Simulation study of anaerobic digestion control

A mixed culture anaerobic digestion model developed previously was applied to the evaluation of several digester control strategies. It was found that pH control by base addition or flow rate manipulation is inadequate. Based on an analysis of digester dynamics, a new control of the total suspended solids concentration at the feed was proposed through the manipulation of the underflow flow rate of the preceeding sedimentation unit. This control was tested in a variety of simulated runs and proved very effective in eliminating most of the usual causes of digester failure.     

Adaptive neuro-fuzzy modelling of anaerobic digestion of primary sedimentation sludge

Modelling of anaerobic digestion systems is difficult because their performance is complex and varies significantly with influent characteristics and operational conditions. In this study, Adaptive Neuro-Fuzzy Inference System (ANFIS) were used for modelling of anaerobic digestion system of primary sludge of Kayseri municipal WasteWater Treatment Plant (WWTP). Effluent Volatile Solid (VS) and methane yield were predicted by the ANFIS. Two stage models were performed. In the first stage, effluent VS concentration was predicted using pH, VS concentration, flowrate of pre-thickened sludge and temperature of the influent as input parameters. In the second stage, effluent VS concentration in addition to first stage input parameters were used as input parameters to predict methane yield. The low Root Mean Square Error (RMSE) and high Index of agreement (IA) values were obtained with subtractive clustering method of a first order Sugeno type inference. The model performance was evaluated with statistical parameters. According to statistical evaluations, the models satisfactorily predict effluent VS concentration and methane yield.     

Hydrogen-dependent control of the continuous anaerobic digestion process

Summary Membrane inlet mass spectrometry was used to directly measure the concentrations of CH₄ and H₂ in a mesophilic (37°C) completely mixed, laboratory scale, anaerobic digester, continuously fed at a retention time of 7 days with a glucose (50 mM) mineral salts medium. When the digester was overloaded by an increase in the influent substrate concentration, equivalent to 15.5 kg (COD) m^-3 (digester) day^-1 the concentrations of H₂ and short chain fatty acids increased with a concomitant decline in the pH: following an initial stimulation methanogenesis was inhibited. Regulation of the H₂ signal from the mass spectrometer in a closed feedback loop by controlled addition of carbon source under a potential overload condition, enabled the H₂ concentration to the controlled around 1M and a high steady state rate of methanogenesis of 42 M min^-1 to be maintained; this is equivalent to 1.4 volumes of CH₄ per culture volume per day. The hydrogen-dependent control system was also used to prevent inhibition of methanogenesis when the digester was subject to volumetric overloading potentially equivalent to a retention time of 1 day.     

Adaptive identification of anaerobic digestion process for biogas production management systems

Bioprocess and Biosystems Engineering - To achieve the goals of sustainable development, supplies of renewable energy must be increased and methods of stable production developed. This study...     

Microbial community in a pilot-scale bioreactor promoting anaerobic digestion and sulfur-driven denitrification for domestic sewage treatment

A pilot-scale reactor treating domestic sewage was operated to promote anaerobic digestion and denitrification using endogenous electron donors. While 55 % of organic matter was removed, nitrogen and sulfur showed a different dynamics during the operation. Pyrosequencing analysis clarified this behavior revealing that specific microbial communities inhabited the anaerobic (47.05 % of OTUs) and anoxic (31.39 % of OTUs) chambers. Analysis of 16S rRNA gene partial sequences obtained through pyrosequencing revealed a total of 1727 OTUs clustered at a 3 % distance cutoff. In the anaerobic chamber, microbial community was comprised of fermentative, syntrophic and sulfate-reducing bacteria. The majority of sequences were related to Aminobacterium and Syntrophorhabdus. In the anoxic chamber, the majority of sequences were related to mixotrophic and strictly autotrophic denitrifiers Arcobacter and Sulfuricurvum, respectively, both involved in sulfur-driven denitrification. These results show that pyrosequencing was a powerful tool to investigate the microbial panorama of a complex system, providing new insights to the improvement of the system.     

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.     

Electrolysis-enhanced anaerobic digestion of wastewater

This study demonstrates enhanced methane production from wastewater in laboratory-scale anaerobic reactors equipped with electrodes for water electrolysis. The electrodes were installed in the reactor sludge bed and a voltage of 2.8-3.5 V was applied resulting in a continuous supply of oxygen and hydrogen. The oxygen created micro-aerobic conditions, which facilitated hydrolysis of synthetic wastewater and reduced the release of hydrogen sulfide to the biogas. A portion of the hydrogen produced electrolytically escaped to the biogas improving its combustion properties, while another part was converted to methane by hydrogenotrophic methanogens, increasing the net methane production. The presence of oxygen in the biogas was minimized by limiting the applied voltage. At a volumetric energy consumption of 0.2-0.3 Wh/L_R, successful treatment of both low and high strength synthetic wastewaters was demonstrated. Methane production was increased by 10-25% and reactor stability was improved in comparison to a conventional anaerobic reactor.     

A review of anaerobic digestion bio-kinetics

Anaerobic digestion is the key to sustainable wastewater management and bioenergy production. Kinetics plays an important role in the design of bioreactors, processes, and process scale-up in anaerobic digestion. This article focuses on a state-of-the-art literature review on the experimental kinetic studies of conventional anaerobic bioreactors and anaerobic membrane bioreactors. Various kinetic models that were used to fit the experimental data and derive the kinetic parameters are summarized and discussed in the literature. The values of the maximum specific growth rate µ_max, half saturation constant K_s, decay co-efficient k_d, sludge yield Y, and methane yield Y_CH4 from experimental studies are summarized for each model. This paper can serve as an updated comprehensive source of anaerobic bio-kinetic studies and digester design.     

Bioremediation of gasoline-contaminated groundwater in a pilot-scale packed-bed anaerobic reactor

This work reports on the anaerobic treatment of gasoline-contaminated groundwater in a pilot-scale horizontal-flow anaerobic immobilized biomass reactor inoculated with a methanogenic consortium. BTEX removal rates varied from 59 to 80%, with a COD removal efficiency of 95% during the 70 days of in situ trial. BTEX removal was presumably carried out by microbial syntrophic interactions, and at the observed concentrations, the interactions among the aromatic compounds may have enhanced overall biodegradation rates by allowing microbial growth instead of co-inhibiting biodegradation. There is enough evidence to support the conclusion that the pilot-scale reactor responded similarly to the lab-scale experiments previously reported for this design.     

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.     

Dynamic simulator for anaerobic digestion processes

A transient, two-culture model simulating methane production from biomass has been developed. The simulator, based partially on the work by Andrews and McCarty, is capable of calculating the hydrolysis products of several common organic materials, accommodating various substrate feeding modes, and simulating the transient physico-biochemical transport and conversion processes occurring in the biological, liquid, and gaseous phases of a well-mixed reactor. The mathematical representation of this bioconversion system consists of a set of 11 coupled, nonlinear first-order rate equations based on the principles of mass conservation and biochemical reaction kinetics. The model can be used in conjunction with laboratory investigations and as a simulator for evaluating process control strategies and cost developments.     

Low-temperature anaerobic digestion for wastewater treatment

Methanogenesis is an important biogeochemical process for the degradation of organic matter within cold environments, and is associated with the release of the potent greenhouse gas, methane. Cold methanogenesis has been harnessed, in engineered systems, as low-temperature anaerobic digestion (LTAD) for wastewater treatment and bioenergy generation. LTAD represents a nascent wastewater treatment biotechnology, which offers an attractive alternative to conventional aerobic and anaerobic processes. Successful, high-rate, LTAD of sewage and industrial wastewaters (e.g. from the brewery, food-processing and pharmaceutical sectors), with concomitant biogas generation, has been demonstrated at laboratory-scale and pilot-scale. A holistic, polyphasic approach, which integrates bioprocess, physiological and molecular biological datasets has been critical to the development of the LTAD concept.     

Pretreatment of municipal waste activated sludge for volatile sulfur compounds control in anaerobic digestion

The effect of combination of mechanical and chemical pretreatment of municipal waste activated sludge (WAS) prior to anaerobic digestion was studied using a laboratory scale system with an objective to decrease volatile sulfur compounds in biogas and digested sludge. Mechanical pretreatment was conducted using depressurization of WAS through a valve from a batch pretreatment reactor pressurized at 75 ± 1 psi, while combined pretreatments were conducted using six different dosages of hydrogen peroxide (H₂O₂) and ferrous chloride (FeCl₂) along with mechanical pretreatment. About 37-46% removal of H₂S in biogas occurred for different combined pretreatment conditions. Sludge solubilization achieved due to the mechanical pretreatment increased total cumulative methane production by 8-10% after 30 days during the biochemical methane potential (BMP) test. The pretreatment also improved dewaterability in terms of time to filter (TTF), and decreased methyl mercaptan generation potential of the digested sludge.     

Hydrogen-dependent control of the continuous thermophilic anaerobic digestion process using membrane inlet mass spectrometry

The control of a thermophilic continuous anaerobic digestion system when subjected to potential inhibitory shock loadings was achieved through the regulation of dissolved H₂, monitored using membrane inlet mass spectrometry, by the controlled addition of carbon source. At a feed pump switching threshold equivalent to 1 μmol/1 H₂ a steady state rate of methanogenesis of approximately 40 μmol/1/min was obtained. Higher H₂ thresholds resulted in an inhibition of methanogenesis, but precise control of H₂ concentration was demonstrated with an oscillatory response of period 2·5–5·0 min.