Kinetics and dynamic modelling of batch anaerobic digestion of municipal solid waste in a stirred reactor

A series of batch, slurry anaerobic digestion experiments were performed where the soluble and insoluble fractions, and unwashed MSW were separately digested in a 200l stirred stainless steel vessel at a pH of 7.2 and a temperature of 38 degrees C. It was found that 7% of the total MSW COD was readily soluble, of which 80% was converted to biogas; 50% of the insoluble fraction was solubilised, of this only 80% was converted to biogas. The rate of digesting the insoluble fraction was about four times slower than the rate of digesting the soluble fraction; 48% of the total COD was converted to biogas and 40% of the total nitrogen was converted to ammonia. Soluble and insoluble fractions were broken down simultaneously. The minimum time to convert 95% of the degradable fraction to biogas was 20 days. The lag phase for the degradation of insoluble fraction of MSW can be overcome by acclimatising the culture with the soluble fraction. The rate of digestion and the methane yield was not affected by particle size (within the range of 2-50mm). A dynamic model was developed to describe batch digestion of MSW. The parameters of the model were estimated using data from the separate digestion of soluble and insoluble fractions and validated against data from the digestion of unwashed MSW. Trends in the specific aceticlastic and formate-utilising methanogenic activity were used to estimate initial methanogenic biomass concentration and bacterial death rate coefficient. The kinetics of hydrolysis of insoluble fraction could be adequately described by a Contois equation and the kinetics of acidogenesis, and aceticlastic and hydrogen utilising methanogenesis by Monod equations.     

Anaerobic biotransformation and methane generation potential of cheese whey in batch and UASB reactors.

Anaerobic treatability and methane generation potential of cheese whey were determined in batch reactors. Furthermore, the effect of nutrient and trace metal supplementation on the batch anaerobic treatment, and the high-rate anaerobic treatability of cheese whey in upflow anaerobic sludge blanket (UASB) reactors were investigated. To this purpose biochemical methane potential experiments were conducted and single- and two-stage UASB reactors with granular cultures were operated. In UASB experiments significance of process staging, operational parameters such as hydraulic retention time (HRT), influent chemical oxygen demand (COD) concentration and loading rate were also investigated. The results revealed that nutrient and trace metal supplementation is vital for the anaerobic treatment of cheese whey; the anaerobic methane generation for the cheese whey studied was found to be 424 ml CH4/g COD (23.4 1 CH4/l cheese whey); undiluted cheese whey could be treated anaerobically at relatively short HRT values (2.06-4.95 days) without any significant stability problems; HRT values as low as 2-3 days can be used for the anaerobic treatment of cheese whey, with a COD removal efficiency of 95-97% at influent COD concentration of 42 700 +/- 141-55 100 +/- 283 mg/l.     

Modelling and experimental investigation of environmental influences on the acetate and methane formation in solid waste

An anaerobic reaction model is represented and used for simulation of the biodegradation of organic compounds and the generation of biogas. The model is based on fundamental relationships among physical, chemical, thermodynamic and microbial processes occurring in municipal landfills. Local microbially mediated degradation processes occurring in municipal landfills are simulated in terms of hydrolysis of readily and inherently degradable organic matter, the formation of acetate as surrogate for intermediary low-molecular carbon substrates, and the generation of the biogases CH4 and CO2. Thus, the overall decomposition of the organic matter has been assumed to follow three sequential anaerobic reactions steps: hydrolysis, acetogenesis and methanogenesis. In order to study the impact of environmental factors on the biological decomposition processes, experiments have been conducted to investigate the effect of temperature and water content. In the degradation model, the impact of temperature and water content was defined as reaction rate influencing factors. Further, waste samples have been taken from four drill holes on a municipal landfill near Wolfsburg (Germany) and used to analyze and to describe the waste composition and prevailing environmental conditions dependent on the depth of the drill hole. The data and waste samples obtained from the landfill have also been used for model development and validation.     

Influence of composition on the biomethanation potential of restaurant waste at mesophilic temperatures

A synthetic waste was used to study the effect of waste composition on anaerobic degradation of restaurant waste. It was made by blending melted pork lard, white cabbage, chicken breast, and potato flakes, to simulate lipids, cellulose, protein, and carbohydrates, respectively. Four blends of the four constituents with an excess of each component were assayed and compared with a fifth blend containing an equal amount of chemical oxygen demand (COD) of each of the four components. The methane production and the time course of soluble COD and volatile fatty acids were assessed in batch assays. A high reduction of volatile solids (between 94% and 99.6%) was obtained in all the assays. The methane yield was between 0.40 m(3) CH(4)/kg VS(initial) (excess of carbohydrates) and 0.49 m(3) CH(4)/kg VS(initial) (excess of lipids). The degradation of the lipid-rich assays differed from the others. Fifty percent of the biochemical methane potential was obtained after 3-6 days for all of the assays, except for the one with excess of lipids which achieved 50% methanation only after 14.7 days of incubation. In the assay with excess of lipids, a considerable fraction of COD remained in the liquid phase, suggesting an inhibition of the methanogenic process that was likely due to the accumulation of long chain fatty acids. The hydrolysis rate constants, assuming first order kinetics, over the first 6 days were between 0.12d(-1) (excess of lipids) and 0.32 d(-1) (excess of carbohydrates). The results indicate that anaerobic digestion facilities with large variations in lipid input could have significant changes in process performance that merit further examination.     

Anaerobic co-digestion of coffee waste and sewage sludge

The feasibility of the anaerobic co-digestion of coffee solid waste and sewage sludge was assessed. Five different solid wastes with different chemical properties were studied in mesophilic batch assays, providing basic data on the methane production, reduction of total and volatile solids and hydrolysis rate constant. Most of the wastes had a methane yield of 0.24-0.28 m3 CH4(STP)/kg VS(initial) and 76-89% of the theoretical methane yield was achieved. Reduction of 50-73% in total solids and 75-80% in volatile solids were obtained and the hydrolysis rate constants were in the range of 0.035-0.063 d(-1). One of the solid wastes, composed of 100% barley, achieved a methane yield of 0.02 m3 CH4(STP)/kg VS(initial), reductions of 31% in total solids, 40% in volatile solids and achieved only 11% of the theoretical methane yield. However, this waste presented the highest hydrolysis rate constant. Considering all the wastes, an inverse linear correlation was obtained between methane yield and the hydrolysis rate constant, suggesting that hydrolysis was not the limiting factor in the anaerobic biodegradability of this type of waste.     

Leachate treatment before injection into a bioreactor landfill: Clogging potential reduction and benefits of using methanogenesis

In this study, an anaerobic sequencing batch reactor (ASBR) was operated with leachate from Brady Road Municipal Landfill in Winnipeg, Manitoba, Canada. Leachate was collected twice from the same cell at the landfill, during the first and 70th day of the study, and then fed into the ASBR. The ASBR was seeded at the start-up with biosolids from the anaerobic digester from Winnipeg’s North End Water Pollution Control Center (NEWPCC). Due to the higher COD and VFA removal rates measured with the second batch of leachate, an increase of approximately 0.3 pH units was observed during each cycle (from pH 7.2 to 7.5). In addition, CO₂ was produced between cycles at constant temperature where a fraction of the CO₂ became dissolved, shifting the CO₂/bicarbonate/carbonate equilibrium. Concurrent with the increase in pH and carbonate, an accumulation of fixed suspend solids (FSS) was observed within the ASBR, indicating a buildup of inorganic material over time. From it, Ca²⁺ and Mg²⁺ were measured within the reactor on day 140, indicating that most of the dissolved Ca²⁺ was removed within cycles. There is precedence from past researches of clogging in leachate-collection systems (Rowe et al., 2004) that changes in pH and carbonate content combined with high concentrations of metals such as Ca²⁺ and Mg²⁺ result in carbonate mineral precipitants. A parallel study investigated this observation, indicating that leachate with high concentration of Ca²⁺ under CO₂ saturation conditions can precipitate out CaCO₃ at the pH values obtained between digestion cycles. These studies presented show that methanogenesis of leachate impacts the removal of organic (COD, VFA) as well as inorganic (FSS, Ca²⁺) clog constituents from the leachate, that otherwise will accumulate inside of the recirculation pipe in bioreactor landfills. In addition, a robust methanogenesis of leachate was achieved, averaging rates of 0.35 L CH₄ produced/g COD removed which is similar to the theoretical removal of 0.4 L CH₄/g COD. Therefore, using methanogenesis of leachate prior to recirculation in bioreactor landfills will help to (1) control clog formation within leachate pipes and (2) produce an important additional source of energy on-site.     

Simultaneous removal of COD and ammonium from landfill leachate using an anaerobic-aerobic moving-bed biofilm reactor system

The performance of a moving-bed biofilm reactor (MBBR) system with an anaerobic-aerobic arrangement was investigated to treat landfill leachate for simultaneous removal of COD and ammonium. It was found that the anaerobic MBBR played a major role in COD removal due to methanogenesis, and the aerobic MBBR acted as COD-polishing and ammonium removal step. The contribution of the anaerobic MBBR to total COD removal efficiency reached 91% at an organic loading rate (OLR) of 4.08 kgCOD/(m3d), and gradually decreased to 86% when feed OLR was increased to 15.70 kgCOD/(m3d). Because of the complementary function of the aerobic reactor, the total COD removal efficiency of the system had a slight decrease from 94% to 92% even though the feed OLR was increased from 4.08 to 15.70 kgCOD/(m3d). Hydraulic retention time (HRT) had a significant effect on NH+4-N removal; more than 97% of the total NH+4-N removal efficiency could be achieved when the HRT of the aerobic MBBR was more than 1.25 days. The anaerobic-aerobic system had a strong tolerance to shock loading. A decrease in COD removal efficiency of only 7% was observed when the OLR was increased by four times and shock duration was 24 h, and the system could recover the original removal efficiency in 3 days. The average sludge yield of the anaerobic reactor was estimated to be 0.0538 gVSS/gCOD rem.     

Enhanced anaerobic treatment of CSTR-digested effluent from chicken manure: The effect of ammonia inhibition

The effect of ammonia inhibition was evaluated during the enhanced anaerobic treatment of digested effluent from a 700 m³ chicken-manure continuous stirred tank reactor (CSTR). A 12.3 L internal circulation (IC) reactor inoculated with an anaerobic granular sludge and operated at 35 ± 1 °C was employed for the investigation. With a corresponding organic loading rate of 1.5-3.5 kg-COD/m³ d over a hydraulic retention time of 1.5 d, a maximum volumetric biogas production rate of 1.2 m³/m³ d and TCOD (total COD) removal efficiency ranging from 70% to 80% was achieved. However, the continual increase in the influent TAN content led to ammonia inhibition in the methanogenesis system. The SCOD/TAN (soluble COD/total ammonia nitrogen) ratio was presented to be the key controlling factor for the anaerobic treatment of semi-digested chicken manure, and further validation through shock loading and ammonia inhibition experiments was conducted. The threshold value of the SCOD/TAN ratio was determined to be 2.4 (corresponding to a TAN of 1250 mg/L) at an influent pH of 8.5-9.     

Kinetics of biochemically driven metal precipitation in synthetic landfill leachate

This study investigates the effectiveness of using metal sulphide and carbonate precipitation mechanisms combined with a landfill‐derived mixed bacterial population. The study was conducted under controlled substrate conditions in anaerobic batch reactors. High chemical oxygen demand (COD):sulphate ratios, butyrate, propionate, and acetate were used anaerobically by bacteria for growth with associated sulphate reduction as well as sulphide and carbonate generation. Propionate and butyrate degradation occurred during sulphate reduction by sulphate‐reducing bacteria while acetate degradation was associated with methanogenesis by methanogenic bacteria. Using low COD, sulphate ratios showed limited acetate utilization, but sulphate reduction still occurred. Precipitation of Cd, Cu, Zn, Ni, and Fe sulphides occurred quickly and was completed in 15 to 30 days, while Ca, Mn, and Mg carbonates formed after 40 to 50 days and some soluble metal remained even after 120 days. The rate of metal precipitation was in the order of Cd>Cu>Zn>Ni>Fe>Mn>Mg>Ca. Bacterially mediated metal precipitation occurred slower than that recognized for chemical precipitation. These findings suggest that contaminant transport models based on chemical equilibrium metal behaviors may over‐predict metal removal by bio‐precipitation. © 2002 Wiley Periodicals, Inc.     

Solubilization and methanogenesis of a particulate industrial waste: Impact of solids loading and temperature

Effective anaerobic treatment of particulate wastes requires solubilization and acid formation prior to methanogenesis. In this case study of a particulate waste from a corn-processing industry, the influence of solids loading in solubilization, acid formation and methanogenesis was studied under mesophilic (35°C) and thermophilic (60°C) conditions. The waste was concentrated by centrifugation to initial suspended solids concentrations (TSS_i) of 150 to 350 g/L (15% to 35%). Anaerobic batch tests were conducted for 20 days, and significant solubilization of the particulate organic matter occurred in all cases. The thermophilic systems were more effective than the mesophilic systems with respect to solubilization of particulates, volatile solids destruction, acetic acid uptake, and methane generation. Methanogenesis appreared to be a rate-limiting step at higher TSS_i values, indicated by accumulation of volatile organic acids in the batch systems. Slower rates of methane production led to identification of the limiting solids loading for both temperature regimes. The results of this study can be used to evaluate the limitations of a single stage system for anaerobic treatment of organic particulate industrial wastes.     

微电解-UASB-接触氧化处理酚醛树脂废水

采用微电解一上流式厌氧污泥床（Upflow Anaerobic Sludge Bed，简称UASB）-接触氧化工艺处理酚醛树脂生产废水（简称废水）。实验结果表明：采用微电解法对废水进行预处理，不仅去除了约36％COD，还大幅度提高了废水的可生化性；微电解出水经UASB厌氧生物处理后，COD去除率达80％；水解酸化COD去除率约为30％；最后经二级接触氧化处理，出水COD为100mg／L以下，达到GB8978--1996《污水综合排放标准》中化工类废水的二级排放标准。     

Biomethanation under psychrophilic conditions

The biomethanation of organic matter represents a long-standing, well-established technology. Although at mesophilic and thermophilic temperatures the process is well understood, current knowledge on psychrophilic biomethanation is somewhat scarce. Methanogenesis is particularly sensitive to temperature, which not only affects the activity and structure of the microbial community, but also results in a change in the degradation pathway of organic matter. There is evidence of psychrophilic methanogenesis in natural environments, and a number of methanogenic archaea have been isolated with optimum growth temperatures of 15–25 °C. At psychrophilic temperatures, large amounts of heat are needed to operate reactors, thus resulting in a marginal or negative overall energy yield. Biomethanation at ambient temperature can alleviate this requirement, but for stable biogas production, a microbial consortium adapted to low temperatures or a psychrophilic consortium is required. Single-step or two-step high rate anaerobic reactors [expanded granular sludge bed (EGSB) and up flow anaerobic sludge bed (UASB)] have been used for the treatment of low strength wastewater. Simplified versions of these reactors, such as anaerobic sequencing batch reactors (ASBR) and anaerobic migrating blanket reactor (AMBR) have also been developed with the aim of reducing volume and cost. This technology has been further simplified and extended for the disposal of night soil in high altitude, low temperature areas of the Himalayas, where the hilly terrain, non-availability of conventional energy, harsh climate and space constraints limit the application of complicated reactors. Biomethanation at psychrophilic temperatures and the contribution made to night-soil degradation in the Himalayas are reviewed in this article.     

Hydrolysis kinetics in anaerobic degradation of particulate organic material: an overview

The applicability of different kinetics to the hydrolysis of particulate organic material in anaerobic digestion is discussed. Hydrolysis has traditionally been modelled according to the first-order kinetics. For complex substrate, the first-order kinetics should be modified in order to take into account hardly degradable material. It has been shown that models in which hydrolysis is coupled to the growth of hydrolytic bacteria work well at high or at fluctuant organic loading. In particular, the surface-related two-phase and the Contois models showed good fits to experimental data from a wide range of organic waste. Both models tend to the first-order kinetics at a high biomass-to-waste ratio and, for this reason, they can be considered as more general models. Examples on different inhibition processes that might affect the degradation of solid waste are reported. Acetogenesis or methanogenesis might be the rate-limiting stages in complex waste. In such cases, stimulation of hydrolysis (mechanically, chemically or biologically) may lead to a further inhibition of these stages, which ultimately affects hydrolysis as well. Since the hydrolysis process is characterized by surface and transport phenomena, new developments in spatially distributed models are considered fundamental to provide new insights in this complex process.     

甲苯二胺对厌氧微生物活性的影响

以相对产甲烷活性(RA)和COD去除率为指标,考察不同浓度甲苯二胺对厌氧污泥微生物活性的影响。实验结果表明:甲苯二胺质量浓度为0~150 mg/L时,RA接近100%甚至高于100%,COD去除率为91%~93%,与对照组无差别,对厌氧微生物几乎没有抑制作用;当甲苯二胺质量浓度为200~400 mg/L时,RA为75%~95%,COD去除率与对照组相比下降了2~6个百分点,属于轻度抑制;当甲苯二胺质量浓度高于400 mg/L时,RA在70%以下,COD去除率降至85%以下,属于中度抑制;且随着甲苯二胺质量浓度增大,RA减小、COD去除率减小,抑制作用增强。因此,为了防止甲苯二胺对厌氧污泥微生物活性的抑制,甲苯二胺质量浓度应控制在0~150 mg/L。     

Regulating the hydrolysis of organic wastes by micro-aeration and effluent recirculation

In this study, the effects of micro-aeration and liquid recirculation on the hydrolysis of vegetable and flower wastes during two-phase solid–liquid anaerobic digestion were assessed. To accomplish this, we evaluated the hydrolysis of five batches of waste that were treated under the following conditions: anaerobic, insufficient micro-aeration (aeration for 5 min every 24 h), and sufficient micro-aeration (aeration for 5 min every 12, 4 and 1 h). Hydrolysis was found to depend on the level of micro-aeration. Specifically, insufficient micro-aeration led to unstable and decreased performance. Conversely, sufficient micro-aeration promoted the hydrolysis of easily biodegradable carbohydrates and proteins, but the microbial activity was later impaired by liquid recirculation using methanogenic effluent. The hydrolysis efficiency under anaerobic conditions was comparable to the efficiency observed under sufficient micro-aeration, while the cumulative TOC of the anaerobic batch was 1.4–2.4 times higher than that of the micro-aerated batches. In addition, liquid recirculation did not have a negative effect on the development of microbial activity under anaerobic conditions, which resulted in the lignocelluloses having a higher hydrolysis efficiency.     

Enzymatic pretreatment of lignocellulosic wastes to improve biogas production

The effect of enzymatic pretreatment of sugar beet pulp and spent hops prior to methane fermentation was determined in this study. These industrial residues were subjected to enzymatic digestion before anaerobic fermentation because of high fiber content (of 85.1% dry matter (DM) and 57.7% DM in sugar beet pulp and spent hops, respectively). Their 24h hydrolysis with a mix of enzymatic preparations Celustar XL and Agropect pomace (3:1, v/v), with endoglucanase, xylanase and pectinase activities, was most effective. Reducing sugars concentrations in hydrolysates of sugar beet pulp and spent hops were by 88.9% and 59.4% higher compared to undigested materials. The highest yield of biogas was obtained from the enzymatic hydrolysate of sugar beet pulp (183.39 mL/d from 1g COD at fermenter loading with organic matter of 5.43 g COD/L × d). Fermentation of sugar beet pulp gave 19% less biogas. Methane fermentation of spent hops hydrolysate yielded 121.47 mL/d biogas from 1g COD (at 6.02 g COD/L × d, 13% more than from spent hops). These results provide evidence that suitable enzymatic pretreatment of lignocellulosic wastes improve biogas yield from anaerobic fermentation.     

Effect of long chain fatty acids removal as a pretreatment on the anaerobic digestion of food waste

This study investigated the effect of long chain fatty acids (LCFAs) removal as a pretreatment prior to anaerobic digestion on the production of methane from food waste. The results showed that the anaerobic digestion of food waste containing 1.6 g COD/L of LCFAs was not inhibited (4 days lag-time, 78.3 % methane recovery in 35 days) compared to that of lipid free food waste (3 days lag time, 72.5 % methane recovery in 35 days); however, some unsaturated LCFAs, which are toxic to microorganism, were accumulated in the batch anaerobic digestion reactor. Meanwhile, in a methanogenic activity study, the activity of methanogens was observed to be linearly inhibited by the presence of more than 1 g COD/L of LCFAs. The possibility of the accumulation of unsaturated LCFAs in the reactor should be considered when operating a large-scale continuous system.     

Fate and transport of phenol in a packed bed reactor containing simulated solid waste

An assessment of the risk to human health and the environment associated with the presence of organic contaminants (OCs) in landfills necessitates reliable predictive models. The overall objectives of this study were to (1) conduct column experiments to measure the fate and transport of an OC in a simulated solid waste mixture, (2) compare the results of column experiments to model predictions using HYDRUS-1D (version 4.13), a contaminant fate and transport model that can be parameterized to simulate the laboratory experimental system, and (3) determine model input parameters from independently conducted batch experiments. Experiments were conducted in which sorption only and sorption plus biodegradation influenced OC transport. HYDRUS-1D can reasonably simulate the fate and transport of phenol in an anaerobic and fully saturated waste column in which biodegradation and sorption are the prevailing fate processes. The agreement between model predictions and column data was imperfect (i.e., within a factor of two) for the sorption plus biodegradation test and the error almost certainly lies in the difficulty of measuring a biodegradation rate that is applicable to the column conditions. Nevertheless, a biodegradation rate estimate that is within a factor of two or even five may be adequate in the context of a landfill, given the extended retention time and the fact that leachate release will be controlled by the infiltration rate which can be minimized by engineering controls.     

Study of the operational conditions for anaerobic digestion of urban solid wastes

This paper describes an experimental evaluation of anaerobic digestion technology as an option for the management of organic solid waste in developing countries. As raw material, a real and heterogeneous organic waste from urban solid wastes was used. In the first experimental phase, seed selection was achieved through an evaluation of three different anaerobic sludges coming from wastewater treatment plants. The methanization potential of these sludges was assessed in three different batch digesters of 500 mL, at two temperature levels. The results showed that by increasing the temperature to 15 degrees C above room temperature, the methane production increases to three times. So, the best results were obtained in the digester fed with a mixed sludge, working at mesophilic conditions (38-40 degrees C). Then, this selected seed was used at the next experimental phase, testing at different digestion times (DT) of 25, 20 and 18 days in a bigger batch digester of 20 L with a reaction volume of 13 L. The conversion rates were registered at the lowest DT (18 days), reaching 44.9 L/kg(-1) of wet wasteday(-1). Moreover, DT also has a strong influence over COD removal, because there is a direct relationship between solids removal inside the reactor and DT.     

微碱解-厌氧水解-SBR好氧生化法处理有机磷农药废水

介绍了微碱解——厌氧水解——SBR好氧生化法处理有机磷农药废水的工艺流程、工艺参数和处理效果。废水处理设施运行结果：废水COD去除率平均为90.1%,BOD5去除率平均为94.2%,TP去除率平均为84.9%。