Pilot-scale Anaerobic Digester Research Articles

Biomethanisation of sewage sludge: Sonication pretreatment and monitoring of microbial communities

The improvement of mesophilic biomethanisation of recalcitrant sewage sludge derived from urban wastewater treatment through the application of a sonication pretreatment was evaluated in parallel in two pilot-scale anaerobic digesters (two biological replicates: reactors RA and RB). The valorisation process was monitored through a novel and holistic approach that related the biomethanisation yield, and its main batch operational parameters, with the abundance of archaeal and bacterial communities in the anaerobic inocula. Sonication allowed achieving a methane yield coefficient derived from sewage sludge of 240 ± 20 mLSTPCH4/g VS (volatile solids) at the load range of 0.8–4.0 g VS/L in both reactors. The process was more stable in reactor B, with a wider range of loads being allowed (up to 5.29 g VS/L). Monitoring the presence of Archaea in the mixing liquor revealed a variation in their abundance throughout the process which was directly related to the availability of organic matter and pH. Advanced metagenomic analysis showed the phylogenetic and functional diversity of the complex microbiome involved. While Bacteria were widely distributed in 35 phyla, Archaea fitted in only two. Euryarchaeota was the majoritarian archaeal phylum (99.5 %) and its more abundant families are linked to methanogenic metabolism. Functional analysis revealed several relevant metabolic pathways that followed similar trends in both reactors. “Methane metabolism” clearly diminished at the end of the process in concordance with the exhaust of methane generation, while “ABC transporters” or “two-component systems”, involved in bacterial survival to changing environments, followed the opposite pattern. This integrated approach could help to increase the methanogenic valorisation of sewage sludge.

Startup performance and microbial communities of a decentralized anaerobic digestion of food waste

The aim of this work was to evaluate the feasibility of treating food waste generated from a hawker centre in a pilot-scale anaerobic digester operating on site in an urban area of Singapore. For this purpose, a 10.4 m3 digester was housed within two 20 feet containerized systems and sited adjacent to the hawker centre. The system reported in this work was during the startup phase, for over 71 days of real and varying food wastes loading rate. The results demonstrated that the decentralized system had an average specific methane yield of 0.55 ± 0.04 m3CH4/kgVS, with methane concentrations of 56.6 ± 2.3%. For the power generation output, the energy assessment revealed an average of 2.05 ± 0.57 m3/kW h consumption rate. Accordingly, on the last day of startup phase, the inoculum of the digester was richer in organisms from the phylum Thermoplasmatota, i.e., genera Candidatus_Methanogranum and Candidatus_Methanoplasma, alongside with other dominant abundance from phyla Bacteroidota, Firmicutes, Synergistota, and Verrucomicrobiota. This study provides new insights into pilot scale decentralized anaerobic digestion with varying food waste relate to the characterizations of digester microbial communities, as well as turning in a typical integrated anaerobic digestion of food waste-to-energy system a reality.

Anaerobic Digestion of Cereal Rye Cover Crop

The rapid growth of cover crop planting area in the U.S. helps with erosion control, soil health, control of greenhouse gases, and also provides abundant biomass for the production of bioenergy and bioproducts. Given the cover crops’ compositional heterogeneity and variability, a tolerate platform technology such as anaerobic digestion (AD) is preferred but has not been widely used for cover crop biorefining. This study evaluated the biogas and methane yields from six cereal rye (Secale cereale L.) cover crops grown in the Midwest, using both bench- and pilot-scale anaerobic digesters. The effects of two critical factors, the total solids (TS) content and ensiling, on digester performance were also investigated. Methane yields of 174.79–225.23 L/kg-VS were obtained from the bench-scale tests using cereal rye as the mono feedstock. The pilot-scale test with no pH adjustment showed a slightly higher methane yield. Ensiling increased the methane yield by 23.08% at 6% TS, but disturbed AD at 8% TS, and failed AD at 10% and 15% TS. Findings from this study would help farmers and the biorefining industry to determine the baseline performance and revenue of cereal rye AD and to develop strategies for process control and optimization.

Lab- and pilot-scale anaerobic digestion of municipal bio-waste and potential of digestate for biogas upgrading sustained by microbial analysis

The challenge of handling the enormous amount of bio-waste that is globally produced on an annual basis can become an opportunity by exploiting such residues for bioenergy production in the form of biogas or biomethane. The application of anaerobic digestion to bio-waste is still limited due to their specific characteristics (e.g., recalcitrant matter, high solids content, etc.) that might lead to process imbalances and microbial toxicity issues. The present study aimed to elucidate microbial ecology in the lab- and pilot-scale biogas reactors fed with bio-waste. The results showed that the indigenous microbial community in the bio-waste was almost extinct during the digestion process. The AD microbiome was mainly composed of members belonging to the orders Clostridiales and Bacteroidales, whose relative abundance dynamically changed to adapt to operational conditions. The increase in organic loading rate led to the proliferation of members associated with the phylum Synergistetes and the transition to versatile acetoclastic/hydrogenotrophic methanogenic pathways. Furthermore, digestate is a suitable biological inoculum and a nutrient supplement to support further biogas upgrading process. The results identified functional microbes that could benefit bio-waste digestion, unveiled insights into microbiome reorganization, and proposed a potential application of the digestate.

Kinetics evaluation of a pilot scale anaerobic biofilm digester treating leachate from a municipal solid waste transfer station

In this study, leachate from municipal solid waste (MSW) transfer station was treated in a pilot scale anaerobic biofilm digester (ABD) system under 30, 25, 20 and 10-day HRT. The maximum COD concentration entering the reactor was 46,944 mg/L and the reactor was under full operation for 163 days. The results showed that COD removal efficiency was above 95 % throughout the operation and measurement of the average daily biogas production yielded a value of 25 ± 1 m3/day with a composition of 57 ± 12 % methane and 26 ± 6 % carbon dioxide. In comparison to the Monod and modified Stover–Kincannon (MSK) models, the second-order Grau model presented the best fit as a substrate-removal kinetic model for this ABD system with its R2, 0.999; while the MSK model for gas generation was proven to be a better fit than the Van der Meer and Heertjes model in predicting the amount of biogas and methane gas produced from leachate treatment via a pilot scale ABD.

Effects of the temperature range on the energy performance of mixed and unmixed digesters with submerged waste: An experimental and CFD simulation study

This study evaluated the effects of temperature range and mixing on the thermal and energetic performance of a batch pilot-scale anaerobic digester with submerged waste (ADSW). This digester was operated under four configurations, namely unmixed mesophilic (UMAD), liquid recirculation-mixed mesophilic (LMAD), unmixed thermophilic (UTAD), and liquid recirculation-mixed thermophilic (LTAD). Anaerobic digestion (AD) experiments indicated that the mixed ADSW produces higher methane yields under the thermophilic temperature range (416.12 Nm3CH4/tVS) compared to the mesophilic range (362.23 Nm3CH4/tVS). However, when the ADSW is not mixed, lower methane yields are measured under the thermophilic range (184.82 Nm3CH4/tVS) compared to the mesophilic range (295.41 Nm3CH4/tVS). In an attempt to understand these opposing results, computational fluid dynamics (CFD) simulations were conducted. Findings showed that while a completely uniform temperature is achieved within the digester when mixing is performed, a temperature stratification occurs under unmixed conditions. This stratification, which is more severe in the thermophilic range, may be the reason behind the reported AD experiments findings. The energy balance calculations indicate that the ADSW achieves its best energy efficiency ratio under UMAD (1.72), followed respectively by LMAD (0.83), LTAD (0.66), and UTAD (0.29).

Fate of antibiotic resistance genes and class 1 integrons during sludge treatment using pilot-scale anaerobic digestion with thermal hydrolysis pretreatment

This work evaluated the fate of antibiotic resistance genes (ARGs) and class 1 integron gene in sewage sludge before and after pilot-scale thermal hydrolysis pretreatment (THP) and subsequent mesophilic anaerobic digestion (AD) treatment. Variables investigated include THP temperatures, feedstock types, and AD solids retention times. Real-time polymerase chain reaction was performed to quantify the ARGs in feedstocks, THP and AD effluent. Results show that THP significantly (t test, p < 0.05) reduced the absolute abundances of most ARGs, with the reduction ranging from 0.03 to 3.09 log units. Rebound effects of ARGs in the subsequent AD were observed and were relevant with tested variables; shorter solids retention time (10 days) and higher THP temperature (165 ℃) can significantly reduce ARGs in AD effluent. These findings provide references about the effects of the THP and AD on the control of ARG spread from sewage sludge to environments.

Pilot-Scale Anaerobic Digestion of Pig Manure with Thermal Pretreatment: Stability Monitoring to Improve the Potential for Obtaining Methane

Monitoring and controlling stability in anaerobic digestion (AD) systems are essential, since it allows to obtain information that helps to take corrective actions in case of deviations in the system and to guarantee a stable performance in the biogas production. In this work, a pilot-scale CSRT reactor (1 m3) was monitored during the anaerobic digestion of pig manure with thermal pretreatment (80 °C) operated at thermophilic temperature (45 °C). The ratio of the volatile organic acids (FOS) to the total inorganic carbonate (TAC) and the pH were the indicators used during the monitoring process to identify deviations in the AD system. Additionally, alkaline solution NaOH (98%) was applied to counteract pH deviations and maintain stability. Chemical oxygen demand (COD) and biogas composition were measured during the AD process. It was found that during the AD process, the FOS/TAC was between the range of 0.5 and 1. The results revealed that, in the anaerobic digestion of pig manure with thermal pretreatment, the pH was kept stable in the range of 6.7–7.4 since no medium acidification occurred. Additionally, the tendency of the chemical oxygen demand decreased from the 10th day of operation, product of the favorable enzymatic activity of the microorganisms, reflected in the stable production of biogas (69% CH4). Finally, it is concluded that thermophilic AD of pig manure with thermal pretreatment is a good option when it is carried out efficiently by employing an adequate energetic integration.

Integration of anaerobic digestion and composting allows safety recovery of energy and nutrients from AFB1 contaminated corn

Occurrence of human carcinogen aflatoxin B1 (AFB1) in feed and foodstuffs is expected to increase in the next years due to climate change. This work aimed to assess the possible recovery of energy and nutrients from AFB1 contaminated corn through the integration of anaerobic digestion and composting. Pilot scale anaerobic digestion and composting of the digestate were carried out using AFB1 contaminated corn (100 µg g-1) and pig slurry. Biomethane yields, compost quality and AFB1 degradation were evaluated during the trials. Results of this research showed that anaerobic digestion and composting integration can represent a suitable solution for the safe valorisation of a hazardous waste like AFB1 contaminated corn. Neither biomethane yields (214 NL kg-1 VS) nor compost quality were affected from AFB1 contamination (i.e., C/N ratio and germination index of the compost were 10.8% and 100.9%, respectively). In addition, the mycotoxin was completely removed at the end of the process (the concentration was under the detection limit already after the active phase of composting). This confirms that AFB1 contaminated feed and foodstuffs might be safely treated in conventional anaerobic and composting facilities, without negative effects on process stability and products quality.

Improvements in the Robustness of Mid-Infrared Spectroscopy Models against Chemical Interferences: Application to Monitoring of Anaerobic Digestion Processes

The monitoring and control of bioprocesses rely on the measurement of the main metabolite concentrations. To this end, infrared spectroscopy (IR) is a good candidate with which to perform rapid and non-destructive measurements. However, IR-based measurements rely on a calibration step linking the measured spectra to the concentrations of the compounds of interest. This calibration may suffer with problems of robustness when the measuring conditions change, such as when some chemicals not present in the calibration spectra are added when using the IR sensor. In this study, a method based on orthogonal projection, dynamic orthogonal projection (DOP), was tested for its ability to cope with the robustness problem caused by the addition of ammonia in a pilot-scale anaerobic digester, whose volatile fatty acid concentrations were monitored by mid-IR spectrometry. The results demonstrate that DOP has significant potential as a form of process analytical technology.

Optimizing ADM1 Calibration and Input Characterization for Effective Co-Digestion Modelling

Anaerobic co-digestion in wastewater treatment plants is looking increasingly like a straightforward solution to many issues arising from the operation of mono-digestion. Process modelling is relevant to predict plant behavior and its sensitivity to operational parameters, and to assess the feasibility of simultaneously feeding a digester with different organic wastes. Still, much work has to be completed to turn anaerobic digestion modelling into a reliable and practical tool. Indeed, the complex biochemical processes described in the ADM1 model require the identification of several parameters and many analytical determinations for substrate characterization. A combined protocol including batch Biochemical Methane Potential tests and analytical determinations is proposed and applied for substrate influent characterization to simulate a pilot-scale anaerobic digester where co-digestion of waste sludge and expired yogurt was operated. An iterative procedure was also developed to improve the fit of batch tests for kinetic parameter identification. The results are encouraging: the iterative procedure significantly reduced the Theil’s Inequality Coefficient (TIC), used to evaluate the goodness of fit of the model for alkalinity, total volatile fatty acids, pH, COD, volatile solids, and ammoniacal nitrogen. Improvements in the TIC values, compared to the first iteration, ranged between 30 and 58%.

Enhancing volatile fatty acids (VFA) production from food waste in a two-phases pilot-scale anaerobic digestion process

The bio-based production of added-value products and energy from waste streams while minimizing environmental impacts is a crucial aspect within the circular economy’s principles. A two-phases anaerobic digestion process to produce volatile fatty acids (VFA) and biogas production in an energetically feasible manner from the organic fraction of municipal solid waste (OFMSW) has been investigated. Mesophilic temperature (T) coupled to 5.0 days as hydraulic retention time (HRT) gave a VFA-rich stream with an overall concentration of 24.4 ± 0.2 g CODVFA/L, dominated by propionic and valeric acids, and high acidification yield, being VFA close to 90% of the soluble COD. The pH was maintained around 7.0 by the digestate recirculation from the second methanation reactor (HRT of 20 days) where biogas production was quantified under mesophilic and thermophilic T. The assessment of energy balance showed the benefits to carry out the second methanation stage under thermophilic condition, having more income from electricity generation without losing the thermal sustainability of the two-phases process as a whole.

The role of oxidation-reduction potential as an early warning indicator, and a microbial instability mechanism in a pilot-scale anaerobic mesophilic digestion of chicken manure

To identify an early warning indicator for the anaerobic digestion of chicken manure and reveal a microbial mechanism of instability, a bench-scale digester with a working volume of 55 L was utilized by raising the organic loading rates (OLRs). Three characteristics were identified and posited as early warning indicators, namely: one-way changes, a longer early response time, and sudden variations. The core early warning indicator set was constructed, which included the oxidation-reduction potential (ORP), individual volatile fatty acids, and CH4/CO2. Considering the convenience, economic nature, and reliability of industrial biogas plants, the ORP was recommended as a final early warning indicator with an inhibition threshold of > −540 mV. The initial instability was caused by the accumulation of ammonia, which suppressed methanogenic activity, although it stimulated the growth of Methanosarcina. The accumulation of free volatile fatty acids, which inhibited both the growth and methanogenic activity of Methanosarcina, was responsible for the final collapse.

Design and Development of Food Waste to Biogas Converter System

The valorisation of food waste via anaerobic digestion has shown to be a promising option for tackling waste accumulation and achieve energy security. In this study, a lab scale and a pilot scale anaerobic digester, with food waste as the feedstock and palm oil mill effluent (POME) as the microbial inoculant, were evaluated for their biogas production. For the lab scale study, the effect of different food waste to POME ratio, ranging from 1:1, 1:2 and 2:1, on the biogas production were investigated for a period of 7 d. From the experimental work, the ratio of food waste to POME at 2:1 gave the highest biogas yield of 0.14 L. For the pilot scale study, a digester was designed based on a daily electricity demand of 9.5 kW of an eco-park. With the use of the Pro Engineer Software, it is estimated that a digester with a volume of 4.27 m3, handling capacity of 200 L of food waste daily, and a hydraulic retention time of 15 d, can generate sufficient biogas to meet the electricity demand for the eco-park.

Pilot-scale anaerobic digestion of by-product liquid (brown juice) from grass protein extraction using an un-heated anaerobic filter

The present study investigated the feasibility of using pilot-scale anaerobic filter to treat by-product liquid (brown juice, BJ) from grass protein extraction in a green biorefinery, and produce biogas via anaerobic digestion without co-digestion substrate. Prior to feed into the un-heated anaerobic filter reactor, the BJ was warmed up to 55−60 °C in order to maintain the digestion temperature. The influence of retention time and feeding frequencies were tested in order to optimize the overall performance. The study demonstrated that anaerobic filter is feasible for converting BJ to biogas. The specific CH production reached 230 mL gCOD−1 d −1 while up to 80 % of influent COD was removed within a hydraulic retention time of 5.5 days. In addition, higher feeding frequencies enhanced the process stability as it narrowed the temperature variation and decreased the instant impact on buffer capacity.

Development of stabilization methods using a pilot scale anaerobic digester for seasonal variations in kitchen wastes for improved methane production with zero breakdowns

Anaerobic digestion of kitchen waste faces the challenge of frequent composition variation and rapid digester acidification due to its highly degradable nature. Considering this challenge, one year long pilot scale anaerobic digestion study was conducted in the anaerobic digester of 25 ​m 3 /day capacity with seasonal kitchen waste (fruits and vegetable waste). The seasonal variation in kitchen waste content was studied by dividing a year into six seasons (each season comprising 60 days), namely, spring, summer, monsoon, autumn, early winter, and prevernal winter. Considering all the properties of kitchen waste, the anaerobic digestion experiment was conducted with the organic loading rate (OLR) of 0.60–0.72 ​kg VS/m 3 /day. The specific biogas production for all the seasons was found to be 0.56–0.68 ​m 3 /kg VS, with the volumetric methane content of 54.3–62.8%. The average seasonal specific methane production was observed in the range of 0.30–0.41 ​m 3 /kg VS. The anaerobic digestion conversion efficiency of 55.3–67.0% was found for the anaerobic digestion of the wastes. The problem of the rapid acidification of kitchen waste has been rectified with the addition of protein-rich de-oiled pongamia cake in place of alkali chemicals. The economic analysis of kitchen waste based anaerobic digestion showed the maximum economic benefits of biogas production as INR 30,783 for its utilization as LPG and INR 37,777 for its utilization as electricity for the summer season compared to all other seasons. The extra economic benefits earned by the utilization of the digested slurry as organic manure, as it contains a good amount of nitrogen (N) 2.3–2.6%, phosphorus (P) 0.6–0.9%, and potassium (K) 0.7–1.1%. • Pilot-scale operation of kitchen waste-based anaerobic digester. • Rapid acidification of kitchen waste hinders the operation of anaerobic digester. • The addition of protein and fat-rich material helps to maintain the appropriate pH of the digester. • The challenge of pH drop in the digester was resolved by using pongamia de-oiled cake instead of any chemicals.

Pilot Scale Solid State Anaerobic Co-digestion of Dairy Manure and Milking Parlor Wastewater

A pilot scale solid-state anaerobic co-digestion study was conducted to elucidate the feasibility of anaerobic digestion as an effective treatment method for dairy manure (DM) and milking parlor wastewater (MPW), and it focusing mainly on the stability of anaerobic digestion process and methane production potential. Dairy manure and milking parlor wastewater were co-digested (1/0.48, w/w) in 11m3 volume reactor for 63days at the mesophilic condition (36°C). In addition, pH, volatile fatty acids and ammonia were periodically analyzed to monitor anaerobic digestion inhibitions. The results showed that the co-digestion of DM and MPW was successfully performed without any inhibition caused by ammonia and volatile fatty acids accumulation. The potential methane production (P) from a modified Gompertz model was about 130.5 mL g-VS-1, and approximately 80% of the methane production was obtained during the 40 days digestion test and the lag phase was 14 days. The pilot scale SSAD (solid-state anaerobic digestion) system showed about 1.3-fold higher methane production than lab-scale system. The results of solid-state anaerobic co-digestion of bedded pack barn dairy manure and milking parlor wastewater using pilot-scale anaerobic digester can be used as a reference for the design of farm-scale anaerobic digester. The pilot scale SSAD system showed about 1.3-fold higher methane production than lab-scale system. The potential methane production (P) was about 130.5 mL g-VS-1, and approximately 80% of the methane production was obtained during the 40 days digestion test and the lag phase was 14 days.

Pilot-scale biogas production in a temperate climate using variable food waste

Low-input, household-scale anaerobic digestion (AD) is practical for rural energy generation, but the heating and maintenance of such systems has prevented the use of this technology in temperate climates. This study quantified the temporal variation of fuel production from an unheated, pilot-scale (2 m3) anaerobic digester in southeastern Ohio, USA. The feedstock for the digester consisted of ground, mixed pre- and post-consumer food waste collected daily and diluted to 10% solids with rainwater collected on site. Fuel production varied from 7.96 × 10−6 to 8.45 × 10−2 m3 CH4 kgVSadded−1 following three separate inoculations over the course of two years. The positive relationship between ambient air temperature, biogas yield, and biomethane production rates for both years was a dominant driver affecting fuel quantity and quality. Biogas quality produced from variable feedstocks in these conditions was poor, with an average volumetric methane (CH4) content of 20% and an average CO2:CH4 ratio of 7.8. Methane yields did reach 50% during the warm seasons, but this yield was not consistently maintained. Despite low energy yields that resulted from the wide range of ambient temperatures (−18 °C–33 °C) and variable feedstocks, we achieved a moderate energy return on investment relative to previously published results describing AD system energy requirements. Pilot-scale, unheated AD systems using mixed food waste can be effective in temperate regions, but the systems should be managed to compensate for seasonal temperature changes and feedstock chemistry.

Towards upscaling the valorization of wheat straw residues: alkaline pretreatment using sodium hydroxide, enzymatic hydrolysis and biogas production.

Lignocellulosic biomass is considered as a recalcitrant substrate for anaerobic digestion due to its complex nature that limits its biological degradation. Therefore, suitable preprocessing for the improvement of the performance of conventional anaerobic digestion remains a challenge in the development of anaerobic digestion technology. The physical and chemical characteristics of wheat straw (WS), as a representative lignocellulosic biomass, have a significant impact on the anaerobic digestion process in terms of quantity and quality of the produced biogas. This study aimed at investigating the enzymatic saccharification and detoxification of straw prior to anaerobic digestion with the final objective of enhancing the performance of conventional anaerobic systems of recalcitrant fractions of agricultural waste. The experimental activity was performed in lab and pilot scale treating WS. Alkaline delignification of straw using sodium hydroxide (NaOH) was studied prior to enzymatic hydrolysis for the production of easily biodegradable sugars. After defining the optimum conditions for the pretreatment scheme, the anaerobic digestability of the effluents produced was measured. Finally, the final liquid effluents were fed to a pilot scale anaerobic digester of 0.5m3 volume, applying an increasing organic loading rate (OLR) regime (in terms of chemical oxygen demand (COD) from 0.2 to 15kg COD/m3/day). The optimum conditions for the delignification and enzymatic hydrolysis of WS were defined as 0.5M NaOH at 50°C for 3-5h and 15μL Cellic CTec2/g pretreated straw at 50°C. It was proven that the resulting liquid effluents could be fed to an anaerobic digester in the ratio that they are produced with satisfactory COD removal efficiencies (over 70%) for OLRs up to 10kg COD/m3/day. This value is correspondent to a hydraulic retention time of around 7.5days, much lower than the respective one for untreated straw (over 12days).

Ultrasound-assisted digestate treatment of manure digestate for increased biogas production in small pilot scale anaerobic digesters

In this study, ultrasound (US) disintegration was evaluated as a digestate treatment strategy. Digestate treatment is the physical, chemical or biological treatment and recirculation AD of the effluents of the anaerobic digesters (i.e., digestate) to the digester as supplementary feed to increase their utilization potential for energy conversion or green chemical production in post-valorization pathways. In this scope, semi-continuous small pilot scale manure digesters were operated in parallel with various operational settings. One of the digesters was fed by adding disintegrated digestate to an equal volume of fresh manure feed (recycle ratio of 1). The obtained results showed that US-assisted digestate treatment at 1500 kJ/kg TS specific energy input with 30 days of hydraulic retention time increased the methane production rate by 18%. The increased methane production rate was found to be related to the applied specific energy and organic loading rate. A basic cost-benefit analysis showed that the energy demand of the US disintegration at lab scale was higher than the energy that can be recovered from the additional biogas produced.