Pilot-scale Anaerobic Digester Research Articles

Dynamic Modelling and Simulation of Pilot Scale Anaerobic Digestion Plant Treating Source Separated Food Waste and Effect of Recycling Sludge

This study investigates the effect of recycling sludge and stability of pilot scale anaerobic digestion plant treating organic fraction of source separated food waste. Pilot plant comprises of pre-treatment, anaerobic digestion and post treatment. Anaerobic digestion is carried out under mesophilic conditions. At flow rate of 0.221 m3/d, slurry of food waste is introduced semi-continuously every day within 2 hr time span. Anaerobic digestion model No.1 (ADM1) was applied for modelling and simulation of continuous stirred tank anaerobic reactor including recycling and was implemented in AQUASIM 2.1f. Input food waste was characterized and parameters were determined as dictated by ADM1. Process parameters were obtained from pilot plant and kinetic parameters are standard parameters given in ADM1. Input Organic Loading Rate(OLR) to reactor is 4.81 kgCOD/m3.d and Hydraulic Retention Time (HRT) was 20 days. Pilot scale AD plant was simulated for 40 days. It was found that biogas production rate and gas composition vary according the intermittent feeding pattern and reactor head space contains average compositions of CH4 and CO2 of 56% and 30% (v/v) respectively. Average biogas production rate under this condition was 16.4 m3/d. Recycling of sludge in terms of biomass was implemented in model as an advective link. When model was simulated at 40 day solid retention time (SRT), average biogas production rate increased by 31%. By varying OLRs to reactor, stability was investigated. When input OLR was doubled in terms of hydraulic load, anaerobic reactor became unstable producing H2 (38% v/v) and CO2 (40%v/v) in reactor head space. Under this condition, bulk liquid phase pH was 4.78. Instability is further confirmed by accumulation of volatile fatty acids and inhibition of strict methanogens. ADM1 can be applied to model and simulate pilot scale anaerobic digestion plant and to screen different options before scaling up into large scale plants.

Tolerance response to in situ ammonia stress in a pilot-scale anaerobic digestion reactor for alleviating ammonia inhibition

The anaerobic digestion (AD) of protein-rich substrates is generally inhibited by ammonia. In this study, ammonia-tolerant acclimation was exposed to a stepwise in situ ammonia stress during the continuous AD of solid residual kitchen waste by using a continuous stirred tank reactor with a 50L active volume. The reactor worked well during the acclimation process, with an average daily biogas production of 58L/d, an effluent soluble chemical oxygen demand of 7238mg/L, a volatile fatty acid (VFA) content of 578mg/L, and a VFA/alkalinity ratio of less than 0.4. Moreover, ammonia stress enhanced the activity of Coenzyme F420. The results of high-throughput 16S rDNA sequencing showed that ammonia stress increased the relative abundance of Firmicutes bacteria and hydrogenotrophic methanogens but decreased the abundance of acetotrophic methanogens. This microbial community shift was proposed to be an in situ response strategy for ammonia stress adaptation.

Seasonal Variation on Microbial Community and Methane Production during Anaerobic Digestion of Cattle Manure in Brazil.

Anaerobic digestion is an alternative method for the treatment of animal manure and wastewater. The anaerobic bioconversion of biomass requires a multi-step biological process, including microorganisms with distinct roles. The diversity and composition of microbial structure in pilot-scale anaerobic digestion operating at ambient temperature in Brazil were studied. Influence of the seasonal and temporal patterns on bacterial and archaeal communities were assessed by studying the variations in density, dynamic and diversity and structure. The average daily biogas produced in the summer and winter months was 18.7 and 16 L day(-1), respectively, and there was no difference in the average methane yield. Quantitative PCR analysis revealed that no differences in abundances and dynamics were found for bacterial communities and the total number of Archaea in different seasons. Analysis of bacterial clone libraries revealed a predominance of Firmicutes (54.5 %/summer and 46.7 %/winter) and Bacteroidetes (31.4 %/summer and 44.4 %/winter). Within the Archaea, the phylum Euryarchaeota was predominant in both digesters. Phylogenetic distribution showed changes in percentage between the phyla identified, but no alterations were recorded in the quality and amount of produced methane or community dynamics. The results may suggest that redundancy of microbial groups may have occurred, pointing to a more complex microbial community in the ecosystem related to this ambient temperature system.

Energy production from anaerobic co-digestion processing of cow slurry, olive pomace and apple pulp

This paper deals with anaerobic co-digestion of cow slurry, apple pulp and olive pomace mixture and results obtained shown that the production of methane by co-digestion of cow slurry, olive pomace and apple pulp is not only possible but also economically and energetically attractive. Tests were performed with a pilot scale anaerobic digester, 128 l in volume, operating under batch and fed-batch condition. The biogas production, methane yield and quality, plus other operating parameters were evaluated under four feeding regimes, to simulate a real situation. Stable biogas production was obtained of about 400 l/kg Volatile Solids at a Hydraulic Retention Time of 40 days in a mixture containing 85% cow slurry, 10% olive pomace and 5% apple pulp (% by volume). The percentage of methane inside the biogas was around 52% and the maximum COD removal was 63%.

Nitrite and nitrate as electron acceptors for biological sulphide oxidation.

Autotrophic denitrification with sulphide using nitrate (R1) and nitrite (R2) as electron acceptor was investigated at bench scale. Different solids retention times (SRT) (5 and 20 d) have been tested in R1 while R2 was operated at SRT=13 d. The results indicated that the process allows complete sulphide removal to be achieved in all tested conditions. Tested sulphide loads were estimated from the H2S produced in a pilot-scale anaerobic digester treating vegetable tannery primary sludge; nitrogen loads originated from the nitrification of the supernatant. Average nitrogen removal efficiencies higher than 80% were observed in all the tested conditions once steady state was reached. A maximum specific nitrate removal rate equal to 0.35 g N-NO3- g VSS(-1) d(-1) was reached in R1. Due to sulphide limitation, incomplete denitrification was observed and nitrite and thiosulphate tend to accumulate especially in the presence of variable environmental conditions in both R1 and R2. Lower SRT caused higher NO2accumulated/NO3reduced ratios (0.22 and 0.24, with SRT of 5 d and 20 d, respectively) using nitrate as electron acceptor in steady-state condition. Temperature decrease caused sudden NO2accumulated/NO3reduced ratio increase in R1 and NO2- removal decrease in R2.

An adaptive observer for operation monitoring of anaerobic digestion wastewater treatment

The operation of anaerobic digestion (AD) has a strong dependence on reliable information about the state variables and kinetic parameters. Such a dependence can lead the AD to restrictions for the implementation of advanced monitoring and automatic control schemes; with consequent implications at operation decisions. An adaptive observer is proposed to advise on organic matter degradation via AD. The underlying idea is to provide a scheme for monitoring simultaneously both the biomass concentration and parameter related to the maximum growth rates. The proposed scheme is based on a dynamical model of two steps wastewater digester which can be written as a general cascade system. The adaptive observer is experimentally evaluated in a pilot-scale anaerobic digester, which is used for the winery wastewater treatment. As a proof-of-principle implementation, the pilot-scale digester is operated under dilution rate with (arbitrary) bounded time-variation. The results show that the proposed estimation approach can provide a reliable information towards the monitoring and full-automation of AD processes.

Dynamics of antibiotic resistance genes and presence of putative pathogens during ambient temperature anaerobic digestion.

This study was focused on evaluating the persistency of antimicrobial resistance (AR) genes and putative pathogenic bacteria in an anaerobic digesters operating at mesophilic ambient temperature, in two different year seasons: summer and winter. Abundance and dynamic of AR genes encoding resistance to macrolides (ermB), aminoglycosides (aphA2) and beta-lactams (blaTEM -1 ) and persistency of potentially pathogenic bacteria in pilot-scale anaerobic digesters were investigated. AR genes were determined in the influent and effluent in both conditions. Overall, after 60days, reduction was observed for all evaluated genes. However, during the summer, anaerobic digestion was more related to the gene reduction as compared to winter. Persistency of potentially pathogenic bacteria was also evaluated by metagenomic analyses compared to an in-house created database. Clostridium, Acinetobacter and Stenotrophomonas were the most identified. Overall, considering the mesophilic ambient temperature during anaerobic digestion (summer and winter), a decrease in pathogenic bacteria detection through metagenomic analysis and AR genes is reported. Although the mesophilic anaerobic digestion has been efficient, the results may suggest medically important bacteria and AR genes persistency during the process. This is the first report to show AR genedynamics and persistency of potentially pathogenic bacteria through metagenomic approach in cattle manure ambient temperature anaerobic digestion.

Importance of scale and hydrodynamics for modeling anaerobic digester performance

In this study it is demonstrated that a correct description of the mixing behavior of an anaerobic digester is necessary to accurately model and predict the reactor performance, especially at a larger scale operation. A lab-scale (3.78 l) and a pilot-scale (120 l) anaerobic digester were operated at both mixing and non-mixing conditions. At lab-scale no significant difference in performance was found between these conditions. In each case about 0.25 lCH4/lreactor/d of biogas was produced. The model predictions were in close agreement with measured values. At pilot-scale, however, the influence of mixing became important. A reduction of about 10% of methane production occurred during unmixed conditions. Furthermore also a build-up of volatile fatty acids (up to 3 gCOD/l) prevailed. This behavior could not be predicted by using a CSTR approach. In contrast, it was accurately predicted with the proposed model, which includes a more rigorous mixing next to common reaction kinetics. Prediction of the daily biogas production assuming a perfectly mixed reactor revealed an overestimation of the biogas production by 10%. It is therefore recommended to use more accurate mixing models in order to predict biogas production from anaerobic digesters.

Comprehensive monitoring of a biogas process during pulse loads with ammonia

A thermophilic pilot scale anaerobic digester treating chicken litter was subjected to pulse loads of ammonia of increasing concentration. A micro gas chromatograph (μ-GC) measured CO2, CH4, N2, H2 and H2S in the biogas. In the liquid, NH4-N was measured manually, volatile fatty acids (VFA) were measured manually and with Near Infrared Spectroscopy (NIRS). Within the first 7–24 h after the pulse, the concentrations of iso-butyric and iso-valeric acid increased rapidly and were the best indicators of process stress due to ammonia pulses, confirmed by multivariate analysis. NIRS was not capable of accurate prediction of VFA iso-forms most likely due to low concentrations. Propionic acid was persistent. The ammonia pulses caused an overall decrease of biogas production. The biogas composition was not a good indicator of imbalances; little correlation with VFA measurements was observed.

Enhanced Methane Production from Pilot-Scale Anaerobic Digester Loaded with Rice Straw

A novel co-digestion approach was evaluated to determine if agricultural and industrial waste residues could efficiently degrade straw without pretreatment. Untreated rice straw was co-digested with pig wastewater and anaerobic sludge from the pulp and paper mill treatment process in a pilot-scale digester (1 m) operated in dry, mesophilic conditions. The total weight ratio of dry straw to pig wastewater to sludge was 1 to1.25 to 0.5. The experiment was performed for a total of 153 days, however, the optimum period to balance the maximum energy output with the minimum retention time was determined to be 93 days. The addition of paper mill sludge accelerated VFA formation and gas production when compared to another pilot-scale digester operated under the same conditions without the sludge material. The straw in the pilot-scale digester with the sludge yielded 231 LCH4/kg VS within a 93-day digestion cycle compared to 189 days without the sludge. Daily leachate recirculation (0.2m/m straw-day), however, was not adequate for internal mixing and homogenization of the digester material. With adequate mixing, this co-digestion approach could enhance methane production and reduce the digestion time for untreated rice straw in a farm-scale digester.

Interval-Based Diagnosis of Biological Systems - a Powerful Tool for Highly Uncertain Anaerobic Digestion Processes

Anaerobic digestion (AD) is a highly nonlinear time-varying process commonly used for biological wastewater treatment, which is subject to large disturbances of both influent concentrations, and flow rates that may lead the process to a breakdown. In order to compensate the effect of these disturbances, the dynamics of the main state variables – including biomass – must be closely monitored and used to improve the process performance. However, AD processes still suffer from a lack of reliable and cheap sensors of key process variables to insure the right process operation. This has led to the development of estimation schemes, which infer the information of such key variables from the available measurements. Nevertheless, reliable measurements are not always possible to get because these readings may be corrupted by noise or erroneous due to sensor failures and as a consequence, they may lead to deteriorated control efforts and the eventual crash of the AD process. In this article, we propose an integrated system for the detection, isolation, and analysis of faults in AD processes by using interval observers (IO). The proposed approach was experimentally implemented on a 1-m3 pilot scale anaerobic digester. Based on the comparison between the measured outputs and their corresponding estimates, results show that this approach was able to detect sensor failures as well as faults in the basic hypotheses made during the design step.

Design considerations for a farm-scale biogas plant based on pilot-scale anaerobic digesters loaded with rice straw and piggery wastewater

Two pilot-scale (1 m3) digesters filled with untreated rice straw and co-digested with raw pig wastewater were operated to obtain design parameters for a farm-scale biogas plant. Both digesters contained 50 kg of dry straw mixed with diluted pig wastewater to create dry digestion conditions (20% TS) and operated for 189 days with leachate recirculation. Digester A was designed for optimum performance (150 L of pig wastewater and mesophilic temperatures) while Digester B was designed to establish minimum inputs (60 L of pig wastewater at ambient temperatures). The pig wastewater provided sufficient buffering capacity to maintain appropriate pH values (between 7.0 and 8.1) and nutrient balances (TOC to TKN ratios of 20 in Digester A and 32 in Digester B). Total biogas production was 22,859 L in Digester A and 1420 L from Digester B, resulting in specific methane yields of 231 and 12 L CH4/kgVS added, respectively. Gas production in Digester A was directly correlated with temperature, but the overall lack of methanogenic activity was caused primarily by the reduced wastewater volume. Two theoretical farm-scale scenarios (considering both untreated and pretreated rice straw) were developed for a 100-ha rice farm. Either scenario can produce 100,000 m3CH4 per year, yielding 328 MWh. Major differences including heat input, space requirements, loading frequency, digester volume, engine size, wastewater quantities, and additives are quantitatively defined. The appropriate choice for a farm-scale operation is the simplest model using untreated rice straw without additives, although six times more heat and twice as much reactor volume is required.

Stable operation during pilot-scale anaerobic digestion of nutrient-supplemented maize/sugar beet silage

Biogas production from maize/sugar beet silage was studied under mesophilic conditions in a continuous stirred tank reactor pilot-scale process. While energy crop mono-digestion is often performed with very long hydraulic retention times (HRTs), the present study demonstrated an efficient process operating with a 50-day HRT and a corrected total solids (TScorr) based organic loading rate of 3.4kg/m3d. The good performance was attributed to supplementation with both macro- and micronutrients and was evidenced by good methane yields (318m3/tonTScorr), which were comparable to laboratory maximum expected yields, plus low total volatile fatty acid concentrations (<0.8g/L). A viscoplastic and thixotropic digester fluid behaviour was observed, and the viscosity problems common in crop mono-digestion were not seen in this study. The effluent also complied with Swedish certification standards for bio-fertilizer for farmland application. Nutrient addition thus rendered a stable biogas process, while the effluent was a good quality bio-fertilizer.

An Innovative Farm Scale Biogas/Composting Facility for a Sustainable Medium Size Dairy Farm

Approach: The amount of energy related costs as a portion of the total farm operating cost can be as high as 29% and the continuing increase of the real cost of energy related farm input has been one of the major factors impacting the cost of agricultural production. However, agricultural has the potential of replacing some of the purchased energy in the form of fossil fuels, commercial fertilizer and field production of animal feed with bioenergy and organic fertilizer from onsite renewable biomass such as animal manure in order to economically and environmentally sustain it. The aim of this study was to develop an innovative energy efficient pilot scale anaerobic digester composting facility. Methodology: A solid/liquid manure separator farm scale anaerobic digester and composting facility for a medium sized dairy farm were designed, constructed and tested. In order to make the anaerobic digestion economically viable under Canadian climatic conditions, the design, installation and operation of the system were based on advantages gained from the digester as a component of the total farm management system. In addition to the biogas production, benefits related to manure handling and storage, environmental quality improvement through odor control and water pollution reduction, fertilizer recovery and water recycling, were considered. Results: The layout of the farm was modified to provide solutions for four environmental problems related to: disposal of milkhouse wastes and overflow from the manure storage facility into the fire pond. The system possesses high energy conversion efficiency at relatively low capital cost and reduced labour requirement and has indirect energy ramifications through the production of organic fertilizer (compost) to replace expensive and energy consuming commercial fertilizer as well as the production of bioenergy (biogas) which will reduce the demand for energy. The overflow from the system (purified water) can be recycled for cleaning the barn, thereby reducing the costs of water use and manure storage facilities on one hand and eliminating pollution problems associated with manure storage and disposal on the other hand. Conclusion: The use of dairy waste as a source of energy and fertilizer resulted in a saving of 6289 kg of fertilizer at a cost of $17 925 annually and additional saving of $20 547 on energy use.

Anaerobic co-digestion of fat, oil, and grease (FOG): A review of gas production and process limitations

The addition of readily available high strength organic wastes such as fats, oils, and grease (FOG) from restaurant grease abatement devices may substantially increase biogas production from anaerobic digesters at wastewater treatment facilities. This FOG addition may provide greater economic incentives for the use of excess biogas to generate electricity, thermal, or mechanical energy. Co-digestion of FOG with municipal biosolids at a rate of 10–30% FOG by volume of total digester feed caused a 30–80% increase in digester gas production in two full scale wastewater biosolids anaerobic digesters (Bailey, 2007; Muller et al., 2010). Laboratory and pilot scale anaerobic digesters have shown even larger increases in gas production. However, anaerobic digestion of high lipid wastes has been reported to cause inhibition of acetoclastic and methanogenic bacteria, substrate, and product transport limitation, sludge flotation, digester foaming, blockages of pipes and pumps, and clogging of gas collection and handling systems. This paper reviews the scientific literature on biogas production, inhibition, and optimal reactor configurations, and will highlight future research needed to improve the gas production and overall efficiency of anaerobic co-digestion of FOG with biosolids from municipal wastewater treatment.

Sulphur fate and anaerobic biodegradation potential during co-digestion of seaweed biomass (Ulva sp.) with pig slurry

Seaweed (Ulva sp.) stranded on beaches were utilized as co-substrate for anaerobic digestion of pig slurry in three-month co-digestion tests in pilot scale anaerobic digesters in the laboratory. The methanogenic potential of Ulva sp. was low compared to that of other potential co-substrates available for use by farmers: 148Nm3CH4/t of volatile solids or 19Nm3CH4/t of crude product. When used as a co-substrate with pig manure (48%/52% w/w), Ulva sp. seaweed did not notably disrupt the process of digestion; however, after pilot stabilisation, biogas produced contained 3.5% H2S, making it unsuitable for energy recovery without treatment. Sequentially addition of the sulphate reduction inhibitor, potassium molybdate, to a final concentration of 3mM, temporarily reduced H2S emissions, but was unable to sustain this reduction over the three-month period. According to these pilot tests, the use of seaweed stranded on beaches as co-substrate in farm-based biogas plants shows some limitations.

Comparison of Methane Production from Bench- and Sub Pilot-Scale Anaerobic Digesters

Design and construction of full-scale anaerobic digesters that co-digest manure with other substrates, such as food processing wastes, is challenging because of the large number of potential mixtures that can be fed to the digester. In this work we examine the relationship between results from bench-scale methods such as biochemical methane potential assays (BMPs) and sub pilot-scale reactors. The baseline feedstock for this study was beef manure from concrete feedlot pens (open and covered) in eastern Iowa. Additional co-digestion substrates tested were short-fiber cardboard, corn processing wastewater, enzyme processing wastewater and lagoon liquid. Substrates were characterized for total solids (TS), volatile solids (VS), chemical oxygen demand (COD), pH, alkalinity, and ammonia, after which BMPs were conducted on all substrates. Based on the BMP and anaerobic toxicity assay (ATA) results, a mixture was created and evaluated using BMPs and tested in 100-L sub pilot-scale reactors. This study showed that results from BMPs of feedstock co-digestion mixtures accurately estimated the range of methane produced from three 100-L, plug flow reactors.

Pilot-scale anaerobic digestion of screenings from wastewater treatment plants

The anaerobic digestion of screenings from a municipal wastewater treatment plant was studied in a 90 L pilot-scale digester operated at 35 degrees C under semi-continuous conditions. In the first 4 weeks, a dry solids residence time of 28 days was applied, but the installation of inhibitory conditions was observed. Feeding was therefore suspended for 4 weeks to allow the digester to recover from inhibition, and then progressively increased up to a constant load of 6 kg of raw waste per week, corresponding to an average residence time of about 35 days of dry solids. At this stage, biogas production stabilized between 513 and 618 Nl/kg VS(added) per week, with methane contents around 61% v/v. The results of this work thereby supported the feasibility of (co-)digestion as a potential alternative treatment of screenings from municipal wastewater treatment plants.

Dynamics of selected pre-existing polybrominated diphenylethers (PBDEs) in municipal wastewater sludge under anaerobic conditions

Despite the rapid, widespread accumulation of polybrominated diphenylethers (PBDEs) in our surroundings, their environmental fate has been largely unknown. In the present study, most common congeners (BDE 47, 99, 100, 138, 153, 154, 183 and 209) were investigated for their dynamics in municipal sewage sludge under mesophilic condition (37 °C). In anaerobic batch cultures, the concentrations of BDE 47, 99, 100 and 209, exhibited significant decreases (by 22–40% from their initial concentration), whereas the levels of the other congeners, BDE 138, 153, 154 and 183, remained stable during a 238-d incubation. However, in a parallel study conducted in a pilot-scale anaerobic sludge digester, loss of all eight congeners was observed. The present study indicates that certain PBDE congeners undergo a significant mass decrease under anaerobic conditions.

Development of a pilot scale anaerobic digester for biogas production from cow manure and whey mix

This paper presents results from anaerobic digestion of cow manure and whey mix. A pilot scale anaerobic digester, 128 l in volume, has been developed, to operate under batch and fed-batch conditions. The versatile and unique characteristics of the instrument allowed testing the methane production directly in the farm. The digester performance was evaluated with two calibration tests, the main for a period of 56 days. The study test was divided into three phases, one for each type of feeding operation (batch, fed-batch, batch). The initial phase of digestion resulted in 57 l-CH 4/kg-VS, the second phase had a yield of 86.6 l-CH 4/kg-VS and the third one had a production of 67 l-CH 4/kg-VS. The total methane yield was equal to 211.4 l-CH 4/kg-VS. Using the obtained pilot plant results to a real scale diary production cycle, it was possible to evaluate an electricity production equal to 8.86 kwh per 1 t/d. The conducted tests did show that there is a good potential to the use of a cow manure and whey biomass mix for biogas production.