Single-stage Anaerobic Digestion Research Articles

Overcoming the bottlenecks of anaerobic digestion of olive mill solid waste by two-stage fermentation

ABSTRACTTwo-stage anaerobic digestion (AD) of two-phase olive mill solid waste (OMSW) was applied for reducing the inhibiting factors by optimizing the acidification stage. Single-stage AD and co-fermentation with chicken manure were conducted coinstantaneous for direct comparison. Degradation of the polyphenols up to 61% was observed during the methanogenic stage. Nevertheless the concentration of phenolic substances was still high; the two-stage fermentation remained stable at OLR 1.5 kgVS/m³day. The buffer capacity of the system was twice as high, compared to the one-stage fermentation, without additives. The two-stage AD was a combined process – thermophilic first stage and mesophilic second stage, which pointed out to be the most profitable for AD of OMSW for the reduced hydraulic retention time (HRT) from 230 to 150 days, and three times faster than the single-stage and the co-fermentation start-up of the fermentation. The optimal HRT and incubation temperature for the first stage were determined to four days and 55°C. The performance of the two-stage AD concerning the stability of the process was followed by the co-digestion of OMSW with chicken manure as a nitrogen-rich co-substrate, which makes them viable options for waste disposal with concomitant energy recovery.

A comprehensive microbial insight into single-stage and two-stage anaerobic digestion of oxytetracycline-medicated cattle manure

At present, there is a distinct lack of understanding to the effect of antibiotic contamination of a substrate on two-stage anaerobic digestion (AD) processes. The aim of this study was to investigate how oxytetracycline (OTC) influences the structure of the microbial community and the production of methane in two-stage anaerobic cattle manure digesters. Ion Torrent sequencing and PCR-DGGE analysis were used to detect microbial community changed in response to the addition of oxytetracycline. Illumina sequencing was also performed to detect antibiotic resistance genes (ARGs). Digestion studies utilized samples of manure produced by cattle which had been therapeutically treated using a standard OTC solution of 50mL per 20mgkg−1 cattle weight. According to the results of the PCR-DGGE and Ion Torrent sequencing, the bacterial diversity of acidogenic digesters was higher than those of single-stage digesters during the digestion period. An assessment of single-stage and two-stage AD data highlighted that Methanosarcina and syntrophic acetate-oxidizing bacteria are critical to the production of biogas. The overall results indicated that changes in the structure of a microbial community lead to changes in biodegradation capacity of antibiotics and occurrence of ARGs.

Comparative assessment of prototype digester configuration for biogas recovery from anaerobic co-digestion of food waste and rain tree leaf as feedstock

A comparative assessment of the developed prototype single-stage and two-stage anaerobic digesters for biogas recovery from anaerobic co-digestion was performed using food waste and rain tree leaf as feed stock. Rain tree or Chamchuri tree is an important tree in Chulalongkorn university and it is recognized as the university symbol. Up to now, Chulalongkorn University has grown more than a hundred of rain trees in the campus. Thus, rain tree leaf is found to be the major gardening wastes. Continuous operation of two prototype systems was implemented to investigate biogas recovery potential. From the obtain results, it was found that higher percentage of organic carbon and total volatile solid reduction were obtained with the prototype two-stage anaerobic digester. Higher COD reduction percentage of 85.74% could be achieved with the prototype two-stage anaerobic digester. Moreover, a maximum biogas production of 0.834 m3/kg of organic waste-day was obtained with the prototype two-stage anaerobic digester at the C/N ratio of 21, while the prototype single-stage anaerobic digester achieved a maximum biogas production of 0.41 m3/kg of organic waste-day at the same C/N ratio of 21.

Treatment of corn ethanol distillery wastewater using two-stage anaerobic digestion

In this study the mesophilic two-stage anaerobic digestion (AD) of corn bioethanol distillery wastewater is investigated in laboratory-scale reactors. Two-stage AD technology separates the different sub-processes of the AD in two distinct reactors, enabling the use of optimal conditions for the different microbial consortia involved in the different process phases, and thus allowing for higher applicable organic loading rates (OLRs), shorter hydraulic retention times (HRTs) and better conversion rates of the organic matter, as well as higher methane content of the produced biogas. In our experiments the reactors have been operated in semi-continuous phase-separated mode. A specific methane production of 1,092 mL/(L·d) has been reached at an OLR of 6.5 g TCOD/(L·d) (TCOD: total chemical oxygen demand) and a total HRT of 21 days (5.7 days in the first-stage, and 15.3 days in the second-stage reactor). Nonetheless the methane concentration in the second-stage reactor was very high (78.9%); the two-stage AD outperformed the reference single-stage AD (conducted at the same reactor loading rate and retention time) by only a small margin in terms of volumetric methane production rate. This makes questionable whether the higher methane content of the biogas counterbalances the added complexity of the two-stage digestion.

Energy conversion of biomass crops and agroindustrial residues by combined biohydrogen/biomethane system and anaerobic digestion

Aim of this study was to evaluate the suitability of ensiled giant reed, ensiled maize, ensiled olive pomace, wheat bran for combined systems (CS: dark fermentation+anaerobic digestion (AD)) producing hydrogen-rich biogas (biohythane), tested in batch under basic operational conditions (mesophilic temperatures, no pH control). Substrates were also analyzed under a single stage AD batch test, in order to investigate the effects of DF on estimated energy recovery (ER) in combined systems. In CS, maize and wheat bran exhibited the highest hydrogen potential (13.8 and 18.9NLkgVS−1) and wheat bran the highest methane potential (243.5NLkgVS−1). In one-stage AD, giant reed, maize and wheat bran showed the highest methane production (239.5, 267.3 and 260.0NLkgVS−1). Butyrate/acetate ratio properly described the dark fermentation, correlating with hydrogen production (r=0.92). Wheat bran proved to be a promising residue for CS in terms of hydrogen/methane potential and ER.

Effect of control of pH in anaerobic digestion of organic fraction of municipal solid waste

Anaerobic digestion often encounters the problem of higher population of the acid-forming bacteria relative to the slower growing methanogenic bacteria which inhibits the methanogenic process leading to problems such as low gas yield and process instability. In this study, anaerobic digestion of organic fraction of municipal solid waste has been carried out in two identical model laboratory-scale single stage anaerobic digesters simultaneously; one without the control of pH and other with the control of pH, with same substrate keeping all other conditions identical. Results indicated that the hydraulic retention time of AD reaction could be reduced by at least seven days and the overall efficiency of gas production of the digester could be increased by more than 21.95% (at 50 days time) by the control of pH in the digester.

Semicontinuous Temperature-Phased Anaerobic Digestion (TPAD) of Organic Fraction of Municipal Solid Waste (OFMSW). Comparison with single-stage processes

The optimization of anaerobic digestion of organic wastes is a challenge to maximize energy production through biogas production process. In this study, semicontinuous TPAD (Temperature Phased Anaerobic Digestion) process has been used for the treatment of the Organic Fraction of Municipal Solid Waste (OFMSW) coming from a mechanical–biological-treatment (MBT) plant. TPAD combines the advantages of operating in different temperature ranges getting better efficiencies of organic matter removal and higher methane productivities than single-stage anaerobic digestion. In this study, the configuration used in the overall process was a thermophilic reactor (55–57°C) for the first phase followed by a mesophilic one (35–37°C) for the second phase. Two TPAD conditions have been tested in this paper: 4:10 and 3:6. The first digit means the SRT used in the first thermophilic phase while the second digit is related to the SRT used in the second mesophilic phase. Moreover, the performance of TPAD processes was compared with those from single-stage digesters operating at similar SRT (i.e., mesophilic and thermophilic reactors operating at 15days SRT and thermophilic reactor operating at 10days).The results showed that achievement reached in TPAD 4:10 was better than the corresponding one in TPAD 3:6, obtaining higher productivity of methane (35–45%) and removal of organic matter (6–19%).Moreover, the results indicate that TPAD processes reach higher efficiencies for organic matter removal (16%, 10% and 30% for DOC, CODsoluble and VS, respectively) and higher methane yields (26–60%) than single-stage systems operating at similar SRT.

Bio-hydrogen and bio-methane potentials of skim latex serum in batch thermophilic two-stage anaerobic digestion

Anaerobic digestion by two-stage process, containing hydrogen-producing (acidogenic) first stage and methanogenic second stage, has been proposed to degrade substrates which are difficult to be treated by single stage anaerobic digestion process. This research was aimed to evaluate the bio-hydrogen and the bio-methane potentials (BHP and BMP) of skim latex serum (SLS) by using sequential batch hydrogen and methane cultivations at thermophilic conditions (55°C) and with initial SLS concentrations of 37.5–75.0% (v/v). The maximal 1.57L H2/L SLS for BHP and 12.2L CH4/L SLS for BMP were both achieved with 60% (v/v) SLS. The dominant hydrogen-producing bacteria in the H2 batch reactor were Thermoanaerobacterium sp. and Clostrdium sp. Meanwhile, the CH4 batch reactor was dominated by the methanogens Methanosarcina mazei and Methanothermobacter defluvii. The results demonstrate that SLS can be degraded by conversion to form hydrogen and methane, waste treatment and bioenergy production are thus combined.

Determination of the archaeal and bacterial communities in two-phase and single-stage anaerobic systems by 454 pyrosequencing

2-Phase anaerobic digestion (AD), where the acidogenic phase was operated at 2day hydraulic retention time (HRT) and the methanogenic phase at 10days HRT, had been evaluated to determine if it could provide higher organic reduction and methane production than the conventional single-stage AD (also operated at 12days HRT). 454 pyrosequencing was performed to determine and compare the microbial communities. The acidogenic reactor of the 2-phase system yielded a unique bacterial community of the lowest richness and diversity, while bacterial profiles of the methanogenic reactor closely followed the single-stage reactor. All reactors were predominated by hydrogenotrophic methanogens, mainly Methanolinea. Unusually, the acidogenic reactor contributed up to 24% of total methane production in the 2-phase system. This could be explained by the presence of Methanosarcina and Methanobrevibacter, and their activities could also help regulate reactor alkalinity during high loading conditions through carbon dioxide production. The enrichment of hydrolytic and acidogenic Porphyromonadaceae, Prevotellaceae, Ruminococcaceae and unclassified Bacteroidetes in the acidogenic reactor would have contributed to the improved sludge volatile solids degradation, and ultimately the overall 2-phase system's performance. Syntrophic acetogenic microorganisms were absent in the acidogenic reactor but present in the downstream methanogenic reactor, indicating the retention of various metabolic pathways also found in a single-stage system. The determination of key microorganisms further expands our understanding of the complex biological functions in AD process.

Effects of sludge recirculation rate and mixing time on performance of a prototype single-stage anaerobic digester for conversion of food wastes to biogas and energy recovery.

Food wastes have been recognized as the largest waste stream and accounts for 39.25% of total municipal solid waste in Thailand. Chulalongkorn University has participated in the program of in situ energy recovery from food wastes under the Ministry of Energy (MOE), Thailand. This research aims to develop a prototype single-stage anaerobic digestion system for biogas production and energy recovery from food wastes inside Chulalongkorn University. Here, the effects of sludge recirculation rate and mixing time were investigated as the main key parameters for the system design and operation. From the results obtained in this study, it was found that the sludge recirculation rate of 100% and the mixing time of 60min per day were the most suitable design parameters to achieve high efficiencies in terms of chemical oxygen demand (COD), total solids (TS), and total volatile solid (TVS) removal and also biogas production by this prototype anaerobic digester. The obtained biogas production was found to be 0.71m(3)/kg COD and the composition of methane was 61.6%. Moreover, the efficiencies of COD removal were as high as 82.9% and TVS removal could reach 83.9% at the optimal condition. Therefore, the developed prototype single-stage anaerobic digester can be highly promising for university canteen application to recover energy from food wastes via biogas production.

Utilising biohydrogen to increase methane production, energy yields and process efficiency via two stage anaerobic digestion of grass

Real time measurement of gas production and composition were used to examine the benefits of two stage anaerobic digestion (AD) over a single stage AD, using pelletized grass as a feedstock. Controlled, parallel digestion experiments were performed in order to directly compare a two stage digestion system producing hydrogen and methane, with a single stage system producing just methane. The results indicated that as well as producing additional energy in the form of hydrogen, two stage digestion also resulted in significant increases to methane production, overall energy yields, and digester stability (as indicated by bicarbonate alkalinity and volatile fatty acid removal). Two stage AD resulted in an increase in energy yields from 10.36MJkg−1 VS to 11.74MJkg−1 VS, an increase of 13.4%. Using a two stage system also permitted a much shorter hydraulic retention time of 12days whilst maintaining process stability.

Production of Biogas Using Maize Silage Supplemented with Residual Glycerine from Biodiesel Manufacturing

Abstract The aim of this study was to investigate the influence of residual glycerine (5 and 10% w/w) from the biodiesel industry, used as a co-substrate, on biogas production from maize silage. The experiments were conducted in a laboratory-scale, single-stage anaerobic digester at 39ºC and hydraulic retention time (HRT) of 60 d. Addition of 5% residual glycerine caused organic load rate (OLR) to increase to 1.82 compared with 1.31 g organic dry matter (ODM) L-1d-1 for maize silage alone. The specific biogas production rate and biogas yield were 1.34 L L-1d-1 and 0.71 L g ODM-1 respectively, i.e. 86% and 30% higher than for maize alone. Increasing the residual glycerine content to 10% increased OLR (2.01 g ODM L-1d-1), but clearly decreased the specific biogas production rate and biogas yield to 0.50 L L-1d-1 and 0.13 L g ODM-1 respectively. This suggested that 10% glycerine content inhibited methanogenic bacteria and organics conversion into biogas. As a result, there was accumulation of propionic and valeric acids throughout the experiment.

Studies on biogas production from vegetable market wastes in a two-phase anaerobic reactor

The organic fraction of the municipal solid waste is not being properly utilized and is disposed in open dumps which leads to environmental pollution. The vegetable market wastes degrade rapidly than other municipal solid waste. In our earlier report, we concluded that single-stage anaerobic digestion systems could not be operated with higher loading rates for the digestion of vegetable market wastes (VMW). Hence, in this investigation, we proposed anaerobic digestion of VMW in a two-phase semi-continuous anaerobic reactor. In this study, the effect of organic loading rates (OLR) and hydraulic retention time (HRT) in methanogenic reactors was investigated. It was found that the specific biogas production increased with an OLR from 1.5 to 4.5 gVS/L/d and decreased with further increase in OLR. Maximum specific biogas production and methane yield of 0.721 ± 0.010 L/gVSadd and 0.469 LCH4/gVSadd were obtained at OLR of 4.5 gVS/L/d and a 25-day HRT in methanogenic reactor. Kinetic parameters of maximum yield and specific growth rate were evaluated by Chen–Hashimoto model.

A prototype single-stage anaerobic digester for co-digestion of food waste and sewage sludge from high-rise building for on-site biogas production

At present, there has been less results from pilot-scale experiments on anaerobic co-digestion system, especially as an on-site system for high-rise building application. The objective of this study was to develop a prototype single-stage anaerobic digester for the co-digestion of food waste (FW) and sewage sludge (SWS) from high-rise building for on-site biogas production. Here, the prototype system was operated at different hydraulic retention times (HRTs) of 27, 22 and 19 days, corresponding to organic loading rates of 7.9, 10.8 and 14.0 kgCOD/m3 d. The feed mixed waste ratio (FW/SWS) of 10:1 by weight was selected from the previous laboratory-scale experiment. The results of this study indicate that higher gas production rate was obtained at shorter hydraulic retention time (HRT) of 19 days; however, higher methane content of the biogas was obtained at longer HRT of 27 days. Therefore, enhancement of methane production from the co-digestion could be achieved with sufficient operating HRT. Moreover, the proposed prototype system could reduce total volatile solid up to 70 percent at HRT of 27 days. Up to now, the biogas from on-site production has been utilized for cooking at Chulalongkorn University as a model case study for high-rise building application.

Should we build "obese" or "lean" anaerobic digesters?

Conventional anaerobic digesters (ADs) treating dairy manure are fed with raw or fermented manure rich in volatile fatty acids (VFAs). In contrast, pre-fermented AD (PF-AD) is fed with the more recalcitrant, fiber-rich fraction of manure that has been pre-fermented and depleted of VFAs. Thus, the substrate of PF-AD may be likened to a lean diet rich in fibers while the pre-fermentation stage fermenter is fed a relatively rich diet containing labile organic substances. Previous results have shown that conventional and pre-fermented ADs fed with raw or pre-fermented manure, respectively, produced comparable methane yields. The primary objective of this study was to characterize, using next-generation DNA sequencing, the bacterial communities in various bioreactors (pre-fermentation stage fermenter; various operational arrangements PF-AD; conventional single-stage AD; and a full scale AD) and compare the Firmicutes to Bacteroidetes (F/B) ratios in these different systems. Firmicutes and Bacteroidetes constituted the two most abundant phyla in all AD samples analyzed, as well as most of the samples analyzed in the fermenters and manure samples. Higher relative abundance of Bacteroidetes, ranging from 26% to 51% of bacteria, tended to be associated with PF-AD samples, while the highest relative abundance of Firmicutes occurred in the fermenter (maximum of 76% of bacteria) and manure (maximum of 66% of bacteria) samples. On average, primary stage fermenters exhibited microbiological traits linked to obesity: higher F/B ratios and a ‘diet’ that is less fibrous and more labile compared to that fed to PF-AD. On the other hand, microbial characteristics associated with leanness (lower F/B ratios combined with fibrous substrate) were associated with PF-AD. We propose that bacterial communities in AD shift depending on the quality of substrate, which ultimately results in maintaining VFA yields in PF-AD, similar to the role of bacterial communities and a high fiber diet in lean mice.

Acidic thermal post-treatment for enhancing anaerobic digestion of sewage sludge

Acidic thermal post-treatment (ATPT) was examined for treatment conditions in a batch study, and was demonstrated to enhance anaerobic digestion of sewage sludge in a continuous study. In the batch study where anaerobically digested sludge was the substrate in view of a post-treatment mode, higher ATPT temperatures between 25 and 180 °C improved volatile suspended solids (VSS) destruction and methane production, but generated color significantly at 180 °C. Lower ATPT pH between 2 and 6 enhanced sludge dewaterability (as capillary suction time), and slightly suppressed color generation. In the continuous study, two single-stage anaerobic digestion processes were operated at 35 °C and 20 days hydraulic retention time. For one of the processes, ATPT at 170 °C and pH 5–6 for 1 h was incorporated in the recycle line. Approximately 75% of VSS destruction was achieved in the ATPT process, which was 2–2.5 times more than that in the control process, 30–37%. The ATPT process also showed 14–21% more methane production and 22–23% better dewaterability, but formed around three times more color, compared to the control process. Sulfuric acid as the acidifying agent caused more release of phosphate from the digested sludge, which enables efficient phosphorus recovery.

Bacterial and methanogenic archaeal communities during the single-stage anaerobic digestion of high-strength food wastewater

Single-stage anaerobic digestion (AD) was operated to treat high-strength food wastewater (FWW) derived from food waste recycling facilities at two different organic loading rates (OLRs) of 3.5 (Phase I) and 7 (Phase II) kgCOD/m3d. Changes in composition of microbial communities were investigated using quantitative real-time PCR (qPCR) and barcoded-pyrosequencing. At the high FWW loading rate, AD showed efficient performance (i.e., organic matter removal and methane production). Bacterial communities were represented by the phyla Bacteroidetes, Firmicutes, Synergistetes and Actinobacteria. During the entire digestion process, the relative abundance phylum Chloroflexi decreased significantly. The qPCR analysis demonstrated that the methanogenic communities shifted from aceticlastic (Methanosarcinales) to hydrogenotrophic methanogens (Methanobacteriales and Methanomicrobiales) with high increase in the proportion of syntrophic bacterial communities. Canonical correspondence analysis revealed a strong relationship between reactor performance and microbial community shifts.

Waste activated sludge treatment based on temperature staged and biologically phased anaerobic digestion system

The concept of temperature staged and biological phased (TSBP) was proposed to enhance the performance of waste-activated sludge anaerobic digestion. Semi-continuous experiments were used to investigate the effect of temperature (35 to 70°C) as well as the hydraulic retention time (HRT) (2, 4 and 6 days) on the acidogenic phase. The results showed that the solubilization degree of waste-activated sludge increased from 14.7% to 30.1% with temperature increasing from 35 to 70°C, while the acidification degree was highest at 45°C (17.6%), and this was quite different from the temperature impact on hydrolysis. Compared with HRT of 2 and 6 days, 4 days was chosen as the appropriate HRT because of its relatively high solubilization degree (24.6%) and acidification degree (20.1%) at 45°C. The TSBP system combined the acidogenic reactor (45°C, 4 days) with the methanogenic reactor (35°C, 16 days) and the results showed 84.8% and 11.4% higher methane yield and volatile solid reduction, respectively, compared with that of the single-stage anaerobic digestion system with HRT of 20 days at 35°C. Moreover, different microbial morphologies were observed in the acidogenic- and methanogenic-phase reactors, which resulted from the temperature control and HRT adjustment. All the above results indicated that 45°C was the optimum temperature to inhibit the activity of methanogenic bacteria in the acidogenic phase, and temperature staging and phase separation was thus accomplished. The advantages of the TSBP process were also confirmed by a full-scale waste-activated sludge anaerobic digestion project which was an energy self-sufficient system.

Economic and environmental analysis of four different configurations of anaerobic digestion for food waste to energy conversion using LCA for: A food service provider case study

The US disposes of more than 34 million tons of food waste in landfills per year. As this food waste decomposes it generates methane gas and negatively contributes to global warming. Diverting theses organic food wastes from landfills and to emerging technologies will prevent these wastes and greenhouse gas emissions while at the same time generating a source renewable energy by collecting the emitted gases. From a waste prevention standpoint, instead of the food waste decomposing at local landfills, it is being converted into an energy source and the by-product may be used as a fertilizer (Fine and Hadas, 2012). The purpose of this study was to compare four different configurations of anaerobic digestion of organic waste to energy technologies from an economic, energy, and emissions standpoint using LCA via a case study at a large food services provider in Northwest Ohio, USA. The technologies studied included two-stage anaerobic digestion system using ultrasound pre-treating, two stage continuous combined thermophilic acidogenic hydrogenesis and mesophilic with recirculation of the digested sludge, long-term anaerobic digestion of food waste stabilized by trace elements, and single stage anaerobic digestion. Using LCA, these scenarios were compared to landfill disposal of the food waste. The findings from the case study indicated that implementing on-site waste to energy systems will result in lower operation costs and lower environmental impacts. In addition, a standardized environmental and economic comparison of competing food waste to energy technologies is provided.

Use of Grasses and Mixtures of Grasses for Energy Purposes

As levels of agricultural productivity increase, there is also an increase in land area not utilized for food production. This area can be used for growing energy crops, including grasses. When land is set aside for grassing, or when the potential of perennial grasses is not utilized due to reductions in cattle herds, there is also an increased amount of grass that can be utilized for energy purposes. Experiments were carried out on the principle of single-stage anaerobic digestion within the mezophyle range. During the experiments, we measured the cumulative production of biogas and its composition. The processed grass was disintegrated by pressing and cutting. This adaptation of the material resulted in increased biogas production. The optimum proportion of grass dry matter is from 35 to 50 % in the total d.m. The results of the experiments proved the suitability of grass phytomass as a material for biogas production.