Specific Biogas Yield Research Articles

Effect of feed slurry dilution and total solids on specific biogas production by anaerobic digestion in batch and semi-batch reactors

Biomass from various sources such as cow dung is a significant source of renewable energy (as biogas) in many regions globally, especially in India, Africa, Brazil, and China. However, biogas production from biomass such as cattle dung is a slow, inefficient biochemical process, and the specific biogas produced per kg of biomass is relatively small. The improvement of specific biogas production efficiency using various dilution ratios (and, hence, total solids [TS]) is investigated in this work. A wide range of feed dilution (FD) ratios of cow dung: water (CD: W) was tested in batch biogas digesters with total solids ranging from 1% to 12.5% and FD ratio ranging from 2:1 to 1:20. To further verify the results from the above batch experiments, semi-batch experiments representative of field-scale biodigesters were conducted. Semi-batch reactors have a steady-state process, unlike batch reactors, which have an unsteady state process. Our results suggested that specific biogas production (mL/g VS) increased continuously when the total solids decreased from 12.5% to 1% (or when dilution increased). Our experiments also indicate that the commonly used 1:1 feed dilution ratio (TS ~ 10% for cow dung) does not produce the maximum specific biogas production. The possible reason for this could be that anaerobic digestion at higher total solids is rate limited due to substrate inhibition, mass transfer limitations, and viscous mixing problems that arise at higher total solids concentration. Hence, a higher feed dilution ratio between 1:2 and 1:4 (TS between 4 and 6.7%) is recommended for a more efficient biomass utilization of cowdung. Empirical relationships were also developed for variation of specific biogas yield with the total solids content of the cow dung slurry.Graphic abstract

Elucidating the kinetics of ammonia inhibition to anaerobic digestion through extended batch experiments and stimulation-inhibition modeling

Ammonia can be accumulated to a level inhibitory to methanogenesis. There are large discrepancies in the reported inhibition thresholds. Through extended batch digestion experiments (up to 110 d) at 6 ammonia concentrations (0.70–13 g N/L), this study discovered sequential occurrence of adaptable and unadaptable inhibition that reveals the discrepancies. Lag phase length representing adaptable inhibition increased exponentially with ammonia concentration. The kinetics of specific biogas yield that reveals unadaptable inhibition was best simulated with the Han & Levenspiel model. The 50% unadaptable inhibition thresholds were 10.7 g N/L with active inoculum and 6.8 g/L with stressed inoculum. The digesters with stressed inoculum had faster adaptation to adaptable inhibition though less resistance to unadaptable inhibition. The inhibition sequence was evidenced by microbial population shifts and confirmed by earlier studies employing short (2–65 d) and long (80–198 d) batch experiments. Distinguishing adaptable from unadaptable inhibition provides precise guidance for mitigating ammonia inhibition.

Biomethane Potential of Selected Organic Waste and Sewage Sludge at Different Temperature Regimes

Sewage sludge (SS) generation and its management still pose a problem in many countries. Anaerobic co-digestion (AcD) of SS with grease trap sludge (GTS) and organic fraction of municipal sewage waste (OFMSW), which are two easily biodegradable substrates, may improve biogas production and AcD kinetics. Algae biomass (AB) of the species Undaria pinnatifida can be the third co-digestion component that may also affect the AcD performance. The aim of the study was therefore to evaluate the performance of mesophilic and thermophilic SS batch AcD with OFMSW, GTS as well as AB through biochemical methane potential (BMP) assay in relation to cumulative specific biogas (YB) and methane yields (Ym). Three kinetic models were applied within the scope of the kinetic study. Results of the study showed that the mixture containing SS, GTS and AB brought the most noticeable improvements in Ym compared to other studied mixtures and in respect to standalone SS digestion, the improvement amounted to 88.37% at mesophilic temperature (260.83 ± 15.02 N mL CH4/g-VSadd and for standalone SS 138.47 ± 4.70 N mL CH4/g-VSadd) and 71.09%, respectively, at the thermophilic one (275.66 ± 4.11 N mL-CH4/g-VSadd and for SS standalone 161.13 ± 13.11 N mL-CH4/g-VSadd).

Links between Process Performance and Microbial Community of Pennisetum Hybrid Co-Digested with Municipal Solid Waste

In this study, the links between performance and microbial communities were investigated with municipal solid waste (MSW) at two feedstock ratios and eight organic loading rates (OLRs). The co-digestion systems were stably operated at OLRs of 2.0–6.0 g VS/(L·d). The performance of the co-digestion system varied with the feedstock ratio. Compared with the 50:50 (hybrid Pennisetum:MSW) system, the 75:50 system, GM31, obtained increases of 1.93–17.68% and 0.29–23.29% for the specific biogas and methane yields, respectively. Whereas a shift in bacterial and methanogen communities occurred as the operating conditions changed, particularly with OLR variations. The genera Saccharofermentans, Prevotella, Clostridium, Syntrophomonas, and Proteiniphilum became the dominant bacteria for the conversion of carbohydrates and nitrogen compounds as the OLR increased. Meanwhile, a shift from acetoclastic to hydrogenotrophic or multifunctional methanogens was observed.

Performance of Food Waste Feeding Pilot Plant Biodigester Operated with Identified Potential Substrate Properties

Anaerobic digestion has been proven as sustainable process technology for organic waste conversion into renewable bio-energy. This study was conducted to evaluate the performance of mono-digestion process for different types of food waste substrates using pilot scale anaerobic bio-digester (1200 L) in terms of biogas production and the chemical oxygen demand (COD) removal efficiency. The biochemical methane potential (BMP) test of rice waste (R), vegetable waste (VW) and coconut meat residue (CMR) were tested at initial volatile solid (VS) loading of 0.1631, 1.1690, 1.0059 g VS/L, respectively at fixed inoculum/substrate (I/S) ratio of 0.5. Further study conducted by using rice waste (R) in pilot plant anaerobic bio-digester (1200L) for 43 days to investigate the reactor performance in term of COD removal efficiency. Interestingly, inoculum used for this study performs very well and able to digest food waste. Results demonstrate that the maximum specific biogas yield (SBY) was observed from rice waste (R) at 0.0587 L/kg VS compared to other substrates. Specific biogas yield (SBY) of rice waste (R) was 16.01% and 11.92% higher than substrate vegetable waste (VW) and coconut meat residue (CMR) respectively. High COD removal efficiency of pilot plant bio-digester (up to 93 %) using rice waste (R) as sole substrate indicates a good performance of reactor in treating food waste. Conversion of food waste to biogas in pilot plant bio-digester is highly potential as one of the sustainable waste treatment technology.

Effects of Vapothermal Pretreatment on Anaerobic Degradability of Common Reed

Residual materials from currently unused plant matter are available locally in abundance and serve as a valuable substrate for biogas production via anaerobic digestion. However, in most cases, a pretreatment is necessary to overcome their recalcitrant behavior. Against this background, earlier ground biomass from Phragmites australis, better known as common reed, is pretreated in a vapothermal atmosphere at temperatures between 122 and 178 °C and residence times between 18 and 102 min. Herein, the influence of such a pretreatment on the specific biogas and methane yield is aimed to be evaluated. In addition, the main causes for changes in degradability are identified by means of accompanying analytics. It is found that vapothermal pretreatment is able to significantly enhance degradation velocity during anaerobic digestion. However, under severe conditions, the biogas and methane yields decrease, which is traced back to an increase in cellulose crystallinity, inaccessibility of pores due to precipitated reaction products, as well as the formation of inhibiting substances and proportional increase in nondegradable lignin.

Study on interchangeability of seed based feedstocks for biogas production in multi-feeding mode

Biogas is an excellent renewable fuel providing gas for clean energy applications and fertilizers for organic farming. In the present study, seeds from four tree species of Acacia nilotica , Albizia lebbeck , Prosopis juliflora , and Leucaena leucocephala have been utilized for biogas production as alternative feedstocks. They have been investigated in a mixed-feeding mode in a single digester to tackle the feedstock availability and seasonality challenges. The investigations were conducted in a 300 ​L anaerobic digester for 120 days under daily feeding mode. The average specific biogas yield and specific methane yield was found to be 0.425 ​m 3 /kg of volatile solids and 0.228 ​m 3 /kg of volatile solids, respectively. The average volumetric methane content in biogas was 54.7%. This paper presents a unique study on anaerobic digestion of selected seeds in a mixed-feeding mode to provide broad opportunities for their efficient and sustainable utilization. • Interchangeability of tree species seeds explored for 120 days in the mixed-feeding mode for biogas production. • Anaerobic digestion parameters for the utilization of these seeds are investigated in detail. • An amount of 3.0–3.5 ​kg of seeds is required to produce one cubic metre of biogas. • Specific biogas yield of 0.425 ​m 3 /kg VS with methane of 54.7% and TVSMRE as 46.0% were found.

Effect of silage maize plant density and plant parts on biogas production and composition

Higher maize plant density represents a possible tool for biomass yield improvement but the effect on biogas production and quality has not been intensively investigated so far. The aim of the study was to compare two different plant densities (90 000 and 130 000 plants ha−1) and their effects on yield and biogas production. Experiments were carried out at two sites in Central Bohemia in 2014 and 2015. Specific biogas yield of maize ears and stover was evaluated in batch tests. In both years, there were significantly higher dry matter yield, volatile solids (VS) degradation, and methane content in biogas from ears in comparison with stover. Stover produced on average 90% of biogas per weight unit (625–719 L kg−1 VS) compared to ears (721–801 L kg−1 VS) depending on locality and year. During the batch tests, ears produced more biogas than stover with the exception of the period from the 4th to 11th day when specific biogas yield of stover was higher. Biogas hectare yield of ears (5039–8962 m3 ha−1) was 1.3–1.8 times higher than biogas hectare yield of stover (3943–4865 m3 ha−1). The influence of plant density on dry matter yield and biogas hectare yield was not significant, but higher plant density supported faster dynamics of specific biogas yield of ears in both years and higher volatile solids degradation of ears in 2014.

Biogas potential of swine manure of different animal classes

ABSTRACT In regions with developed animal husbandry, there is a problem of utilization of animal waste, which could be used as raw material for biogas production. In Russia most of the stations are designed and built by foreign companies taking into account generalized data on the properties of substrate. The particular characteristics of the local raw materials and substrates are not considered in the design process. Subsequently, this factor negatively affects the efficiency of biogas plants. In order to solve the problem, it is necessary to study the biogas potential of local substrates. The novelty of this work is to identify the peculiarities of pig manure as a substrate for biogas production depending on the classes of animals and the place of origin of raw materials. The goal of the present study of biogas potential of swine manure of different classes of animals (pregnant sows, lactating sows, piglets, fattening pigs) in Belgorod region of Russia. The experiment was carried out in small-scale batch reactors with the volume of 0.3 L each. Specific biogas yield from pig manure of different classes of animals ranged from 76.73 ± 11.77 to 151.42 ± 20.18 mn 3/t оDM, methane content – from 29.79 to 66.88%. In general, pig manure showed low biogas productivity. The highest rates were achieved at manure of sows. Taking into account the developed industry of pig breeding, biogas technologies, however, are the most optimal option of liquid manure utilization for the Belgorod region of Russia and are promising for many regions of the world with intensive animal breeding. When designing biogas plants, the specifics of local substrates should be taken into account.

Preliminary evaluation of the effect of chicken feather with no major pre-treatment on biogas production from horse dung

This study examined the effect of the anaerobic co-digestion of chicken feather (CF) and horse dung (HD) on biogas production and yield kinetics. It was carried out in eight (8) experimental designs using eight (8) identical digesters with replication. While Digesters A and B contained 100 % HD (6 kg) and 100 % CF (6 kg) respectively, C, D and E contained 75 % HD, 50 % HD and 25 % HD (4.5 kg, 3 kg and 1.5 kg) respectively. The last three Digesters F, G and H contained 75 % HD: 25 % CF, 50 % HD: 50 % CF and 25 % HD: 75 % CF respectively. Average biogas production for digesters with 100 % HD, 100 % CF, 75 % HD: 25 % CF, 75 % HD: 50 % HD: 50 % CF, 50 % HD and 25 % HD: 75 % CF are 0.36, 8.22876E-04, 0.20, 0.27, 0.14, 0.18, 0.097 and 0.09 L/gVS respectively. The paired t-test result showed a significant difference for 75 % HD and 50 % HD when co-digested with CF but no significant difference for 25 % HD at 95 % confidence level. The modified Gompertz model revealed that chicken feather had negative effects of 6.89 %, 14.84 % and 27.75 % on total biogas produced, maximum specific biogas yield (μm) and latency (λ) respectively. The study concluded that that CF without chemical and or enzymatic pre-treatment was not a good co-substrate of HD for enhanced biogas production.

Impact of C/N ratios and organic loading rates of paper, cardboard and tissue wastes in batch and CSTR anaerobic digestion with food waste on their biogas production and digester stability

The C/N ratio and organic loading rates (OLRs) are the main constraint factors of anaerobic digestion (AD) which symbols in digestion mortifications due to the rapid accumulation of volatile fatty acids (VFAs) and total ammonia nitrogen (TAN). In this study, the addition of high C-content wastes; paper waste (PW), cardboard waste (CW), and tissue waste (TW) into food waste (FW) feedstock is individually taken into consideration. At first, batch anaerobic digestion is conducted at a balanced C/N ratio of waste materials and then CSTR anaerobic digestion is carried out at different OLRs to investigate the potential of biogas production and process stability, under mesophilic conditions. In batch anaerobic digestion, co-substrate (PW + FW) feedstock is outperformed the other co-substrate feedstocks in terms of biogas production and process stability due to slow formation of VFAs and stable microbial conversion of VFAs into biogas fuel. The highest specific biogas yield (SBY) of 651 mL/gVS and specific methane yield (SMY) of 350 mL/gVS with a 57% rate of biodegradability are obtained from (PW + FW) feedstock at OLR of 15 gVS/L d. Furthermore, (PW + FW) feedstock with superb performance is then subjected to investigate the optimum OLR in CSTR digester for 30 days. The SBY of 22 L/gVS and SMY of 13 L/gVS is obtained with the utmost rate of VS reduction of 96%. The pH, alkalinity, COD, TS reduction, VS reduction, VFAs, and TAN concentration are all in the optimal range at the C/N ratio of 25 and OLR of 2 gVS/L d. This study provides new information regarding the C/N ratio and OLR for AD own great potential in improving the methane yield and productivity of PW and FW.

Biogas yield assessment from the anaerobic co-digestion of food waste and cymbopogon citratus

This study, aimed at maximizing the biogas yield from the co-digestion of food waste (FW) and cymbopogon citratus (CC) was done in reactors (Rs) I–V at temperature of 36 ± 2 °C. Rs I, II, III, IV and V with respective FW: CC of 100:0; 80:20; 60:40; 40:60; 0:100 had mean biogas yield of 0.41 ± 0.02, 0.68 ± 0.05, 0.49 ± 0.02, 0.29 ± 0.01, and 0.22 ± 0.03 L/gVS, respectively. From the modeling result, while R I had the least latency (λ) of 3.9, RV had the highest value of 11.2 days. RII also had a reasonably low latency (λ) of 5.2 days. Top maximum specific biogas yield (Rm) of 0.48 L/gVS/day and maximum biogas production potential (A) of 19.9 L/gVS were obtained from RII. The high R2 values of 0.99–0.89 obtained from the simulation analysis showed a good fit with the modified Gompertz model. The study has revealed that FW: CC of 80: 20% could be most appropriate for optimal biogas production.

Multiple Substrates Anaerobic Co-Digestion: A Farm-Scale Biogas Project and the GHG Emission Reduction Assessment

UGlobal warming has become an increasingly inconvenient situation, indicating the importance of reducing greenhouse gas (GHG) emissions. Anaerobic co-digestion of multiple substrates is an effective way to utilize wastes and reduce GHG emissions. In this study, batch anaerobic co-digestion of pig manure, corn straw, and fruit and vegetable wastes (FVW) was performed in a laboratory and extended to a farm-scale biogas project. Laboratory test results showed a specific biogas yield of 420.18 mL/g VS when the ratio of pig manure, corn straw, and FVW was 100:20:1 (TS). A farm-scale biogas project was improved on the basis of laboratory studies, and an average specific volumetric biogas production rate of 1.22 m3/m3 d was obtained. For GHG emission reduction assessment, the open anaerobic pond of pig manure, solid waste disposal site of FVW, and uncontrolled burning of corn straw were used as baselines, obtaining a total GHG emission reduction of 303.08 t CO2 annually. The farm-scale biogas project could treat corn straw, pig manure, and fruits and vegetable wastes, achieving considerable environmental benefits.

Utilization of de-oiled rice bran as a feedstock for renewable biomethane production

This research aims to study biogas production using de-oiled rice bran as a renewable source of energy. The measured volatile solids content of de-oiled rice bran revealed its significant potential for biogas production. In the first phase of the experimentation, the hydrolytic behavior was studied to determine the optimal total solids (TS) concentration of the substrate, required for maintaining the favourable environmental condition for efficient anaerobic digestion. Out of four TS concentration taken (5%, 7.5%, 10%, and 15%), the most significant result was found for the substrate maintained at 5% TS. As per the finding of the hydrolysis experiment, further investigation was conducted using 60 L plastic made digester under continuous feed mode and at mesophilic temperature condition in three replicates. The results revealed average specific biogas yield of 0.470 L/g TS and 0.547 L/g VS with an average specific methane production yield of 0.232 L/g TS and 0.270 L/g VS. The most encouraging finding of this study was the high material conversion efficiency (maximum 67.54%), which makes de-oiled rice bran more potential feedstock than other conventional biomass. The novelty of the present study lies in the development of efficient ‘secondary inoculum’ through a continuous system. The inoculum developed through continuous mode gives a higher degree of control than batch mode. Along with the microbial growth, the secondary metabolites produced during the anaerobic digestion process, (which plays a crucial role in process continuation) can also be sustained simultaneously in the developed continuous system.

Trace Element Supplementation and Enzyme Addition to Enhance Biogas Production by Anaerobic Digestion of Chicken Litter

Anaerobic digestion (AD) of chicken litter (CL) is a viable alternative to disposal. However, methane yields from this primarily organic waste are quite low when mono-digested. This paper discusses the effect of an enzyme cocktail, trace element (TE) supplementation and selenium (Se) addition in small-scale batch biomethane potential (BMP) assays to enhance the AD of CL. Eleven different assays were set up in triplicate including assays containing only inoculum (blank), only CL (negative control) and cellulose and inoculum (positive control). The results indicate that both enzyme treatment and trace element supplementation enhanced the biogas and methane yield. The highest specific biogas and methane yields were noted for 1% enzyme-treated CL of 835.2 L/kg volatile solids (VS) and 460.8 L/kg VS, respectively. Usually, mono-digestion of CL is low due to high nitrogen content and the presence of recalcitrant lignocellulosic material from the bedding material. Enzyme treatment performed better than the addition of the TE mix and Se.

Anaerobic co-digestion of waste microalgal biomass with cattle dung in a pilot-scale reactor: effect of seasonal variations and long-term stability assessment

The present work is first of its kind to attempt on year-round biogas production utilizing microalgal biomass from lake water. The analyses showed that the lake water contain ample nutrients (TAN: 3.6–16; TDP: 4.88–20.38 and NO3-N: 0.3–19.63 mg L−1) to support microalgal growth, with an average biomass concentration of ≈ 2.0 g L−1, throughout the year. Significant correlation (p < 0.05) was observed between microalgal biomass and nutrient concentration in the lake water. Further, co-digestion studies in the pilot-scale plant (0.5 m3) showed good performance with a volumetric and specific biogas yield in the range of 0.45 to 0.65 m3 day−1 and 0.72 to 1.04 m3 kg−1 VSfed day−1 respectively, during the summer. However, during winters, the volumetric and specific biogas yield were found to be in the range of 0.06 and 0.21 m3 day−1 and 0.096–0.336 m3 kg−1 VSfed day−1, respectively, with an average 55–65% CH4. Detailed analyses of digestate indicated long-term stability of the pilot-scale process. This study established the feasibility of utilizing waste algal biomass from eutrophic lake water for year-round biogas production.

Suitability of paludiculture biomass as biogas substrate − biogas yield and long-term effects on anaerobic digestion

Abstract Fen plants cultivated on wet peatlands might be an environmentally friendly alternative biogas substrate to maize and grass grown on drained peatlands. This study demonstrates that if Typha latifolia, Phragmites australis, and Phalaris arundinacea were harvested in mid-June, then their specific biogas yields (SBY) reached values of up to 581 LN kg−1 volatile solids (VS), which is similar to the SBY of grass, but lower than the SBY, of 670 LN kg−1 VS, for maize. Mixtures with equal or more than 10% T. latifolia or 40% P. arundinacea (VS-base) exhibited a reduced SBY compared to 100% maize silage in a batch-test. From the composition of the substrates, it remains unclear why fen plants degraded that poorly. However, during the semi-continuous long-term experiment, this effect led to an accumulation of non-degraded material, which destabilized the degradation process at loading rates above 3 kg VS m−3 d−1. Destabilization became apparent with substantial increases in the viscosity of the fermenter content, enrichment of acids and a worsened methane formation. Our findings suggest that only small proportions of maize could be replaced by fen plants as substrate for biogas plants.

By-products of fish-oil refinery as potential substrates for biogas production in Norway: A preliminary study

Norway is one of the biggest fish exporters in the world, and the fish industry is the second biggest economy of the country after oil and natural gas. As fish industries expand rapidly, the resulting fish-oil refining by-products are also receiving an upsurge interest, giving an enormous window of opportunity in generating bioenergy through this route. A number of fish-oil refinery by-products such as soapstock, glycerol, ethyl monoesters (light and dark) were utilized in the present study where the anaerobic digestion feasibility of these by-products as co-substrates with fish ensilage (acidified fish waste) was investigated. The method utilized was biomethane potential experiment by employing a series of 0.5 ​L batch bottles as anaerobic digesters operated at total feeding of about 7.0 ​g with co-substrates feeding ranging between 0.6 and 2.0 ​g per bottle. The digestion environment was set at extreme mesophilic having temperature of 39 ​± ​1 ​°C. During the ~65 days of experiment, it was found that accumulated volumetric biogas yield (in mL) from co-digestion of monoesters and fish ensilage was much higher than that of the mono digester operated with fish ensilage alone. The accumulated yield from light and dark monoesters co-digesters amounted to ~2100 ​mL and ~1950 ​mL respectively. In terms of specific biogas yield, highest yield was obtained from the soapstock co-digesters with an average peaking to 775 mL/gTS. The average methane content in biogas for all the digesters over the course of the experiment was found as ca 61%. The study concludes that fish-oil refinery by-products as co-substrates with fish ensilage upon utilization to anaerobic digestion plants can potentially offer Norwegian fish refineries an excellent opportunity to contribute heavily in local bioeconomy.

Assessment of organic loading rate by using a water tank digester for biogas production in Bangladesh

The anaerobic digestion technology has many advantages and is a sustainable way to produce energy, solve the crisis on energy shortage, and reduce waste stream and carbon footprint. In this field experiment, a mixture of food, fruits, and vegetable waste, dairy manure, and inoculum was anaerobically digested in a constructed water tank digester. This study was performed at ambient temperature under different organic loading rates (OLRs), which were increased every two weeks. Results showed that the optimum feedstock-to-inoculum ratio is 0.4, which resulted in highest specific biogas and methane yields (0.44 and 0.28 Nm3/kg VS, respectively). With the OLRs, the highest yield was observed at 1.4 kg VS/m3 average feeding. When the feedstock-to-inoculum ratio was increased from 0.4 to 0.8, the effect on biogas yield and the volatile solids decreased. Hence, the total cumulative biogas production was approximately 18.5 Nm3 per reactor volume. The pathogen concentration, which was determined in the feedstock and the effluent samples, revealed that the reduction efficiency was relatively low. On the basis of the performance of this field study, emphasizing household organic waste composed with other feed materials to produce bioenergy was feasible. Anaerobic co-digestion produces biogas, contributes to sustainable waste management, and reduces environmental pollution. This scientific study can play a fundamental role in implementing this technology at the household level for efficient biogas production in Bangladesh and worldwide.

Reversibility of propionic acid inhibition to anaerobic digestion: Inhibition kinetics and microbial mechanism

Anaerobic digestion is a technology that simultaneously treats waste and generates energy in the form of biogas. Unfortunately, when a high organic loading rate is applied, anaerobic digestion can suffer from volatile fatty acid accumulation that results in pH drop and decreased biogas production. In particular, propionic acid has shown to inhibit biogas production even at a very low concentration. Therefore, the kinetics of biogas production in relation to propionic acid concentration needs to be investigated. In batch experiments on anaerobic co-digestion of food waste and dairy manure in the present study, cumulative biogas production showed little inhibition by propionic acid in the concentration range of 6.5–14.6 mM, but a lag phase of 9.4, 16.3 and 60.8 d was detected in the digesters with initial propionic acid concentrations of 22.7, 36.2, and 56.4 mM, respectively. After the lag phase, these digesters accelerated to specific biogas yields of 0.59–0.70 L g-VS−1. The similar specific biogas yields across all of the digesters at initial propionic acid concentrations of 6.5–56.4 mM indicated reversibility of the inhibition. The reversibility was made possible by microbial acclimation and the shift to hydrogenotrophic methanogenesis in syntrophy with acetogenic bacteria. Evidently, an increase of hydrogenotrophic Methanobacterium and Methanoculleus abundances was found at 36.2 and 56.4 mM. Batch digestion experiments must be extended beyond the lag phase in order to fully reveal the inhibition kinetics. This paper highlights the need for a standard protocol that experimentally evaluates inhibition in anaerobic digestion.