Improvement of activated sludge stabilisation and filterability during anaerobic digestion by fruit and vegetable waste addition

Anaerobic co-digestion of fruit and vegetable waste (FVW) and activated sludge (AS) was investigated using anaerobic sequencing batch reactors (ASBRs). The effects of AS:FVW ratio and the organic loading rate (OLR) on digesters performances were examined. The mixtures having AS:FVW ratios of 100:00, 65:35, 35:65, by a total solid (TS) basis were operated at an hydraulic retention time (HRT) of 20d. However, 30:70, 20:80, 15:85, 10:90 and 0:100 ratios were tested at an HRT of 10d. To investigate effects of aerobic and anaerobic digestion on the sludge filterability, specific resistance to filtration (R) was also determined. Increasing FVW proportions in the feedstock significantly improved the biogas production yield. The reactor that was fed with a 30:70 ratio showed the highest VS removal and biogas production yield of 88% and 0.57 L g(-1) VS added, respectively. The filterability results showed that the anaerobic effluent was characterised by a slightly better filterability efficiency of 1.6 x 10(16) m kg(-1) than 1.74 x 10(16) m kg(-1) of aerobic effluent. However, FVW addition improved the anaerobic co-digestion effluent filterability (5.52 x 10(14) m kg(-1)).     

Effect of temperature on the performance of an anaerobic tubular reactor treating fruit and vegetable waste

This study compares the performance of anaerobic digestion of fruit and vegetable waste (FVW) in the thermophilic (55 °C) process with those under psychrophilic (20 °C) and mesophilic (35 °C) conditions in a tubular anaerobic digesters on a laboratory scale. The hydraulic retention time (HRT) ranged from 10 to 20 days, and raw fruit and vegetable waste was supplied in a semi-continuous mode at various concentrations of total solids (TS) (4, 6, 8 and 10% on dry weight). Biogas production from the experimental thermophilic digester was higher on average than from psychrophilic and mesophilic digesters by 144 and 41%, respectively. The net energy production in the thermophilic digester was 195.7 and 49.07 kJ per day higher than that for the psychrophilic and mesophilic digesters, respectively. The relation between the daily production of biogas and the temperature indicates that for the same produced quantity of biogas, the size of the thermophilic digester can be reduced with regard to that of the psychrophilic and the mesophilic digesters.     

Anaerobic digestion of solid slaughterhouse waste (SHW) at laboratory scale: Influence of co-digestion with the organic fraction of municipal solid waste (OFMSW)

Mesophilic anaerobic digestion of slaughterhouse waste (SHW) and its co-digestion with the organic fraction of municipal solid waste (OFMSW) have been evaluated. These processes were carried out in a laboratory plant semi-continuously operated and two set-ups were run. The first set-up, with a hydraulic retention time (HRT) of 25 days and organic loading rate (OLR) of 1.70 kg VS m⁻³ day⁻¹ for digestion, and 3.70 kg VS m⁻³ day⁻¹ for co-digestion, was not successful. The second set-up was initiated with an HRT of 50 days and an OLR of 0.9 kg VS m⁻³ day⁻¹ for digestion and 1.85 kg VS m⁻³ day⁻¹ for co-digestion. Under these conditions, once the sludge had been acclimated to a medium with a high fat and ammonia content, it was possible to decrease the HRT while progressively increasing the OLR to the values used in the first set-up until an HRT of 25 days and OLRs of 1.70 and 3.70 kg VS m⁻³ day⁻¹, for digestion and co-digestion, respectively (the same conditions of the digesters failures previously). These digesters showed a highly stable performance, volatile fatty acids (VFAs) were not detected and long chain fatty acids (LCFAs) were undetected or only trace levels were measured in the analyzed effluent. Fat removal reached values of up to 83%. Anaerobic digestion was thus found to be a suitable technology for efficiently treating lipid and protein waste.     

Robustness of archaeal populations in anaerobic co-digestion of dairy and poultry wastes

The objective of this study is to investigate the responses of methanogen populations to poultry waste addition by comparing the archaeal microbial populations in continuous anaerobic digesters with or without the addition of poultry waste as a co-substrate. Poultry waste was characterized as an organic/nitrogen-rich substrate for anaerobic digestion. Supplementing dilute dairy waste with poultry waste for anaerobic co-digestion to increase organic loading rate by 50% resulted in improved biogas production. Elevated ammonia derived from poultry waste did not lead to process inhibition at the organic loadings tested, demonstrating the feasibility of the anaerobic co-digestion of dairy and poultry wastes for improved treatment efficiency. The stability of the anaerobic co-digestion process was linked to the robust archaeal microbial community, which remained mostly unchanged in community structure following increases in organic loading and ammonia levels. Surprisingly, Crenarchaeota archaeal populations, instead of the Euryarchaeota methanogens, dominated the archaeal communities in the anaerobic digesters. The ecological functions of these abundant non-methanogen archaeal populations in anaerobic digestion remain to be identified.     

Degradation of aflatoxin B1 during anaerobic digestion and its effect on process stability

The effect of aflatoxin B1 (AFB1) on an anaerobic digestion process (AD) was studied. Batch anaerobic digestion trials were performed with both non-contaminated AFB1 corn grain (Control A) and contaminated-AFB1 corn grain at different doses (AFB1 contents of 0.54, 66.2, and 110 μg kg⁻¹ wet weight). Both cumulative biogas production and the degradation rate of AFB1 were studied. Results indicated that no adverse effects on AD were detected during the processes which could be attributed to the presence of AFB1. AFB1 degradation ranged from 69% to 87% of the total initial AFB1 content.Anaerobic digestion trials using Completely Stirred Tank Reactors (CSTR) were also carried out, comparing the biogas production of a mix of contaminated corn grain plus pig slurry (AFB1 content of 7.2 μg kg⁻¹ wet weight) with a mix of non-contaminated corn grain plus pig slurry (Control B). No adverse effect of AFB1 on biogas production was detected. The CSTR trial resulted in an average degradation of AFB1 of 42%. The further storage of the digestate for 30 days resulted in an overall degradation (CSTR plus storage) of AFB1 of 61% of the starting content.     

Biodegradability and mesophilic co-digestion of municipal sludge and scum

The objective of this study is to investigate and optimize the co-digestion of scum with thickened waste activated sludge (TWAS) and primary sludge (PS) undergoing mesophilic anaerobic digestion. The effect of scum loading on the co-digestion of PS, TWAS and scum has shown to have a significant impact on the ultimate cumulative biogas production and on the specific biogas production between 20 and 40 days of digestion, while the effects of the scum holding time within the scum concentrator and temperature of the scum concentrator did not demonstrate a significant effect on the ultimate or specific biogas production. The study demonstrates that care must be taken to avoid inhibitory effects and potential souring of digesters due to scum overloading and specifically scum overloading in combination with long holding times of scum within the scum concentrator at elevated temperatures.     

Effect of mixing on biogas production during mesophilic anaerobic digestion of screened dairy manure in a pilot plant

The effect of mixing on biogas production of a 1.5‐m³ pilot continuous stirred tank reactor (CSTR) processing screened dairy manure was evaluated. Mixing was carried out by recirculation of reactor content with a mono pump. The experiment was conducted at a controlled temperature of 37±1°C and hydraulic retention times (HRTs) of 20 and 10 days. The effect of continuous and intermittent operation of the recirculation pump on biogas production was studied. At 10 days of HRT, the results showed a minimal influence of recirculation rate on biogas production and that continuous recirculation did not improve reactor performance. At 20 days of HRT, the recirculation rate did not affect reactor performance. Combination of low solid content in feed animal slurry and long HRTs results in minimal mixing requirements for anaerobic digestion.     

The use of ultrasound and gamma-irradiation as pre-treatments for the anaerobic digestion of waste activated sludge at mesophilic and thermophilic temperatures

The effect of ultrasound and gamma-irradiation used as pre-treatments for the anaerobic digestion of waste activated sludge at both mesophilic and thermophilic temperatures was examined. Untreated activated sludge was also subjected to anaerobic digestion at these temperatures as a control. The sonication time was 90 s using a Soniprep 150 (MSE Scientific Instruments) which operated at 23 kHz and had been adjusted to give an output of 47 W and the gamma-irradiation dose was 500 krad. The digesters were operated in a semi-continuous mode, being fed with fresh sludge every 24 h at hydraulic retention times (HRT) of 8, 10 and 12 days. Over the 24 h period the differences between the digesters, in terms of volatile solids (VS) reductions and biogas production, were not statistically significant for any particular set of conditions. Thermophilic digestion performed better than mesophilic digestion in terms of biogas production, VS reductions (except at HRT of 8 days) and specific methane yields and the optimum retention time was 10 days, at both temperatures. When gas production over the initial eight hours (probably the hydrolytic stage) was examined, it was found that the gas production rates for pre-treated sludges were higher than those for untreated sludges. This was most pronounced at thermophilic temperatures and a HRT of 10 days. Sonication did not affect the numbers of faecal coliforms in the sludge. However, gamma-radiation caused a 3-log reduction and, when coupled with mesophilic digestion, gave a product which contained < 100 g(-1) TS. Thermophilic anaerobic digestion produced sludges which contained < 1 g(-1) TS irrespective of any pre-treatment.     

A half-submerged integrated two-phase anaerobic reactor for agricultural solid waste codigestion

Anaerobic digestion is widely used in bioenergy recovery from waste. In this study, a half-submerged, integrated, two-phase anaerobic reactor consisting of a top roller acting as an acidogenic unit and a recycling bottom reactor acting as a methanogenic unit was developed for the codigestion of wheat straw (WS) and fruit/vegetable waste (FVW). The reactor was operated for 21 batches (nearly 300 d). Anaerobic granular sludge was inoculated into the methanogenic unit. The residence time for the mixed waste was maintained as 10 d when the operation stabilized, and the temperature was kept at 35 °C. The highest organic loading rate was 1.37 kg VS/(m³ d), and the maximum daily biogas production was 328 L/d. Volatile solid removal efficiencies exceeded 85%. WS digestion could be confirmed, and efficiency was affected by both the ratio of WS to FVW and the loading rate. The dominant bacteria were Bacteroides-like species, which are involved in glycan and cellulose decomposition. Methanogenic community structures, pH levels, and volatile fatty acid concentrations in the acidogenic and methanogenic units differed, indicating successful phase separation. This novel reactor can improve the mass transfer and microbial cooperation between acidogenic and methanogenic units and can efficiently and steady codigest solid waste.     

Anaerobic digestion of secondary residuals from an anaerobic bioreactor at a brewery to enhance bioenergy generation

Many beer breweries use high-rate anaerobic digestion (AD) systems to treat their soluble high-strength wastewater. Biogas from these AD systems is used to offset nonrenewable energy utilization in the brewery. With increasing nonrenewable energy costs, interest has mounted to also digest secondary residuals from the high-rate digester effluent, which consists of yeast cells, bacteria, methanogens, and small (hemi)cellulosic particles. Mesophilic (37 °C) and thermophilic (55 °C) lab-scale, low-rate continuously-stirred anaerobic digestion (CSAD) bioreactors were operated for 258 days by feeding secondary residuals at a volatile solids (VS) concentration of ∼40 g l⁻¹. At a hydraulic retention time (HRT) of 15 days and a VS loading rate of 2.7 g VS l⁻¹ day⁻¹, the mesophilic bioreactor showed an average specific volumetric biogas production rate of 0.88 l CH₄ l⁻¹ day⁻¹ and an effluent VS concentration of 22.2 g VS l⁻¹ (43.0% VS removal efficiency) while the thermophilic bioreactor displayed similar performances. The overall methane yield for both systems was 0.21 l CH₄ g⁻¹ VS fed and 0.47–0.48 l CH₄ g⁻¹ VS removed. A primary limitation of thermophilic digestion of this protein-rich waste is the inhibition of methanogens due to higher nondissociated (free) ammonia (NH₃) concentrations under similar total ammonium (NH₄ ⁺) concentrations at equilibrium. Since thermophilic AD did not result in advantageous methane production rates or yields, mesophilic AD was, therefore, superior in treating secondary residuals from high-rate AD effluent. An additional digester to convert secondary residuals to methane may increase the total biogas generation at the brewery by 8% compared to just conventional high-rate digestion of brewery wastewater alone. JIMB-2008: BioEnergy—Special issue.     

Alkalinity ratios to identify process imbalances in anaerobic digesters treating source-sorted organic fraction of municipal wastes

Two 5 L anaerobic reactors were used to monitor the mesophilic anaerobic digestion of source sorted organic fraction of municipal solid wastes (SS-OFMSW) focusing the attention on the response of alkalinity ratios. Intermediate/partial alkalinity (IA/PA) ratio can be used as a simple and cheaper alternative to VFAs analysis when digester's stability needs to be assessed in full-scale plants treating these organic wastes. However, lab-scale studies in order to establish a specific limit value of IA/PA referred to SS-OFMSW had not been conducted. In this study, a reference reactor (R1) was operated at low organic loading rates (OLR) and high hydraulic retention times (HRT) during 165 days. Besides, severe disturbances were applied to a second reactor (R2) during 281 days by means of increasing both HRT and OLR in order to assess the digester response under continuous overload conditions. The obtained results show that an IA/PA ratio of below 0.3 is recommended to maintain total VFAs between 2.5 and 3.5 kg m⁻³ and achieve a stable reactor performance treating SS-OFMSW in a range of total alkalinity (TA) between 13 and 15 kg CaCO₃ m⁻³. These results provide a starting point to develop further works in full-scale digesters, in order to improve the monitoring and process control of full-scale anaerobic reactors treating SS-OFMSW.     

Emergy analysis of biogas production and electricity generation from small-scale agricultural digesters

An emergy analysis was performed to assess the relative sustainability and environmental impact of small-scale energy production using Taiwanese model plug-flow anaerobic digesters to treat livestock manure in Costa Rica. Emergy analysis quantifies all inputs to a system by converting them to solar energy equivalents, thus allowing for direct comparison of the diverse inputs of renewable energies, human labor and economic goods needed to construct and maintain anaerobic digestion systems. The digesters were located on the campus of EARTH University, Costa Rica, and the biogas was utilized to power a 40 kW generator that supplies electricity for farm operations. Separate emergy analyses were performed for the biogas production and the combination of biogas production and generation of electricity. Manure was the largest input in both analyses, accounting for 85.3% of the annual emergy input for biogas production and 66.9% for electricity generation from the biogas. The fraction of emergy inputs from renewable sources (ΦR) was 66% for biogas production and 52% for electricity generation from the biogas. The transformities of biogas and electricity generation from the biogas were 5.23E+04 sej/J and 1.01E+06 sej/J respectively. The emergy yield ratios (EYR) were 2.93 for biogas production and 2.07 for electricity generation indicating that these digesters efficiently match purchased resources and renewable energies to produce energy from livestock manure. The generation of electricity from the biogas resulted in a decrease in the emergy sustainability index (ESI) from 5.67 to 2.22 and an increase in the environmental loading ratio (ELR) from 0.52 to 0.93. Using a generator to convert the biogas to electricity does decrease the sustainability of the system, largely due to the high emergy value associated with the electrical generation equipment and machinery, but these results demonstrate that the production of biogas and the generation of electricity from Taiwanese model digesters in Costa Rica are environmentally sustainable processes that result in the production of energy that is largely dependent on renewable and recycled energies.     

Quantifying electricity generation and waste transformations in a low-cost, plug-flow anaerobic digestion system

Methane production, electricity production, and wastewater transformations were quantified for a digestion system that combines biogas from a swine digester and dairy digester in Costa Rica. The low-cost, plug-flow digesters were not heated and were operated in the lower portion of the mesophilic range (25–27 °C).The dairy digester produced 27.5 m³/day of biogas with 62.6% methane and reduced organic matter (COD) by 86%. The swine digester produced 6.0 m³/day of biogas with 76.4% methane and reduced COD by 92%. Combining biogas from a swine and dairy digester, increased electricity production due to the higher biogas production rate of the dairy farm and the higher quality biogas obtained from the swine farm. The farm’s 2-h peak electricity demand (12.9 kW/day) was 81.8% met. The electricity was produced using manure equivalent to the quantity excreted by 5 dairy cows and 40 pigs remaining in corrals 100% of the time.The $21,000 capital cost of the digester project will be recovered in 10.1 years through electricity savings and reductions in wastewater fines. If the generator were more appropriately sized for the farm, the capital recovery time would have been 7.6 years.     

The effect of temperature and retention time on methane production and microbial community composition in staged anaerobic digesters fed with food waste

Background

Food waste is a large bio-resource that may be converted to biogas that can be used for heat and power production, or as transport fuel. We studied the anaerobic digestion of food waste in a staged digestion system consisting of separate acidogenic and methanogenic reactor vessels. Two anaerobic digestion parameters were investigated. First, we tested the effect of 55 vs. 65 °C acidogenic reactor temperature, and second, we examined the effect of reducing the hydraulic retention time (HRT) from 17 to 10 days in the methanogenic reactor. Process parameters including biogas production were monitored, and the microbial community composition was characterized by 16S amplicon sequencing.

Results

Neither organic matter removal nor methane production were significantly different for the 55 and 65 °C systems, despite the higher acetate and butyrate concentrations observed in the 65 °C acidogenic reactor. Ammonium levels in the methanogenic reactors were about 950 mg/L NH₄ ⁺ when HRT was 17 days but were reduced to 550 mg/L NH₄ ⁺ at 10 days HRT. Methane production increased from ~ 3600 mL/day to ~ 7800 when the HRT was decreased. Each reactor had unique environmental parameters and a correspondingly unique microbial community. In fact, the distinct values in each reactor for just two parameters, pH and ammonium concentration, recapitulate the separation seen in microbial community composition. The thermophilic and mesophilic digesters were particularly distinct from one another. The 55 °C acidogenic reactor was mainly dominated by Thermoanaerobacterium and Ruminococcus, whereas the 65 °C acidogenic reactor was initially dominated by Thermoanaerobacterium but later was overtaken by Coprothermobacter. The acidogenic reactors were lower in diversity (34–101 observed OTU_0.97, 1.3–2.5 Shannon) compared to the methanogenic reactors (472–513 observed OTU_0.97, 5.1–5.6 Shannon). The microbial communities in the acidogenic reactors were > 90% Firmicutes, and the Euryarchaeota were higher in relative abundance in the methanogenic reactors.

Conclusions

The digestion systems had similar biogas production and COD removal rates, and hence differences in temperature, NH₄ ⁺ concentration, and pH in the reactors resulted in distinct but similarly functioning microbial communities over this range of operating parameters. Consequently, one could reduce operational costs by lowering both the hydrolysis temperature from 65 to 55 °C and the HRT from 17 to 10 days.

     

Pretreatment of pulp mill secondary sludge for high-rate anaerobic conversion to biogas

Three pretreatment methods were compared based on their ability to increase the extent and rate of anaerobic bioconversion of pulp mill secondary sludge to biogas. The pretreatment technologies used in these experiments were: (i) thermal pretreatment performed at 170 °C; (ii) thermochemical (caustic) pretreatment performed at pH 12 and 140 °C; and (iii) sonication performed at 20 kHz and 1 W mL⁻¹. Sludge samples were obtained from a sulfite and a kraft pulp mill, and biochemical methane potential (BMP) assays were performed using microbial granules obtained from a high-rate anaerobic digester operating at a pulp mill. Biogas production from untreated sludge was 0.05 mL mg⁻¹ of measured chemical oxygen demand (COD) and 0.20 mL mg⁻¹ COD for kraft and sulfite sludge, respectively. Thermal pretreatment had the highest impact on sludge biodegradability. In this case, biogas yield and production rate from sulfite sludge increased by 50% and 10 times, respectively, while biogas yield and production rate from kraft sludge increased by 280% and 300 times, respectively. Biogas yield correlated to soluble carbohydrate content better than soluble COD.     

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.     

Anaerobic co-digestion of desugared molasses with cow manure; focusing on sodium and potassium inhibition

Desugared molasses (DM), a syrup residue from beet-molasses, was investigated for biogas production in both batch and in continuously-stirred tank reactor (CSTR) experiments. DM contained 2-3 times higher concentration of ions than normal molasses, which could inhibit the biogas process. The effect of sodium and potassium concentration on biogas production from manure was also investigated. Fifty percent inhibition occurred at sodium and potassium concentration of 11 and 28 g/L, respectively. The reactor experiments were carried out to investigate the biogas production from DM under different dilutions with water and co-digestion with manure. Stable operation at maximum methane yield of 300 mL-CH₄/gVS-added was obtained at a mixture of 5% DM in cow manure. The biogas process was inhibited at DM concentrations higher than 15%. Manure was a good base substrate for co-digestion, and a stable anaerobic digestion could be achieved by co-digesting DM with manure at the concentration below 15% DM.     

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.     

Mesophilic biogas production from fruit and vegetable waste in a tubular digester

A semi-continuously mixed mesophilic tubular anaerobic digester was tested for the conversion of fruit and vegetable waste (FVW) into biogas. The effect of hydraulic retention time (HRT) and the feed concentration on the extent of the degradation of the waste was examined. Varying the HRT between 12 and 20 days had no effect on the fermentation stability and pH remained between 6.8 and 7.6, but an inhibition of methanogenic bacteria was observed at HRT below 12 days. The overall performance of the reactor was depressed by changing the feed concentration from 8% to 10% TS (dry weight). By applying a feed concentration of 6% and HRT of 20 days in the tubular digester, 75% conversion efficiency of FVW into biogas with a methane content of 64% was achieved.     

Anaerobic co-digestion of food waste and piggery wastewater: focusing on the role of trace elements

The objective of this study was to evaluate the feasibility of anaerobic co-digestion of food waste and piggery wastewater, and to identify the key factors governing the co-digestion performance. The analytical results indicated that the food waste contained higher energy potential and lower concentrations of trace elements than the piggery wastewater. Anaerobic co-digestion showed a significantly improved biogas productivity and process stability. The results of co-digestion of the food waste with the different fractions of the piggery wastewater suggested that trace element might be the reason for enhancing the co-digestion performance. By supplementing the trace elements, a long-term anaerobic digestion of the food waste only resulted in a high methane yield of 0.396 m³/kg VS_added and 75.6% of VS destruction with no significant volatile fatty acid accumulation. These results suggested that the typical Korean food waste was deficient with some trace elements required for anaerobic digestion.