Anaerobic batch co-digestion of sisal pulp and fish wastes

Co-digestion of various wastes has been shown to improve the digestibility of the materials and biogas yield. Batchwise digestion of sisal pulp and fish waste was studied both with the wastes separately and with mixtures in various proportions. While the highest methane yields from sisal pulp and fish waste alone were 0.32 and 0.39 m3 CH4/kg volatile solids (VS), respectively, at total solid (TS) of 5%, co-digestion with 33% of fish waste and 67% of sisal pulp representing 16.6% of TS gave a methane yield of 0.62 m3 CH4/kg VS added. This is an increase of 59-94% in the methane yield as compared to that obtained from the digestion of pure fractions at 5% TS.     

Thermophilic anaerobic digestion of source-sorted organic fraction of municipal solid waste

The influence of different organic fraction of municipal solid wastes during anaerobic thermophilic (55 degrees C) treatment of organic matter was studied in this work: food waste (FW), organic fraction of municipal solid waste (OFMSW) and shredded OFMSW (SH_OFMSW). All digester operated at dry conditions (20% total solids content) and were inoculated with 30% (in volume) of mesophilic digested sludge. Experimental results showed important different behaviours patterns in these wastes related with the organic matter biodegradation and biogas and methane production. The FW reactor showed the smallest waste biodegradation (32.4% VS removal) with high methane production (0.18 LCH4/gVS); in contrast the SH_OFMSW showed higher waste biodegradation (73.7% VS removal) with small methane production (0.05 LCH4/g VS). Finally, OFMSW showed the highest VS removal (79.5%) and the methane yield reached 0.08 LCH4/g VS. Therefore, the nature of organic substrate has an important influence on the biodegradation process and methane yield. Pre-treatment of waste is not necessary for OFMSW.     

Integration of on-farm biodiesel production with anaerobic digestion to maximise energy yield and greenhouse gas savings from process and farm residues

Anaerobic co-digestion of residues from the cold pressing and trans-esterification of oilseed rape (OSR) with other farm wastes was considered as a means of enhancing the sustainability of on-farm biodiesel production. The study verified the process energy yields using biochemical methane potential (BMP) tests and semi-continuous digestion trials. The results indicated that high proportions of OSR cake in the feedstock led to a decrease in volatile solids destruction and instability of the digestion process. Co-digestion with cattle slurry or with vegetable waste led to acceptable specific and volumetric methane productions, and a digestate low in potentially toxic elements (PTE). The results were used to evaluate energy balances and greenhouse gas emissions of the integrated process compared with biodiesel production alone. Co-digestion was shown to provide energy self-sufficiency and security of supply to farms, with sufficient surplus for export as fuel and electricity.     

Biological nutrients removal from the supernatant originating from the anaerobic digestion of the organic fraction of municipal solid waste

Abstract

This study critically evaluates the biological processes and techniques applied to remove nitrogen and phosphorus from the anaerobic supernatant produced from the treatment of the organic fraction of municipal solid waste (OFMSW) and from its co-digestion with other biodegradable organic waste (BOW) streams. The wide application of anaerobic digestion for the treatment of several organic waste streams results in the production of high quantities of anaerobic effluents. Such effluents are characterized by high nutrient content, because organic and particulate nitrogen and phosphorus are hydrolyzed in the anaerobic digestion process. Consequently, adequate post-treatment is required in order to comply with the existing land application and discharge legislation in the European Union countries. This may include physicochemical and biological processes, with the latter being more advantageous due to their lower cost. Nitrogen removal is accomplished through the conventional nitrification/denitrification, nitritation/denitritation and the complete autotrophic nitrogen removal process; the latter is accomplished by nitritation coupled with the anoxic ammonium oxidation process. As anaerobic digestion effluents are characterized by low COD/TKN ratio, conventional denitrification/nitrification is not an attractive option; short-cut nitrogen removal processes are more promising. Both suspended and attached growth processes have been employed to treat the anaerobic supernatant. Specifically, the sequencing batch reactor, the membrane bioreactor, the conventional activated sludge and the moving bed biofilm reactor processes have been investigated. Physicochemical phosphorus removal via struvite precipitation has been extensively examined. Enhanced biological phosphorus removal from the anaerobic supernatant can take place through the sequencing anaerobic/aerobic process. More recently, denitrifying phosphorus removal via nitrite or nitrate has been explored. The removal of phosphorus from the anaerobic supernatant of OFMSW is an interesting research topic that has not yet been explored. At the moment, standardization in the design of facilities that treat anaerobic supernatant produced from the treatment of OFMSW is still under development. To move toward this direction, it is first necessary to assess the performance of alternative treatment options. It study concentrates existing data regarding the characteristics of the anaerobic supernatant produced from the treatment of OFMSW and from their co-digestion with other BOW. This provides data documenting the effect of the anaerobic digestion operating conditions on the supernatant quality and critically evaluates alternative options for the post-treatment of the liquid fraction produced from the anaerobic digestion process.     

Production and characterization of β-glucosidase from Aspergillus niger fermentation: Application for organic fraction of municipal solid waste hydrolysis and methane enhancement

The anaerobic digestion of the organic fraction of municipal solid waste (OFMSW) is currently an attractive treatment process with energy production in the form of biogas. Hydrolysis is the rate-limiting step for the anaerobic digestion of solid wastes. Thus, in the present study fungal enzymatic pretreatment of OFMSW was applied to enhance biogas production. Two enzyme cocktails rich on β-glucosidase were produced from submerged fermentation of Aspergillus niger on basal medium using OFMSW as carbon source and urea (Urea cocktail) and Ulva rigida as nitrogen source (Ulva cocktail). Ulva cocktail displayed an important effect on OFMSW solubilization. Therefore, an increase of reducing sugar concentration about 60% was obtained which was in correlation with chemical oxygen demand (COD) increase. The performance of enzymatic pretreatment on anaerobic digestion of OFMSW was studied by conducting biochemical methane potential tests. Results showed that the enzymatic pretreatment improved methane yield of OFMSW even at high solid concentration. High methane yield about 500 ml/g total volatile solid was obtained, which corresponds up to 68% enhancement over the control.     

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.     

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.     

Application of the IWA ADM1 model to simulate anaerobic co-digestion of organic waste with waste activated sludge in mesophilic condition

Anaerobic digestion model no. 1 model of international water association was applied to a full scale anaerobic co-digestion process for the treatment of the organic fraction of municipal solid wastes along with activated sludge wastes originating from a municipal wastewater treatment plant. This operation was carried out in a digester of 2000 m(3) in volume. It is operates at an average hydraulic retention time of 26.9 days with an average organic loading rate of 1.01 kg TVS/m(3) day, at a temperature of 37 degrees C with an average gas production rate of 0.296 m(3)/m(3) day. The aim of the present study is to compare the results obtained from the simulation with the experimental values. The simulated results showed a good fit for pH, methane and carbon dioxide percentages, biogas volume, chemical oxygen demand, total volatile fatty acids, inorganic nitrogen and inorganic carbon.     

Feasibility of anaerobic co-digestion as a treatment option of meat industry wastes

The anaerobic biodegradability of meat industry wastes was investigated in mesophilic batch reactors and combined with a mathematical model for describing their biodegradable fractions. The characteristics and methane yield achieved when digesting waste sludge, suggested the use of this as co-substrate for enhancing the biodegradability of the other wastes. The co-digestion experiments showed that it would be feasible to co-digest cow manure or ruminal waste with waste sludge, but biodegradability of pig/cow slurries was not improved, being strongly influenced by the ammonium concentration of co-digestion mixture. By applying the mathematical model, it was observed that when increasing the amount of waste sludge in the co-digestion mixtures, the amount of inert and slowly biodegradable fractions decreased leading to an increase in readily biodegradable fractions, volatile solid removal efficiencies and methane yields. These results suggest that using readily biodegradable wastes as co-substrate, the anaerobic biodegradability of complex organic wastes can be improved.     

Anaerobic co-digestion of mechanically biologically treated municipal waste with primary sewage sludge - a feasibility study

This bench scale study investigated the suitability of MBT material for treatment by anaerobic digestion and the impacts of co-digestion of these wastes with sewage sludge. The results suggest that MBT material is amenable to anaerobic digestion with sewage sludge. The main problems for scale-up are related to the physical composition of the MBT material, the accumulation of heavy metals and other inert contaminants and the impact of both of these factors on final sludge quality. Full-scale trials would be required to assess the long-term impacts of MBT waste on anaerobic digestion, if this form of co-digestion were to be pursued. The material contamination that presents a barrier to the direct recycling of MBT material in land-applications is also a major hurdle in commercial co-digestion. Better quality input material would be likely to result in higher methane yields and fewer restrictions on the utilisation of the product in recycling.     

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.     

Anaerobic co-digestion of algal sludge and waste paper to produce methane

The unbalanced nutrients of algal sludge (low C/N ratio) were regarded as an important limitation factor to anaerobic digestion process. Adding high carbon content of waste paper in algal sludge feedstock to have a balanced C/N ratio was undertaken in this study. The results showed adding 50% (based on volatile solid) of waste paper in algal sludge feedstock increased the methane production rate to 1170+/-75 ml/l day, as compared to 573+/-28 ml/l day of algal sludge digestion alone, both operated at 4 g VS/l day, 35 degrees C and 10 days HRT. The maximum methane production rate of 1607+/-17 ml/l day was observed at a combined 5 g VS/l day loading rate with 60% (VS based) of paper adding in algal sludge feedstock. Results suggested an optimum C/N ratio for co-digestion of algal sludge and waste paper was in the range of 20-25/1.     

Fermentative production of hydrogen from a wheat flour industry co-product

The global flour industry produces 96 million ton/year of wheatfeed, which is mainly used for animal feed. This co-product is high in carbohydrates and potentially a significant substrate for biohydrogen production. A 10 l bioreactor, inoculated with sewage sludge, was operated on wheatfeed (10 g l(-1)) at pH 5.5 and 35 degrees C in batch and semi-continuous mode (15 h hydraulic retention time (HRT)). Wheatfeed hydrolysate was also investigated in continuous mode (15 h HRT). NaOH-H2O2 treatment of 25 g l(-1) wheatfeed resulted in hydrolysate containing on average 8.1 g l(-1) total sugar. Hydrogen yields of 64 and 56 m3 H(2) per ton dry weight were produced from wheatfeed in batch and 56 m3 H2 per ton dry weight of wheatfeed in semi-continuous mode. Hydrogen yields from hydrolysate were only 22 and 31 m3 H2 per ton dry weight, (or 0.9 mol H2 per mol hexose degraded, assuming all sugar is hexose). Fermentation of unhydrolysed wheatfeed is therefore recommended. It is calculated that approximately 264 m3/ton of CH4 can be produced from a subsequent anaerobic digestion stage. The biohydrogen produced (diesel equivalents) from the 1.2 million ton/year of wheatfeed in the UK would be more than twice that required for transportation by the UK flour industry.     

Composting potential of different inoculum sources in the modified SEBAC system treatment of municipal solid wastes

The aim of the work described here was to analyse the biomethanization process for three types of organic fraction of municipal solid waste (OFMSW) using two different inoculum sources in a sequential leach-bed anaerobic reactor under thermophilic (55 degrees C) and dry conditions (20%TS). The OFMSWs studied were: separately collected food waste (SC_OFMSW) from restaurants, synthetic waste (ST_OFMSW) and mechanically selected municipal waste (MS_OFMSW). The inoculum studied were digested mesophilic sludge (SLUDGE) and digested SC_OFMSW. The results show that SLUDGE gave the best performance and this was therefore selected for the rest of the experiments. Three assays were performed to analyse the biomethanization processes of three types of organic waste. The results suggest that all three wastes give rise to an acclimation stage with acidogenic/acetogenic activity between days 5 and 20-30 and a stabilization phase associated with methanogenic activity. In conclusion, a mixture of OFMSW (regardless of its origin) and SWINE arranged in layers in the reactor, as well as SLUDGE used with an inoculum source, enhanced the fast start up phase of a modified sequential leach-bed system under dry thermophilic conditions.     

Effects of Temperature and Carbon-Nitrogen (C/N) Ratio on the Performance of Anaerobic Co-Digestion of Dairy Manure,Chicken Manure and Rice Straw: Focusing on Ammonia Inhibition

Anaerobic digestion is a promising alternative to disposal organic waste and co-digestion of mixed organic wastes has recently attracted more interest. This study investigated the effects of temperature and carbon-nitrogen (C/N) ratio on the performance of anaerobic co-digestion of dairy manure (DM), chicken manure (CM) and rice straw (RS). We found that increased temperature improved the methane potential, but the rate was reduced from mesophilic (30∼40°C) to thermophilic conditions (50∼60°C), due to the accumulation of ammonium nitrogen and free ammonia and the occurrence of ammonia inhibition. Significant ammonia inhibition was observed with a C/N ratio of 15 at 35°C and at a C/N ratio of 20 at 55°C. The increase of C/N ratios reduced the negative effects of ammonia and maximum methane potentials were achieved with C/N ratios of 25 and 30 at 35°C and 55°C, respectively. When temperature increased, an increase was required in the feed C/N ratio, in order to reduce the risk of ammonia inhibition. Our results revealed an interactive effect between temperature and C/N on digestion performance.     

Effect of pre-aeration and inoculum on the start-up of batch thermophilic anaerobic digestion of municipal solid waste

In this study, a short pre-aeration step was investigated as pre-treatment for thermophilic anaerobic digestion of the organic fraction of municipal solid waste (OFMSW). It was found that pre-aeration of 48 h generated enough biological heat to increase the temperature of bulk OFMSW to 60 °C. This was sufficient self-heating of the bulk OFMSW for the start-up of thermophilic anaerobic digestion without the need for an external heat source. Pre-aeration also reduced excess easily degradable organic compounds in OFMSW, which were the common cause of acidification during the start-up of the batch system. Careful consideration however must be taken to avoid over aeration as this consumes substrate, which would otherwise be available to methanogens to produce biogas. To accelerate methane production and volatile solids destruction, the anaerobic digestion in this study was operated as a wet process with the anaerobic liquid recycled through the OFMSW. Appropriate anaerobic liquid inoculum was found to be particularly beneficial. It provided high buffer capacity as well as suitable microbial inoculum. As a result, acidification during start-up was kept to a minimum. With volatile fatty acids (VFAs-acetate in particular) and H₂ accumulation typical of hydrolysis and fermentation of the easily degradable substrates during start-up, inoculum with high numbers of hydrogenotrophic methanogens was critical to not only maximise CH₄ production but also reduce H₂ partial pressure in the system to allow VFAs degradation. In a lab-scale bioreactor, the combined pre-aeration and wet thermophilic anaerobic digestion was able to stabilise the OFMSW within a period of only 12 days. The stabilised inert residual material can be used as a soil amendment product.     

Biochemical methane potential and biodegradability of complex organic substrates

The biomethane potential and biodegradability of an array of substrates with highly heterogeneous characteristics, including mono- and co-digestion samples with dairy manure, was determined using the biochemical methane potential (BMP) assay. In addition, the ability of two theoretical methods to estimate the biomethane potential of substrates and the influence of biodegradability was evaluated. The results of about 175 individual BMP assays indicate that substrates rich in lipids and easily-degradable carbohydrates yield the highest methane potential, while more recalcitrant substrates with a high lignocellulosic fraction have the lowest. Co-digestion of dairy manure with easily-degradable substrates increases the specific methane yields when compared to manure-only digestion. Additionally, biomethane potential of some co-digestion mixtures suggested synergistic activity. Evaluated theoretical methods consistently over-estimated experimentally-obtained methane yields when substrate biodegradability was not accounted. Upon correcting the results of theoretical methods with observed biodegradability data, an agreement greater than 90% was achieved.     

Dry-thermophilic anaerobic digestion of organic fraction of the municipal solid waste: focusing on the inoculum sources

The effect of inoculum source on anaerobic thermophilic digestion of separately collected organic fraction of municipal solid wastes (SC_OFMSW) has been studied. Performance of laboratory scale reactors (V: 1.1 L) were evaluated using six different inoculums sources: (1) corn silage (CS); (2) restaurant waste digested mixed with rice hulls (RH_OFMSW); (3) cattle excrement (CATTLE); (4) swine excrement (SWINE); (5) digested sludge (SLUDGE); and (6) SWINE mixed with SLUDGE (1:1) (SWINE/SLUDGE). The SC_OFMSW was separately and collected from university restaurant. The selected conditions were: 25% of inoculum, 30% of total solid and 55 degrees C of temperature, optimum in the thermophilic range. The six inoculum sources showed an initial start-up phase in the range between 2 and 4 days and the initial methane generation began over 10 days operational process. Results indicated that SLUDGE is the best inoculum source for anaerobic thermophilic digestion of the treatment of organic fraction of municipal solid waste at dry conditions (30%TS). Over 60 days operating period, it was confirmed that SLUDGE reactor can achieve 44.0%COD removal efficiency and 43.0%VS removal. In stabilization phase, SLUDGE reactor showed higher volumetric biogas generated of 78.9 mL/day (or 35.6 mLCH(4)/day) reaching a methane yield of 0.53 LCH(4)/gVS. Also, SWINE/SLUDGE and SWINE were good inoculums at these experimental conditions.     

Modeling solid waste decomposition

The hydrolysis rate coefficients of sorted municipal waste were evaluated from the biochemical methane potential tests using non-linear regression. A distributed mathematical model of anaerobic digestion of rich (food) and lean (non-food) solid wastes with greatly different rates of polymer hydrolysis/acidogenesis was developed to describe the balance between the rates of hydrolysis/acidogenesis and methanogenesis. The model was calibrated using previously published experimental data [Biores. Technol. 52 (1995) 245] obtained upon various initial food waste loadings. Simulations of one- and two-stage digestion systems were carried out. The results showed that initial spatial separation of food waste and inoculum enhances methane production and waste degradation in a one-stage solid-bed digester at high waste loading. A negative effect of vigorously mixing at high waste loading reported in some papers was discussed. It was hypothesized that the initiation methanogenic centers developing in time and expanding in space under minimal mixing conditions might be a key factor for efficient anaerobic conversion of solid waste into methane.     

Methane production by anaerobic digestion of organic waste from vegetable processing facilities

The article concerns converting waste from vegetable processing facilities into methane in anaerobic reactors with a small amount of inoculum (8.4%). Anaerobic digestion of vegetable waste with a high content of organic acids and carbohydrates makes it possible to achieve a methanogenesis productivity of 273–436 L CH₄/kg of volatile solidis, which is comparable to or higher than the productivity of such reactors in the world (according to the literature). The contents of ammonia nitrogen and soluble phosphorus in the form of on undiluted substrate basis in the digested vegetable wastes ranged from 3.39 to 5.06 and from 0.78 to 1.03 g/L respectively. Thus, mineralized vegetable waste can be used as an organic fertilizer with a high nutrient content. The results show the feasibility of the technology of conversion of organic waste from vegetable processing facilities into methane and organic fertilizer in anaerobic fermenters (digesters).