Effect of low solids retention time and focused pulsed pre-treatment on anaerobic digestion of waste activated sludge

The interacting effects of Focused Pulsed (FP) treatment and solids retention time (SRT) were evaluated in laboratory-scale digesters operated at SRTs of 2-20 days. Anaerobic digestion and methanogenesis of waste activated sludge (WAS) were stable for SRT ? 5 days, but the effluent soluble organic compounds increased significantly for SRT = 2 days due to a combination of faster hydrolysis kinetics and washout of methanogens. FP treatment increased the CH₄ production rate and TCOD removal efficiency by up to 33% and 18%, respectively, at a SRT of 20 days. These effects were the result of an increase in the hydrolysis rate, since the concentrations of soluble components remained low for SRT ? 5 days. Alternately, FP pre-treatment of WAS allowed the same conversion of TCOD to CH₄ with a smaller SRT and digester size: e.g., 40% size savings with a CH₄ conversion of 0.23 g CH₄-COD/g COD_in.     

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.     

Using cheese whey for hydrogen and methane generation in a two-stage continuous process with alternative pH controlling approaches

This study focuses on the exploitation of cheese whey as a source for hydrogen and methane, in a two-stage continuous process. Mesophilic fermentative hydrogen production from undiluted cheese whey was investigated at a hydraulic retention time (HRT) of 24 h. Alkalinity addition (NaHCO₃) or an automatic pH controller were used, to maintain the pH culture at a constant value of 5.2. The hydrogen production rate was 2.9 ± 0.2 L/Lreactor/d, while the yield of hydrogen produced was approximately 0.78 ± 0.05 mol H₂/mol glucose consumed, with alkalinity addition, while the respective values when using pH control were 1.9 ± 0.1 L/Lreactor/d and 0.61 ± 0.04 mol H₂/mol glucose consumed. The corresponding yields of hydrogen produced were 2.9 L of H₂/L cheese whey and 1.9 L of H₂/L cheese whey, respectively. The effluent from the hydrogenogenic reactor was further digested to biogas in a continuous mesophilic anaerobic bioreactor. The anaerobic digester was operated at an HRT of 20d and produced approximately 1 L CH₄/d, corresponding to a yield of 6.7 L CH₄/L of influent. The chemical oxygen demand (COD) elimination reached 95.3% demonstrating that cheese whey could be efficiently used for hydrogen and methane production, in a two-stage process.     

Evaluation of bioenergy recovery processes treating organic residues from ethanol fermentation process

This study evaluates a two-stage bioprocess for recovering bioenergy in the forms of hydrogen and methane while treating organic residues of ethanol fermentation from tapioca starch. A maximum hydrogen production rate of 0.77 mmol H₂/g VSS/h can be achieved at volumetric loading rate (VLR) of 56 kg COD/m³/day. Batch results indicate that controlling conditions at S₀/X₀ = 12 with X₀ = 4000 mg VSS/L and pH 5.5-6 are important for efficient hydrogen production from fermentation residues. Hydrogen-producing bacteria enriched in the hydrogen bioreactor are likely utilizing lactate and acetate for biohydrogen production from ethanol-fermentation residues. Organic residues remained in the effluent of hydrogen bioreactor can be effectively converted to methane with a rate of 0.37 mmol CH₄/g VSS/h at VLR of 8 kg COD/m³/day. Approximately 90% of COD in ethanol-fermentation residues can be removed and among that 2% and 85.1% of COD can be recovered in the forms of hydrogen and methane, respectively.     

Optimization of food waste hydrolysis in leach bed coupled with methanogenic reactor: effect of pH and bulking agent

The effects of pH and bulking agents on hydrolysis/acidogenesis of food waste were studied using leach bed reactor (LBR) coupled with methanogenic up-flow anaerobic sludge blanket (UASB) reactor. The hydrolysis rate under regulated pH (6.0) was studied and compared with unregulated one during initial experiment. Then, the efficacies of five different bulking agents, i.e. plastic full particles, plastic hollow sphere, bottom ash, wood chip and saw dust were experimented under the regulated pH condition. Leachate recirculation with 50% water replacement was practiced throughout the experiment. Results proved that the daily leachate recirculation with pH control (6.0) accelerated the hydrolysis rate (59% higher volatile fatty acids) and methane production (up to 88%) compared to that of control without pH control. Furthermore, bottom ash improved the reactor alkalinity, which internally buffered the system that improved the methane production rate (0.182 l CH₄/g VS_added) than other bulking agents.     

Effect of pig manure to grass silage ratio on methane production in batch anaerobic co-digestion of concentrated pig manure and grass silage

Anaerobic co-digestion of concentrated pig manure (PM) with grass silage (GS) at five different PM to GS volatile solid (VS) ratios of 1:0, 3:1, 1:1, 1:3 and 0:1 was evaluated by examining operation stability and methane (CH₄) production potentials. The highest specific CH₄ yields were 304.2 and 302.8 ml CH₄/g VS at PM to GS ratios of 3:1 and 1:1, respectively. The digestion systems failed at the ratio of 0:1. The lag phase lasted 29.5, 28.1, 24.6 and 21.3 days at the ratios of 1:0, 3:1, 1:1 and 1:3, respectively. The daily methane yield was linearly correlated with the acetic acid concentration, indicating methane production was probably associated with acetoclastic methanogenesis. The hydrolysis constant linearly decreased with increasing the fraction of GS in the feedstock. This study recommends applying the PM to GS ratio of 1:1 in practice due to a high specific methane yield and a short lag phase.     

Effect of pH on phase separated anaerobic digestion

A pilot scale experiment was performed for a year to develop a two-phase anaerobic process for piggery wastewater treatment (COD: 6,000 mg/L, BOD: 4,000 mg/L, SS: 500 gm/L, pH 8.4, alkalinity 6,000 mg/L). The acidogenic reactor had a total volume of 3 m³, and the methanogenic reactor, an, anaerobic up-flow sludge filter, combining a filter and a sludge bed, was also of total volume 3 m³ (1.5 m³ of upper packing material). Temperatures of the acidogenic and methanogenic reactors kept at 20°C and 35°C., respectively. When the pH of the acidogenic reactor was controlled at 6.0–7.0 with HCl, the COD removal efficiency increased from 50 to 80% over a period of six months, and as a result, the COD of the final effluent fell in the range of 1,000–1,500 mg/L. BOD removal efficiency over the same period was above 90%, and 300 to 400 mg/L was maintained in the final effluent. The average SS in the final effluent was 270 mg/L. The methane production was 0.32 m³ CH₄/kg COD_removed and methane content of the methanogenic reactor was high value at 80–90%., When the pH of the acidogenic reactor was not controlled over the final two months, the pH reached 8.2 and acid conversion decreased compared with that of pH controlled, while COD removal was similar to the pH controlled operation. Without pH control, the methane content in the gas from methanogenic reactor improved to 90%, compared to 80% with pH control.     

Effect of the solid content on anaerobic digestion of meat and bone meal

The effect of the solid content on anaerobic digestion of meat and bone meal (MBM) was investigated in batch reactors at MBM solid contents of 1%, 2%, 5% and 10%. There was no significant difference in the specific methane (CH₄) production potential with respect to the total volatile MBM solids (TVS) applied at these solid contents, which ranged from 351 to 381 ml CH₄/g TVS. However, the highest CH₄ yield with respect to the removed volatile MBM solids (RVS) was 482 ml CH₄/g RVS at the MBM solid content of 5%; the CH₄ yields were 384–448 ml CH₄/g RVS at the other MBM solid contents. The lag time of CH₄ production rose with the increase in the solid content. The longer lag time at MBM solid contents of 5% and 10% was due to inhibition caused by high concentrations of volatile fatty acids (VFAs) and free ammonia in the reactors, but the inhibition was reversible. The production of VFAs during the digestion varied with solid contents: at the solid content of 1%, only acetic acid was detected; at 2%, both acetic and propionic acids were detected; and at 5% and 10%, acetic, propionic, butyric and valeric acids were detected. After 93-day digestion, the volatile MBM solid reduction was 92%, 91%, 79% and 80% at MBM solid contents of 1%, 2%, 5% and 10%, respectively.     

Performance and spatial community succession of an anaerobic baffled reactor treating acetone-butanol-ethanol fermentation wastewater

An anaerobic baffled reactor with four compartments (C1-C4) was successfully used for treatment of acetone-butanol-ethanol fermentation wastewater and methane production. The chemical oxygen demand (COD) removal efficiency was 88.2% with a CH₄ yield of 0.25 L/(g COD_removed) when organic loading rate (OLR) was 5.4 kg COD m⁻³ d⁻¹. C1 played the most important role in solvents (acetone, butanol and ethanol) and COD removal. Community structure of C2 was similar to that in C1 at stage 3 with higher OLR, but was similar to those in C3 and C4 at stages 1-2 with lower OLR. This community variation in C2 was consistent with its increased role in COD and solvent removal at stage 3. During community succession from C1 to C4 at stage 3, abundance of Firmicutes (especially OTUs ABRB07 and ABRB10) and Methanoculleus decreased, while Bacteroidetes and Methanocorpusculum became dominant. Thus, ABRB07 coupled with Methanoculleus and/or acetogen (ABRB10) may be key species for solvents degradation.     

Comparative study of the effect of ultrasonication on the anaerobic biodegradability of food waste in single and two-stage systems

Five different mesophilic systems were evaluated in this study for the anaerobic treatment of food waste. Systems A and B were one stage methane with unsonicated and sonicated feeds, respectively, while, systems C and D were two-stage hydrogen and methane with unsonicated and sonicated feeds, respectively. System E comprised a novel sonicated biological hydrogen reactor (SBHR) followed by methane reactor. The results showed that sonication inside the reactor in the first stage (system E) showed superior results compared to all other systems. Overall VSS removal efficiencies of 67%, 59%, 51%, 44%, and 36% were achieved in systems E, D, C, B, and A, respectively. Volumetric hydrogen production rates of 4.8, 3.3, and 2.6 L H₂/L_reactor d were achieved in the SBHR, CSTR with and without sonicated feed, respectively, while, methane production rates of 1.6, 2.1, 2.3, 2.6, and 3.2 L CH₄/L_reactor d were achieved in systems A-E, respectively.     

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.     

Fermentative hydrogen production from Laminaria japonica and optimization of thermal pretreatment conditions

As a sustainable biofuel feedstock, marine algae have superior aspects to terrestrial biomass such as less energy and water requirement for cultivation, higher CO₂ capture capacity, and negligible lignin content. In this study, various marine algae were tested for fermentative hydrogen production (FHP). Among them, Laminaria japonica exhibited the best performance, showing the highest H₂ yield of 69.1 mL H₂/g COD_added. It was attributed to its high carbohydrate content and main constituents of polysaccharides, laminarin and alginate, which were found to posses higher H₂ production potential than agar and carrageenan. To enhance the H₂ production from L. japonica, thermal pretreatment was applied at various conditions. At 170 °C and 20 min, H₂ yield was maximized to 109.6 mL H₂/g COD_added. The experimental results suggested that marine algae, especially L. japonica, could be used for FHP, and future works would be focused on gaining more energy from the H₂ fermentation effluent.     

Biogas production from potato-juice, a by-product from potato-starch processing, in upflow anaerobic sludge blanket (UASB) and expanded granular sludge bed (EGSB) reactors

In this study, the utilization of potato-juice, the organic by-product from potato-starch processing, for biogas production was investigated in batch assay and in high rate anaerobic reactors. The maximum methane potential of the potato-juice determined by batch assay was 470 mL-CH₄/gVS-added. Anaerobic digestion of potato-juice in an EGSB reactor could obtain a methane yield of 380 mL-CH₄/gVS-added at the organic loading rate of 3.2 gCOD/(L-reactor.d). In a UASB reactor, higher organic loading rate of 5.1 gCOD/(L-reactor.d) could be tolerated, however, it resulted in a lower methane yield of 240 mL-CH₄/gVS-added. The treatment of reactor effluent was also investigated. By acidification with sulfuric acid to pH lower than 5, almost 100% of the ammonia content in the effluent could be retained during the successive up-concentration process step. The reactor effluent could be up-concentrated by evaporation to minimize its volume, and later be utilized as fertilizer.     

Hydrogen and methane production through two-stage mesophilic anaerobic digestion of olive pulp

The present study focused on the anaerobic biohydrogen production from olive pulp (two phase olive mill wastes, TPOMW) and the subsequent anaerobic treatment of the effluent for methane production under mesophilic conditions in a two-stage process. Biohydrogen production from water-diluted (1:4) olive pulp was investigated at hydraulic retention times (HRT) of 30 h, 14.5 h and 7.5 h while methane production from the effluent of hydrogenogenic reactor was studied at 20 d, 15 d, 10 d and 5 d HRT. In comparison with previous studies, it has been shown that the thermophilic hydrogen production process was more efficient than the mesophilic one in both hydrogen production rate and yield. The methanogenic reactor was successfully operated at 20, 15 and 10 days HRT while it failed when an HRT of 5 days was applied. Methane productivity reached the maximum value of 1.13 ± 0.08 L/L/d at 10 days HRT whereas the methane yield increased with the HRT. The Anaerobic Digestion Model no. 1 (ADM1) was applied to the obtained experimental data from the methanogenic reactor to simulate the digester response at all HRT tested. The ability of the model to predict the experimental results was evident even in the case of the process failure, thus implying that the ADM1 could be a valuable tool for process design even in the case of a complex feedstock. In general, the two-stage anaerobic digestion proved to be a stable, reliable and effective process for energy recovery and stabilization treatment of olive pulp.     

Methane emissions from a full-scale A/A/O wastewater treatment plant

Methane (CH₄) emissions from a full-scale anaerobic/anoxic/oxic (A/A/O) wastewater treatment plant (WWTP) (Jinan, China) were investigated during spring and summer of 2010. Results showed that the major emission sources of CH₄ performed the following descending order: anaerobic tanks, oxic tanks, aerated grit chambers and sludge concentration tanks. The total annual fluxes of CH₄ emissions from the Jinan WWTP were 1.69 × 10⁴ kg yr⁻¹, with the emission factors of per capita emissions of 11.3 g CH₄ person⁻¹ yr⁻¹ and flow-based emissions of 1.55 × 10⁻⁴ g CH₄ (L of wastewater)⁻¹. The estimated source strength of methane for all WWTPs in China was 6.2 Gg yr⁻¹ (1 Gg = 10⁹ g). The most significant factors influencing methane emissions were dissolved oxygen concentration in aerated grit chamber and oxic tank and water temperature in high density settler tanks.     

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.     

Effects of nitrate on methane production, fermentation, and microbial populations in in vitro ruminal cultures

The objective of this study was to examine the effects of nitrate on methane production, important fermentation characteristics, Fibrobacter succinogenes, Ruminococcus albus, Ruminococcus flavefaciens, total bacteria, and methanogens using in vitro ruminal cultures. Potential adaptation of the above microbes and persistency of nitrate to mitigate CH₄ production were also evaluated. Methane production was reduced by 70% at 12 μmol ml⁻¹ and nearly completely at ?24 μmol ml⁻¹ nitrate. Production of volatile fatty acids (VFAs) was affected to different extents at different nitrate concentrations. Over a series of six consecutive cultures receiving 12 μmol ml⁻¹nitrate, production of CH₄ and VFA did not change significantly. R. albus and R. flavefaciens seemed to adapt to nitrate, while F. succinogenes and methanogens did not. Nitrate may be used in achieving persistent mitigation of CH4 production by ruminants.     

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.     

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.