Generation of soluble lignin-derived compounds during degradation of barley straw in an artificial rumen reactor

Summary The supernatants of effluents from an artificial rumen reactor degrading barley straw have been shown to contain lignin-derived compounds by UV spectral characteristics and pyrolysis mass spectrometry (PYMS). Most of these compounds were shown to be released by the action of rumen microorganisms. The compounds were quantified by measuring absorbance at 280 nm using bamboo-milled wood lignin as a standard. The concentration of the compounds rose from 0.5 mg·ml^–1 at solid and liquid retention times (SRT and HRT) of 60 and 12 h, respectively, and a loading rate (LR) of 25 g total solids (TS)·l^–1 per day to 3.5 mg·ml^–1 at a SRT of 144 h, an HRT of 20 days and an LR of 15 g TS·1^–1 per day. The highest concentration was below the level known to be toxic to rumen microorganisms in vitro. No indications were found for anaerobic lignin degradation in the rumen reactor. Offprint requests to: H. J. M. Op den Camp     

High-Rate two-phase process for the anaerobic degradation of cellulose, employing rumen microorganisms for an efficient acidogenesis

A novel two-stage anaerobic process for the microbial conversion of cellulose into biogas has been developed. In the first phase, a mixed population of rumen bacteria and ciliates was used in the hydrolysis and fermentation of cellulose. The volatile fatty acids (VFA) produced in this acidogenic reactor were subsequently converted into biogas in a UASB-type methanogenic reactor.A stepwise increase of the loading rate from 11.9 to 25.8 g volatile solids/L reactor volume/day (g VS/L/day) did not affect the degradation efficiency in the acidogenic reactor, whereas the methanogenic reactor appeared to be overloaded at the highest loading rate. Cellulose digestion was almost complete at all loading rates applied. The two-stage anaerobic process was also tested with a closed fluid circuit. In this instance total methane production was 0.438 L CH(4)g VS added, which is equivalent to 98% of the theoretical value. The application of rumen microorganisms in combination with a high-rate methane reactor is proposed as a means of efficient anaerobic degradation of cellulosic residues to methane. Because this newly developed two-phase system is based on processes and microorganisms from the ruminant, it will be referred to as "Rumen Derived Anaerobic Digestion" (RUDAD-) process.     

A novel process configuration for anaerobic digestion of source-sorted household waste using hyper-thermophilic post-treatment

A novel reactor configuration was investigated for anaerobic digestion (AD) of the organic fraction of municipal solid waste (OFMSW). An anaerobic hyper-thermophilic (68 degrees C) reactor R68 was implemented as a post-treatment step for the effluent of a thermophilic reactor R1 (55 degrees C) in order to enhance hydrolysis of recalcitrant organic matter, improve sanitation and ease the stripping of ammonia from the reactor. The efficiency of the combined system was studied in terms of methane yield, volatile solids (VS) reduction, and volatile fatty acid (VFA) production at different hydraulic retention times (HRT). A single-stage thermophilic (55 degrees C) reactor R2 was used as control. VS reduction and biogas yield of the combined system was 78-89% and 640-790 mL/g VS, respectively. While the VS reduction in the combined system was up to 7% higher than in the single-stage treatment, no increase in methane yield was observed. Shifting the HRT of the hyper-thermophilic reactor from 5 days to 3 days resulted in a drop in the methanogenic activity in the hydrolysis reactor to a minimum. Operation of R68 at HRTs of 24-48 h was sufficient to achieve high VS conversion into VFAs. Removal of pathogens was enhanced by the hyper-thermophilic post-treatment. 7% of the ammonia load was removed in the hyper-thermophilic reactor with a flow of headspace gas through the reactor equivalent to four times the biogas flow produced in reactor R1.     

Biogas production from different substrates in an experimental Continuously Stirred Tank Reactor anaerobic digester

Different mixtures were digested in a single-stage, batch, mixed, laboratory scale mesophilic anaerobic digester at the Biomass Research Centre Laboratory (University of Perugia). The yield and the composition of biogas from the different substrates were evaluated and the cumulative curves were estimated. Two experimental campaigns were carried out, the first on three mixtures (chicken, pig and bovine manures), the second on animal and vegetal biomasses (chicken and cow manure, olive husk) with different inocula (rumen fluid and digested sludge). In the first campaign pig manure mixture showed the maximum biogas production (0.35 Nm³/kg) and energy content (1.35 kWh/kg VS); in the second one the differences in produced biogas from the different inocula were analyzed: olive husk with piggery manure anaerobically digested as inoculum showed the higher biogas (0.28 Nm³/kg VS) and methane yield (0.11 Nm³/kg VS), corresponding to an energetic content of 1.07 kWh/kg VS. All data obtained from the laboratory scale anaerobic digester are comparable to the values in literature for several biomass and in particular for olive husk, dairy manure and chicken manure.     

Conversion of cereal residues into biogas in a rumen-derived process

A recently developed high-rate, two-phase process, which employs rumen microorganisms for efficient acidogenesis, was tested for anaerobic degradation of barley straw, rye straw, and maize stover. Under conditions similar to those of the rumen and loading rates varying between 9.8 and 26.0 g of organic matter/I/day in the first phase (acidogenic reactor), total fibre degradation efficiencies ranged between 42% and 57%, irrespective of the loading rate applied. Average specific production of volatile fatty acids and biogas/g volatile solid digested in the acidogenic reactor varied between 6.9 and 11.2 mmol and 0.10 and 0.25 l, respectively.The effect of varying solid retention times on the extent of degradation of barley straw was examined. Changing of retention times in the range of 60 to 156 h had no effect on degradation efficiency, but a decrease in efficiency was observed at retention times below 60 h.By connecting the acidogenic reactor in series to an Upflow Anaerobic Sludge Blanket (UASB) methanogenic reactor the volatile fatty acids were converted into biogas. Average methane contents of the gases produced in the acidogenic reactor and in the UASB reactor were 30±3% and 78±3%, respectively.     

Continuous anaerobic conversion of sugar beet pulp to biogas

Summary A continuous two step anaerobic digestion of sugar beet pulp as carbon and energy source was carried out. The loading rates in the acidification reactor were varied from 5 g·1^–1·d^–1 to 15 g·1^–1·d^–1 while the hydraulic retention time was in the range of 10 hours to 30 hours; the corresponding values for the methane reactor varied from 2 days to nearly 6 days.With this reactor configuration a biogas yield of more than 85% of the theoretical value for total carbon conversion was achieved resulting in a corresponding COD reduction.     

Comparative evaluation of biogas production from dairy manure and co‐digestion with maize silage by CSTR and new anaerobic hybrid reactor

This study aimed to investigate potential methane production through anaerobic digestion of dairy manure and co‐digestion with maize silage. Two different anaerobic reactor configurations (single‐stage continuously stirred tank reactor [CSTR] and hybrid anaerobic digester) were used and biogas production performances for each reactor were compared. The HR was planned to enable phase separation in order to improve process stability and biogas production under higher total solids loadings (≥4%). The systems were tested under six different organic loading rates increased steadily from 1.1 to 5.4 g VS/L.d. The CSTR exhibited lower system stability and biomass conversion efficiency than the HR. The specific biogas production of the hybrid system was between 440 and 320 mL/gVS with 81–65% volatile solids (VS) destruction. The hybrid system provided 116% increase in specific biogas production and VS destruction improved by more than 14%. When MS was co‐digested together with dairy manure, specific biogas production rates increased about 1.2‐fold. Co‐digestion was more beneficial than mono‐material digestion. The hybrid system allowed for generating methane enriched biogas (>75% methane) by enabling phase separation in the reactor. It was observed that acidogenic conditions prevailed in the first two compartments and the following two segments as methanogenic conditions were observed. The pH of the acidogenic part ranged between 4.7 and 5.5 and the methanogenic part was between 6.8 and 7.2.     

Effect of reactor configuration on biogas production from wheat straw hydrolysate

The potential of wheat straw hydrolysate for biogas production was investigated in continuous stirred tank reactor (CSTR) and up-flow anaerobic sludge bed (UASB) reactors. The hydrolysate originated as a side stream from a pilot plant pretreating wheat straw hydrothermally (195 °C for 10–12 min) for producing 2nd generation bioethanol [Kaparaju, P., Serrano, M., Thomsen, A.B., Kongjan, P., Angelidaki, I., 2009. Bioethanol, biohydrogen and biogas production from wheat straw in a biorefinery concept. Bioresource Technology 100 (9), 2562–2568]. Results from batch assays showed that hydrolysate had a methane potential of 384 ml/g-volatile solids (VS)_added. Process performance in CTSR and UASB reactors was investigated by varying hydrolysate concentration and/or organic loading rate (OLR). In CSTR, methane yields increased with increase in hydrolysate concentration and maximum yield of 297 ml/g-COD was obtained at an OLR of 1.9 g-COD/l d and 100% (v/v) hydrolysate. On the other hand, process performance and methane yields in UASB were affected by OLR and/or substrate concentration. Maximum methane yields of 267 ml/g-COD (COD removal of 72%) was obtained in UASB reactor when operated at an OLR of 2.8 g-COD/l d but with only 10% (v/v) hydrolysate. However, co-digestion of hydrolysate with pig manure (1:3 v/v ratio) improved the process performance and resulted in methane yield of 219 ml/g-COD (COD removal of 72%). Thus, anaerobic digestion of hydrolysate for biogas production was feasible in both CSTR and UASB reactor types. However, biogas process was affected by the reactor type and operating conditions.     

Anaerobic acidogenesis of primary sludge: the role of solids retention time

This research investigates the effect of solids retention time (SRT) on the acid-phase anaerobic digestion of primary sludge. A series of experiments were conducted using two continuous-flow 3-L units with the following configuration: a completely mixed reactor (CMR) with clarifier and solids recycle and an upflow anaerobic sludge blanket (UASB) reactor. Results show that C(2) to C(5) volatile fatty acids (VFA) were the predominant compounds formed. At a constant hydraulic retention time (HRT) of 12 h, variation in SRT from 10 to 20 days resulted in a slight increase in VFA production in both systems, but at a shorter SRT (5 days) a drastic drop in acid production was observed. In addition, the percent distribution of VFA was to some extent affected by the change in SRT. On the other hand, organic matter degradation [measured by the chemical oxygen demand (COD) specific solubilization rate or the percent volatile suspended solids (VSS) reduction] appeared to be independent of SRT, at least in the range investigated. The percent soluble COD in the form of VFA, however, increased steadily with increasing SRT, approaching the 90% level at 20 days. The remaining soluble COD in the effluent from these systems may be mainly attributed to metabolic intermediates and unused soluble substrate. (c) 1994 John Wiley & Sons, Inc.     

Characteristics and biogas production potential of municipal solid wastes pretreated with a rotary drum reactor

This study was conducted to determine the characteristics and biogas production potential of organic materials separated from municipal solid wastes using a rotary drum reactor (RDR) process. Four different types of wastes were first pretreated with a commercial RDR system at different retention times (1, 2 and 3 d) and the organic fractions were tested with batch anaerobic digesters with 2.6 g VS L(-1) initial loading. The four types of waste were: municipal solid waste (MSW), a mixture of MSW and paper waste, a mixture of MSW and biosolids, and a mixture of paper and biosolids. After 20 d of thermophilic digestion (50+/-1 degrees C), it was found that the biogas yields of the above materials were in the range of 457-557 mL g VS(-1) and the biogas contained 57.3-60.6% methane. The total solid and volatile solid reductions ranged from 50.2% to 65.0% and 51.8% to 66.8%, respectively. For each material, the change of retention time in the RDR from 1 to 3d did not show significant (alpha=0.05) influence on the biogas yields of the recovered organic materials. Further studies are needed to determine the minimum retention time requirements in the RDR system to achieve effective separation of organic from inorganic materials and produce suitable feedstock for anaerobic digesters.     

Stimulation of biomethanation by Clostridium sp. PXYL1 in coculture with a Methanosarcina strain PMET1 at psychrophilic temperatures

Aim:  Bioaugumentation of low temperature biogas production was attempted by addition of cold-adapted Clostridium and a methanogen.
Methods and Results:  A psychrotrophic xylanolytic acetogenic strain Clostridium sp. PXYL1 growing optimally at 20°C and pH 5·3 and a Methanosarcina strain, PMET1, growing optimally on acetate and producing methane at 15°C were isolated from a cattle manure digester. Anaerobic conversion of xylose at 15°C with the coculture of the two strains was performed, and batch culture methane production characteristics indicated that methanogenesis occurred via acetate through 'acetoclastic' pathway. Stimulation studies were also undertaken to evaluate the effect of exogenous addition of the coculture on biogas yields at 15°C. Addition of 3 ml of PXYL1 at the rate of 12 × 10² CFU ml⁻¹ increased the biogas 1·7-fold (33 l per kg cowdung) when compared to control (19·3 l per kg cowdung) as well as increased the volatile fatty acid (VFA) levels to 3210 mg l⁻¹ when compared to 1140 mg l⁻¹ in controls. Exogenous of addition of 10 ml PMET1 inoculum at the rate of 6·8 ± 10² CFU ml⁻¹ in addition to PXYL1 served to further improve the biogas yields to 46 l kg⁻¹ as well as significantly brought down the VFA levels to 1350 mg l⁻¹.
Conclusions:  Our results suggest that the rate-limiting methanogenic step at low temperatures could be overcome and that biogas yields improved by manipulating the population of the acetoclastic methanogens.
Significance and Impact of the Study:  Stimulation of biomethanation at low temperature by coculture.     

Feeding approaches for biogas production from animal wastes and industrial effluents

Different feeding approaches were applied to a 5 l anaerobic digester in order to improve the biogas production. During operation, the reactor was fed with a mixture (9.7% w/v total solids (TS) and 7.6% w/v volatile solids (VS) in average) of pig manure with fish oil waste and waste from bentonite of edible oil filtration process, at different intervals of 24, 12 and 4 h at 15 days of hydraulic retention time. Production and quality of the biogas were practically constant at 183.7 ml (average) of biogas per gram of volatile solids available in the reactor per day, and the best biogas composition was 73.6% v/v CH₄ and 26.4% v/v CO₂.     

Anaerobic digestion of cellulose fraction of domestic refuse by means of rumen microorganisms

The anaerobic digestion of a cellulose-enriched fraction of domestic refuse by means of rumen microorganisms in an "artificial rumen" digester was studied. Various combinations of solid and liquid retention times and loading rates were applied to establish optimum conditions for the acidogenic phase digestion of the refuse fraction. An optimal substrate conversion of about 72% was obtained at a loading rate of 23.4 g volatile solids (VS)/L d and a solids retention time of 90 h. Variation of dilution rate between 1.04 and 3.14 fermentor volume turnovers per day had no effect on degradation efficiency. At a loading rate of 23.4 g VS/L d a differential removal rate of solids and liquids appeared to be necessary to obtain an effective degradation of the refuse fraction.     

Stimulation of conversion rates and bacterial activity in a silage-fed two-phase biogas process by initiating liquid recirculation

The effects of liquid recirculation on a liquefaction-acidogenic reactor in an anaerobic two-phase digesting system operating with grass-clover silage was studied during 40 days after initiating recirculation of effluent from the methanogenic reactor to the liquefaction-acidogenic reactor. An increase in alkalinity and, thus, an increase in pH from 5.2 to 6.0 occurred in the liquefaction-acidogenic reactor. During the same period, a 10-fold increase (from 0.2 to 1.9 g·l^–1·h^–1) in the degradation rate of mannitol and an almost 9-fold increase in the activity of hydrogenotrophic methanogens was observed. The estimated number of these bacteria increased by one order of magnitude. The average degradation rate of lactate increased 3-fold, probably as a consequence of the more efficient hydrogen consumption by the hydrogenotrophic methanogens. An observed increase in net mineralization of organic nitrogen compounds was probably the main reason for an enhanced net production of organic acids (from 0.2 to 0.9 g·l^–1·d^–1). The liquefaction of cellulose and hemicellulose was low from the start of recirculation (3% and 20% reduction, respectively) and did not seem to be affected by the liquid recirculation. This was in accordance with the low number of cellulose degraders (4.0·10² counts·ml^–1) observed. The results from this investigation show that the initiation of liquid recirculation in silage-fed two-phase biogas processes will stimulate the activity of hydrogenotrophic methanogens in the liquefaction-acidogenic reactor. This will lead to more thermodynamically favourable conditions for acidification reactions which are dependent upon interspecies transfer of reducing equivalents.Abbreviations COD chemical oxygen demand - CSTR completely stirred tank reactor - HRT hydraulic retention time - LA-reactor liquefaction-acidogenic reactor - M-reactor methanogenic reactor - MPN most probable number - OLR organic loading rate - SMA specific methanogenic activity - SRT solids retention time - TKN total Kjeldahl nitrogen - ts total solids - tss total suspended solids - vs volatile solids - vss volatile suspended solids     

Influence of trace elements on biogas production from mango processing waste in 1.5 m3 KVIC digesters

Summary The influence of trace elements (Co²⁺, Ni²⁺ and Fe³⁺) in varying concentrations and combination, was studied in 1.5 m³ Khadi & Village Industries Commission (KVIC) digesters for biogas generation from mangopeel. Addition of these trace metals enhanced the biogas yield and methane content moderately, the maximum being with the iron fed digester. The digesters were always found to be stable without much variation in total volatile fatty acids (VFA), pH, total alkalinity and other parameters. A methane content of 62% and biogas yield of 0.49 m³/kg VS added was obtained with 4000 mg/L FeCl₃ supplemented mangopeel fed digester as compared to control having biogas yield of 0.22 m³/kg VS added with a methane content of about 48–50%.     

High rate anaerobic digestion of piggery manure with polyurethane sponges as support material

Summary The use of polyurethane foam sponges to colonize methanogenic associations for the digestion of piggery manure has been investigated. Fermentors containing polyurethane pads as colonization matrix reached a biogas production rate of ca. 2.0 litres per litre reactor per day (30–33°C), hydraulic retention time 7.5 daysl and a biogas yield of 16 litres per litre piggery manure (7–9% TS). Corresponding control fermentors containing no pads reached a gas production rate of 1.3 litres per litre reactor per day and only about 10 litres biogas per litre piggery manure.     

Thermophilic anaerobic digestion of source-sorted household solid waste: the effects of enzyme additions

Thermophilic (55° C) methanation of source-sorted household solid waste (HSW) was studied in batch and in continuous experiments. Furthermore, the effects of additions of xylanase, lipase, protease and a mixture of these on the methanation were tested. In the batch studies, comparative assays with active and inactive enzymes were used to elucidate the role of the added enzymes. The results showed that the HSW was readily digestible, up to 400–590 mlCH₄·g^–1 volatile solids (VS) was produced. Only with protease added, at a concentration of 1.1 Anson protease units·kg^–1 VS was a higher specific methanogenic activity found with active enzymes compared to inactive (autoclaved) enzymes or without enzyme addition. The methane yield by conversion of the HSW in the batch assays and in the reactor studies was not increased by enzyme additions (enzyme mixture). Correspondence to: B. K. Ahring     

Anaerobic digestion of processed municipal solid waste using a novel high solids reactor: Maximum solids levels and mixing requirements

Summary Novel, laboratory-scale, high solids reactors operated under mesophilic conditions were used to study the anaerobic fermentation of processed municipal solid waste (MSW) to methane. The anaerobic digestion consortium was introduced to high solids levels through gradual adaptation. The maximum sludge solids level for stable anaerobic fermentation performance was identified as approximately 36% wt/wt. Recovery of the anaerobic consortium, following dilution of inhibitory high solids levels, was swift. Reactor mixing requirements were also studied. No significant difference in fermentation performance was observed between agitator speeds of 1 and 25 rpm. Preliminary fermentation performance tests showed that solids loading rates as high as 9.5 g VS (volatile solids) feed/L sludge^.d, at 32% solids within the reactor, were possible. Under these conditions, operation was stable with an average pH of 7.8–8.0, total volatile fatty acid pools of <20 mM, and a biogas composition of 55%–60% methane.     

Experiences with the dual digestion of municipal sewage sludge

The dual digestion process was investigated using sludge samples collected at the WWTP of Tomaszow Mazowiecki (Poland). Mixed sludge was treated in a laboratory setup under batch and semi-continuous conditions. Dual digestion with a 1d SRT aerobic thermophilic pretreatment followed by an anaerobic step with 20 d of SRT turned out to be optimal, since a 44-46% VS reduction and a biogas yield of 480 dm(3)/kg VS fed were achieved. In the course of the process, the concentration of nitrogen in supernatant increased over 5 times and its major portion was converted into ammonia. Phosphorus also entered the supernatant, reaching over 200 g/m(3). The dual digestion noticeably deteriorated the sludge dewaterability. Following completion of the process, capillary suction time measurements averaged 64 s for the raw sludge, 400 s for aerobically pretreated sludge and 310-360 for the anaerobically digested sludge. Aerobic pretreatment consistently reduced Enterobacteriaceae content to below detectable limits.     

Inactivation of virus during anaerobic digestion of manure in laboratory scale biogas reactors

Reduction of porcine parvovirus, bovine enterovirus and faecal enterococci were measured in biogas reactors continuously run on manure and manure supplemented with household waste at 35°C and 55°C and in batch test run at 70°C. The aim of the experiments was to study the sanitation effect of anaerobic digestion and to evaluate the use of faecal enterococci as an indicator of sanitation. Parallel studies on the reduction of virus and faecal enterococci were done in physiological saline solution. Heat ínactivation was found to be an important factor in thermophilic biogas plants and the overall dominant factor at 70°C. However, other environmental factors with a substantial virucidal and bactericidal effect were involved in inactivation. The death rates for faecal enterococci were generally higher than for porcine parvovirus and lower than for bovine enterovirus. For faecal enterococci, a logarithmic reduction of 4 (corresponding to the recommended minimum guaranteed retention time) was obtained after 300 hours at 35°C and after 1–2 hours at 55°C. In continuously-fed reactors, a high reduction rate was initially seen for the virus tested, followed by a reduction in the rate. For porcine parvovirus, a minimum guaranteed retention time of 11–12 hours is necessary at 55°C in the initial phase (0–4 hours) and 54 hours hereafter (4–48 h). Correspondingly, for bovine enterovirus a MGRT of 23 hours was necessary at 35°C and < 0.5 hours at 55°C. The data indicate that faecal enterococci measurements give a good indication of inactivation of enterovirus and other more heat sensitive virus, especially under thermophilic conditions. Parvovirus is very suitable for comparative investigations on inactivation in the temperature range of 50–80°C, due to the extreme thermal resistance of this virus. However, in stipulating sanitation demands for biogas reactors it seems more reasonable to use less resistant virus, such as a reovirus or picornavirus, which better represents the pathogenic animal virus.Abbreviations BEV bovine enterovirus - CFU colony forming unit - FE faecal enterococci - HRT hydraulic retention time - MGRT minimum guaranteed retention time - ND not detected - PPV porcine parvovirus - TCID₅₀ tissue cell infective dose 50 % - VFA volatile fatty acids - VS volatile solids