Anaerobic co-digestion of organic wastes

Over the last years anaerobic digestion has been successfully established as technology to treat organic wastes. The perspective of turning, through a low-cost process, organic wastes into biogas, a source of renewable energy and profit, has certainly increased the interest around this technology and has required several studies aimed to develop methods that could improve the performance as well as the efficiency of this process. The present work reviews the most interesting results achieved through such studies, mainly focusing on the following three aspects: (1) the analysis of the organic substrates typically co-digested to exploit their complementary characteristics; (2) the need of pre-treating the substrates before their digestion in order to change their physical and/or chemical characteristics; (3) the usefulness of mathematical models simulating the anaerobic co-digestion process. In particular these studies have demonstrated that combining different organic wastes results in a substrate better balanced and assorted in terms of nutrients, pre-treatments make organic solids more accessible and degradable to microorganisms, whereas mathematical models are extremely useful to predict the co-digestion process performance and therefore can be successfully used to choose the best substrates to mix as well as the most suitable pre-treatments to be applied.     

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.     

Anaerobic bioprocessing of organic wastes

Anaerobic digestion of dissolved, suspended and solid organics has rapidly evolved in the last decades but nevertheless still faces several scientific unknowns. In this review, some fundamentals of bacterial conversions and adhesion are addressed initially. It is argued in the light of G-values of reactions, and in view of the minimum energy quantum per mol, that anaerobic syntrophs must have special survival strategies in order to support their existence: redistributing the available energy between the partners, reduced end-product fermentation reactions and special cell-to-cell physiological interactions. In terms of kinetics, it appears that both reaction rates and residual substrate thresholds are strongly related to minimum G-values. These new fundamental insights open perspectives for efficient design and operation of anaerobic bioprocesses. Subsequently, an overview is given of the current anaerobic biotechnology. For treating wastewaters, a novel and high performance new system has been introduced during the last decade; the upflow anaerobic sludge blanket system (UASB). This reactor concept requires anaerobic consortia to grow in a dense and eco-physiologically well-organized way. The microbial principles of such granular sludge growth are presented. Using a thermodynamic approach, the formation of different types of aggregates is explained. The application of this bioprocess in worldwide wastewater treatment is indicated. Due to the long retention times of the active biomass, the UASB is also suitable for the development of bacterial consortia capable of degrading xenobiotics. Operating granular sludge reactors at high upflow velocities (5–6 m/h) in expanded granular sludge bed (EGSB) systems enlarges the application field to very low strength wastewaters (chemical oxygen demand < 1 g/l) and psychrophilic temperatures (10°C). For the treatment of organic suspensions, there is currently a tendency to evolve from the conventional mesophilic continuously stirred tank system to the thermophilic configuration, as the latter permits higher conversion rates and easier sanitation. Integration of ultrafiltration in anaerobic slurry digestion facilitates operation at higher volumetric loading rates and at shorter residence times. With respect to organic solids, the recent trend in society towards source separated collection of biowaste has opened a broad range of new application areas for solid state anaerobic fermentation.W. Verstraete and D. de Beer are with the Center for Environmental Sanitation, University of Gent, Coupure L 653, B-9000 Gent, Belgium; D. de Beer is also with the Max Plank Institut für Marine Mikrobiologie-Microzensor Group, Fahrenstrasse 1, 28359 Bremen, Germany. M. Pena is with the Groupo de Biotechnologia Ambiental, Departamento de Ingenieria Quimica, Universidad de Valladolid, Prado de la Magdalena, 47005 Valladolid, Spain. G. Lettinga is with the Department of Environmental Technology, Wageningen Agricultural University, Bomenweg 2, 6703 HD Wageningen, The Netherlands. P. Lens is with the Environmental Research Unit. Department of Microbiology, University College Galway, Galway, Ireland.     

Anaerobic co-digestion of swine manure with energy crop residues

Anaerobic co-digestion involves the treatment of different substrates with the aim of improving the production of biogas and the stability of the process. In this research, co-digestion of swine manure (SM) and energy crop residues (ECRs) was studied. The mixtures evaluated contained SM combined with maize (Mz), rapeseed (Rs) or sunflower (Sf) residues. Batch and semi-continuous experiments were performed to determine methane (CH₄) yields and the behavior of reactors while co-digesting agricultural wastes. Three different proportions of ECRs were tested in batch experiments for co-digestion with SM: 25, 50, and 75% volatile solids (VS). On the basis of the results obtained from batch tests, a mixture with a 50% ECR content was selected for the second stage of the study. Mesophilic reactors with a 3 L working volume were used for semi-continuous experiments. The hydraulic retention time (HRT) was set at 30 days and the reactors were kept under these operational conditions over four HRTs. The addition of ECR to the co-digestion system resulted in a major increase in the amount of biogas produced daily. The highest biogas yield was obtained when co-digesting Rs (3.5 L/day), although no improvement was observed in specific gas production from the addition of the co-substrate.     

Anaerobic digestion of native cellulosic wastes

Summary Anaerobic digestion is an extensively used bioconversion process to produce gaseous fuel from native lignocellulosic materials. It consists essentially of two steps; acidogenesis and methanogenesis. Most conventional anaerobic digesters are single-stage systems. Animal waste, agricultural residues, sewage sludge and industrial effluents are suitable as feedstock. Biogas productivity in single stage digestion ranges from 0.5–1.5 m³/m³/day at mesophilic (20–40°C) and 1.0–2.5 m³/m³/day at thermophilic (40–60°C) temperatures, and about 30–50% of the volatile solids are converted to biogas. In two-stage systems, acidogenesis is separated from methanogenesis, which improves cellulose hydrolysis and process efficiency. Recent advances in digester design include the introduction of the upflow anaerobic sludge blanket, and fluidized-bed and fixed-film bioreactors, which are operated at much higher loading. Process efficiency can be as high as 97%. An overview of these technologies is presented.

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Resumen La digestión anaeróbica es un proceso de bioconversión muy utilizado para producir gas combustible a partir de materiales de tipo lignocelulósico. El proceso consta esencialmente de dos etapas acidogénesis y metaogénesis. La mayoría de los digestores de tipo convencional son de una sola etapa. Como material de partida pueden utilizarse residuos dé ganado, agrícolas, lodos de depuradoras y efluentes industriales. La producción de biogás en un digestor de una sola etapa va desde 0.5–1.5 m³/m³ día cuando el proceso transcurre a temperaturas mesófilas hasta 1.0–2.5 m³/m³ día cuando lo hace a temperaturas termófilas, transformándose entre un 30 y un 50% de los sólidos volátiles en biogás. En los sistemas con dos etapas la acidogénesis se separa de la metanogénesis con lo cual se mejora la hidrólisis de la celulosa y por lo tanto la eficiencia del proceso. Entre los recientes adelantos en el diseño de digestores cabe incluir la introducción de una manta de flujo ascendente para fangos anaeróbicos y bioreactores con camas fluidas y de lámina fija que pueden usarse con cargas mayores. La eficiencia del proceso puede llegar entonces hasta 97%. En este trabajo se pasa revista a todas estas nuevas tecnologías.

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Résumé La fermentation anaérobie est un procédé de bio-conversion très utilisé pour produire des carburants gazeux à partir de ligno-cellulose. Elle comporte essentiellement deux étapes: acidogenèse et méthanogenèse. La plupart des digesteurs usuels sont des systèmes à un seul étage. Les déchets animaux, les résidus agricoles, les boues d'épandage et certains effluents industriels sont des substrats appropriés. Dans les fermentations à un seul étage, la productivité en bio-gaz varie entre 0,5 et 1,5 m³ par m³ et par jour pour les températures mésophiles (20–40°C) et de 1,0 à 2,5 pour les températures thermophiles (40–50°C); d'autre part environ 30 à 50% des solides volatils sont convertis en bio-gaz. Dans les systèmes à deux étages, l'acidogenèse est séparée de la méthanogenèse, ce qui améliore l'hydrolyse de la cellulose et l'efficacité du procédé. Les progrès récents concernant la structure des fermenteurs comprennent la circulation du dépôt anaérobie et la réalisation de bioréacteurs à lit fluidisé et à film-fixé, ce qui permet d'opérer avec des charges trés supérieures. L'efficacité peut atteindre 97%. Un aperçu de ces nouvelles technologies est présenté dans l'article.

     

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.     

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.     

Anaerobic biodegradation of sugar beet pulp

Sugar beet pulp is a by-product of sugar production and consists mainly of cellulose, hemicellulose and pectin. Its composition is suitable for biological degradation. A possible alternative for the utilization of this material (besides cattle feeding) can be anaerobic methanogenic degradation. It has an additional advantage – biogas production. Beet pulp was treated by a two-step anaerobic process. The first step consisted of hydrolysis andacidification. The second step was methanogenesis. In this paper, observation ofthe process of anaerobic degradation and determination of optimal parameters is discussed. A laboratory-scale model for sugar beet pulp anaerobic biodegradation was operated. Results of model performance have shown very good pulp digestion characteristics. In addition, high efficiency removal of organic matter was achieved. Methane yield was over 0.360 m³ kg^-1 dried pulp and excess sludge production was 0.094 g per gram COD added.     

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 degradation of benzoate: batch studies

Response of benzoate along with phenol to different anaerobic inocula has been investigated in batch reactors. In Phase I, the anaerobic biodegradability of benzoate and phenol were evaluated using (a) washed acclimatized granular sludge (WAGS) collected from a passive phenol fed bench-scale up-flow anaerobic sludge blanket reactor (UASBR) and (b) unacclimatized flocculent sludge (UFS) from a UASB based sewage treatment plant (STP). The effect of varying concentrations of benzoate has been investigated in Phase II using acclimatized granular sludge (AGS) from a bench-scale UASBR. Extent of degradation of benzoate was more than the phenol. Increasing benzoate COD from 2500 to 11,700mg/L, resulted in decrease in (i) rate constant, k from 0.79 to 0.11/d and (ii) ultimate biochemical methane potential (microb, g CH4-COD formed/g benzoate COD) from 84% to 60%. Temporal trend conforming to logistic S-curve indicated stressed conditions at higher benzoate concentration. Benzoate degradation was found to be sensitive to nature as well as quantity i.e. food to microorganism (F/M) of inocula used.     

Anaerobic co-digestion of sewage sludge and grease trap: Assessment of enzyme addition

Anaerobic co-digestion of grease trap and sewage sludge from a wastewater treatment plant is evaluated. Enzyme-lipase application, both addition and dosage, are evaluated by fitting the methane production of biochemical potential tests with the first order model. The enzyme addition effect, at 2, 5 and 10% of grease trap (%GT VS_FED^?1) and the enzymes doses, between 0.25 and 1.67% (v/v), without and with grease trap presence were studied. Grease trap addition showed a negative effect on the waste biodegradability, which was completely overcome by the addition of lipase. Enzyme addition improved notably the methane production for all grease trap fractions studied. In regards to the dosage, the best result was achieved between 0.33 and 0.83% (v/v) of enzyme. The co-digestion of sewage sludge and grease trap may be a feasible process by using lipases due to the saving in operational costs and the increase in the biogas production     

Anaerobic co-digestion of table olive debittering & washing effluent, cattle manure and pig manure in batch and high volume laboratory anaerobic digesters: effect of temperature

The prospective of table olive debittering & washing Effluent (DWE) as feed stock wastewater for anaerobic digestion (AD) systems was investigated in batch and continuous systems together with cattle and pig manures. While DWE considered unsuitable for biological treatment methods due to its unbalanced nature, the co-digestion of the wastewaters resulted in a 50% increase in the methane production/gram volatile solids_added (CH₄/gVS_added), accompanied by 30% phenol reduction and 80% total organic carbon removal (TOC). pH increase during the co-digestion period was not identified as an inhibitory factor and all reactors were able to withstand this operational condition change. Moreover, no volatile fatty acid (VFA) accumulation was observed, indicating that the reactors were not operating under stress-overloading state. Under thermophilic conditions a 7% increase on the TOC removal efficiency was achieved when compared to the mesophilic systems while, under mesophilic conditions phenolic compounds reduction was 10% higher compared to the thermophilic systems.     

Anaerobic digestion and co-digestion processes of vegetable and fruit residues: process and microbial ecology

This study evaluated the feasibility of methane production from fruit and vegetable waste (FVW) obtained from the central food distribution market in Mexico City using an anaerobic digestion (AD) process. Batch systems showed that pH control and nitrogen addition had significant effects on biogas production, methane yield, and volatile solids (VS) removal from the FVW (0.42 m(biogas)(3)/kg VS, 50%, and 80%, respectively). Co-digestion of the FVW with meat residues (MR) enhanced the process performance and was also evaluated in a 30 L AD system. When the system reached stable operation, its methane yield was 0.25 (m(3)/kg TS), and the removal of the organic matter measured as the total chemical demand (tCOD) was 65%. The microbial population (general Bacteria and Archaea) in the 30 L system was also determined and characterized and was closely correlated with its potential function in the AD system.     

Anaerobic hydrolysis of beet pulp — Discontinuous experiments

Pressed beet pulp has been fermented in batch experiments by adapted mixed cultures under anaerobic conditions. Hydrolysis of the constituents proceeds in two stages, first pectin and pentosans, then cellulose are degraded. These reactions limit the overall process, whereas the formation of organic acids, mainly acetic, propionic and butyric acid, proceeds more readily.     

Studying the effects of laccase-catalysed grafting of ferulic acid on sisal pulp fibers

Functionalization of sisal specialty pulp fibers by laccase-catalysed grafting of ferulic acid (FRC) was investigated. To this end, the extent of phenol coupling to fibers under different reaction conditions (laccase and FRC rates, and time) was evaluated in terms of pulp properties including kappa number (expressed as the combined contributions of lignin and hexenuronic acids), brightness, Klason lignin and surface anionic charge after Soxhlet extraction of acetone-treated pulp. The specific treatment resulting in the highest degree of grafting was then used in a comparative study of the effects of applying the laccase-FRC system to refined and unrefined pulp with a view to confirming whether the increased surface area obtained by effect of fibrillation would lead to enhanced grafting. Based on the results, refining the pulp prior to the enzyme treatment resulted in increased grafting which in turn led to handsheets with improved strength-related properties (particularly wet tensile strength) relative to control samples.     

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.     

Anaerobic co-digestion of dairy manure with mulched switchgrass for improvement of the methane yield

The owners of farm-scale anaerobic digesters are relying on off-farm wastes or energy crops as a co-digestion feedstock with animal manure in order to increase their production of methane and thus revenues. Switchgrass represents an interesting feedstock for Canadian digesters owners as it is a high-yielding low-maintenance perennial crop, well adapted to northern climate. Methane potential assays in batch tests showed methane production of 19.4 ± 3.6, 28.3 ± 1.7, 37.3 ± 7.1 and 45.7 ± 0.8 L kg⁻¹, for raw manure, blended manure, manure and mulched switchgrass, manure and pretreated switchgrass, respectively. Two 6-L lab-scale anaerobic digesters were operated for 130 days in order to assess the benefit of co-digesting switchgrass with bovine manure (digester #2), at a 20% wet mass fraction, compared with a manure-only operation (digester #1) The digesters were operated at an hydraulic retention time of 37 ± 6 days and at loads of 2.4 ± 0.6 and 2.6 ± 0.6 kg total volatile solids (TVS) L⁻¹ day⁻¹ for digesters #1 (D1) and #2 (D2), respectively. The TVS degradation reached 25 and 39%, which resulted in a methane production of 1.18 ± 0.18 and 2.19 ± 0.31 L day⁻¹ for D1 and D2, respectively. The addition of 20% on a wet mass ratio of switchgrass to a manure digester increased its methane production by 86%. The co-digestion of switchgrass in a 500 m³ manure digester could yield up to 10.2 GJ day⁻¹ of purified methane or 1.1 MWh day⁻¹ of electricity.     

Anaerobic treatability of waste water from pulp and paper industries

A black liquor evaporator condensate from a Kraft mill and a waste water from production of corrugating medium were anaerobically treated on a laboratory scale. The composition of the waste waters was determined before and after treatment in fixed bed reactors.

Toxicity studies by the Microtox-method showed that both waste waters were highly toxic and a slight decrease in toxicity was achieved by anaerobic treatment. Despite the toxicity efficient anaerobic treatment was obtained.

Major components of the condensate were methanol, ethanol, acetone, guaiacol, hydrogen sulfide and dimethyl disulfide. Anaerobic treatment reduced the concentration of the major components considerably with one exception. The concentration of hydrogen sulfide was unchanged.

Organic overloading of the fixed bed reactor or a temperature drop resulted in an accumulation of acetone, although methanol and ethanol were degraded.

Major components of the waste water from the production of corrugating medium were: Klason-lignin, acid-soluble lignin, carbohydrates, extractives and ash. When the fixed bed reactor was operated at a volumetric load of 1.6 kg COD/m³.d the following reductions were obtained:

Klason - lignin (solids fraction 84%; soluble and colloidal fraction 76%), acid-soluble lignin (solids fraction 56%; soluble and colloidal fraction 7%), carbohydrates (100%), extractives (71%), total-S (80%), COD (73%) and BOD₇ (78%).

Kinetic studies showed that condensate was more easily degraded anaerobically than corrugating medium waste water.