Effect of feed to inoculum ratios on biogas yields of food and green wastes

Biogas and methane yields of food and green wastes and their mixture were determined using batch anaerobic digesters at mesophilic (35 ± 2 °C) and thermophilic (50 ± 2 °C) temperatures. The mixture was composed of 50% food waste and 50% green waste, based on the volatile solids (VS) initially added to the reactors. The thermophilic digestion tests were performed with four different feed to inoculum (F/I) ratios (i.e., 1.6, 3.1, 4.0 and 5.0) and the mesophilic digestion was conducted at one F/I (3.1). The results showed that the F/I significantly affected the biogas production rate. At four F/Is tested, after 25 days of thermophilic digestion, the biogas yield was determined to be 778, 742, 784 and 396 mL/g VS for food waste, respectively; 631, 529, 524 and 407 mL/g VS for green waste, respectively; and 716, 613, 671 and 555 mL/g VS for the mixture, respectively. About 80% of the biogas production was obtained during the first 10 days of digestion. At the F/I of 3.1, the biogas and methane yields from mesophilic digestion of food waste, green waste and their mixture were lower than the yields obtained at thermophilic temperature. The biogas yields were 430, 372 and 358 mL/g VS, respectively, and the methane yields were 245, 206, and 185 mL/g VS, respectively.     

Biohydrogen and methane production by co-digestion of cassava stillage and excess sludge under thermophilic condition

Thermophilic anaerobic hydrogen and methane production by co-digestion of cassava stillage (CS) and excess sludge (ES) was investigated in this study. The improved hydrogen and subsequent methane production were observed by co-digestion of CS with certain amount of ES in batch experiments. Compared with one phase anaerobic digestion, two phase anaerobic digestion offered an attractive alternative with more abundant biogas production and energy yield, e.g., the total energy yield in two phase obtained at VS_CS/VS_ES of 3:1 was 25% higher than the value of one phase. Results from continuous experiments further demonstrated that VS_CS/VS_ES of 3:1 was optimal for hydrogen production with the highest hydrogen yield of 74 mL/g total VS added, the balanced nutrient condition with C/N ratio of 1.5 g carbohydrate-COD/g protein-COD or 11.9 g C/g N might be the main reason for such enhancement. VS_CS/VS_ES of 3:1 was also optimal for continuous methane production considering the higher methane yield of 350 mL/g total VS added and the lower propionate concentration in the effluent.     

Enhanced methane and hydrogen production from municipal solid waste and agro-industrial by-products co-digested with crude glycerol

The effects of crude glycerol on the performance of single-stage anaerobic reactors treating different types of organic waste were examined. A reactor treating the organic fraction of municipal solid waste produced 1400 mL CH₄/d before the addition of glycerol and 2094 mL CH₄/d after the addition of glycerol. An enhanced methane production rate was also observed when a 1:4 mixture of olive mill wastewater and slaughterhouse wastewater was supplemented with crude glycerol. Specifically, by adding 1% v/v crude glycerol to the feed, the methane production rate increased from 479 mL/d to 1210 mL/d. The extra glycerol-COD added to the feed did not have a negative effect on the reactor performance in either case. Supplementation of the feed with crude glycerol also had a significant positive effect on anaerobic fermentation reactors. Hydrogen yield was 26 mmole H₂/g VS added and 15 mmole H₂/g VS added in a reactor treating the organic fraction of municipal solid waste and a 1:4 mixture of olive mill and slaughterhouse wastewater. The addition of crude glycerol to the feed enhanced hydrogen yield at 2.9 mmole H₂/g glycerol added and 0.7 mmole H₂/g glycerol added.     

Potential for methane production from anaerobic co-digestion of swine manure with winery wastewater

This work examines the methane production potential for the anaerobic co-digestion of swine manure (SM) with winery wastewater (WW). Batch and semi-continuous experiments were carried out under mesophilic conditions. Batch experiments revealed that the highest specific methane yield was 348 mL CH₄ g⁻¹ COD added, obtained at 85.4% of WW and 0.7 g COD g⁻¹ VS. Specific methane yield from SM alone was 27 mL CH₄ g⁻¹ COD added d⁻¹. Furthermore, specific methane yields were 49, 87 and 107 mL CH₄ g⁻¹ COD added d⁻¹ for the reactors co-digesting mixtures with 10% WW, 25% WW and 40% WW, respectively. Co-digestion with 40% WW improved the removal efficiencies up to 52% (TCOD), 132% (SCOD) and 61% (VSS) compared to SM alone. These results suggest that methane can be produced very efficiently by the co-digestion of swine manure with winery wastewater.     

Characteristics of hydrogen and methane production from cornstalks by an augmented two- or three-stage anaerobic fermentation process

This paper presents the co-production of hydrogen and methane from cornstalks by a two- or three-stage anaerobic fermentation process augmented with effective artificial microbial community. Two-stage fermentation by using the anaerobic sludge and DGGE analysis showed that effective and stable strains should be introduced into the system. We introduced Enterobacter aerogens or Clostridium paraputrificum into the hydrogen stage, and C. paraputrificum was proven to be more effective. In the three-stage process consisting of the improved hydrolysis, hydrogen and methane production stages, the highest soluble sugars (0.482 kg/kg cornstalks) were obtained after the introduction of Clostridium thermocellum in the hydrolysis stage, under the thermophilic (55 °C) and acidic (pH 5.0) conditions. Hydrolysates from 1 kg of cornstalks could produce 2.61 mol (63.7 l) hydrogen by augmentation with C. paraputrificum and 4.69 mol (114.6 l) methane by anaerobic granular sludge, corresponding to 54.1% energy recovery.     

Eliminating methanogenic activity in hydrogen reactor to improve biogas production in a two-stage anaerobic digestion process co-digesting municipal food waste and sewage sludge

Laboratory scale two-stage anaerobic digestion process model was operated for 280 days to investigate the feasibility to produce both hydrogen and methane from a mixture feedstock (1:1 (v/v)) of municipal food waste and sewage sludge. The maximum hydrogen and methane yields obtained in the two stages were 0.93 and 9.5 mL/mL feedstock. To eliminate methanogenic activity and obtain substantial hydrogen production in the hydrogen reactor, both feedstock and mixed liquor required treatment. The heat treatment (100 °C, 10 min) for feedstock and a periodical treatment (every 2-5 weeks, either heating, removal of biomass particles or flushing with air) for mixed liquor were effective in different extent. The methane production in the second stage was significantly improved by the hydrogen production in the first stage. The maximum methane production obtained in the period of high hydrogen production was more than 2-fold of that observed in the low hydrogen production period.     

Combined thermochemical and fermentative destruction of municipal biosolids: a comparison between thermal hydrolysis and wet oxidative pre-treatment

In this study the comparative destruction of municipal biosolids using thermal hydrolysis (140 or 165 °C) and wet oxidation (220 °C) was followed by biological degradation via mesophilic anaerobic digestion (36 °C). Wet oxidation (WO) destroyed more than 93% of the VSS, while thermal hydrolysis (TH) at 140 and 165 °C destroyed 9% and 22%, respectively. Combined TH and anaerobic digestion resulted in approximately 50% VSS destruction. The ultimate methane potential of the combined fractions from the thermal hydrolysis at 140 and 165 °C improved by 12-13% relative to the untreated control sample. Methane production from the WO material was 53% of the control yield and wholly attributable to soluble organic carbon in the liquid fraction, indicating that the WO destroyed all putrescible carbon from the solids fraction. Point sampling during the BMP assay revealed that methanogenic development, not solids hydrolysis, was the kinetic barrier during anaerobic digestion in this study.     

Optimization of thermal-dilute sulfuric acid pretreatment for enhancement of methane production from cassava residues

In this study, the pretreatment of cassava residues by thermal-dilute sulfuric acid (TDSA) hydrolysis was investigated by means of a statistically designed set of experiments. A three-factor central composite design (CCD) was employed to identify the optimum pretreatment condition of cassava residues for methane production. The individual and interactive effects of temperature, H₂SO₄ concentration and reaction time on increase of methane yield (IMY) were evaluated by applying response surface methodology (RSM). After optimization, the resulting optimum pretreatment condition was 157.84 °C, utilizing 2.99% (w/w TS) H₂SO₄ for 20.15 min, where the maximum methane yield (248 mL/g VS) was 56.96% higher than the control (158 mL/g VS), which was very close to the predict value 56.53%. These results indicate the model obtained through RSM analysis is suit to predict the optimum pretreatment condition and there is great potential of using TDSA pretreatment of cassava residues to enhance methane yield.     

Mesophilic batch anaerobic co-digestion of pulp and paper sludge and monosodium glutamate waste liquor for methane production in a bench-scale digester

This paper presented results from anaerobic co-digestion of pulp and paper sludge (PPS) and monosodium glutamate waste liquor (MGWL). A bench-scale anaerobic digester, 10 L in volume was developed, to operate under mesophilic (37 ± 2 °C) batch condition. Under versatile and reliable anaerobic conduct, high efficiency for bioconversion of PPS and MGWL were obtained in the system. The accumulative methane yield attained to 200 mL g⁻¹ VS_added and the peak value of methane daily production was 0.5 m³/(m³ d). No inhibitions of volatile fatty acids (VFAs) and ammonia on anaerobic co-digestion were found. pH 6.0-8.0 and alkalinity 1000-4000 mg CaCO₃/L were got without adjustment. This work showed that there was a good potential to the use of PPS and MGWL to anaerobic co-digestion for methane production.     

Low temperature thermo-chemical pretreatment of dairy waste activated sludge for anaerobic digestion process

An investigation into the influence of low temperature thermo-chemical pretreatment on sludge reduction in a semi-continuous anaerobic reactor was performed. Firstly, effect of sludge pretreatment was evaluated by COD solubilization, suspended solids reduction and biogas production. At optimized condition (60 °C with pH 12), COD solubilization, suspended solids, reduction and biogas production was 23%, 22% and 51% higher than the control, respectively. Secondly, semi-continuous process performance was studied in a lab-scale semi-continuous anaerobic reactor (5 L), with 4 L working volume. With three operated SRTs, the SRT of 15 days was found to be most appropriate for economic operation of the reactor. Combining pretreatment with anaerobic digestion led to 80.5%, 117% and 90.4% of TS, SS and VS reduction respectively, with an improvement of 103% in biogas production. Thus, low temperature thermo-chemical can play an important role in reducing sludge production.     

An integrated UASB-sludge digester system for raw domestic wastewater treatment in temperate climates

To improve the performance of an upflow anaerobic sludge blanket (UASB) reactor treating raw domestic wastewater under temperate climates conditions, the addition of a sludge digester to the process was investigated. With the decrease in temperature, the COD removal decreased from 78% at 28 °C to 42% at 10 °C for the UASB reactor operating alone at a hydraulic retention time of 6 h. The decrease was attributed to low hydrolytic activity at lower temperatures that reduced suspended matter degradation and resulted in solids accumulation in the top of the sludge blanket. Solids removed from the upper part of the UASB sludge were treated in an anaerobic digester. Based on sludge degradation kinetics at 30 °C, a digester of 0.66 l per liter of UASB reactor was design operating at a 3.20 days retention time. Methane produced by the sludge digester is sufficient to maintain the temperature at 30 °C.     

Comparison of the effects of microwave irradiation with different intensities on the biodegradability of sludge from the dairy- and meat-industry

Microwave (MW) irradiation is a relatively new possibility of conditioning and pretreating for wastewater sludge. Following its application in the telecommunications and food-industries, the environmental use of this technique has come into the limelight in recent years, and has become increasingly popular. Various publications have dealt with the examination of the effects of MW irradiation in municipal sludge-handling processes. We focused on the effects of MW irradiation at different power levels on solubilization (sCOD/tCOD), biodegradation and anaerobic digestion of sludge from the food-industry. For evaluating the efficiency of MW pre-treatment, the changes in the soluble fraction of the organic matter, the VS/TS ratio, the biogas yield, the methane content in the biogas, and the rate of batch mesophilic digestion were used as control parameters. Additionally, the energetic efficiency of MW pre-treatment was also examined. The results were compared with those of conventional heat (CH) treatments of the same sludge. The MW treatment proved to increase both the sCOD/tCOD and the VS/TS ratio. Furthermore, the biogas and methane yields increased during the digestion of the MW-pretreated food-industry sludge. A higher MW power level generally enhanced the biogas and methane production. Energetically, the most economic pre-treatment of sludge from dairy and meat processing was at a power level of 1.5 Wg⁻¹ and 2.5 Wg⁻¹ MW respectively; the surplus energy content of the enhanced biogas product could not compensate the extra energy demand of the stronger MW pre-treatments.     

Improving pig manure conversion into biogas by thermal and thermo-chemical pretreatments

Thermal (70–190 °C) and thermo-chemical (pH = 10 and 12, 25 °C and 90–190 °C) treatments were investigated in order to maximise the production of methane from pig manure. Methane production from treated and raw manure was assessed from batch mesophilic biochemical methane potential tests. Methane potential of manure soluble fraction increased with the temperature of thermal treatments whereas temperatures higher than 135 °C were necessary to improve the methane potential of the total fraction. The best results were obtained with the highest temperature (190 °C).     

Dry-thermophilic anaerobic digestion of simulated organic fraction of Municipal Solid Waste: Process modeling

Solid retention time (SRT) is a very important operational variable in continuous and semicontinuous waste treatment processes since the organic matter removal efficiency - expressed in terms of percentage of Dissolved Organic Carbon (% DOC) or Volatile Solids (% VS) removed - and the biogas or methane production are closely related with the SRT imposed. Optimum SRT is depending on the waste characteristics and the microorganisms involved in the process and, hence, it should be determined specifically in each case.In this work a series of experiments were carried out to determine the effect of SRT, from 40 to 8 days, on the performance of the dry (30% Total Solids) thermophilic (55 °C) anaerobic digestion of organic fraction of Municipal Solid Wastes (OFMSW) operating at semicontinuous regime of feeding.The experimental results show than 15 days is the optimum SRT (the best between all proved) for this process. Besides, data of organic matter concentration and methane production versus SRT have been used to obtain the kinetic parameters of the kinetic model of Romero García (1991): the maximum specific growth rate of the microorganisms (μ_max = 0.580 days⁻¹) and the fraction of substrate non-biodegradable (α = 0.268).     

Experimental studies and mathematical modeling of an up-flow biofilm reactor treating mustard oil rich wastewater

Bioremediation of lipid-rich model wastewater was investigated in a packed bed biofilm reactor (anaerobic filter). A detailed study was conducted about the influence of fatty acid concentration on biomethanation of the high-fat liquid effluent of edible oil refineries. The biochemical methane potential (BMP) of the liquid waste was reported and maximum cumulative methane production at the exit of the reactor is estimated to be 785 ml CH₄ (STP)/(g VSS added). The effects of hydraulic retention time (HRT), organic loading rate (OLR) and bed porosity on the cold gas efficiency or energy efficiency of the bioconversion process were also investigated. Results revealed that the maximum cold gas efficiency of the process is 42% when the total organic load is 2.1 g COD/l at HRT of 3.33 days. Classical substrate uninhibited Monod model is used to generate the differential system equations which can predict the reactor behavior satisfactorily.     

A comparison study on the high-rate co-digestion of sewage sludge and food waste using a temperature-phased anaerobic sequencing batch reactor system

Assessing contemporary anaerobic biotechnologies requires proofs on reliable performance in terms of renewable bioenergy recovery such as methane (CH₄) production rate, CH₄ yield while removing volatile solid (VS) effectively. This study, therefore, aims to evaluate temperature-phased anaerobic sequencing batch reactor (TPASBR) system that is a promising approach for the sustainable treatment of organic fraction of municipal solid wastes (OFMSW). TPASBR system is compared with a conventional system, mesophilic two-stage anaerobic sequencing batch reactor system, which differs in operating temperature of 1st-stage. Results demonstrate that TPASBR system can obtain 44% VS removal from co-substrate of sewage sludge and food waste while producing 1.2 m³CH₄/m³_system/d (0.2 m³CH₄/kgVS_added) at organic loading rate of 6.1 gVS/L/d through the synergy of sequencing-batch operation, co-digestion, and temperature-phasing. Consequently, the rapid and balanced anaerobic metabolism at thermophilic stage makes TPASBR system to afford high organic loading rate showing superior performance on OFMSW stabilization.     

Effect of seed sludge and operation conditions on performance and archaeal community structure of low-temperature anaerobic solvent-degrading bioreactors

Two laboratory-scale expanded granular sludge bed (EGSB) anaerobic bioreactors (R1 and R2) were inoculated with biomass from different mesophilic (37 °C) treatment plants, and used for the treatment of an organic solvent-based wastewater at 9–14 °C at applied organic loading rates (OLRs) of 1.2–3.6 kg chemical oxygen demand (COD) m⁻³ d⁻¹. Replicated treatment performance was observed at 10–14 °C, which suggested the feasibility of the process at pilot-scale. Stable and efficient COD removal, along with high methane productivity, was demonstrated at 9 °C at an applied OLR of 2.4 kg COD m⁻³ d⁻¹. Clonal libraries and fluorescence in situ hybridization (FISH) indicated that the seed sludges were dominated (>60%) by acetoclastic Methanosaeta-like organisms. Specific methanogenic activity (SMA) profiles indicated shifts in the physiological profiles of R1 and R2 biomass, including the development of psychrotolerant methanogenic activity. Acetoclastic methanogenesis represented the primary route of methane production in R1 and R2, which is in contrast with several previous reports from low-temperature bioreactor trials. A reduction in the abundance of Methanosaeta-like clones (R2), along with the detection of hydrogenotrophic methanogenic species, coincided with altered granule (sludge) morphology and the development of hydrogenotrophic SMA after prolonged operation at 9 °C.     

Co-digestion of intermediate landfill leachate and sewage sludge as a method of leachate utilization

This study examines the co-digestion of intermediate landfill leachate and sewage sludge from a municipal wastewater treatment plant. Application of leachate as a co-fermentation component increased the concentrations of soluble organic compounds (expressed as total organic carbon), ammonium nitrogen, and alkalinity in the digester influents.The biogas yield obtained from the co-fermentation of a 20:1 sewage sludge: intermediate leachate mixture was 1.30 m³ per kg of removed volatile solids (VS), while that from a 10:1 mixture was 1.24 m³ per kg of removed VS. These values exceeded the biogas yield for the sludge alone by 13% and 8%, respectively. The leachate addition influenced the proportion of methane to a minor extent. Increased methane yields of 16.9% and 6.2% per kg of removed VS were found for the two sewage sluge:intermediate leachate mixtures, respectively.     

Fate of organic carbon in UAFB treating raw sewage: impact of moderate to low temperature

The anaerobic biological treatment of raw sewage under low ambient temperatures was investigated in an up-flow anaerobic fixed bed (UAFB) reactor, over a 220-day trial period. The operating temperature was lowered stepwisely from 35 to 25, 20 and 15 °C. Obvious accumulation of acetic acid was observed in the effluent as the temperature dropped, which almost tripled at 15 °C compared to that at 35 °C. An interesting psychrophilic-forward-shift trend of methane production rate following the decrease of temperature was discovered. The methane yield at each trial was 169, 201, 256 and 269 L_STP CH₄/kg COD_removed. For each temperature trial, about 32.6%, 28.3%, 34.4% and 34.9% were removed in form of methane. Based on PCR-DGGE results, the band corresponded to Methanosphaera spp. only showed up on 15 °C profile. Analysis of both methane production and DGGE results indicated that psychro-tolerant microorganisms dominated the archaeal populations in the UAFB under low-temperature.