Integration of mixing, heat transfer, and biochemical reaction kinetics in anaerobic methane fermentation

An extensive investigation of anaerobic methane fermentation requires identifying the relationship between the physical environment and biological process. In this study, a computational fluid dynamics (CFD) technique was used to characterize bacterial fermentation mechanisms intertwined with mixing and heat transfer in anaerobic digesters. The results demonstrate that the methane yield remains almost unchanged while the energy efficiency decreases with increasing mixing power in a complete‐mix digester, and that the energy output increases nonlinearly with the increase in heating energy in a plug‐flow digester. The CFD method can be applied to other bioreactors to gain valuable insights into their behavior as well. Integrating flow and temperature with kinetic behavior for anaerobic digestion not only solves the controversy about how mixing influences the digestive process, but also assists in optimizing the digester design and increasing the efficiency of energy conversion, and additionally, provides a reference for improving the mixing guidelines recommended by the U.S. Environmental Protection Agency. Biotechnol. Bioeng. 2012; 109: 2864–2874. © 2012 Wiley Periodicals, Inc.     

Analysis of the performance of an anaerobic digestion system at the Regina Wastewater Treatment Plant

From the performance analysis of the anaerobic digestion system at the Regina Wastewater Treatment Plant, it was found that the anaerobic digestion system at the Regina plant was generally operated in a stable condition as indicated by pH, volatile acids and alkalinity levels. The operation of the anaerobic digestion system was not optimal because of the low volatile solids concentration and low volatile solids loading rate, especially because of high HRT. Two options, thickening the primary sludge and increasing the volatile solids loading rate, were recommended for the optimal operation of the digestion system. After examining a number of kinetic models, it was found that the Chen-Hashimoto model could be used to predict the volumetric methane production rate and the first-order model could be used to predict the efficiency of volatile solids reduction. The study showed that utilization of digester gas for power production was the best alternative for the excess digester gas. 13.3% of the electrical demand and 35.5% of the plant's total energy could be met based on digester gas wasted, assuming 25% as the conversion efficiency.     

CFD simulation of mixing in anaerobic digesters

A three-dimensional CFD model incorporating the rheological properties of sludge was developed and applied to quantify mixing in a full-scale anaerobic digester. The results of the model were found to be in good agreement with experimental tracer response curve. In order to predict the dynamics of mixing, a new parameter, UI (uniformity index) was defined. The visual patterns of tracer mixing in simulation were well reflected in the dynamic variation in the value of UI. The developed model and methods were applied to determine the required time for complete mixing in a full-scale digester at different solid concentrations. This information on mixing time is considered to be useful in optimizing the feeding cycles for better digester performance.     

ANAMMOX process start up and stabilization with an anaerobic seed in Anaerobic Membrane Bioreactor (AnMBR)

ANaerobic AMMonium OXidation (ANAMMOX) process, an advanced biological nitrogen removal alternative to traditional nitrification--denitrification removes ammonia using nitrite as the electron acceptor without oxygen. The feasibility of enriching anammox bacteria from anaerobic seed culture to start up an Anaerobic Membrane Bioreactor (AnMBR) for N-removal is reported in this paper. The Anammox activity was established in the AnMBR with anaerobic digester seed culture from a Sewage Treatment Plant in batch mode with recirculation followed by semi continuous process and continuous modes of operation. The AnMBR performance under varying Nitrogen Loading Rates (NLR) and HRTs is reported for a year, in terms of nitrogen transformations to ammoniacal nitrogen, nitrite and nitrate along with hydrazine and hydroxylamine. Interestingly ANAMMOX process was evident from simultaneous Amm-N and nitrite reduction, consistent nitrate production, hydrazine and hydroxylamine presence, notable organic load reduction and bicarbonate consumption.     

Short-term and long-term effects of increasing temperatures on the stability and the production of volatile sulfur compounds in full-scale thermophilic anaerobic digesters

This study compares the effect of a rapid increase of the digester temperature (from 54 degrees C to 58 degrees C in 2 weeks) with a slow increase (from 53.9 degrees C to 57.2 degrees C at a rate of 0.55 degrees C per month) on full-scale thermophilic anaerobic digestion at Hyperion Treatment Plant. The short-term test demonstrated that rapidly increasing the digester temperature caused elevated production of volatile sulfur compounds, most notably methyl mercaptan, but volatile solids destruction and methane production were not significantly affected. The increase of the volatile fatty acid to alkalinity ratio from 0.1 to over 0.3 indicated a transient change in digester biochemistry, which was reversed by lowering the temperature. In the long term-test, a slow increase of digester temperature, the production of hydrogen sulfide increased above temperatures of 56.1 degrees C, but was controlled by increased injection of ferrous chloride. Methyl mercaptan was detected in trace amounts at the highest temperature tested (57.2 degrees C). This test showed insignificant effects on other digestion parameters, although some temperature-independent changes were observed that could have been seasonal effects over the year that the long-term test lasted. Thus a slow temperature increase was preferable. This observation contrasts with previous results showing the desirability of a rapid temperature rise to first establish a thermophilic culture when converting from mesophilic operation. Further research is warranted on temperature limits and process changes to optimize thermophilic anaerobic digestion.     

Numerical simulation of mechanical mixing in high solid anaerobic digester

Computational fluid dynamics (CFD) was employed to study mixing performance in high solid anaerobic digester (HSAD) with A-310 impeller and helical ribbon. A mathematical model was constructed to assess flow fields. Good agreement of the model results with experimental data was obtained for the A-310 impeller. A systematic comparison for the interrelationship of power number, flow number and Reynolds number was simulated in a digester with less than 5% TS and 10% TS (total solids). The simulation results suggested a great potential for using the helical ribbon mixer in the mixing of high solids digester. The results also provided quantitative confirmation for minimum power consumption in HSAD and the effect of share rate on bio-structure.     

CFD simulation of non-Newtonian fluid flow in anaerobic digesters

A general mathematical model that predicts the flow fields in a mixed-flow anaerobic digester was developed. In this model, the liquid manure was assumed to be a non-Newtonian fluid, and the flow governed by the continuity, momentum, and k-epsilon standard turbulence equations, and non-Newtonian power law model. The commercial computational fluid dynamics (CFD) software, Fluent, was applied to simulate the flow fields of lab-scale, scale-up, and pilot-scale anaerobic digesters. The simulation results were validated against the experimental data from literature. The flow patterns were qualitatively compared for Newtonian and non-Newtonian fluids flow in a lab-scale digester. Numerical simulations were performed to predict the flow fields in scale-up and pilot-scale anaerobic digesters with different water pump power inputs and different total solid concentration (TS) in the liquid manure. The optimal power inputs were determined for the pilot-scale anaerobic digester. Some measures for reducing dead and low velocity zones were proposed based upon the CFD simulation results.     

An evaluation of aerobic and anaerobic sludges as start-up material for microbial fuel cell systems

The operation of microbial fuel cells (MFCs) seeded with the same quantities of aerobic or anaerobic sludge has been compared. The two sludges consisted of mixed cultures obtained from the aerobic reactor and anaerobic digester, respectively, of a municipal Wastewater Treatment Plant (WWTP). Both the sludges were diluted with their sedimentation supernatant to avoid modifying their metabolism. The results show that the type of sludge has a major impact on the performance of the system. Seeding an MFC with anaerobic acclimated sludge leads to a more rapid start-up of electricity production and the absence of a lag period. In the MFC seeded with anaerobic sludge, the steady-state operation conditions were achieved in less than 10 days, while in the aerobic sludge-seeded MFC more than 20 days were necessary to achieve this regime. The anaerobic sludge also led to better performance of the MFC. Thus, maximum power densities above 300mWm(-2) were obtained for such systems (i.e. two times higher than that achieved with the aerobic sludge-seeded MFC in the same setup). This better performance is a direct consequence of the greater availability of Chemical Oxygen Demand (COD) in anaerobic sludge. However, the performance is not a consequence of the coulombic efficiency in the use of the COD to produce electricity because the aerobic sludge-seeded MFC doubles this figure with respect to the anaerobic sludge-seeded system.     

Dehalogenation and mineralization of 2,4,6-trichlorophenol by the sequential activity of anaerobic and aerobic microbial populations

Summary A sequential anaerobic-aerobic treatment process that can mineralize 2,4,6-trichlorophenol has been developed. The process uses diluted anaerobic digester fluid as a culture medium, and a single microbial population enriched from the digester fluid for both the anaerobic and aerobic steps.     

Novel membrane bioreactor: Able to cope with fluctuating loads, poorly water soluble VOCs, and biomass accumulation

We determined the effect of different mixing intensities on the performance, methanogenic population dynamics, and juxtaposition of syntrophic microbes in anaerobic digesters treating cow manure from a dairy farm. Computer automated radioactive particle tracking in conjunction with computational fluid dynamics was performed to quantify the shear levels locally. Four continuously stirred anaerobic digesters were operated at different mixing intensities of 1,500, 500, 250, and 50 revolutions per min (RPM) over a 260-day period at a temperature of 34 +/- 1 degrees C. Animal manure at a volatile solids (VS) concentration of 50 g/L was fed into the digesters daily at five different organic loading rates between 0.6 and 3.5 g VS/L day. The different mixing intensities had no effect on the biogas production rates and yields at steady-state conditions. A methane yield of 0.241 +/- 0.007 L CH(4)/g VS fed was obtained by pooling the data of all four digesters during steady-state periods. However, digester performance was affected negatively by mixing intensity during startup of the digesters, with lower biogas production rates and higher volatile fatty acids concentrations observed for the 1,500-RPM digester. Despite similar methane production yields and rates, the acetoclastic methanogenic populations were different for the high- and low-intensity mixed digesters with Methanosarcina spp. and Methanosaeta concilii as the predominant methanogens, respectively. For all four digesters, epifluorescence microscopy revealed decreasing microbial floc sizes beginning at week 4 and continuing through week 26 after which no microbial flocs remained. This decrease in size, and subsequent loss of microbial flocs did not, however, produce any long-term upsets in digester performance.     

Evaluation of a new fixed-bed digester design utilizing large media for flush dairy manure treatment

A new anaerobic digester design for the treatment of diluted (<2% solids) flush dairy manure was evaluated. The new design was developed as an economic alternative for enhancing the performance of anaerobic lagoon systems in cold weather areas. The digester employed used automobile tires as fixed-bed media to improve bacterial retention. The digester was heated by steam injection and built underground to enhance insulation. The tires were sorted in a unique pattern for improving mixing and uniform temperature distribution. The system was tested on a pilot-scale. The treatment mechanism was explored by mathematical modeling. The observed treatment efficiency of the new design was comparable to that of conventional digesters operating at higher total solids concentrations (>4%). With a hydraulic retention time (HRT) of 17 days, the measured removal rates were 30-50% and 40-60% of TVS and COD, respectively. The new digester maintained longer solids retention time (SRT) as estimated using the model, supported by the observed thick biofilm formation and resistance to hydraulic overload. The model was used to analyze different operation scenarios varying both the organic and hydraulic loads.     

Mixing characteristics of fixed film anaerobic reactors

Summary Since the mixing characteristics of anaerobic reactors would appear to have an effect on their operational performance, lithium tracer studies were made on two different digester types; the upflow sludge blanket and the expanded bed reactors. The mixing characteristics of both types of reactor, defined by this technique, were found to be of the intermediate type with a bias towards good mixing.     

Microbial community dynamics in mesophilic anaerobic co-digestion of mixed waste

This paper identifies key components of the microbial community involved in the mesophilic anaerobic co-digestion (AD) of mixed waste at Rayong Biogas Plant, Thailand. The AD process is separated into three stages: front end treatment (FET); feed holding tank and the main anaerobic digester. The study examines how the microbial community structure was affected by the different stages and found that seeding the waste at the beginning of the process (FET) resulted in community stability. Also, co-digestion of mixed waste supported different bacterial and methanogenic pathways. Typically, acetoclastic methanogenesis was the major pathway catalysed by Methanosaeta but hydrogenotrophs were also supported. Finally, the three-stage AD process means that hydrolysis and acidogenesis is initiated prior to entering the main digester which helps improve the bioconversion efficiency. This paper demonstrates that both resource availability (different waste streams) and environmental factors are key drivers of microbial community dynamics in mesophilic, anaerobic co-digestion.     

Concurrent microscopic observations and activity measurements of cellulose hydrolyzing and methanogenic populations during the batch anaerobic digestion of crystalline cellulose

This study compares process data with microscopic observations from an anaerobic digestion of organic particles. As the first part of the study, this article presents detailed observations of microbial biofilm architecture and structure in a 1.25-L batch digester where all particles are of an equal age. Microcrystalline cellulose was used as the sole carbon and energy source. The digestions were inoculated with either leachate from a 220-L anaerobic municipal solid waste digester or strained rumen contents from a fistulated cow. The hydrolysis rate, when normalized by the amount of cellulose remaining in the reactor, was found to reach a constant value 1 day after inoculation with rumen fluid, and 3 days after inoculating with digester leachate. A constant value of a mass specific hydrolysis rate is argued to represent full colonization of the cellulose surface and first-order kinetics only apply after this point. Additionally, the first-order hydrolysis rate constant, once surfaces were saturated with biofilm, was found to be two times higher with a rumen inoculum, compared to a digester leachate inoculum. Images generated by fluorescence in situ hybridization (FISH) probing and confocal laser scanning microscopy show that the microbial communities involved in the anaerobic biodegradation process exist entirely within the biofilm. For the reactor conditions used in these experiments, the predominant methanogens exist in ball-shaped colonies within the biofilm.     

Two-phase anaerobic digestion for production of hydrogen-methane mixtures

An anaerobic digestion process to produce hydrogen and methane in two sequential stages was investigated, using two bioreactors of 2 and 15 L working volume, respectively. This relative volume ratio (and shorter retention time in the second, CH(4)-producing reactor) was selected, in part, to test the assumption that separation of phase can enhance metabolism in the second methane producing reactor. The reactor system was seeded with conventional anaerobic digester sludge, fed with a glucose-yeast extract--peptone medium and operated under conditions of relatively low mixing, to simulate full scale operation. A total of nine steady states were investigated, spanning a range of feed concentrations, dilution rates, feed carbon to nitrogen ratios and degree of integration of the two stages. The performance of this two-stage process and potential practical applications for the production of clean-burning hydrogen-methane mixtures are discussed.     

Medium for the Enumeration and Isolation of Bacteria from a Swine Waste Digester

A habitat-simulating medium was developed for the enumeration and isolation of bacteria from a swine waste digester. A roll tube medium with growth factors for strict anaerobes from previously studied anaerobic ecosystems was used to evaluate the effects of deletion, addition, or level of digester fluid, digester fluid treated with acid or base, rumen fluid, fecal extract, anaerobic pit extract, tissue extract, carbohydrates, peptones, short-chain fatty acids, minerals, vitamins, N and P sources, reducing and solidifying agents, buffers, and gases on colony counts. Decreasing the agar concentration from 2.5 to 1.0% increased the counts twofold. Blending increased the counts 1.7-fold. With a medium (174) containing digester fluid, peptones, minerals, cysteine, sodium carbonate, and agar, colony counts were 60% of the microscopic count and improved yields 2.5 to 20 times those obtained with media previously used for digesters or developed for other anaerobic ecosystems. Colony counts continued to increase for up to 4 weeks of incubation. Medium 174 permits the enumeration of total, methanogenic, and, with deletion of reducing agent, aerotolerant bacteria. The results suggest that the predominant bacteria grow slowly and have requirements different from those of bacteria from other ecosystems.     

An economical bioreactor for evaluating biogas potential of particulate biomass

An economical bioreactor designed for evaluating the biogas potential of particulate biomass is described. The bioreactor uses a simple stirring apparatus, called the Bordeaux stirrer, to enable gas-tight mixing of fermentation cultures. The apparatus consists of a low-rpm motor connected to a bent steel stir rod, which is placed in a length of flexible plastic tubing inserted through a rubber stopper in a gas-tight manner. This stirrer is suitable for providing intermittent or continuous mixing in bench-scale anaerobic cultures containing particulate biomass. The reactor system may be operated as a batch-fed or semi-continuously fed digester. This communication documents the advantages of the stirring apparatus, describes the details of reactor fabrication and operation, and outlines the type of experimental work for which the bioreactor is suitable.     

Identification of para-Cresol as a Growth Factor for Methanoplanus endosymbiosus

Growth of the methanogenic bacterium Methanoplanus endosymbiosus is dependent on the presence of ruminal fluid. Ruminal fluid could be replaced by the eluate of a rumen-derived anaerobic digester. From the eluate of the digester, a growth-stimulatory component was purified and identified as p-cresol. Authentic p-cresol supported a half-maximal growth rate of the organism at 50 nM concentration.     

Mixed culture model of anaerobic digestion: application to the evaluation of startup procedures

The successful operation of anaerobic digestion depends on the balanced growth of many bacterial species. The functions of the main groups of microorganisms present in a digester have been analyzed and a mathematical model constructed that describes the interactions among the microbial populations and their effect on the digester performance. The model was validated by comparing its predictions with actual digester operation. Several startup procedures were evaluated in the light of the model predictions and improvements on current operational practices suggested in order to minimize startup time.