Feasibility of biogas production from anaerobic co-digestion of herbal-extraction residues with swine manure

The objective of this work was to examine the feasibility of biogas production from the anaerobic co-digestion of herbal-extraction residues with swine manure. Batch and semi-continuous experiments were carried out under mesophilic anaerobic conditions. Batch experiments revealed that the highest specific biogas yield was 294 mL CH₄ g⁻¹ volatile solids added, obtained at 50% of herbal-extraction residues and 3.50 g volatile solids g⁻¹ mixed liquor suspended solids. Specific methane yield from swine manure alone was 207 mL CH₄ g⁻¹ volatile solid added d⁻¹ at 3.50 g volatile solids g⁻¹ mixed liquor suspended solids. Furthermore, specific methane yields were 162, 180 and 220 mL CH₄ g⁻¹ volatile solids added d⁻¹ for the reactors co-digesting mixtures with 10%, 25% and 50% herbal-extraction residues, respectively. These results suggested that biogas production could be enhanced efficiently by the anaerobic co-digestion of herbal-extraction residues with swine manure.     

Pentachlorophenol (PCP) dechlorination in horizontal-flow anaerobic immobilized biomass (HAIB) reactors

This study verifies the potential applicability of horizontal-flow anaerobic immobilized biomass (HAIB) reactors to pentachlorophenol (PCP) dechlorination. Two bench-scale HAIB reactors (R1 and R2) were filled with cubic polyurethane foam matrices containing immobilized anaerobic sludge. The reactors were then continuously fed with synthetic wastewater consisting of PCP, glucose, acetic acid, and formic acid as co-substrates for PCP anaerobic degradation. Before being immobilized in polyurethane foam matrices, the biomass was exposed to wastewater containing PCP in reactors fed at a semi-continuous rate of 2.0 μg PCP g⁻¹ VS. The applied PCP loading rate was increased from 0.05 to 2.59 mg PCP l⁻¹ day⁻¹ for R1, and from 0.06 to 4.15 mg PCP l⁻¹ day⁻¹ for R2. The organic loading rates (OLR) were 1.1 and 1.7 kg COD m⁻³ day⁻¹ at hydraulic retention times (HRT) of 24 h for R1 and 18 h for R2. Under such conditions, chemical oxygen demand (COD) removal efficiencies of up to 98% were achieved in the HAIB reactors. Both reactors exhibited the ability to remove 97% of the loaded PCP. Dichlorophenol (DCP) was the primary chlorophenol detected in the effluent. The adsorption of PCP and metabolites formed during PCP degradation in the packed bed was negligible for PCP removal efficiency.     

Performance evaluation and phylogenetic characterization of anaerobic fluidized bed reactors using ground tire and pet as support materials for biohydrogen production

This study evaluated two different support materials (ground tire and polyethylene terephthalate [PET]) for biohydrogen production in an anaerobic fluidized bed reactor (AFBR) treating synthetic wastewater containing glucose (4000 mg L⁻¹). The AFBR, which contained either ground tire (R1) or PET (R2) as support materials, were inoculated with thermally pretreated anaerobic sludge and operated at a temperature of 30 °C. The AFBR were operated with a range of hydraulic retention times (HRT) between 1 and 8 h. The reactor R1 operating with a HRT of 2 h showed better performance than reactor R2, reaching a maximum hydrogen yield of 2.25 mol H₂ mol⁻¹ glucose with 1.3 mg of biomass (as the total volatile solids) attached to each gram of ground tire. Subsequent 16S rRNA gene sequencing and phylogenetic analysis of particle samples revealed that reactor R1 favored the presence of hydrogen-producing bacteria such as Clostridium, Bacillus, and Enterobacter.     

Saline adaptation of granules in mesophilic UASB reactors

We exposed mesophilic up-flow anaerobic sludge blanket (UASB) reactors to high concentrations of NaCl to elucidate the saline adaptation capacity of their granular sludge. We operated 10 lab-scale UASB reactors at 37 °C and added NaCl to the influent either abruptly or gradually. With abrupt addition, NaCl concentrations were increased from 0 g L⁻¹ to 20, 30, 35, 40, 45, or 50 g L⁻¹. With gradual addition, the NaCl concentrations were gradually increased from 0 to 64 g L⁻¹ or 0 to 40 g L⁻¹. We successfully saline-adapted the granules up to 32 g NaCl L⁻¹, while maintaining high reactor performance, suggesting that 32 g NaCl L⁻¹ is a practical level for system operation. In the UASB reactors gradually exposed to 32 g L⁻¹ NaCl, methane production decreased by only 13%. We also learned that combining abrupt and gradual salinity increases could shorten the adaptation period. Thus we were able to shorten the adaptation period to only 30 days by increasing the salinity abruptly to 20 g L⁻¹, followed by gradual adaptation to 30 g NaCl L⁻¹.     

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.     

Effect of increasing the surface area of primary sludge on anaerobic digestion at low temperature

Two sets of reactors were operated at 15 °C and at sludge retention times (SRTs) of 65 days and 30 days, respectively. Each set was operated at six different mixing velocities. Shear forces provided by mixing affected particle size distribution and resulted in different sludge surface areas. The aim was to investigate the effect of increasing primary sludge surface area on anaerobic digestion at low temperature. The maximum surface areas at the applied mixing velocities were 5926 cm²/cm³of sludge and 4672 cm²/cm³ of sludge at SRTs of 65 days and 30 days, respectively. The corresponding calculated methanogenesis were 49% and 15% at SRTs of 65 days and 30 days, respectively. Maximum total solids (TS) reductions were 26% and 11% at 65 days and 30 days SRTs, respectively. Lipase activity increased with increasing SRT and sludge surface area. Results clearly showed that increasing sludge surface area improved sludge digestion at 15 °C.     

Biomass adaptation over anaerobic co-digestion of sewage sludge and trapped grease waste

The feasibility of sewage sludge co-digestion using intermediate waste generated inside a wastewater treatment plant, i.e. trapped grease waste from the dissolved air flotation unit, has been assessed in a continuous stirred lab reactor operating at 35 °C with a hydraulic retention time of 20 days. Three different periods of co-digestion were carried out as the grease waste dose was increased. When the grease waste addition was 23% of the volatile solids fed (organic loading rate 3.0 kg_COD m⁻³ d⁻¹), an increase in methane yield of 138% was reported. Specific activity tests suggested that anaerobic biomass had adapted to the co-substrate. The adapted inoculum showed higher acetoclastic methanogenic and β-oxidation synthrophic acetogenic activities but lower hydrogenotrophic methanogenic activity. The results indicate that a slow increase in the grease waste dose could be a strategy that favours biomass acclimation to fat-rich co-substrate, increases long chain fatty acid degradation and reduces the latter’s inhibitory effect.     

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.     

Improved volatile fatty acids production from proteins of sewage sludge with anthraquinone-2,6-disulfonate (AQDS) under anaerobic condition

Organic matters in sewage sludge can be converted into volatile fatty acids (VFAs) as renewable carbon sources. This work for the first time applied anthraquinone-2,6-disulfonate (AQDS) for enhancing VFA production from sewage sludge. With 0.066 or 0.33 g AQDS g⁻¹ dried solids (DS), the yields for VFAs peak at 403 or 563 mg l⁻¹, 1.9- or 2.7-fold to the control. The accumulated VFAs were principally composed of acetate and propionate. The AQDS enhances degradation rates of model proteins (bovine serum albumin), but had little enhancement on that of model polysaccharides (dextrans). The acidification step is proposed the rate-limiting step for VFA production from sewage sludge, in which the AQDS molecules shuttle electrons to accelerate the redox reactions associated with amino acid degradation. Methanogenic activities are inhibited in the presence of AQDS. The AQDS-assisted VFAs are renewable organic carbon sources, although their direct use for anaerobic digestion is not advised.     

Performance of a pilot-scale sewage treatment: an up-flow anaerobic sludge blanket (UASB) and a down-flow hanging sponge (DHS) reactors combined system by sulfur-redox reaction process under low-temperature conditions

Performance of a wastewater treatment system utilizing a sulfur-redox reaction of microbes was investigated using a pilot-scale reactor that was fed with actual sewage. The system consisted of an up-flow anaerobic sludge blanket (UASB) reactor and a down-flow hanging sponge (DHS) reactor with a recirculation line. Consequently, the total COD_Cr (465 ± 147 mg L⁻¹; total BOD of 207 ± 68 mg L⁻¹) at the influent was reduced (70 ± 14 mg L⁻¹; total BOD of 9 ± 2 mg L⁻¹) at the DHS effluent under the conditions of an overall hydraulic retention time of 12 h, a recirculation ratio of 2, and a low-sewage temperature of 7.0 ± 2.8 °C. A microbial analysis revealed that sulfate-reducing bacteria contributed to the degradation of organic matter in the UASB reactor even in low temperatures. The utilized sulfur-redox reaction is applicable for low-strength wastewater treatment under low-temperature conditions.     

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.     

The effect of electrodialytic treatment and Na2H2EDTA addition on methanogenic activity of copper-amended anaerobic granular sludge: treatment costs and energy consumption

The effect of electrodialytic treatment in terms of a current density, pH and Na₂H₂EDTA addition on the methanogenic activity of copper-amended anaerobic granular sludge taken from the UASB reactor from paper mill was evaluated. Moreover, the specific energy consumption and simplified operational and treatment costs were calculated. Addition of Na₂H₂EDTA (at pH 7.7) to copper-amended sludge resulted in the highest microbial activity (62 mg CH₄-COD g VSS⁻¹ day⁻¹) suggesting that Na₂H₂EDTA decreased the toxic effects of copper on the methanogenic activity of the anaerobic granular sludge. The highest methane production (159 %) was also observed upon Na₂H₂EDTA addition and simultaneous electricity application (pH 7.7). The energy consumption during the treatment was 560, 840, 1400 and 1680 kW h m⁻³ at current densities of 0.23, 0.34, 0.57 and 0.69 mA cm⁻², respectively. This corresponded to a treatment costs in terms of electricity expenditure from 39.2 to 117.6 € per cubic meter of sludge.     

Electricity generation in single-chamber microbial fuel cells using a carbon source sampled from anaerobic reactors utilizing grass silage

Production of electricity from samples obtained during anaerobic digestion of grass silage was examined using single-chamber air-cathode mediator-less microbial fuel cells (MFCs). The samples were obtained from anaerobic reactors at start-up conditions after 3 and 10 days of operation under psychrophilic (15 °C) and mesophilic (37 °C) temperatures. Electricity was directly produced from all samples at a concentration of 1500 mg COD L⁻¹. Power density obtained from the samples, as a sole carbon source, ranged from 56 ± 3 W m⁻³ to 31 ± 1 W m⁻³ for the mesophilic and psychrophilic samples, respectively. Coulombic efficiencies ranged from 18 ± 1% to 12 ± 1% for the same samples. The relationship between the maximum voltage output and initial COD concentration appeared to follow saturation kinetics at the external resistance of 217 Ω. Chemical oxygen demand (COD) removal was over 90% and total phenolics removal was in the range of 30-75% for all samples tested, with a standard amount of 60 mg L⁻¹ total phenolics removed for every sample. Our results indicate that generating electricity from solution samples of anaerobic reactors utilizing grass silage is possible, opening the possibility for combination of anaerobic digestion with MFC technology for energy generation.     

Limitations of thermophilic anaerobic wastewater treatment and the consequences for process design

Thermophilic anaerobic digestion offers an attractive alternative for the treatment of medium- and high-strength wastewaters. However, literature reports reveal that thermophilic wastewater treatment systems are often more sensitive to environmental changes than the well-defined high-rate reactors at the mesophilic temperature range. Also, in many cases a poorer effluent quality is experienced while the carry over of suspended solids in the effluent is relatively high. In this paper recent achievements are discussed regarding the process stability of thermophilic anaerobic wastewater treatment systems. Laboratory experiments reveal a relatively low sensitivity to temperature changes if high-rate reactors with immobilized biomass are used. Other results show that if a staged process is applied, thermophilic reactors can be operated for prolonged periods of time under extreme loading conditions (80–100 kg chemical oxygen demand.m^-3.day^-1), while the concentrations of volatile fatty acids in the effluent remain at a low level.     

Temperature dependent (37-15°C) anaerobic digestion of a trichloroethylene-contaminated wastewater

The impact of a trichloroethylene (TCE) contaminated wastewater on the microbial community structure of an anaerobic granular biomass at 15 °C compared to 37 °C was investigated. Four expanded granular sludge bed (EGSB) bioreactors (R1-R4) were employed in pairs at 37 and 15 °C. The influents of one of each pair were supplemented with increasing concentrations of TCE (max. 60 mg l⁻¹). At 37 °C, stable operation was maintained with 88% COD removal and >99% TCE removal at maximum influent TCE concentrations. R3 performance decreased at influent TCE concentration of 60 mg l⁻¹, although TCE removal rates of >97% were recorded. Archaeal community analysis via clone library and quantitative polymerase chain reaction (qPCR) analysis, and bacterial community analysis via denaturing gradient gel electrophoresis (DGGE), indicated that temperature resulted in a greater change in community structure than the presence of TCE, and clones related to cold adaptation of biomass were identified at 15 °C.     

Influence of carbon source and inoculum type on anaerobic biomass adhesion on polyurethane foam in reactors fed with acid mine drainage

This paper analyzes the influence of carbon source and inoculum origin on the dynamics of biomass adhesion to an inert support in anaerobic reactors fed with acid mine drainage. Formic acid, lactic acid and ethanol were used as carbon sources. Two different inocula were evaluated: one taken from an UASB reactor and other from the sediment of a uranium mine. The values of average colonization rates and the maximum biomass concentration (C_max) were inversely proportional to the number of carbon atoms in each substrate. The highest C_max value (0.35 g TVS g⁻¹ foam) was observed with formic acid and anaerobic sludge as inoculum. Maximum colonization rates (v_max) were strongly influenced by the type of inoculum when ethanol and lactic acid were used. For both carbon sources, the use of mine sediment as inoculum resulted in a v_max of 0.013 g TVS g⁻¹ foam day⁻¹, whereas 0.024 g TVS g⁻¹ foam day⁻¹ was achieved with anaerobic sludge.     

Two-stage anaerobic digestion of biodegradable municipal solid waste using a rotating drum mesh filter bioreactor and anaerobic filter

A rotating drum mesh filter bioreactor (RDMFBR) with a 100 μm mesh coupled to an anaerobic filter was used for the anaerobic digestion of biodegradable municipal solid waste (BMW). Duplicate systems were operated for 72 days at an organic loading rate (OLR) of 7.5 gVS l⁻¹ d⁻¹. Early in the experiment most of the methane was produced in the 2nd stage. This situation gradually reversed as methanogenesis became established in the 1st stage digester, which eventually produced 86–87% of the total system methane. The total methane production was 0.2 l g⁻¹ VS_added with 60–62% volatile solids destruction. No fouling was experienced during the experiment at a transmembrane flux rate of 3.5 l m⁻² h⁻¹. The system proved to be robust and stably adjusted to a shock loading increase to 15 gVS l⁻¹ d⁻¹, although this reduced the overall methane production to 0.15 l g⁻¹ VS_added.     

Laboratory scale examination of the effects of overloading on the anaerobic digestion by glycerol

The anaerobic digestion of pure glycerol, which produces a baseline acetic acid to propionic acid ratio of 0.2, was studied in laboratory scale reactors (3 l working volume) at mesophilic temperature (37 °C) with 3000 mg chemical oxygen demand (COD) l⁻¹d⁻¹. During the experiment tVFA and C2-C6 VFA analysis and daily biogas yield measurement were carried out. Following 10 days of a 15% d⁻¹ increase in the organic loading rate (OLR) of 3.0-10.5 g COD l⁻¹d⁻¹, the concentration of propionic acid increased to 6200-8000 mg l⁻¹. Then the inoculum was divided into three parts feeding with 100% glycerol, 50% glycerol + 50% acetic acid, and 50% glycerol + 50% thick stillage, (presented in % of 2.60 g COD l⁻¹d⁻¹ OLR), respectively. The application of co-substrates reduced the recovery period by 5 days compared to feeding with pure glycerol. When the reactors were loaded with glycerol again (10% OLR raise per day) the previously applied co-substrates had a positive effect on the VFA composition and the biogas yield as well.     

Synergistic effects of natural and modified zeolites on the methanogenesis of acetate and methanol

The addition of Mg²⁺ modified zeolite at 0.1 g g^–1 of volatile suspended solids (VSS) increased the potential specific methanogenic activity (PSMA) and the apparent kinetic constant, k ₀, showing PSMA and k ₀ values 15 and 2 times higher than those observed for control reactors during the methanogenesis of acetate and methanol, respectively. In addition, the population of hydrolytic, acetogenic and methanogenic bacteria increased 100, 10 and 100 times compared to that observed in the controls. The anaerobic degradation of methanol and acetate occurred in one and two stages, respectively.     

Vermistabilization of primary sewage sludge

An integrated composting-vermicomposting process has been developed for utilization of primary sewage sludge (PSS). Matured vermicompost was used as bulking material and a source of active microbial culture during aerobic activated composting (AAC). AAC resulted in sufficient enrichment of bulking material with organic matter after 20 cycles of recycling and mixing with PSS and produced materials acceptable for vermicomposting. Vermicomposting caused significant reduction in pH, volatile solids (VS), specific oxygen uptake rate (SOUR), total organic carbon (TOC), C/N ratio and pathogens and substantial increase in electrical conductivity (EC), total nitrogen (TN) and total phosphorous (TP) as compared to compost. Environmental conditions and stocking density have profound effects on vermicomposting. Temperature of 20 °C with high humidity is favorable environmental condition for vermicomposting employing Eisenia fetida. Favorable stocking density range for vermiculture is 0.5-2.0 kg m⁻² (optimum: 0.5 kg m⁻²) and for vermicomposting is 2.0-4.0 kg m⁻² (optimum: 3.0 kg m⁻²), respectively.