Mesophilic Digestion Research Articles

Mechanism of zero valent iron and anaerobic mesophilic digestion combined with hydrogen peroxide pretreatment to enhance sludge dewaterability: Relationship between soluble EPS and rheological behavior

This study proposed a novel two-step conditioning strategy to enhance activated sludge (AS) dewatering performance. The method involved a zero valent iron (ZVI), anaerobic mesophilic digestion (AMD) process, and hydrogen peroxide (H2O2) oxidation. Response surface methodology (RSM) was applied to achieve optimum dewatering conditions. After the combined conditioning, dewatering was significantly better in the treated sludge compared to the raw AS. The specific resistance of filtration (SRF) of the treated sludge decreased to 2.48 × 1011 m/kg; this SRF level was 93.60% lower compared to the raw AS. The bound water content (BWC) decreased to 1.19 g/g dry solid (DS); this BWC level was 15.2% lower compared to the raw AS. The water content of the treated sludge cake decreased to 44.18 ± 0.46%. An economic analysis shows that ZVI-AMD-H2O2 can be used in real-world settings. Investigations of the underlying mechanisms showed that small block structures were formed after conditioning; viscosity and the colloidal forces of the sludge decreased; and organic matter and BWC were released from inner extracellular polymeric substances (EPS) layers to form soluble (SB)-EPS. This study illuminated the relationship between SB-EPS and the rheological behavior of AS. There is a high correlation coefficient between rheological parameter τy and N-containing substances in SB-EPS (R = −0.993, p < 0.05). The ZVI-AMD-H2O2 process effectively changed the EPS content, especially protein materials. This led to a decrease in AS viscosity and an increase in sludge dewaterability.

Evaluating the level of ammonia and sulfide in the liquid phase during anaerobic digestion of slaughterhouse waste operating at mesophilic scale digester—the impact of inhibition and process performance

The performance of experimental batch-reactor loaded with slaughterhouse waste at mesophilic temperature was investigated as well as the inhibition of both ammonia and sulfide concentration in the aqueous phase. The digester was operated for 68 days by evaluating the process stability basing on controlling parameters such as pH, volatile fatty acids and alkalinity in relation to the methane produced. The maximum CH4 content of 69.6% was achieved at 0.37 VFA/Alkalinity ratio and pH of 7.51 during day 37 of anaerobic digestion. However, a sudden increase of ammonia nitrogen in the digester from day 44 to day 68 decreased the methane content about 62.15% from 65% to 24.6%. Similarly, as the amount of sulfide content decreased in the liquid phase, gaseous H2S was elevated up to 132 ppm in the 68th day. During this time, it was observed that the ratio of VFA/Alkalinity decreased to 0.16, with a very low concentration of VFA, which was 150.92 mg/L. This phenomenon indicated that all the acids produced were consumed by methanogens and ammonia inhibition was at the highest rate due to the increase of ammonia nitrogen concentration in the last days of digestion. Furthermore, among of peculiar characteristic shown by slaughterhouse waste is the ability to maintain the pH above 7 without the addition of any buffering agent throughout the AD process. Meanwhile, the evaluation of the level of both ammonia and sulfide in the aqueous phase revealed that the inhibitory effect of ammonia concentration was higher than sulfide concentration.

Effect of viscosity on process stability and microbial community composition during anaerobic mesophilic digestion of Maotai-flavored distiller’s grains

To investigate the effects of viscosity on the mesophilic digestion of Maotai-flavored distiller’s grains, a continuous experiment was conducted in a 70 L reactor at organic loading rates of 3, 4, 5, and 6 g VS/(L·d) with and without effluent recirculation. High organic loading rates and continuous effluent recirculation increased the digestate viscosity, and high viscosity caused severe foaming, which blocked the biogas outlet pipe. Moreover, a viscosity above 782 mPa·s was proposed as an early warning indicator for foaming. A maximum volumetric biogas production rate of 1.72 L/(L·d) was accomplished by diluting the feed without effluent recirculation at a recommended organic loading rate of 5 g VS/(L·d). Proteiniphilum, Ruminococcus_2, norank_f_Synergistaceae, norank_o__DTU014, Syntrophomonas, Methanosarcina, Methanobacterium, and Methanosaeta were the dominant acidogens, syntrophic bacteria, and methanogens existed in both low and high viscosity groups. Candidatus_Methanofastidiosum capable of employing the methylated thiol reduction pathway was found only in the high viscosity system.

Differences of methanogenesis between mesophilic and thermophilic in situ biogas-upgrading systems by hydrogen addition

To investigate the differences in microbial community structure between mesophilic and thermophilic in situ biogas-upgrading systems by H2 addition, two reactors (35°C and 55°C) were run for four stages according to different H2 addition rates (H2/CO2 of 0:1, 1:1, and 4:1) and mixing mode (intermittent and continuous). 16S rRNA gene-sequencing technology was applied to analyze microbial community structure. The results showed that the temperature is a crucial factor in impacting succession of microbial community structure and the H2 utilization pathway. For mesophilic digestion, most of added H2 was consumed indirectly by the combination of homoacetogens and strict aceticlastic methanogens. In the thermophilic system, most of added H2 may be used for microbial cell growth, and part of H2 was utilized directly by strict hydrogenotrophic methanogens and facultative aceticlastic methanogens. Continuous stirring was harmful to the stabilization of mesophilic system, but not to the thermophilic one.

Analysis of uncontrolled phosphorus precipitation in anaerobic digesters under thermophilic and mesophilic conditions

ABSTRACT This study compares the operation of mesophilic and thermophilic anaerobic digestion of sewage sludge and their effects in uncontrolled phosphorus precipitation. The research has been carried out using a pilot plant consisting of two digesters of 1.6 m3 working volume, treating the mixed sludge of Alzira WWTP (Valencia, Spain). The digesters were operated in parallel, at different conditions: mesophilic (38 ± 2.0°C) and thermophilic (55 ± 2.5°C) temperatures and organic loading rates (OLR) ranging from 1.1 to 1.7 kg volatile solids (VS) m−3 d−1 and different hydraulic retention times (HRT) 20, 15 and 12 days. Uncontrolled precipitation was evaluated through P, Mg and Ca mass balances in both digesters. The results revealed that up to 82% of the available P and 81% of the available Mg precipitated in the mesophilic digester at HRT = 20 days which suggests the possible formation of struvite in both digesters. At lower HRT (HRT = 12 days) Mg and Ca precipitation was negligible and P fixation has been attributed to the possible formation of iron phosphates or adsorption processes on solid surfaces.

Responses of antibiotics, antibiotic resistance genes, and mobile genetic elements in sewage sludge to thermal hydrolysis pre-treatment and various anaerobic digestion conditions

Sewage sludge from wastewater treatment plants (WWTPs) harbours large amounts of antibiotics, antibiotic resistance genes (ARGs), and mobile genetic elements (MGEs), the variation and fate of these emerging pollutants during sludge treatment processes must be thoroughly studied to reduce their potential risks to human health. In this study, 7 pilot-scale CSTR anaerobic digesters were established with the same seed sludge and fed with the same thermal hydrolysis pre-treated sewage sludge, while operating under different conditions. High-throughput quantitative PCR, UPLC-MS/MS and Illumina Hiseq-sequencing were used to systematically evaluate the responses of antibiotics, ARGs, and MGEs in sewage sludge to thermal hydrolysis pre-treatment and various anaerobic digestion (AD) conditions. The results showed that thermal hydrolysis effectively reduced the abundance (>94%) of almost all subtypes of ARGs and MGEs, and it was a powerful technology for reducing tetracyclines, macrolides, and lincosamides. Besides, the abundance of ARGs and MGEs in thermophilic digesters was lower than that in mesophilic digesters, suggesting that thermophilic digesters could be used to avoid the ARGs rebounding. In addition, the thermophilic system further reduced the concentrations of quinolones. For the digesters operated under the mesophilic conditions, a longer hydraulic retention time (HRT) facilitated the removal of antibiotics, ARGs, and MGEs. Furthermore, the microbial community and MGEs had important effects on the persistence and proliferation of ARGs in AD process. The findings of this study provide effective clues for controlling the spread of antibiotic resistance and suggest the optimal operating conditions of digesters.

Novel insight into sludge dewaterability mechanism using polymeric aluminium ferric chloride and anaerobic mesophilic digestion treatment under ultrahigh pressure condition

There are significant challenges associated with treating sewage sludge with high water content. In this study, polymeric aluminium ferric chloride (PAFC)/anaerobic mesophilic digestion (AMD) conditioning combined with ultrahigh pressure filter press (UPFP) dewatering was investigated as a novel and feasible technique to improve sludge dewatering. Response surface methodology experiments found that the optimal operating conditions were at an AMD temperature of 42.6 °C, an AMD reaction time of 240.00 min, a PAFC dose of 150.00 mg/g dry solid, and filter pressure of 5.37 MPa. Under these optimal conditions, the water content of the dewatered cake was reduced by 54.54 wt%. The study explored the detailed mechanisms involved with the conditioning and dewatering process, by tracing the sludge, filter cloth, and filtrate characteristics. First, during the combined PAFC/AMD conditioning process, the treatment effectively removed the protein-like and polysaccharide-like materials in the extracellular polymeric substances of sludge, forming large flocs. Over time, the sludge had a lower viscosity and a higher flowability, promoting the release of bound water. Second, during the UPFP dewatering process, the filter cloth maintained its high permeability, and water easily passed through it. Third, high levels of dissolved organic matter were released after the dewatering process, ultimately forming a porous and rigid sludge cake. Furthermore, the study found strong correlations between sludge dewaterability and sludge, filter cloth, and filtrate characteristics. The experiment involving recycling filter cloth demonstrated that the combined treatment effectively prevented and reduced clogging of the filter cloth, reducing costs.

Engineered addition of slag fines for the sequestration of phosphate and sulfide during mesophilic anaerobic digestion.

The potential for weathered steel slag fines (D50 <1.6mm) to sequester dissolved phosphorus and sulfur from fermenting biosolids was determined. Batch studies showed that sorption equilibrium between phosphates and sulfides was reached in less than48hr after adding basic oxygen furnace (BOF) slag grains to municipal digester sludge. When these same particles were added to pilot digesters (75L) operating at a 30-day SRT, a classic dose-response was observed with respect to PO4 and H2 S sequestration, and an inhibitory performance threshold emerged above 20gBOF/L. Only a fraction of the BOF slag solids contributed to the digester TS mass, because a substantial portion dissolved into the supernatant, some of which contributed to alkalinity. After 160days of continuous operation, approximately 63% of the total phosphate was sequestered from the supernatant and 78% of the hydrogen sulfide from the biogas, when a steady-state BOF slag dose of 10gBOF/L was applied. At or below this level, BOF slag fines had no significant effect on mesophilic digester performance as judged by conventional operational metrics. This study demonstrated that upcycling BOF slag directly into the anaerobic digestion process could offer wastewater resource recovery facilities a novel nutrient management tool. PRACTITIONER POINTS: Basic Oxygen Furnace (BOF) Slag fines can be engineered as a cost-effective alternative to conventional phosphorus and sulfur control strategies. A rapid phosphate and sulfide removal from anaerobic digestates occurs within hours after BOF slag addition. 63% of phosphate and 78% of hydrogen sulfide were removed when digester sludge was dosed with 10gBOF/L, without impacting digester performance. Only a fraction of the dosed BOF slag contributed to digester TS, likely due to partial dissolution into the digestate. BOF slag can potentially increase the nutrient content of biosolids, facilitate biogas recovery, and reduce odors.

Life cycle assessment (LCA) of end‐of‐life dairy products (EoL‐DPs) valorization via anaerobic co‐digestion with agro‐industrial wastes for biogas production

AbstractBACKGROUNDThe aim of the present study was to assess the environmental impacts of end‐of‐life dairy products (EoL‐DPs) management via their co‐treatment with agro‐industrial wastes (AgW) in a centralized biogas facility located in Cyprus using a gate‐to‐gate life cycle assessment (LCA) approach. Two different scenarios were examined under the framework of this project. In the first one, co‐treatment of EoL‐DPs with various AgW in a 20:80 (w/w) ratio was evaluated in a one‐stage mesophilic anaerobic digestion (AD) process. In the second scenario, the same amount of EoL‐DPs were acidified before methanogenesis with AgW in order to improve biogas production.RESULTSPrior acidification of EoL‐DPs showed a better environmental performance compared with the results obtained upon direct co‐digestion in a mesophilic digester, having a total impact of 52.44 Pt against 57.13 Pt respectively. Biogas production upon acidification, and therefore energy yield, was higher, reaching up to 22.88 m3 CH4 ton−1 feed (229.25 kWh ton−1 feed), compared with 17.45 m3 CH4 ton−1 feed (174.85 kWh ton−1 feed) for the case where no pretreatment was performed.CONCLUSIONThe acidification of EoL‐DPs enhanced the environmental performance of the process by reducing its impact by 8.2% (in Pt equivalents). The energy consumption of the biogas plant mixing equipment was identified as the process hotspot. However, further analysis of the environmental performance of the proposed process is required by extending the system's boundaries towards a cradle‐to‐grave approach. © 2019 Society of Chemical Industry

A snapshot of microbial community structures in 20 different field-scale anaerobic bioreactors treating food waste

This study aimed to identify significant factors shaping the microbial populations in biogas plants treating food waste (FW). Twenty full-scale anaerobic acidogenic/methanogenic bioreactors, located at 11 FW treatment facilities, were compared to find patterns in their microbial community structures and potential interactions with the process parameters. Temperature, hydraulic retention time, and organic loading rate were design parameters that systematically influenced the microbial communities. The latter two clearly separated the acidogenic and methanogenic bioreactors. Lactobacillus was the dominant (69.7 ± 19.8%) bacteria in the acidogenic reactors, while hydrogen-utilizing methanogens, such as Methanoculleus (65.1 ± 33.5%), were the dominant archaea in most methanogenic digesters. Defluiviitoga was the dominant (82.7 ± 1.4%) bacteria in the thermophilic digesters, but was also the most abundant (33.1–33.6%) bacteria in dry mesophilic digesters. The two bioreactor categories had lower bacterial diversities, and also higher propionate concentrations (>5 g/L in 4 out of 5 cases), which may impose potential risks for the management of such digesters. The current ‘snapshot’ of the microbial communities suggests several bacterial and archaeal taxa as potential indicators of bioreactor categories and/or process variables.

Fate of Antibiotic Resistance Genes and Virulence Genes in Enterococci During Anaerobic Digestion Process of Thermal Hydrolyzed Sludge

Municipal sludge contains large amounts of enterococci, which can harbor antibiotic resistance genes (ARGs) and virulence genes (VGs). ARG- and VG-containing enterococci therefore present potential resistance and virulence and, as a consequence, represent a significant health risk to humans. Therefore, the resistance phenotype of enterococci and the prevalence of ARGs and VGs in the enterococci isolated from the mesophilic (40℃) and thermophilic (55℃) anaerobic digestion of thermal hydrolyzed sludge was investigated. Results showed that the enterococci isolated from thermal hydrolyzed sludge showed significantly higher resistance to azithromycin than that to spiramycin and tetracycline. Thermophilic anaerobic digestion resulted in a greater reduction of enterococci abundance (by two orders of magnitude and one order of magnitude, respectively), and a greater reduction in the antibiotic resistance rates of the enterococci. However, thermophilic digestion can promote the expression of tetracycline resistance genes in the enterococci. Furthermore, both mesophilic and thermophilic digestion can facilitate horizontal genes transfer (HGT) between enterococci, which might result in an increase in the occurrence of double- or multiple-resistance. Mesophilic digestion reduced the prevalence of co-occurring ARGs and VGs in enterococci, while thermophilic digestion had the opposite effect. This research improves understanding of the occurrence and fate of ARGs and VGs in potential pathogens during the treatment of municipal sludge.

Anaerobic mesophilic digestion of sludge with extra-thermophilic and high ph pre-treatment

In this study different methods of sludge pre-treatment with elevated temperatures andpH have been selected for investigation. Five sets of sludge samples were pretreated asfollows: heating at 70°C for 30 min, at 80°C for 20 min, at 90°C for 10 min, at 100°C forWWTP5 min and NaOH-treatment (pH 12 for 4 hours). For comparison a sample from Tallinn(3:1 mixture of primary (dry solids 5.2%) and activated sludge (dry solids 0.5%))was used. Extra-thermophilic pre-treatment increased the degree of hydrolyses of sludge,enabling the following anaerobic digestion process to proceed faster than that of rawsludge. However, extra-thermophilic pre-treatment was insufficient for removal ofnitrogen and phosphorous. Nitrogen can be easily solubilized during extra-thermophilicpre-treatment but solubilization of phosphorous occurs only through digestion. Theproduction of biogas during anaerobic digestion was also dependant on the quality of theinput of pre-treated sludge. The highest cumulative volume of biogas (124 mUg CODadded) was achieved by pre-treatment at 80°C. The percentage of mineralization of pretreated sludges on mesophilic digestion was the highest with 90°C sample - 65.9%.

Enhanced methanogenic performance and metabolic pathway of high solid anaerobic digestion of chicken manure by Fe2+ and Ni2+ supplementation

High solid anaerobic digestion (AD) is a promising technology for the treatment of organic waste in terms of efficient bioenergy production and digestate volume reduction. However, the high solid AD of chicken manure has been a challenge due to the inhibition effects caused by high ammonia levels. In this study, the addition of 280 mg/L Fe2+ and 2 mg/L Ni2+ has been proven to enhance the microbial activity and to overcome inhibition under the stressed environment. The mesophilic digesters feeding with total solid around 150 g/kg operated at an organic loading rate of 4.8 g/(L d) based on volatile solid and a 20-day hydraulic retention time. The total ammonia-nitrogen reached around 6.8 g/L in control and Fe-Ni digesters. The results from the 147-day continuously-fed experiment provided an increment of 34% methane production and a 29% reduction of volatile fatty acids against control. The acetoclastic and hydrogenotrophic methanogenic activity was increased by 89% and 40% respectively. The high throughout sequencing results showed an increased percentage of Methanosarcina sp., which may have contributed to the shifting of the methanogenic pathway towards acetoclastic methanogenesis. The positive effects of Fe2+ and Ni2+ supplementation obtained in this study lay the foundation for its use in AD of nitrogen rich materials.

Reduction of viable Mycobacterium avium ssp. paratuberculosis in slurry subjected to anaerobic digestion in biogas plants

Cattle infected with Mycobacterium avium ssp. paratuberculosis (MAP) shed the bacterium in their feces. This may lead to considerable concentrations of MAP in slurry, which has been postulated to contribute to MAP transmission when this slurry is used as fertilizer. For other bacterial species, anaerobic digestion has been shown to reduce bacterial load and to increase the safety of organic waste. Therefore, the objective of this study was to investigate the effects of anaerobic digestion in biogas plants on MAP survival in slurry from 16 dairy farms with a history of MAP infection. Presence of MAP was determined using MAP culture and a commercial MAP IS900 quantitative PCR (qPCR) applied on untreated slurry samples, slurry samples after primary fermentation, and digestate. Unfermented slurry samples from most enrolled farms tested positive for MAP, via both culture and qPCR. After the fermentation process, MAP could no longer be cultured in most samples, with the exception of 2 samples from farms where high numbers of MAP-shedding cows were kept at the time of sampling. A Bayesian binomial model predicted a probability of 93% for a MAP-negative culture result after fermentation. In most samples, MAP DNA was still detectable when using the IS900 qPCR. The probability of a negative result in qPCR was estimated to be 27%. Results of this study indicate that subjecting MAP-positive slurry to anaerobic digestion in biogas plants leads to a reduction of viable MAP below the detection limit; however, MAP DNA remained detectable. It remains undetermined whether MAP DNA detected in fermentation products is a residue of MAP degradation or belongs to viable MAP below the detection limit or in a dormant state. In conclusion, subjecting MAP-positive slurry to anaerobic mesophilic digestion reduces viable MAP concentration below the detection limit. The use of digestion products as fertilizer on pasture and agricultural soils instead of untreated slurry may therefore reduce the risk of MAP transmission.

Effects of Enzymes Addition on Biogas Production From Anaerobic Digestion of Agricultural Biomasses

The hydrolytic effect of a commercial enzymes mixture, containing mainly cellulose-degrading and hemicellulose-degrading enzymes, was studied during the mesophilic anaerobic digestion of two different mixtures of maize (Zea mays L.) and triticale (× Triticosecale Wittm. ex A. camus). Laboratory-scale trials demonstrated that, for the crop mixtures studied, the enzymatic pretreatment had limited practical value as enhancement agent in terms of both, specific gas production and viscosity reduction. Then, a complete set of biomethane potential essays was conducted aiming to build a more robust base of knowledge regarding hydrolytic powerfulness of enzyme supplementation. Two commercial enzymatic products were applied in different supply modes to several lignocellulosic substrates which are typically employed in real biogas plants. Results confirmed that neither enzymatic pretreatment nor direct application of enzymes in the reactor contribute meaningfully during mesophilic digestion of substrates with original high degradability such as maize and triticale. Nevertheless, significant enhancements of the degradation and conversion into biogas were observed when enzymes were supplied to digestion of agricultural biomasses with complex lignocellulosic structure: both enzymes tested improved the degradation performance of sorghum (Sorghum bicolor L. Moench) silage, straw or corn cob flour leading to increases over 30% of biogas production.

Powdered biochar doubled microbial growth in anaerobic digestion of oil

Oil is a desirable substrate for anaerobic digestion (AD) for its high energy recovery potential. However, low solubility of oil and long-chain fatty acids (LCFAs) inhibition hinder microbial growth and further reduce the biogas production in most cases. Biochar is a cheap but reliable additive in AD. Its functions to enrich microbial enrichment and enhance biogas production were confirmed previously. In present study, effects of biochars with different particle sizes on oil AD were investigated under thermophilic and mesophilic conditions. Results from the quantification and space distribution of microbes indicate that powdered biochar (<5 μm) aggregated more microbes than granular biochar (0.5–1 mm). Especially in the mesophilic digesters, powdered biochar doubled the number of microbes in the whole digester, which was a satisfactory result of acclimation. In addition, powdered biochar was able to adjust the microbial communities and further increased CH4 production by 13.3% in thermophilic digesters. Granular biochar enhanced the maximum CH4 potential by 32.5% under mesophilic condition, which was most promising from the perspective of energy recovery. Biochar of both particle sizes relieved propionic acid accumulation in thermophilic digesters. Hence, biochar addition might be an ideal approach to elevate energy recovery efficiency by overcoming the barriers of anaerobic microbes to utilize LCFAs.

Dynamic modeling of a full-scale anaerobic mesophilic digester start-up process for the treatment of primary sludge

The current start-up procedures for anaerobic sludge digesters offer limited early biogas production. This increases the cost of heating since natural gas must be purchased until methane can be produced onsite. In this research, three simulations were carried out using the BioWin 5.2 software package to expedite the production of biogas during the start-up of a full-scale anaerobic digester treating primary sludge. The first set of simulations was conducted to assess the predictive capabilities of the software when used to model this start-up process. The second set of simulations was conducted to identify which kinetic factors most significantly affect the aforementioned model. The third set of simulations was conducted to evaluate strategies designed to accelerate biogas production. The start-up strategies were developed for use in wastewater treatment plants with limited availability to seed sludge. These strategies aimed to determine the minimum amount of seed sludge, the initial sludge feed rate, and the daily sludge feed rate (with and without the addition of a pH control agent), for early biogas production. BioWin was able to reliably simulate the start-up of a full-scale digester with a relatively good fit to the plant measured data. The overall mean absolute percentage error was less than 25%, and the overall Willmott index was greater than 0.82. The results of the sensitivity analysis indicated that the BioWin outputs were more sensitive to changes in the hydrolysis rate than the acetoclastic maximum specific growth rate or the acetoclastic anaerobic decay rate.

Effects of low-temperature dry anaerobic digestion on methane production and pathogen reduction in dairy cow manure

Cow manure with bedding is a renewable organic biomass, available around the year on dairy farms. However, its reuse is hampered by many factors including existence of potential pathogens. Efficient and cost-effective dry anaerobic digestion at low temperatures (20 °C and 28 °C) can be used to eliminate pathogens. At the same time, it can provide energy and income to dairy farms. A dry anaerobic digestion process was used in this study to investigate its effect on methane production and removal of pathogens. This operational feasibility study showed that (i) the digester operating at 28 °C obtained 50% higher specific methane yield (0.229–0.286 LCH4/gVSfed) compared to 20 °C. This value was similar to those obtained by mesophilic (35–38 °C) digesters (0.228–0.302 LCH4/gVS), while the net heat energy requirement to maintain the digester temperature was lower in our digesters; (ii) approximately 90–100% pathogens (E. coli, streptococcus, total gram-negative bacteria, Salmonella and Klebsiella) were eliminated in the dry anaerobic digestion process. These results are encouraging to reuse digested material as an alternate economical bedding source for dairy cows.

Performance and Microbial Community Dynamics in Anaerobic Digestion of Waste Activated Sludge: Impact of Immigration

Waste activated sludge (WAS) is a byproduct of municipal wastewater treatment. WAS contains a large proportion of inactive microbes, so when it is used as a substrate for anaerobic digestion (AD), their presence can interfere with monitoring of active microbial populations. To investigate how influent cells affect the active and inactive microbial communities during digestion of WAS, we operated model mesophilic bioreactors with conventional conditions. Under six different hydraulic retention times (HRTs; 25, 23, 20, 17, 14, and 11.5 d), the chemical oxygen demand (COD) removal and CH4 production of the AD were within a typical range for mesophilic sludge digesters. In the main bacteria were proteobacteria, bacteroidetes, and firmicutes in both the WAS and the bioreactors, while in main archaeal methanogen group was Methanosarcinales in the WAS and methanomicrobiales in the bioreactors. Of the 106 genera identified, the estimated net growth rates were negative in 72 and positive in 34. The genera with negative growth included many aerobic taxa. The genera with positive growth rates included methanogens and syntrophs. In some taxa, the net growth rate could be positive or negative, depending on HRT, so their abundance was also affected by HRT. This study gives insights into the microbial dynamics of a conventional sludge anaerobic digester by distinguishing potentially active (growing) and inactive (non-growing, dormant) microbes and by correlating population dynamics with process parameters.

Analysis of the relationship of extracellular polymeric substances to the dewaterability and rheological properties of sludge treated by acidification and anaerobic mesophilic digestion

The initial pH value of sludge affects sludge anaerobic digestion and dewatering performance. In order to determine the suitable pH value and decrease the sludge digestion time, in this study, the effect and the mechanism of combined acidification and anaerobic mesophilic digestion (acid-AMD) on sludge dewaterability were investigated. The changes and relationships among the extracellular polymeric substances (EPS), physicochemical properties, and rheological behavior of the treated sludge were analyzed. The results indicated that the combined acid-AMD treatment improved sludge dewaterability approximately 36.08% and 30.28% compared to the acid conditioning and AMD treatment, respectively. The factors improving sludge dewaterability include a lower sludge pH value and appropriate duration of AMD, changes in particle size, surface properties and distribution of the EPS fractions. The acid-AMD treatment hydrolyzed the EPS, loosening the sludge structure. These changes reflected in the rheological properties of the sludge. After the treatment, the network strength and colloid force of the sludge weakened. The linear viscoelastic region contracted, and the sludge system became sensitive to shear. These results demonstrated that rheological analysis can help explain the sludge dewatering mechanism. The acid-AMD treatment effectively changed the distribution of EPS that play a vital role in sludge dewaterability.