Influence of temperature on the performance of anaerobic fixed-film reactors utilizing screened dairy manure

Thermophilic (55°C) and mesophilic (35°C and 22°C) anaerobic digestions in laboratory scale (4 litre) fixed-film reactors fed with screened dairy manure were successfully operated over a range of hydraulic retention times, from 1 to 20 days. Maximum methane production rates of 1·82, 1·68 and 1·28 litres CH₄ litre⁻¹ day⁻¹ occurred at 1, 1·5 and 1 days HRT for the respective 55°C, 35°C and 22°C reactors. Both thermophilic and mesophilic digestions achieved maximum biodegradation efficiency at 10 days HRT. The thermophilic fixed-film reactor performed better than completely-mixed reactors in terms of methane production at HRTs shorter than 2 days. From the results, mesophilic fixed-film reactor operated at 35°C provided optimum methane production and net energy output between 1 and 5 days HRT.     

Acclimation Strategy of a Biohydrogen Producing Population in a Continuous‐Flow Reactor with Carbohydrate Fermentation

Poor startup of biological hydrogen production systems can cause an ineffective hydrogen production rate and poor biomass growth at a high hydraulic retention time (HRT), or cause a prolonged period of acclimation. In this paper a new startup strategy was developed in order to improve the enrichment of the hydrogen‐producing population and the efficiency of hydrogen production. A continuously‐stirred tank reactor (CSTR) and molasses were used to evaluate the hydrogen productivity of the sewage sludge microflora at a temperature of 35 °C. The experimental results indicated that the feed to microorganism ratio (F/M ratio) was a key parameter for the enrichment of hydrogen producing sludge in a continuous‐flow reactor. When the initial biomass was inoculated with 6.24 g of volatile suspended solids (VSS)/L, an HRT of 6 h, an initial organic loading rate (OLR) of 7.0 kg chemical oxygen demand (COD)/(m³ × d) and an feed to microorganism ratio (F/M) ratio of about 2–3 g COD/(g of volatile suspended solids (VSS) per day) were maintained during startup. Under these conditions, a hydrogen producing population at an equilibrium state could be established within 30 days. The main liquid fermentation products were acetate and ethanol. Biogas was composed of H₂ and CO₂. The hydrogen content in the biogas amounted to 47.5 %. The average hydrogen yield was 2.01 mol/mol hexose consumed. It was also observed that a special hydrogen producing population was formed when this startup strategy was used. It is supposed that the population may have had some special metabolic pathways to produce hydrogen along with ethanol as the main fermentation products.     

The influence of anaerobic pretreatment on the nitrogen removal from biosynthetic pharmaceutical wastewaters

The C:N ratio of the pharmaceutical wastewaters is usually suitable for a combination of the anaerobic pretreatment with the high COD removal and aerobic posttreatment with the efficient biological N removal. This kind of anaerobic-aerobic process was tested in semipilot scale by using a UASB reactor and an activated sludge system with a predenitrification (total volume 100 1). It was found that at a total HRT of 2.3 days an average of 97.5% of COD and 73.5% of total N was removed. The UASB reactor was operated at 30°C with a volumetric loading rate of 8.7 kg.m^-3.d^-1, the efficiency of COD removal was 92.2%. The processes, which take part in the biological removal of nitrogen, especially the nitrification, were running with lower rates than usually observed in aerobic treatment systems.Abbreviations AAO anaerobic anoxic oxic configuration - AOO anaerobic oxic oxic configuration - B _V volumetric organic loading rate (kg COD.m^-3. d^-1) - dB _x specific COD removal rate (mg COD. g^-1 VSS. d^-1) - DNR denitrification rate (mg N–NO₃. g^-1 VSS. h^-1) - E_COD efficiency of COD removal (%) - HRT hydraulic retention time (d) - NR nitrification rate (mg N–NO₃. g^-1 VSS. h^-1) - R recirculation ratio (%) - SBP specific biogas production (m³.kg^-1 removed COD) - SRT solids retention time; sludge age (d) - SS suspended solids (g.1^-1) - UASB upflow anaerobic sludge blanket reactor - VSS volatile suspended solids (g.1^-1)     

Dual anaerobic co-digestion of sewage sludge and confectionery waste

Three configurations for a dual digestion system were examined. The units were based on three 5 l completely stirred tank reactors (CSTR). A first-stage thermophilic digester was used to provide the feed to each of the two second-stage mesophilic (35°C) digesters. Using a mixture of sewage sludge and strong confectionery waste, the thermophilic digester was operated at 55°C with a hydraulic retention time of 4 h. The mesophilic digesters were operated at hydraulic retention times of 8, 12 and 15 days. In terms of the reduction of volatile solids (VS), the three dual digestion configurations were similar but were more effective than the single-stage reactor which was used as a control. However, based on the specific methane yield (m³ CH₄/kg VS removed), the configuration with a first stage operating at 55°C and a secondary digester at 35°C with a hydraulic retention time of 12 days was the most effective. This configuration also maintained a more stable pH, irrespective of the quality of the feed sludge.     

Operational temperature regulates anodic biofilm growth and the development of electrogenic activity

The operational temperature of microbial fuel cell reactors influences biofilm development, and this has an impact on anodic biocatalytic activity. In this study, we compared three microbial fuel cell (MFC) reactors acclimated at 10°C, 20°C and 35°C to investigate the effect on biomass development, methanogenesis and electrogenic activity over time. The start-up time was inversely influenced by temperature, but the amount of biomass accumulation increased with increased temperatures, the 10°C, 20°C and 35°C acclimated biofilms resulted in 0.57, 0.82 and 5.43 g biomass (volatile suspended solids) per litre respectively at 56 weeks of operation. Biofilm build-up on the 35°C anode was further demonstrated by scanning electron microscopy, which showed large aggregations of biomass accumulating on the anode when compared to 10°C and 20°C biofilms. Biomass accumulation had a direct impact on biocatalytic performance, with the maximum power at 35°C after 60 weeks of operation being 2.14 W m⁻³ and power densities for the 10°C and 20°C reactors being and 4.29 W m⁻³. Methanogenic activity was also shown to be higher at 35°C, with a rate of 10.1 mmol CH₄ biofilm per gram of volatile suspended solid (VSS) per day, compared to 0.28 mmol CH₄ per gram of VSS per day produced at 20°C. These results demonstrate that higher MFC operating temperatures could be detrimental to the biocatalytic performance of electrochemically active bacteria in anodic biofilms due to biomass accumulation with enhanced development of non-electrogenic communities (e.g. methanogens and fermenters), meaning that, over time, psychro- or mesophilic operation can have beneficial effects for the development of electrogenically active populations in the reactor.     

Evaluation of an on-site pilot static granular bed reactor (SGBR) for the treatment of slaughterhouse wastewater

An on-site pilot-scale static granular bed reactor (SGBR) system was evaluated for treating wastewater from a slaughterhouse in Iowa. The study evaluated SGBR reactor suitability for slaughterhose wastewater having high particulate COD concentration (7.9 ± 4.3 g COD/L) at 0.3–1.4 m³/m²/day of the surface loading rates. High organic removal efficiency (over 95% of TSS and VSS removal) was obtained due to the consistent treatability of SGBR system during operation at HRTs of 48, 36, 30, 24, and 20 h. The average effluent TSS, VSS, COD, soluble COD, and BOD₅ concentrations were 84, 71, 301,197, and 87 mg/L, respectively. An effective backwash procedure was performed once every 7–14 days to waste a portion of the accumulated solids in the system. This procedure limited the increase in hydraulic head loss and maintained the system stability. COD removal efficiencies greater than 95% were achieved at organic loading rates ranging from 0.77 to 12.76 kg/m³/day.     

Anaerobic treatment of synthetic medium-strength wastewater using a multistage biofilm reactor

A laboratory-scale multistage anaerobic biofilm reactor of three compartments with a working volume of 54-L was used for treating a synthetic medium-strength wastewater containing molasses as a carbon source at different influent conditions. The start-up period, stability and performance of this reactor were assessed at mesophilic temperature (35 °C). During the start-up period, pH fluctuations were observed because there was no microbial selection or zoning, but as the experiment progressed, results showed that phase separation had occurred inside the reactor. COD removal percentages of 91.6, 91.6, 90.0 and 88.3 were achieved at organic loading rates of 3.0, 4.5, 6.75 and 9.0 kg COD/m³ day, respectively. A decrease in HRT from 24 to 16 h had no effect on COD removal efficiency. When HRT decreased to 8 h, COD removal efficiency was still 84.9%. Recirculation ratios of 0.5 and 1.0 had no effect on COD removal but other factors such as the volatile fatty acid (VFA) content were affected. The effect of toxic shock was also investigated and results showed that the main advantage of using this bioreactor lies in its compartmentalized structure.     

Liquefaction and gasification during anaerobic digestion of coffee waste by two-phase methane fermentation with slurry-state liquefaction

In order to improve the gas evolution rate during anaerobic digestion of coffee waste by two-phase methane fermentation with slurry-state liquefaction, the liquefaction and gasification processes were separately investigated. In the liquefaction process (including the acidification process), treatment at a pH above 6 had no major effects on the liquefaction and acidification rates. However, the VFA production rates were 880 and 320 mg/l·d during mesophilic (37°C) and thermophilic (53°C) liquefaction, respectively. Mesophilic conditions were superior to thermophilic conditions in the liquefaction. With respect to the gasification process, a high TOC volumetric loading rate of 21 g/l·d was achieved during thermophilic gasification. However, the mesophilic gasification did not yield stable data, even at a low TOC volumetric loading rate of 2 g/l·d. The gas yield was 1.7 l/g TOC consumed during thermophilic gasification. The thermophilic liquefaction and thermophilic gasification reactors were connected in series and a two-phase experiment was conducted with the reactors at various volumetric ratios. The maximum gas evolution rate of 1.43 l/l·d was achieved with a combination of a gasification reactor with a 0.45l working volume and liquefaction reactor with a 2l working volume. This rate was 1.7 times higher than the rate obtained in a previous study.     

Factors affecting the performance of horizontal flow constructed treatment wetland vegetated with Cyperus papyrus for municipal wastewater treatment

The effect of hydraulic loading rate (HLR) and hydraulic retention time (HRT) on the bioremediation of municipal wastewater using a pilot scale subsurface horizontal flow constructed treatment wetland (HFCTW) vegetated with Cyprus papyrus was investigated. Different HLRs were applied to the treatment system namely 0.18, 0.10, and 0.07 m³/m². d with corresponding HRTs of 1.8, 3.2, and 4.7 days, respectively. The flow rate was 8 m³/d, and the average organic loading rate (OLR) was 0.037 kg BOD/m³/d. Results showed that the performance of the HFCTW was linearly affected by decreasing the HLR and increasing the HRT. The highest treatment efficiency was achieved at HRT (4.7 days) and HLR (0.07 m³/m². d). The percentage reductions of chemical oxygen demand (COD), biochemical oxygen demand (BOD), and total suspended solids (TSS) were 86%, 87%, and 80%, respectively. Satisfactory nutrient removal was obtained. Also, removal of 2–3 logs of bacterial indicators of pollution was achieved. The dry biomass of Cyperus was 7.7 kg/m² and proved to be very efficient in nitrification processes due to high diversity of the roots that increase the treatment surface area.     

Effect of organic loading rate on the performance of an up-flow anaerobic sludge blanket reactor treating olive mill effluent

The primary objective of this study was to evaluate the effects of the organic loading rate on the performance of an up-flow anaerobic sludge blanket (UASB) reactor treating olive mill effluent (OME), based on the following indicators: (i) chemical oxygen demand (COD) removal efficiency; and (ii) effluent variability (phenol, suspended solids, volatile fatty acids, and pH stability). The UASB reactor was operated under different operational conditions (OLRs between 0.45 and 32 kg COD/m³·day) for 477 days. The results demonstrated that the UASB reactor could tolerate high influent COD concentrations. Removal efficiencies for the studied pollution parameters were found to be as follows: COD, 47∼92%; total phenol, 34∼75%; color, 6∼46%; suspended solids, 34∼76%. The levels of VFAs in the influent varied between 310 and 1,750 mg/L. Our measurements of the VFA levels indicated that some of the effluent COD could be attributed to VFAs (principally acetate, butyrate, iso-butyrate, and propionate) in the effluent, which occurred at levels between 345 and 2,420 mg/L. As the OLRs were increased, more VFAs were measured in the effluent. A COD removal efficiency of 90% could be achieved as long as OLR was kept at a level of less than 10 kg COD/m³·day. However, a secondary treatment unit for polishing purposes is necessary to comply with discharge standards.     

Anaerobic thermophilic digestion of cutting oil wastewater: Effect of co-substrate

This paper describes the thermophilic (55 °C) anaerobic biodegradation of a mixed feed composed of vinasses and cutting oil wastewater (COW) in a laboratory upflow anaerobic fixed-film reactor (UAFF) with a porous support medium. The experimental protocol was defined to examine the effect of increasing the percentage of cutting oil wastewater in the feed.The UAFF reactor was initially started-up with vinasses as the only carbon source at an organic loading rate of 22.3 kg COD/m³ day and HRT of 0.8 days using porous particles as the support (SIRAN). The percentage of organic matter composed of vinasses was subsequently reduced while increasing the amount of cutting oil until 100% of cutting oil wastewater was added in the feed. Four stages were considered in the study (0, 42.4, 66.6 and 100% COW). HRT was adjusted in order to maintain an approximately constant organic loading rate applied to the system. Under theses conditions, the UAFF reactor was subjected to a programme of steady-state operation with hydraulic retention times (HRT) in the range 0.8–0.15 days and organic loading rates (OLR) between 22.3 and 14.9 kg COD/m³ day in order to evaluate the treatment capacity of the system.The COD removal efficiency was found to be 87% COD and 94.6% TOC in the reactor when treating vinasses at 22.3 kg COD/m³ day. The volumetric methane level produced in the digester reached 0.45 m³/m³ day. After an operating period of 120 days, the reactor was fed with cutting oil wastewater (COW) as the only source of carbon. An OLR of 16.7 kg COD/m³ day was achieved with 85.8% COD removal efficiency (58.1%TOC) in the experimental UAFF reactor. Under these conditions the volumetric methane produced in the digester was negligible.Hence, COW can be removed, if not degraded, by anaerobic treatment in the presence of a biodegradable co-substrate. Wine vinasses degradation creates conditions for non-biological removal of COW constituents. More studies are necessary in order to test the mechanisms of organic removal when biodegradation apparently had ceased. Also, toxicity assays of COW are necessary to evaluate the toxicity to the methanogenic community.     

Use of the upflow sludge blanket (USB) reactor concept for biological wastewater treatment,especially for anaerobic treatment

In recent years considerable effort has been made in the Netherlands toward the development of a more sophisticated anaerobic treatment process, suitable for treating low a strength wastes and for applications at liquid detention times of 3–4 hr. The efforts have resulted in new type of upflow anaerobic sludge blanket (UASB) process, which in recent 6 m³ pilot-plant experiments has shown to be capable of handling organic space loads of 15–40 kg chemical oxygen demand (COD)·m^?3/day at 3–8 hr liquid detention times. In the first 200 m³ full-scale plant of the UASB concept, organic space loadings of up to 16 kg COD·m^?3/day could be treated satisfactorily at a detention times of 4 hr, using sugar beet waste as feed. The main results obtained with the process in the laboratory as well as in 6 m³ pilot plant and 200 m³ full-scale experiments are presented and evaluated in this paper. Special attention is given to the main operating characteristics of the UASB reactor concept. Moreover, some preliminary results are presented of laboratory experiments concerning the use of the USB reactor concept for denitrification as well as for the acid formation step in anaerobic treatment. For both purposes the process looks feasible because very satisfactory results with respect to denitrification and acid formation can be achieved at very high hydraulic loads (12 day^?1) and high organic loading rates, i.e., 20 kg COD·m^?3/day in the denitrification and 60–80 kg COD·m^?3/day in the acid formation experiments.     

Organic Matter Degradation Kinetics in an Anaerobic Thermophilic Fluidised Bed Bioreactor

This paper describes the thermophilic anaerobic biodegradation of wine distillery wastewater (vinasses) in a laboratory fluidised bed reactor (AFB) with a porous support medium. The experimental protocol was defined to examine the effect of increasing organic loading rate on the efficiency of AFB and to report on its steady-state performance. Moreover, in order to evaluate treatment efficiency and to investigate fermentation kinetics in an AFB reactor, experimental data were used to estimate the ‘active biomass’ concentration using an autocatalytic kinetic model proposed in this paper, since viable biomass in AFB reactors is very difficult to measure experimentally. The AFB reactor was subjected to a program of steady-state operation over a range of hydraulic retention time (HRTs) of 2.5–0.37 days and organic loading rate (OLRs) up to 5.88 kgCOD/m³/day in order to evaluate its treatment capacity. The AFB reactor was initially operated with organic loading rate of 5.88 kgCOD/m³/day and HRT of 2.5 days. The chemical oxygen demand (COD) removal efficiency was found to be 96.5% in the reactor while the methane content of biogas produced in the digester reached 1.08 m³/m³digester/day. Over 94 days operating period, an OLR of 32 kgCOD/m³/day at a food-to-micro-organisms (F:M) ratio of 0.55 kgCOD/kgVS_att/day was achieved with 81.5% COD removal efficiency in the experimental AFB reactor. At this moment, the methane content of biogas produced in the digester reached 9.0 m³/m³digester/day. The proposed kinetic model is able to estimate kinetic constants of the biodegradation process: non-biodegradable substrate (S_nb) and active adhered biomass concentration (X_a). The parameters of the model were obtained by the curve-fitting method to the proposed kinetic model using the COD as substrate of the anaerobic process and assuming a maximum specific μ_max: 0.72 per day. The comparison of the measured concentration of volatile attached solids (VS_att) with the estimated ‘active’ biomass concentration indicated that extremely high ‘active biomass’ concentrations can be maintained in the system because biofilm thickness is limited by the liquid flow rate applied. This is due to the fact that the anaerobic fluidised bed system retains the growth support medium in suspension by drag forces exerted by upflowing wastewater, and the distribution of biomass holdup (in the form of a biofilm) is thus relatively uniform.     

Behavior of methanogens during start-up of farm-scale anaerobic digester treating swine wastewater

The aim of this study was to monitor the changes in methanogenic community structures in an anaerobic digester (250 m³ working volume) during start-up including prolonged starvation periods. Redundancy analysis was performed to investigate the correlations between environmental variables and microbial community structures. The anaerobic digester was operated for 591 days at alternating operating temperatures. In initial start-up period at stage I (35 °C), growth of various species of mesophilic aceticlastic methanogens (AMs) and hydrogenotrophic methanogens (HMs) was observed. Methanobacteriales species survived better than other methanogens under long-term starvation conditions. In stage II (50 °C), HMs became dominant over AMs as the operating temperature changed from mesophilic to thermophilic due to increase of ammonia inhibition. In stage III (35 °C), only the Methanomicrobiales population significantly increased during 50 days of HRT while Methanobacteriales dominated over 15 days of HRT. The influent pH negatively correlated with all methanogenic populations especially in stage II.     

Operating a full-scale poultry manure anaerobic digester

A full-scale, completely-mixed digester, with a liquid capacity of 587 m³, was constructed to process the manure from 70 000 caged layers. Biogas from the digester was used as fuel for an engine/generator set. The operating temperature was maintained at 35°C using waste heat from the engine. The digester was operated on a 22–24 day HRT. Digester influent averaged 5·90% TS, 5250 ppm TKN, and 3790 ppm NH₃N. Digester effluent averaged 3·11% TS, 5090 ppm TKN, and 4060 ppm NH₃N.Sustained operation of the digester was achieved during the period of study (8/83−4/85). During this period biogas production averaged 0·38 m³ kg⁻¹ VS added (0·58 m³ kg⁻¹ VS destroyed). The CH₄ content averaged 58·0%. The major operational problem encountered was grit accumulation in the digester. This problem was reduced by settling most of the grit from the manure prior to the digester. Biogas production was reduced when concentrated lagoon-liquid was used as make-up water. Approximately 22% of the electricity produced was required for operating the system.     

Granulation in anaerobic sludge bed reactors treating food industry wastes

Three parallel laboratory reactors, R1, R2 and R3, received food industry wastewater: R1 unadulterated; R2 supplemented with calcium and phosphate; R3 supplemented with ferric chloride and traces of nickel and cobalt. Reactors were packed with active granular sludge from a large scale pilot reactor treating the same wastewater. Addition of calcium and phosphate was found to be detrimental to the granule formation at naturally established reactor pH = 6·9–7·4 in R2 while iron promoted granulation in R3. Conditions of upflow velocities of 1·5–6 m h⁻¹, rapid increase of loads up to 15 kg COD m⁻³ day⁻¹ and ratios of recycle to raw waste feed of 20:1–80:1 were imposed on all reactors. The granules in R1 and R2 disintegrated, from 70–100 g liter⁻¹ VSS to a flocculant sludge at 1·5–3 g liter⁻¹. In spite of such severe washout, reactors R1 and R2 were able to maintain a steady COD removal of over 90% at a load of 10kg m⁻³ day⁻¹. R3 retained a VSS concentration around 100 g liter⁻¹ and maintained COD removal at over 95%. R3 exhibited a more stable performance and was less vulnerable to the shock treatment to which all reactors were subjected.     

Microbial communities involved in biogas production from wheat straw as the sole substrate within a two-phase solid-state anaerobic digestion

Microbial communities involved in biogas production from wheat straw as the sole substrate were investigated. Anaerobic digestion was carried out within an up-flow anaerobic solid-state (UASS) reactor connected to an anaerobic filter (AF) by liquor recirculation. Two lab-scale reactor systems were operated simultaneously at 37 °C and 55 °C. The UASS reactors were fed at a fixed organic loading rate of 2.5 g L⁻¹ d⁻¹, based on volatile solids. Molecular genetic analyses of the bacterial and archaeal communities within the UASS reactors (digestate and effluent liquor) and the AFs (biofilm carrier and effluent liquor) were conducted under steady-state conditions. The thermophilic UASS reactor had a considerably higher biogas and methane yield in comparison to the mesophilic UASS, while the mesophilic AF was slightly more productive than the thermophilic AF. When the thermophilic and mesophilic community structures were compared, the thermophilic system was characterized by a higher Firmicutes to Bacteroidetes ratio, as revealed by 16S rRNA gene (rrs) sequence analysis. The composition of the archaeal communities was phase-separated under thermophilic conditions, but rather stage-specific under mesophilic conditions. Family- and order-specific real-time PCR of methanogenic Archaea supported the taxonomic distribution obtained by rrs sequence analysis. The higher anaerobic digestion efficiency of the thermophilic compared to the mesophilic UASS reactor was accompanied by a high abundance of Firmicutes and Methanosarcina sp. in the thermophilic UASS biofilm.     

Effective anaerobic treatment of fresh leachate from MSW incineration plant and dynamic characteristics of microbial community in granular sludge

We investigated the treatment of fresh leachate from municipal solid waste incineration plants with high-strength organics using a lab-scale expanded granular sludge bed (EGSB) reactor. The reactor was operated at a mesophilic temperature (33 °C) for 118 days. The influent chemical oxygen demand (COD) of the leachate gradually increased to over 70,000 mg/L, and the organic loading rate increased to 18 kg COD/(m³?day). An average COD removal efficiency of 86.7 % was achieved when the reactor was fed with raw leachate, which suggests the feasibility of the EGSB process for leachate treatment. The microbial communities in the sludge from the reactor during the trial operation were constructed by denaturing gradient gel electrophoresis, clone libraries, and real-time quantitative polymerase chain reaction. The dominant group for archaea was Methanosaeta, with 68.4 % proportion at the start of the operation, and then changed to Methanosarcina, with a proportion of 62.3 %, after 118 days of operation. The dominant group of eubacteria was confirmed to be Firmicutes throughout the operation process, with the proportion increasing from >50 to 81.2 %. Almost all the operational taxonomic units of Firmicutes belonged to the order Clostridiales, with characteristic spore formation. The microbial diversity of the population was low under raw leachate as feed in the reactor. The dynamics of the microbial community in the anaerobic granular sludge was discussed relating with the operating status of the EGSB reactor.     

Methane fermentation of brewery by-products

A laboratory-scale research program was undertaken to investigate the kinetics of the mesophilic (37°C) anaerobic digestion of brewery industry by-product. The purpose was to develop data for the design and operation of full-scale units which could be used to generate methane fuel gas from these materials. This is important because the brewery industry has been susceptible to shortages of natural gas in recent years. The minimum SRT is 2.3 days, although for design purposes as much as ten days is recommended. The biomass yield is 0.512 g volatile suspended solids (VSS)/g volatile solids (VS) or 0.421 g VSS/g chemical oxygen demand (COD). The maintenance requirement is 0.052 g VS/g VSS per day or 0.061 g COD/g VSS per day. The specific methane yield is 2.51 liter/g VSS, and the methane productivity is 0.32–0.41 liter/g dry substrate added or 0.69–0.91 liter/g destroyed. The maximum loading rate for which substrate inhibition is not observed is 6 g dry substrate added per liter per day. The results of the entire program indicate that processing brewery by-product in this manner is both technically feasible and economically attractive.     

Methanosarcinaceae and Acetate-Oxidizing Pathways Dominate in High-Rate Thermophilic Anaerobic Digestion of Waste-Activated Sludge

This study investigated the process of high-rate, high-temperature methanogenesis to enable very-high-volume loading during anaerobic digestion of waste-activated sludge. Reducing the hydraulic retention time (HRT) from 15 to 20 days in mesophilic digestion down to 3 days was achievable at a thermophilic temperature (55°C) with stable digester performance and methanogenic activity. A volatile solids (VS) destruction efficiency of 33 to 35% was achieved on waste-activated sludge, comparable to that obtained via mesophilic processes with low organic acid levels (<200 mg/liter chemical oxygen demand [COD]). Methane yield (VS basis) was 150 to 180 liters of CH₄/kg of VS_added. According to 16S rRNA pyrotag sequencing and fluorescence in situ hybridization (FISH), the methanogenic community was dominated by members of the Methanosarcinaceae, which have a high level of metabolic capability, including acetoclastic and hydrogenotrophic methanogenesis. Loss of function at an HRT of 2 days was accompanied by a loss of the methanogens, according to pyrotag sequencing. The two acetate conversion pathways, namely, acetoclastic methanogenesis and syntrophic acetate oxidation, were quantified by stable carbon isotope ratio mass spectrometry. The results showed that the majority of methane was generated by nonacetoclastic pathways, both in the reactors and in off-line batch tests, confirming that syntrophic acetate oxidation is a key pathway at elevated temperatures. The proportion of methane due to acetate cleavage increased later in the batch, and it is likely that stable oxidation in the continuous reactor was maintained by application of the consistently low retention time.