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.     

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.     

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.     

Mixing in anaerobic digestion

The mixing of the anaerobic digester contents significantly influences the efficiency of this operation; in particular, hydraulic dead zones are extremely detrimental to the reaction kinetics involved in anaerobic digestion. An analysis of the relative importance of thermal fluid movement in the digester to those caused by fluid inflow and outflow is presented. As an example, these principles are applied to a digester at the South Bend Wastewater Treatment Plant. Experimental measurements, which have general applicability for the measurement of digester mixing volume, confirm the theoretical conjectures. Various types of optimizations can be attempted on this mixing operation. One such optimization applied to gas lift mixers, as employed in the South Bend Treatment Plant, is illustrated.     

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.     

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.     

Phenylacetic acid in an anaerobic swine manure digester

Summary The presence of phenylacetic acid (PAA) in an anaerobic swine manure digester was determined by gas chromatography of the butyl ester and confirmed by mass spectroscopy. PAA concentration increased during start-up of a digester and with low carbon, high nitrogen loading. Unlike acetate, propionate and butyrate, the concentration of PAA varied little through the day in a stable digester loaded once per day. The laboratory scale digester was loaded at 4 g of swine manure solids/liter digester volume per day. The retention time and temperature were 15 days and 37°C. PAA is a microbial intermediate which is produced by one group of anaerobic bacteria and converted to methane by other members of the bacterial community in the digester. As such, it may be a useful indicator of the relative metabolic activity of the bacterial groups and thus of the overall stability of the anaerobic process.     

Development of an efficient process for the treatment of residual sludge discharged from an anaerobic digester in a sewage treatment plant

In order to reduce the discharge of residual sludge from an anaerobic digester, pre-treatment methods including low-pressure wet-oxidation, Fenton oxidation, alkali treatment, ozone oxidation, mechanical destruction and enzymatic treatment were evaluated and compared. VSS removal efficiencies of greater than 50% were achieved in cases of low-pressure wet-oxidation, Fenton oxidation and alkali treatment. Residual sludge from an anaerobic digester was pre-treated and subjected to thermophilic anaerobic digestion. As a result, the process of low-pressure wet-oxidation followed by anaerobic digestion achieved the highest VSS removal efficiency of 83%. The total efficiency of VSS removal of sewage sludge consisting of primary and surplus sludge would be approximately 92%, assuming that the VSS removal efficiency of sewage sludge is 50% in the anaerobic digester of the sewage treatment plant.     

Survival of pathogenic bacteria during mesophilic anaerobic digestion of animal waste

The survival of pathogenic bacteria was investigated during the operation of a full-scale anaerobic digester which was fed daily and operated at 28°C. The digester had a mean hydraulic retention time of 24 d. The viable numbers of Escherichia coli, Salmonella typhimurium, Yersinia enterocolitica, Listeria monocytogenes and Campylobacter jejuni were reduced during mesophilic anaerobic digestion. Echerichia coli had the smallest mean viable numbers at each stage of the digestion process. Its mean T₉₀ value was 76.9 d. Yersinia enterocolitica was the least resistant to the anaerobic digester environment; its mean T₉₀ value was 18.2 d. Campylobacter jejuni was the most resistant bacterium; its mean T₉₀ value was 438.6 d. Regression analysis showed that there were no direct relationships between the slurry input and performance of the digester and the decline of pathogen numbers during the 140 d experimental period.     

Anaerobic and complementary treatment of domestic sewage in regions with hot climates--a review

This study presents a literature review on the treatment of domestic sewage in controlled environments having the anaerobic process and specifically the upflow anaerobic sludge blanket (UASB) concept as the core, under natural hot conditions. The UASB process application is however beset by the preponderance of suspended solids, and the paper looks at its optimization via pre- and post-treatments to curb the prevailing problems, in the light of possible discharge and re-use/recycling/resource recovery, leading to efficient environmental protection. Pre-treatment clarification could be done with ferric chloride/polyelectrolyte, so that phosphate precipitates during the process. The pre-treated liquid phase can be submitted to a high rate anaerobic process, using the simple and robust UASB technology. In a subsequent post-treatment step, ammonium can be removed by ion exchange using a zeolite column through which the wastewater percolates after leaving the anaerobic digester. The various stages can also eliminate a large fraction of the pathogens present in the raw wastewater, mainly through the pre-treatment sedimentation and the ion exchange filtration. The sludge produced in the precipitation stage can be stabilized in a conventional anaerobic digester. Integration of the different treatment steps provides a sustainable technology to treat domestic sewage under hot climate conditions.     

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.     

Application of flowcell technology for monitoring biofilm development and cellulose degradation in leachate and rumen systems

In this study, a flat plate flowcell was modified to provide a reactor system that could maintain anaerobic, cellulolytic biofilms while providing the data needed to carry out a chemical oxygen demand mass balance to determine the cellulose digestion rates. The results showed that biofilms could be observed to grow and develop on cellulose particle surfaces from both anaerobic digester leachate and rumen fluid inocula. The observations suggest that the architecture of rumen and leachate derived biofilms may be significantly different with rumen derived organisms forming stable, dense biofilms while the leachate derived organisms formed less tenacious surface attachments. This experiment has indicated the utility of flowcells in the study of anaerobic biofilms.     

Survival of Salmonella spp. in a simulated acid-phase anaerobic digester treating sewage sludge

The presence of pathogenic microorganisms in municipal waste sludge may create a serious outbreak of water borne diseases if the sludge is used for agricultural purpose. An attempt to decrease the number of pathogenic microorganisms, Salmonella spp. using a simulated acid-phase anaerobic digester was tested in a laboratory-scale batch experiment. Reduction of Salmonella spp. was demonstrated in a mixture of sludge and organic acid, simulating an acid digester of a two-phase anaerobic digestion process. A high concentration of organic acid at a pH value of 5.5-6.0 prevents a decrease in Salmonella spp. concentration. Almost complete destruction of Salmonella spp. is observed within two days if the pH value is maintained below 5.5.     

Survival of Salmonella spp. in a simulated acid-phase anaerobic digester treating sewage sludge

The presence of pathogenic microorganisms in municipal waste sludge may create a serious outbreak of water borne diseases if the sludge is used for agricultural purpose. An attempt to decrease the number of pathogenic microorganisms, Salmonella spp. using a simulated acid-phase anaerobic digester was tested in a laboratory-scale batch experiment. Reduction of Salmonella spp. was demonstrated in a mixture of sludge and organic acid, simulating an acid digester of a two-phase anaerobic digestion process. A high concentration of organic acid at a pH value of 5.5-6.0 prevents a decrease in Salmonella spp. concentration. Almost complete destruction of Salmonella spp. is observed within two days if the pH value is maintained below 5.5.     

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.     

Dissolved hydrogen concentration as an on-line control parameter for the automated operation and optimization of anaerobic digesters

The use of dissolved hydrogen as an early warning signal of digester failure and a control parameter to operate anaerobic digesters was investigated. A sensitive, on-line method was developed for measuring trace levels of dissolved hydrogen in a semi-permeable membrane, situated within the biomass of a 1 L laboratory anaerobic digester, using trace reduction gas analysis. At normal operating conditions, the dissolved hydrogen partial pressure (2 to 8 Pa) was found to be linearly correlated with the loading rate of the digester, and was a sensitive indicator of the effect of shockloads as well as gradual overloading. An increase in hydrogen partial pressure above a critical concentration of 6.5-7 Pa indicated the initial stage of digester overloading (i.e., volatile fatty acids accumulation). A H(2)-based computer control system, using a critical hydrogen partial pressure of 6.5 Pa as the setpoint, was found to be effective for the safe operation of a laboratory digester close to its maximum sustainable loading rate. The existence of a relationship between hydrogen level and organic loading rate was also confirmed on a 600 m(3) industrial digester, with digester overloading occurring at hydrogen concentrations above 7 Pa. The results suggest that the dissolved hydrogen concentration is capable of being a sensitive on-line parameter for the automated management of anaerobic digesters near their maximum sustainable loading capacity. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 56: 626-634, 1997.     

Effect of temperature on the performance of an anaerobic tubular reactor treating fruit and vegetable waste

This study compares the performance of anaerobic digestion of fruit and vegetable waste (FVW) in the thermophilic (55 °C) process with those under psychrophilic (20 °C) and mesophilic (35 °C) conditions in a tubular anaerobic digesters on a laboratory scale. The hydraulic retention time (HRT) ranged from 10 to 20 days, and raw fruit and vegetable waste was supplied in a semi-continuous mode at various concentrations of total solids (TS) (4, 6, 8 and 10% on dry weight). Biogas production from the experimental thermophilic digester was higher on average than from psychrophilic and mesophilic digesters by 144 and 41%, respectively. The net energy production in the thermophilic digester was 195.7 and 49.07 kJ per day higher than that for the psychrophilic and mesophilic digesters, respectively. The relation between the daily production of biogas and the temperature indicates that for the same produced quantity of biogas, the size of the thermophilic digester can be reduced with regard to that of the psychrophilic and the mesophilic digesters.     

Microbiological aspects of anaerobic digestion of swine slurry in upflow fixed-bed digesters with different packing materials

Samples taken from liquid influents, liquid effluents, bottom sediments and biofilms attached to the supports of three laboratory anaerobic, fixed-bed, upflow digesters, filled with wood chips, PVC or expanded-clay support media and fed with swine slurry, were tested microbiologically. The numbers of anaerobic heterotrophic, anaerobic cellulolytic, acidogenic-peptone-glucose fermenting and methanogenic bacteria were determined. For each digester the biogas production was monitored.The highest biogas production, referred to the volatile solids concentration in the feed, was obtained in the digester with wood chips, while production was almost nil in the digester with expanded clay.     

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.     

Influence of the microorganism support on the kinetics of anaerobic fermentation of condensation water from thermally concentrated olive mill wastewater

Two materials of different structure, sepiolite and bentonite, evaluated as supports for the microorganisms effecting anaerobic fermentation, behaved differently towards condensation water from thermally concentrated olive mill wastewater from a kinetic point of view. Assuming the overall anaerobic digestion process to conform to first-order kinetics, the apparent kinetic constant for the digester including sepiolite as support was 1.12 day^-1, while that of the digester using the bentonite support was 0.73 day^-1. Thus, the apparent kinetic constant of the process was increased by 35% with the use of sepiolite. The yield coefficient, Y_p/s, was 0.344 and 0.318 litres CH₄ STP/g COD for the sepiolite and bentonite supports respectively.