Co-digestion of grease trap sludge and sewage sludge

Redirection of organic waste, from landfilling or incineration, to biological treatment such as anaerobic digestion is of current interest in the Malmö-Copenhagen region. One type of waste that is expected to be suitable for anaerobic digestion is sludge from grease traps. Separate anaerobic digestion of this waste type and co-digestion with sewage sludge were evaluated. The methane potential was measured in batch laboratory tests, and the methane yield was determined in continuous pilot-scale digestion. Co-digestion of sludge from grease traps and sewage sludge was successfully performed both in laboratory batch and continuous pilot-scale digestion tests. The addition of grease trap sludge to sewage sludge digesters was seen to increase the methane yield of 9–27% when 10–30% of sludge from grease traps (on VS-basis) was added. It was also seen that the grease trap sludge increases the methane yield without increasing the sludge production. Single-substrate digestion of grease trap sludge gave high methane potentials in batch tests, but could not reach stable methane production in continuous digestion.     

Anaerobic digestion of microalgal biomass after ultrasound pretreatment

High rate algal ponds are an economic and sustainable alternative for wastewater treatment, where microalgae and bacteria grow in symbiosis removing organic matter and nutrients. Microalgal biomass produced in these systems can be valorised through anaerobic digestion. However, microalgae anaerobic biodegradability is limited by the complex cell wall structure and therefore a pretreatment step may be required to improve the methane yield. In this study, ultrasound pretreatment at a range of applied specific energy (16–67 MJ/kg TS) was investigated prior to microalgae anaerobic digestion. Experiments showed how organic matter solubilisation (16–100%), hydrolysis rate (25–56%) and methane yield (6–33%) were improved as the pretreatment intensity increased. Mathematical modelling revealed that ultrasonication had a higher effect on the methane yield than on the hydrolysis rate. A preliminary energy assessment indicated that the methane yield increase was not high enough as to compensate the electricity requirement of ultrasonication without biomass dewatering (8% VS).     

污泥热水解技术研究进展

热水解可以改善污泥的生物降解性能和脱水性能,提高后续厌氧消化系统中有机物的去除率。本文阐述了热水解对污泥物理特性主要包括流变特性和脱水行为的影响;分析了热水解对污泥中氨氮浓度及对厌氧消化的作用;同时介绍了热水解技术对整个污泥处理系统能量需求的影响。指出：热水解改善了污泥的厌氧消化性能,提高了沼气等再生资源的产量,且厌氧消化产生的沼气量与热水解技术所需的能量基本平衡。     

Need for improvements in physical pretreatment of source-separated household food waste

The aim of the present study was to investigate the efficiency in physical pretreatment processes of source-separated solid organic household waste. The investigation of seventeen Swedish full-scale pretreatment facilities, currently receiving separately collected food waste from household for subsequent anaerobic digestion, shows that problems with the quality of produced biomass and high maintenance costs are common. Four full-scale physical pretreatment plants, three using screwpress technology and one using dispergation technology, were compared in relation to resource efficiency, losses of nitrogen and potential methane production from biodegradable matter as well as the ratio of unwanted materials in produced biomass intended for wet anaerobic digestion. Refuse generated in the processes represent 13–39% of TS in incoming wet waste. The methane yield from these fractions corresponds to 14–36 Nm³/ton separately collected solid organic household waste. Also, 13–32% of N-tot in incoming food waste is found in refuse. Losses of both biodegradable material and nutrients were larger in the three facilities using screwpress technology compared to the facility using dispersion technology.¹ Thus, there are large potentials for increase of both the methane yield and nutrient recovery from separately collected solid organic household waste through increased efficiency in facilities for physical pretreatment. Improved pretreatment processes could thereby increase the overall environmental benefits from anaerobic digestion as a treatment alternative for solid organic household waste.     

Wet explosion of wheat straw and codigestion with swine manure: Effect on the methane productivity

The continuously increasing demand for renewable energy sources renders anaerobic digestion to one of the most promising technologies for renewable energy production. Twenty-two (22) large-scale biogas plants are currently under operation in Denmark. Most of these plants use manure as the primary feedstock but their economical profitable operation relies on the addition of other biomass products with a high biogas yield. Wheat straw is the major crop residue in Europe and the second largest agricultural residue in the world. So far it has been used in several applications, i.e. pulp and paper making, production of regenerated cellulose fibers as an alternative to wood for cellulose-based materials and ethanol production. The advantage of exploiting wheat straw for various applications is that it is available in considerable quantity and at low-cost. In the present study, the codigestion of swine manure with wheat straw in a continuous operated system was investigated, as a method to increase the efficiency of biogas plants that are based on anaerobic digestion of swine manure. Also, the pretreatment of wheat straw with the wet explosion method was studied and the efficiency of the wet explosion process was evaluated based on (a) the sugars release and (b) the methane potential of the pretreated wheat straw compared to that of the raw biomass. It was found that, although a high release of soluble sugars was observed after wet explosion, the methane obtained from the wet-exploded wheat straw was slightly lower compared to that from the raw biomas s. On the other hand, the results from the codigestion of raw (non-pretreated) wheat straw with swine manure were very promising, suggesting that 4.6 kg of straw added to 1 t of manure increase the methane production by 10%. Thus, wheat straw can be considered as a promising, low-cost biomass for increasing the methane productivity of biogas plants that are based mainly on swine manure.     

Anaerobic digestion of the liquid fraction of dairy manure in pilot plant for biogas production: residual methane yield of digestate

The performance of the only dairy manure biogas plant in Cantabria (Northern coast of Spain) was evaluated in terms of liquid-solid separation and anaerobic digestion of the liquid fraction. Screened liquid fraction was satisfactorily treated in a CSTR digester at HRTs from 20 to 10 days with organic loading rates ranging from 2.0 to 4.5 kg VS/(m(3)d). Stable biogas productions from 0.66 to 1.47 m(3)/(m(3)d) were achieved. Four anaerobic effluents collected from the digester at different HRTs were analyzed to measure their residual methane potentials, which ranged from 12.7 to 102.4 L/gVS. These methane potentials were highly influenced by the feed quality and HRT of the previous CSTR anaerobic digestion process. Biomethanization of the screened liquid fraction of dairy manure from intensive farming has the potential to provide up to 2% of total electrical power in the region of Cantabria.     

Optimization of anaerobic digestion of municipal solid waste in combined process and sequential staging.

The optimization of anaerobic digestion aims to maximize organic waste stabilization after a short digestion period. This paper presents the optimization performance of the combined anaerobic digestion and sequential staging concept in a thermophilic, solid-state batch system as a treatment technology prior to landfill. The former involves enhanced pre-stage flushing with the addition of microaeration and inoculum in the methane phase. The latter involves leachate cross-recirculation between the mature and fresh waste reactors without conducting a pre-stage operation. The optimized process for combined anaerobic digestion showed that reducing the pre-stage operation with the maximum removal of organics from the waste bed is beneficial. Moreover, the sequential staging concept offers an improved process over the combined anaerobic digestion wherein the specific methane yield of 11.9 and 7.2 L CH4 kg(-1) volatile solids (VS) per day was achieved, respectively. After 28 days of operation, the sequential staging process showed an improved waste stabilization with 86 and 79% mass and volume reduction, respectively. A higher methane yield of 334 L CH4 kg(-1) VS with 86% VS reduction, which is equivalent to 84% process efficiency was obtained.     

Degradation of lignocellulosic materials under sulfidogenic and methanogenic conditions

Biochemical methane potential (BMP) assays, typically used to assess anaerobic biodegradability of liquid wastes with added nutrients and bacteria, were adapted to compare hydrolysis of lignocellulosic material under sulfidogenic and methanogenic environments. A method based on selective inhibition of microorganism activity, by 3% toluene, was used to measure the hydrolysis rate of lignocellulosic material and the accumulation of sugar. The neutral sugars, galactose, glucose, and xylose, which were released from lignocellulosic material such as office paper and newspaper in the presence 3% toluene, clearly accumulated over time under sulfidogenic conditions. The accumulation rates of sugars, glucose, and xylose, were higher in the sulfidogenic condition than in the methanogenic condition, indicating a faster degradation of lignocellulosic materials under the sulfidogenic condition.     

Evaluating the biogas potential of the dry fraction from pretreatment of food waste from households

At the waste handling company NSR, Helsingborg, Sweden, the food waste fraction of source separated municipal solid waste is pretreated to obtain a liquid fraction, which is used for biogas production, and a dry fraction, which is at present incinerated. This pretreatment and separation is performed to remove impurities, however also some of the organic material is removed. The possibility of realising the methane potential of the dry fraction through batch-wise dry anaerobic digestion was investigated. The anaerobic digestion technique used was a two-stage process consisting of a static leach bed reactor and a methane reactor. Treatment of the dry fraction alone and in a mixture with structural material was tested to investigate the effect on the porosity of the leach bed. A tracer experiment was carried out to investigate the liquid flow through the leach beds, and this method proved useful in demonstrating a more homogenous flow through the leach bed when structural material was added. Addition of structural material to the dry fraction was needed to achieve a functional digestion process. A methane yield of 98 m³/ton was obtained from the dry fraction mixed with structural material after 76 days of digestion. This was in the same range as obtained in the laboratory scale biochemical methane potential test, showing that it was possible to extract the organic content in the dry fraction in this type of dry digestion system for the production of methane.     

Anaerobic co-digestion of food waste and landfill leachate in single-phase batch reactors

In order to investigate the effect of raw leachate on anaerobic digestion of food waste, co-digestions of food waste with raw leachate were carried out. A series of single-phase batch mesophilic (35 ± 1 °C) anaerobic digestions were performed at a food waste concentration of 41.8 g VS/L. The results showed that inhibition of biogas production by volatile fatty acids (VFA) occurred without raw leachate addition. A certain amount of raw leachate in the reactors effectively relieved acidic inhibition caused by VFA accumulation, and the system maintained stable with methane yield of 369–466 mL/g VS. Total ammonia nitrogen introduced into the digestion systems with initial 2000–3000 mgNH₄–N/L not only replenished nitrogen for bacterial growth, but also formed a buffer system with VFA to maintain a delicate biochemical balance between the acidogenic and methanogenic microorganisms. UV spectroscopy and fluorescence excitation–emission matrix spectroscopy data showed that food waste was completely degraded.We concluded that using raw leachate for supplement water addition and pH modifier on anaerobic digestion of food waste was effective. An appropriate fraction of leachate could stimulate methanogenic activity and enhance biogas production.     

Anaerobic digestion potential of urban organic waste: a case study in Malm?.

A study of existing organic waste types in Malm?, Sweden was performed. The purpose was to gather information about organic waste types in the city to be able to estimate the potential for anaerobic treatment in existing digesters at the wastewater treatment plan (WWTP). The urban organic waste types that could have a significant potential for anaerobic digestion amount to about 50 000 tonnes year(-1) (sludge excluded). Some of the waste types were further evaluated by methane potential tests and continuous pilot-scale digestion. Single-substrate digestion and co-digestion of pre-treated, source-sorted organic fraction of municipal solid waste, wastewater sludge, sludge from grease traps and fruit and vegetable waste were carried out. The experiments showed that codigestion of grease sludge and WWTP sludge was a better way of making use of the methane potential in the grease trap sludge than single-substrate digestion. Another way of increasing the methane production in sludge digesters is to add source-sorted organic fraction of municipal solid waste (SSOFMSW). Adding SSOFMSW (20% of the total volatile solids) gave a 10-15% higher yield than could be expected by comparison with separate digestion of sludge respective SSOFMSW. Co-digestion of sludge and organic waste is beneficial not just for increasing gas production but also for stabilizing the digestion process. This was seen when co-digesting fruit and vegetable waste and sludge. When co-digested with sludge, this waste gave a better result than the separate digestion of fruit and vegetable waste. Considering single-substrate digestion, SSOFMSW is the only waste in the study which makes up a sufficient quantity to be suitable as the base substrate in a full-scale digester that is separated from the sludge digestion. The two types of SSOFMSW tested in the pilot-scale digestion were operated successfully at mesophilic temperature. By adding SSOFMSW, grease trap sludge and fruit and vegetables waste to sludge digesters at the wastewater treatment plant, the yearly energy production from methane could be expected to increase from 24 to 43 GWh.     

Methane production potential of leachate generated from Korean food waste recycling facilities: a lab-scale study

This paper examines the applicability of food waste leachate (FWL) in bioreactor landfills or anaerobic digesters to produce methane as a sustainable solution to the persisting leachate management problem in Korea. Taking into account the climatic conditions in Korea and FWL characteristics, the effect of key parameters, viz., temperature, alkalinity and salinity on methane yield was investigated. The monthly average moisture content and the ratio of volatile solids to total solids of the FWL were found to be 84% and 91%, respectively. The biochemical methane potential experiment under standard digestion conditions showed the methane yield of FWL to be 358 and 478 ml/g VS after 10 and 28 days of digestion, respectively, with an average methane content of 70%. Elemental analysis showed the chemical composition of FWL to be C(13.02)H(23.01)O(5.93)N(1). The highest methane yield of 403 ml/g VS was obtained at 35 degrees C due to the adaptation of seed microorganisms to mesophilic atmosphere, while methane yields at 25, 45 and 55 degrees C were 370, 351 and 275 ml/g VS, respectively, at the end of 20 days. Addition of alkalinity had a favorable effect on the methane yield. Dilution of FWL with salinity of 2g/l NaCl resulted in 561 ml CH(4)/g VS at the end of 30 days. Considering its high biodegradability (82.6%) and methane production potential, anaerobic digestion of FWL in bioreactor landfills or anaerobic digesters with a preferred control of alkalinity and salinity can be considered as a sustainable solution to the present emergent problem.     

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.     

Use Of Peat Extract In The Ensiling Of Fisheries Wastes

Fish processing wastes, namely minced cod (Gadus morhua) offal and minced male capelin (Mallotus villosus), were used in two kinds of ensiling (silage making) processes. In one process they were ensiled by the direct addition of acids, and in the other they were ensiled using various carbohydrate sources and bacterial cultures to produce organic acids in situ. Ensiled material was analysed for various components at times throughout its storage period, and monitored for onset of spoilage. It was found that neither prior freezing nor initial heat treatment of capelin had an effect on the preservation effectiveness of the ensiling process. It was concluded that both the addition of formic acid to cod offal and the fermentation of lactic acid bacteria are effective methods of ensiling this material. Peat extracts with a relatively high carbohydrate content were found to be a satisfactory source of carbohydrates for the fermentation of lactic acid bacteria in cod offal silage. However, peat extracts with a relatively low pH were found to be unsatisfactory as a direct source of acid for ensiling.     

Effect of alkaline pretreatment on anaerobic digestion of solid wastes

The introduction of the anaerobic digestion for the treatment of the organic fraction of municipal solid waste (OFMSW) is currently of special interest. The main difficulty in the treatment of this waste fraction is its biotransformation, due to the complexity of organic material. Therefore, the first step must be its physical, chemical and biological pretreatment for breaking complex molecules into simple monomers, to increase solubilization of organic material and improve the efficiency of the anaerobic treatment in the second step. This paper describes chemical pretreatment based on lime addition (Ca(OH)2), in order to enhance chemical oxygen demand (COD) solubilization, followed by anaerobic digestion of the OFMSW. Laboratory-scale experiments were carried out in completely mixed reactors, 1 L capacity. Optimal conditions for COD solubilization in the first step of pretreatment were 62.0 mEq Ca(OH)2/L for 6.0 h. Under these conditions, 11.5% of the COD was solubilized. The anaerobic digestion efficiency of the OFMSW, with and without pretreatment, was evaluated. The highest methane yield under anaerobic digestion of the pretreated waste was 0.15 m3CH4/kg volatile solids (VS), 172.0% of the control. Under that condition the soluble COD and VS removal were 93.0% and 94.0%, respectively. The results have shown that chemical pretreatment with lime, followed by anaerobic digestion, provides the best results for stabilizing the OFMSW.     

Effects of acclimated sludge used as seeding material in the start-up of anaerobic digestion of glycerol

Methods for improving the anaerobic digestion of glycerol (propane-1,2,3-triol) were investigated, particularly the effects of using acclimated sludge as seeding material during start-up. Glycerol was supplied to the anaerobic digester at an organic loading rate of 2.5 g-COD L^?1 day^?1. Four experimental runs were carried out with varying mixing ratios of acclimated sludge to unacclimated sludge (0, 10, 20, and 33%). Calculations were performed by employing a numerical model, whose parameters were determined by experimental measurements. Methane production rate (MPR) for all runs attained similar stable values around 21.4 mmol L^?1 day^?1, though more time was required for attaining stable state of methane production with lower mixing ratios of acclimated sludge. The initial MPR calculated was proportional to the mixing ratio of acclimated sludge. Furthermore, molecular biological methods showed that the types of microorganisms observed in all runs were similar. These results indicate that the seeding with different mixing ratios of acclimated sludge did not affect the microbial consortia in the anaerobic digestion approaching stable state, but did affect the cell density of the useful microorganisms at the start of methane fermentation. Consequently, it was confirmed that at a higher mixing ratio of acclimated sludge, the start of methane production became more vigorous.     

有机固体废物生物法制氢的研究进展

综述了利用有机固体废物生物法制氢的原理和研究现状。城市有机固体废物、农业有机废物、工业有机废物是生物法制氢的主要原料。暗发酵制氢是利用有机废物厌氧消化的产酸阶段而产氢，pH、温度、水力停留时间、氢气分压、原料性质、微量元素含量、产甲烷微生物抑制剂等均影响氢气产率。光发酵制氢是利用光合厌氧细菌将挥发性有机酸转化为氢气和二氧化碳。暗发酵和光发酵制氢时，生物固定化有利于高速连续产氢。在有机废物处理和生物法制氢方面，暗发酵一光发酵、暗发酵一微生物燃料电池的组合工艺是具有前景的技术。今后的研究方向是原料的预处理技术、选育高效产氢菌株、发明高效反应器、优化处理工艺和处理条件等。     

Effects of a gradually increased load of fish waste silage in co-digestion with cow manure on methane production

This study examined the effects of an increased load of nitrogen-rich organic material on anaerobic digestion and methane production. Co-digestion of fish waste silage (FWS) and cow manure (CM) was studied in two parallel laboratory-scale (8 L effective volume) semi-continuous stirred tank reactors (designated R1 and R2). A reactor fed with CM only (R0) was used as control. The reactors were operated in the mesophilic range (37 °C) with a hydraulic retention time of 30 days, and the entire experiment lasted for 450 days. The rate of organic loading was raised by increasing the content of FWS in the feed stock. During the experiment, the amount (volume%) of FWS was increased stepwise in the following order: 3% – 6% – 13% – 16%, and 19%. Measurements of methane production, and analysis of volatile fatty acids, ammonium and pH in the effluents were carried out. The highest methane production from co-digestion of FWS and CM was 0.400 L CH4 gVS^?1, obtained during the period with loading of 16% FWS in R2. Compared to anaerobic digestion of CM only, the methane production was increased by 100% at most, when FWS was added to the feed stock. The biogas processes failed in R1 and R2 during the periods, with loadings of 16% and 19% FWS, respectively. In both reactors, the biogas processes failed due to overloading and accumulation of ammonia and volatile fatty acids.     

Current EU-27 technical potential of organic waste streams for biogas and energy production

Anaerobic digestion of organic waste generated by households, businesses, agriculture, and industry is an important approach as method of waste treatment – especially with regard to its potential as an alternative energy source and its cost-effectiveness. Separate collection of biowaste from households or vegetal waste from public green spaces is already established in some EU-27 countries. The material recovery in composting plants is common for biowaste and vegetal waste. Brewery waste fractions generated by beer production are often used for animal feeding after a suitable preparation. Waste streams from paper industry generated by pulp and paper production such as black liquor or paper sludge are often highly contaminated with toxic substances. Recovery of chemicals and the use in thermal processes like incineration, pyrolysis, and gasification are typical utilization paths. The current utilization of organic waste from households and institutions (without agricultural waste) was investigated for EU-27 countries with Germany as an in-depth example. Besides of biowaste little is known about the suitability of waste streams from brewery and paper industry for anaerobic digestion. Therefore, an evaluation of the most important biogas process parameters for different substrates was carried out, in order to calculate the biogas utilization potential of these waste quantities. Furthermore, a calculation of biogas energy potentials was carried out for defined waste fractions which are most suitable for anaerobic digestion. Up to 1% of the primary energy demand can be covered by the calculated total biogas energy potential. By using a “best-practice-scenario” for separately collected biowaste, the coverage of primary energy demand may be increased above 2% for several countries. By using sector-specific waste streams, for example the German paper industry could cover up to 4.7% and the German brewery industry up to 71.2% of its total energy demand.     

Anaerobic digestion of cattail by rumen cultures

The anaerobic digestion of aquatic plants could serve the dual roles for producing renewable energy and reducing waste. In this study, the anaerobic digestion of cattail (Typha latifolia linn), a lignocellulosic aquatic plant, by rumen microorganisms in batch cultures was investigated. At a substrate level of 12.4 g/l volatile solids (VS) and pH 6.7, maximum VS conversion of 66% was achieved within an incubation time of 125 h. However, a decrease in pH from 6.7 to 5.8 resulted in a marked reduction in VS conversion. The total volatile fatty acids (VFAs) yield was about 0.56 g/g VS digested. Acetate and propionate were the major aqueous fermentation products, while butyrate, i-butyrate and valerate were also formed in smaller quantities. Biogas that was produced was composed of carbon dioxide, methane and hydrogen. A modified Gompertz equation was developed to describe substrate consumption and product formation. The hydrolysis of insoluble components was the rate-limiting step in the anaerobic digestion of cattail.