation, whereas other digesters with poorly adaptable microbial communities may benefit greatly.

Methanogen community structure-activity relationship and bioaugmentation of overloaded anaerobic digesters.

Accumulation of acids in anaerobic digesters after organic overload can inhibit or stop CH4 production. Therefore, methods to reduce acid concentrations would be helpful. One potential method to improve recovery involves bioaugmentation, addition of specific microorganisms to improve performance. In this study, transiently overloaded digesters were bioaugmented with a propionate-degrading enrichment culture in an effort to decrease recovery time. Biomass samples from 14 different, full-scale anaerobic digesters were screened for specific methanogenic activity (SMA) against propionate; the microbial communities were also compared. SMA values spanned two orders of magnitude. Principal component analysis of denaturing gradient gel electrophoresis (DGGE) banding patterns for a functional gene (mcrA) suggested an underlying community structure-activity relationship; the presence of hydrogenotrophic methanogens closely related to Methanospirillum hungatei and Methanobacterium beijingense was associated with high propionate SMA values. The biomass sample demonstrating the highest SMA was enriched for propionate degrading activity and then used to bioaugment overloaded digesters. Bioaugmented digesters recovered more rapidly following the organic overload, requiring approximately 25 days (2.5 solids retention times (SRTs)) less to recover compared to non-bioaugmented digesters. Benefits of bioaugmentation continued for more than 12 SRTs after organic overload. Bioaugmentation is a promising approach to decrease recovery time after organic overload.

Stoichiometry of combined aerobic and methanogenic COD transformation

Under oxygen-limited conditions, both aerobic respiration and methanogenesis were accomplished by a mixed culture developed from a municipal anaerobic digester sludge and maintained under batch conditions. Oxygen-limited systems were maintained for over 130 days, producing methane and biomass while consuming oxygen under stable conditions, with chemical oxygen demand (COD) removal averaging 92%. Typically, the dissolved oxygen in the culture medium was zero. However, short periods of greater-than-zero dissolved oxygen were evident. In this case, methanogens were not killed by dissolved oxygen, but may have been inhibited during oxic periods. Electron equivalent balances were employed to construct stoichiometric equations for propionate- and ethanol-fed cultures that received enough oxygen to oxidize approximately 0, 10, and 30% of the added substrate. Oxygen-limited cultures exhibited maximum yield coefficients (Y) of between 0.06 and 0.11 g volatile suspended solids per gram COD. These values are more typical of methanogenic, rather than aerobic systems. Under some conditions, it may be possible to achieve low COD values associated with aerobic systems and produce the minimum waste biomass associated with methanogenic systems in a single-sludge, oxygen-limited process. Other possible benefits of methanogenic, oxygen-limited systems include: degradation of specific organics and mixtures of organics that proceed through both reductive and oxidative biological reactions; degradation of specific organics through cometabolic, methanotrophic reactions; concurrent nitrification and denitrification; and description of natural systems under oxygen-limited conditions.

Methyl Coenzyme M Reductase (mcrA) Gene Abundance Correlates with Activity Measurements of Methanogenic H2/CO2-Enriched Anaerobic Biomass

Biologically produced methane (CH4) from anaerobic digesters is a renewable alternative to fossil fuels, but digester failure can be a serious problem. Monitoring the microbial community within the digester could provide valuable information about process stability because this technology is dependent upon the metabolic processes of microorganisms. A healthy methanogenic community is critical for digester function and CH4 production. Methanogens can be surveyed and monitored using genes and transcripts of mcrA, which encodes the α subunit of methyl coenzyme M reductase – the enzyme that catalyses the final step in methanogenesis. Using clone libraries and quantitative polymerase chain reaction, we compared the diversity and abundance of mcrA genes and transcripts in four different methanogenic hydrogen/CO2 enrichment cultures to function, as measured by specific methanogenic activity (SMA) assays using H2/CO2. The mcrA gene copy number significantly correlated with CH4 production rates using H2/CO2, while correlations between mcrA transcript number and SMA were not significant. The DNA and cDNA clone libraries from all enrichments were distinctive but community diversity also did not correlate with SMA. Although hydrogenotrophic methanogens dominated these enrichments, the results indicate that this methodology should be applicable to monitoring other methanogenic communities in anaerobic digesters. Ultimately, this could lead to the engineering of digester microbial communities to produce more CH4 for use as renewable fuel.

Bioaugmentation of overloaded anaerobic digesters restores function and archaeal community.

Adding beneficial microorganisms to anaerobic digesters for improved performance (i.e. bioaugmentation) has been shown to decrease recovery time after organic overload or toxicity upset. Compared to strictly anaerobic cultures, adding aerotolerant methanogenic cultures may be more practical since they exhibit higher methanogenic activity and can be easily dried and stored in ambient air for future shipping and use. In this study, anaerobic digesters were bioaugmented with both anaerobic and aerated, methanogenic propionate enrichment cultures after a transient organic overload. Digesters bioaugmented with anaerobic and moderately aerated cultures recovered 25 and 100 days before non-bioaugmented digesters, respectively. Increased methane production due to bioaugmentation continued a long time, with 50-120% increases 6 to 12 SRTs (60-120 days) after overload. In contrast to the anaerobic enrichment, the aerated enrichments were more effective as bioaugmentation cultures, resulting in faster recovery of upset digester methane and COD removal rates. Sixty days after overload, the bioaugmented digester archaeal community was not shifted, but was restored to one similar to the pre-overload community. In contrast, non-bioaugmented digester archaeal communities before and after overload were significantly different. Organisms most similar to Methanospirillum hungatei had higher relative abundance in well-operating, undisturbed and bioaugmented digesters, whereas organisms similar to Methanolinea tarda were more abundant in upset, non-bioaugmented digesters. Bioaugmentation is a beneficial approach to increase digester recovery rate after transient organic overload events. Moderately aerated, methanogenic propionate enrichment cultures were more beneficial augments than a strictly anaerobic enrichment.

Instrumentation and control of anaerobic digestion processes : A review and some research challenges

AbstractTo enhance energy production from methane or resource recovery from digestate, anaerobic digestion processes require advanced instrumentation and control tools. Over the years, research on these topics has evolved and followed the main fields of application of anaerobic digestion processes: from municipal sewage sludge to liquid—mainly industrial—then municipal organic fraction of solid waste and agricultural residues. Time constants of the processes have also changed with respect to the treated waste from minutes or hours to weeks or months. Since fast closed loop control is needed for short time constant processes, human operator is now included in the loop when taking decisions to optimize anaerobic digestion plants dealing with complex solid waste over a long retention time. Control objectives have also moved from the regulation of key variables—measured on-line—to the prediction of overall process performance—based on global off-line measurements—to optimize the feeding of the processes. Additionally, the need for more accurate prediction of methane production and organic matter biodegradation has impacted the complexity of instrumentation and should include a more detailed characterization of the waste (e.g., biochemical fractions like proteins, lipids and carbohydrates) and their bioaccessibility and biodegradability characteristics. However, even if in the literature several methodologies have been developed to determine biodegradability based on organic matter characterization, only a few papers deal with bioaccessibility assessment. In this review, we emphasize the high potential of some promising techniques, such as spectral analysis, and we discuss issues that could appear in the near future concerning control of AD processes.

Relating Anaerobic Digestion Microbial Community and Process Function

Anaerobic digestion (AD) involves a consortium of microorganisms that convert substrates into biogas containing methane for renewable energy. The technology has suffered from the perception of being periodically unstable due to limited understanding of the relationship between microbial community structure and function. The emphasis of this review is to describe microbial communities in digesters and quantitative and qualitative relationships between community structure and digester function. Progress has been made in the past few decades to identify key microorganisms influencing AD. Yet, more work is required to realize robust, quantitative relationships between microbial community structure and functions such as methane production rate and resilience after perturbations. Other promising areas of research for improved AD may include methods to increase/control (1) hydrolysis rate, (2) direct interspecies electron transfer to methanogens, (3) community structure–function relationships of methanogens, (4) methanogenesis via acetate oxidation, and (5) bioaugmentation to study community–activity relationships or improve engineered bioprocesses.

Triclocarban Influences Antibiotic Resistance and Alters Anaerobic Digester Microbial Community Structure

Triclocarban (TCC) is one of the most abundant organic micropollutants detected in biosolids. Lab-scale anaerobic digesters were amended with TCC at concentrations ranging from the background concentration of seed biosolids (30 mg/kg) to toxic concentrations of 850 mg/kg to determine the effect on methane production, relative abundance of antibiotic resistance genes, and microbial community structure. Additionally, the TCC addition rate was varied to determine the impacts of acclimation time. At environmentally relevant TCC concentrations (max detect = 440 mg/kg), digesters maintained function. Digesters receiving 450 mg/kg of TCC maintained function under gradual TCC addition, but volatile fatty acid concentrations increased, pH decreased, and methane production ceased when immediately fed this concentration. The concentrations of the mexB gene (encoding for a multidrug efflux pump) were higher with all concentrations of TCC compared to a control, but higher TCC concentrations did not correlate with increased mexB abundance. The relative abundance of the gene tet(L) was greater in the digesters that no longer produced methane, and no effect on the relative abundance of the class 1 integron integrase encoding gene (intI1) was observed. Illumina sequencing revealed substantial community shifts in digesters that functionally failed from increased levels of TCC. More subtle, yet significant, community shifts were observed in digesters amended with TCC levels that did not inhibit function. This research demonstrates that TCC can select for a multidrug resistance encoding gene in mixed community anaerobic environments, and this selection occurs at concentrations (30 mg/kg) that can be found in full-scale anaerobic digesters (U.S. median concentration = 22 mg/kg, mean = 39 mg/kg).

Biochar from Pyrolysis of Biosolids for Nutrient Adsorption and Turfgrass Cultivation.

At water resource recovery facilities, nutrient removal is often required and energy recovery is an ever-increasing goal. Pyrolysis may be a sustainable process for handling wastewater biosolids because energy can be recovered in the py-gas and py-oil. Additionally, the biochar produced has value as a soil conditioner. The objective of this work was to determine if biochar could be used to adsorb ammonia from biosolids filtrate and subsequently be applied as a soil conditioner to improve grass growth. The maximum carrying capacity of base modified biochar for NH3-N was 5.3 mg/g. Biochar containing adsorbed ammonium and potassium was applied to laboratory planters simulating golf course putting greens to cultivate Kentucky bluegrass. Planters that contained nutrient-laden biochar proliferated at a statistically higher rate than planters that contained biosolids, unmodified biochar, peat, or no additive. Nutrient-laden biochar performed as well as commercial inorganic fertilizer with no statistical difference in growth rates. Biochar from digested biosolids successfully immobilized NH3-N from wastewater and served as a beneficial soil amendment. This process offers a means to recover and recycle nutrients from water resource recovery facilities.

Emerging investigators series: pyrolysis removes common microconstituents triclocarban, triclosan, and nonylphenol from biosolids

Reusing biosolids is vital for the sustainability of wastewater management. Pyrolysis is an anoxic thermal degradation process that can be used to convert biosolids into energy rich py-gas and py-oil, and a beneficial soil amendment, biochar. Batch biosolids pyrolysis (60 minutes) revealed that triclocarban and triclosan were removed (to below quantification limit) at 200 °C and 300 °C, respectively. Substantial removal (>90%) of nonylphenol was achieved at 300 °C as well, but 600 °C was required to remove nonylphenol to below the quantification limit. At 500 °C, the pyrolysis reaction time to remove >90% of microconstituents was less than 5 minutes. Fate studies revealed that microconstituents were both volatilized and thermochemically transformed during pyrolysis; microconstituents with higher vapor pressures were more likely to volatilize and leave the pyrolysis reactor before being transformed than compounds with lower vapor pressures. Reductive dehalogenation products of triclocarban and suspected dehalogenation products of triclosan were identified in py-gas. Application of biosolids-derived biochar to soil in place of biosolids has potential to minimize organic microconstituents discharged to the environment provided appropriate management of py-gas and py-oil.