Energy recovery from grass using two-phase anaerobic digestion

Biotechnology to reduce logistics burden and promote environmental stewardship for Air Force civil engineering requirements

This review provides discussion of advances in biotechnology with specific application to civil engineering requirements for airfield and airbase operations. The broad objectives are soil stabilization, waste management, and environmental protection. The biotechnology focal areas address (1) treatment of soil and sand by biomineralization and biopolymer addition, (2) reduction of solid organic waste by anaerobic digestion, (3) application of microbes and higher plants for biological processing of contaminated wastewater, and (4) use of indigenous materials for airbase construction and repair. The consideration of these methods in military operating scenarios, including austere environments, involves comparison with conventional techniques. All four focal areas potentially reduce logistics burden, increase environmental sustainability, and may provide energy source, or energy-neutral practices that benefit military operations.

Effect of Zeolite on the Methane Production from Chicken Manure Leachate

This study demonstrates the leachate characteristics derived from bench-scale leach-bed reactors (LBRs) filled with chicken manure (CM) and zeolite. Zeolite was used to maintain the necessary porosity for the leaching process and to adsorb ammonia. Fresh water was added for leachate production and removed daily, in order to estimate the readily leachable organic and nitrogen matter of the CM. Tests were conducted at two ratios of zeolite to bed (10% and 3.5% v/v CMbed). Other operating parameters studied were the amount of water added in the LBRs, the leachate recirculation rate, and the hydraulic retention time (HRT). A control LBR with river pebbles at a similar size and ratio (10% v/v) with zeolite was also studied. Some experiments were repeated with CM, which had different characteristics. Compared to the control test, the LBR with zeolite at 10% v/v yielded leachate with less NH3 and a higher biochemical methane potential (BMP). However, free ΝH3 in the control experiment was below the inhibition threshold, proving that zeolite contributes to the higher BMP of leachate, and that this effect is not only due to NH3 adsorption.

Localized mixing of anaerobic plug flow reactors

An innovative localized-mixing concept was tested in an Anaerobic Plug Flow Reactor (AnPFR) treating Food Waste (FW) mixed with municipal Wastewater (WW). The proposed concept consists of placing propellers along the shaft of the AnPFR at key points that represent the mid-region of each of the anaerobic digestion stages: hydrolysis, acidogenesis, and methanogenesis. First, the need for and efficiency of localized mixing (the new concept suggested by the authors) were investigated. While the main benefit of localized mixing is the reduction of energy demand associated with (conventional) uniform mixing (i.e., throughout the longitudinal axis), the system can also benefit from synergetic reactions in non-mixed zones. In fact, at a Total Solid (TS) content of 15% (Organic Loading Rate (OLR) of 4.2 g VS.L ^- 1.d ^- 1) and a Hydraulic Retention Time (HRT) of 28 days, the mixing pattern was sufficient to maintain stable operation, with high removal rates (up to 96% of solids) and high biogas generation (1128 ± 55 ml.g VS_fed^-1, of which 68.9% consisted of CH₄); but when mixing was halted, the system's performance deteriorated. Second, the loading capacity of the locally-mixed AnPFR was investigated by subjecting it to different TS content (10%, 15%, 20%, and 22.5%, corresponding to OLRs of 2.8, 4.2, 6.3, and 7.9 g VS.L ^- 1.d ^- 1, respectively) while operating under the same HRT. It was found that the system can adequately sustain a feed with a maximum TS of 20% while achieving removal rates up to 92% for solids and a CH₄ yield of 613 ml.g VS_fed^-1. The digester was simulated using computational fluid dynamics. The outputs revealed: (1) highest radial mixing at the center of the methanogenesis zone where the propeller is located and (2) low longitudinal mixing before and after the propeller of the methanogenesis stage, implying the presence of sedimentation zones that was visually verified. The former is assumed to favor better dispersion of inhibitors and improved stability, while the latter is expected to provide stagnant areas for enhanced biochemical synergies.

Biogas Production from Physicochemically Pretreated Grass Lawn Waste: Comparison of Different Process Schemes

Various pretreatment methods, such as thermal, alkaline and acid, were applied on grass lawn (GL) waste and the effect of each pretreatment method on the Biochemical Methane Potential was evaluated for two options, namely using the whole slurry resulting from pretreatment or the separate solid and liquid fractions obtained. In addition, the effect of each pretreatment on carbohydrate solubilization and lignocellulossic content fractionation (to cellulose, hemicellulose, lignin) was also evaluated. The experimental results showed that the methane yield was enhanced with alkaline pretreatment and, the higher the NaOH concentration (20 g/100 gTotal Solids (TS)), the higher was the methane yield observed (427.07 L CH₄/kg Volatile Solids (VS), which was almost 25.7% higher than the BMP of the untreated GL). Comparing the BMP obtained under the two options, i.e., that of the whole pretreatment slurry with the sum of the BMPs of both fractions, it was found that direct anaerobic digestion without separation of the pretreated biomass was favored, in almost all cases. A preliminary energy balance and economic assessment indicated that the process could be sustainable, leading to a positive net heat energy only when using a more concentrated pretreated slurry (i.e., 20% organic loading), or when applying NaOH pretreatment at a lower chemical loading.

Exploitation of Mowed Grass from Green Areas by Means of Anaerobic Digestion: Effects of Grass Conservation Methods (Drying and Ensiling) on Biogas and Biomethane Yield

Grass from landscape management or from agricultural practices is currently destined mainly for composting, with the production of a valuable product; however, this process demands energy. Anaerobic digestion, instead, represents an energy-positive process that results in the production of fuel, biogas, and a fertilizer, namely digestate. Previous tests for the evaluation of biogas yield from freshly harvested grass gave promising results. However, for a practical exploitation of this resource, appropriate conservation is necessary in order to enable the daily load of digesters while reducing the loss of organic matter. The present work is focused on the evaluation of biogas and methane yield from dried and ensiled grass (without conditioning) in order to assess eventual biogas potential losses in comparison to digested fresh grass. Tests were performed with grass collected from riverbanks (Veneto, Northern Italy) in batch, lab scale digesters. Dry and ensiled grass showed a good potential for exploitation in the anaerobic digestion process, reaching biogas yields of 565.9 and 573.4 NL∙kgVS−1, respectively. Compared to the biogas yield of 639.7 NL∙kgVS−1 of the fresh grass, the conservation treatment determined yield reductions of 11.5% and 10.4% for dried and ensiled grass, respectively. However, considering the methane yields, conservation treatments showed lower reductions, amounting to 4.8% for dry grass and 0.5% for ensiled grass; presumably the higher concentration of organic acids in ensiled grass determined a higher methane content in biogas and the consequently lower reduction of methane yield.

A Review on Anaerobic Co-Digestion with a Focus on the Microbial Populations and the Effect of Multi-Stage Digester Configuration

Recent studies have shown that anaerobic co-digestion (AnCoD) is superior to conventional anaerobic digestion (AD). The benefits of enhanced bioenergy production and solids reduction using co-substrates have attracted researchers to study the co-digestion technology and to better understand the effect of multi substrates on digester performance. This review will discuss the results of such studies with the main focus on: (1) generally the advantages of co-digestion over mono-digestion in terms of system stability, bioenergy, and solids reduction; (2) microbial consortia diversity and their synergistic impact on biogas improvement; (3) the effect of digester mode, i.e., multi-stage versus single stage digestion on AnCoD. It is essential to note that the studies reported improvement in the synergy and diverse microbial consortia when using co-digestion technologies, in addition to higher biomethane yield when using two-stage mode. A good example would be the co-digestion of biodiesel waste and glycerin with municipal waste sludge in a two-stage reactor resulting in 100% increase of biogas and 120% increase in the methane content of the produced biogas with microbial population dominated by Methanosaeta and Methanomicrobium.

Anaerobic digestion of thin stillage of corn ethanol plant in a novel anaerobic baffled reactor

In this study, the performance of a conventional anaerobic baffled reactor (ABR) and a novel configuration of hybrid ABR for the treatment of thin stillage was evaluated. The hybrid ABR achieved the chemical oxygen demand (COD) removal, sulfate removal and methane yield of 97-94%, 94-97% and 294-310 mL CH₄ g^-1 COD_removed, respectively at organic loading rate (OLR) of 1-3.5 kg COD m^-3 d^-1. On the other hand, the value of COD and sulfate removal and methane yield for the conventional ABR were 75-94%, 67-76% and 140-240 mL CH₄ g^-1 COD_removed, respectively at OLR range of 1.1-1.8 kg COD m^-3 d^-1. The enhanced performance and robustness of the novel ABR was demonstrated to be the result of incorporation of solid/liquid/gas separators into the configuration of the conventional ABR, leading to reduced biomass washout, higher solid retention time and significantly improved phase separation.

Biomethanation of Harmful Macroalgal Biomass in Leach-Bed Reactor Coupled to Anaerobic Filter: Effect of Water Regime and Filter Media

Ulva is a marine macroalgal genus which causes serious green tides in coastal areas worldwide. This study investigated anaerobic digestion as a way to manage Ulva waste in a leach-bed reactor coupled to an anaerobic filter (LBR-AF). Two LBR-AF systems with different filter media, blast furnace slag grains for R1, and polyvinyl chloride rings for R2, were run at increasing water replacement rates (WRRs). Both achieved efficient volatile solids reduction (68.4–87.1%) and methane yield (148–309 mL/g VS fed) at all WRRs, with the optimal WRR for maximum methane production being 100 mL/d. R1 maintained more stable methanation performance than R2, possibly due to the different surface properties (i.e., biomass retention capacity) of the filter media. Such an effect was also noted in the different behaviors of the LBR and AF between R1 and R2. The molecular analysis results revealed that the development of the microbial community structure in the reactors was primarily determined by the fermentation type, i.e., dry (LBR) or wet (AF).

Effects of digestate recirculation on a two-stage anaerobic digestion system, particularly focusing on metabolite correlation analysis

Single-stage (S-N treatment) and two-stage anaerobic digestion with (T-R treatment) and without digestate recirculation (T-N treatment) for methane production using food waste (FW) were comparatively evaluated to examine the effects of digestate recirculation on anaerobic digestion (AD). Digestate recirculation positively affected the methane yield and organic loading rate (OLR). Metabolite correlation analysis revealed that a systematic hydrolysis degree of greater than 75% is crucial to achieve the complete recoverable yield of methane from FW. Digestate recirculation also markedly increased the system alkalinity, maintaining an optimum pH for methanogens. However, the ammonium accumulated by T-R treatment would destroy the metabolic balance between the hydrolytic bacteria and methanogens, especially at a critical OLR. Therefore, the appropriate control of two-stage AD systems with digestate recirculation is limited not only to OLR regulation but also to the prevention of ammonium accumulation.

Intrinsic gas production kinetics of selected intermediates in anaerobic filters for demand-orientated energy supply

As a consequence of a growing share of solar and wind power, recent research on biogas production highlighted a need for demand-orientated, flexible gas production to provide grid services and enable a decentralized stabilization of the electricity infrastructure. Two-staged anaerobic digestion is particularly suitable for shifting the methane production into times of higher demand due to the spatio-temporal separation of hydrolysis and methanogenesis. To provide a basis for predicting gas production in an anaerobic filter, kinetic parameters of gas production have been determined experimentally in this study. A new methodology is used, enabling their determination during continuous operation. An order in methane production rate could be established by comparing the half lives of methane production. The order was beginning with the fastest: acetic acid>ethanol>butyric acid>iso-butyric acid>valeric acid>propionic acid>1,2propanediol>lactic acid. However, the mixture of a natural hydrolysate from the acidification tank appeared to produce methane faster than all single components tested.

Methanogens: biochemical background and biotechnological applications

Since fossil sources for fuel and platform chemicals will become limited in the near future, it is important to develop new concepts for energy supply and production of basic reagents for chemical industry. One alternative to crude oil and fossil natural gas could be the biological conversion of CO2 or small organic molecules to methane via methanogenic archaea. This process has been known from biogas plants, but recently, new insights into the methanogenic metabolism, technical optimizations and new technology combinations were gained, which would allow moving beyond the mere conversion of biomass. In biogas plants, steps have been undertaken to increase yield and purity of the biogas, such as addition of hydrogen or metal granulate. Furthermore, the integration of electrodes led to the development of microbial electrosynthesis (MES). The idea behind this technique is to use CO2 and electrical power to generate methane via the microbial metabolism. This review summarizes the biochemical and metabolic background of methanogenesis as well as the latest technical applications of methanogens. As a result, it shall give a sufficient overview over the topic to both, biologists and engineers handling biological or bioelectrochemical methanogenesis.

Kinetics and microbial community analysis for hydrogen production using raw grass inoculated with different pretreated mixed culture

In this study, five pretreatment methods (heat shock, acid, base, aeration and gamma radiation) were applied for enriching hydrogen producers from anaerobically digested sludge, aiming to compare their hydrogen fermentation performance using raw ryegrass as substrate. Results showed that various pretreatment methods caused great variations on grass hydrogen fermentation performance. Acid pretreatment was most efficient compared with other tested pretreatment methods, with relevant hydrogen yield of 64.4mL/g dry grass and organics removal of 31.4%. Kinetics results showed that the first-order kinetic model fitted hydrogen evolution better than the modified Gompertz model. Microbiological analysis showed that various pretreatment methods caused great variations on microbial activity and microbial community composition. Clostridium and Enterococcus were two dominant genera, while relative abundances of these two genera varied greatly for different pretreated samples. Difference in microbial activity and microbial community distribution induced by the pretreatment methods might directly cause different ryegrass fermentation performance.

Co-digestion of agricultural and municipal waste to produce energy and soil amendment

In agriculture, manure and cotton gin waste are major environmental liabilities. Likewise, grass is an important organic component of municipal waste. These wastes were combined and used as substrates in a two-phase, pilot-scale anaerobic digester to evaluate the potential for biogas (methane) production, waste minimisation, and the digestate value as soil amendment. The anaerobic digestion process did not show signs of inhibition. Biogas production increased during the first 2 weeks of operation, when chemical oxygen demand and volatile fatty acid concentrations and the organic loading rate to the system were high. Chemical oxygen demand from the anaerobic columns remained relatively steady after the first week of operation, even at high organic loading rates. The experiment lasted about 1 month and produced 96.5 m³ of biogas (68 m³ of CH₄) per tonne of waste. In terms of chemical oxygen demand to methane conversion efficiency, the system generated 62% of the theoretical methane production; the chemical oxygen demand/volatile solids degradation rate was 62%, compared with the theoretical 66%. The results showed that co-digestion and subsequent digestate composting resulted in about 60% and 75% mass and volume reductions, respectively. Digestate analysis showed that it can be used as a high nutrient content soil amendment. The digestate met Class A faecal coliform standards (highest quality) established in the United States for biosolids. Digestion and subsequent composting concentrated the digestate nitrogen, phosphorus, and potassium content by 37%, 24%, and 317%, respectively. Multi-substrate co-digestion is a practical alternative for agricultural waste management, minimisation of landfill disposal, and it also results in the production of valuable products.

Anaerobic Codigestion of Grass and Sewage Sludge: Laboratory Experiments and Feasibility Analysis

  Grass is drawing attention for its high potential for biogas production. Anaerobic digesters in wastewater treatment plants can be used for producing biogas by processing grass without having to invest in the construction of new treatment facilities. Batch and continuous digestion experiments were conducted to assess the feasibility of codigestion of sewage sludge and grass. The focus was on a thermophilic condition, starting-up from existing mesophilic anaerobic digestion of sewage sludge only. The batch and continuous experiments used two different sewage sludges. Results demonstrated a methane generation of approximately 0.2 normal liter-methane/g volatile solids-grass. The addition of grass did not affect the ammonia concentration and improved the dewaterability of the digested sludge. The start-up performance from mesophilic digested sewage sludge to thermophilic codigestion with grass was confirmed. A feasibility analysis demonstrated that codigestion is applicable in terms of energy recovery and greenhouse gas emission, depending on the transport distance of grass.

A fast linear predictive adaptive model of packed bed coupled with UASB reactor treating onion waste to produce biofuel

BACKGROUND

Agro-industrial wastes are an energy source for different industries. However, its application has not reached small industries. Previous and current research activities performed on the acidogenic phase of two-phase anaerobic digestion processes deal particularly with process optimization of the acid-phase reactors operating with a wide variety of substrates, both soluble and complex in nature. Mathematical models for anaerobic digestion have been developed to understand and improve the efficient operation of the process. At present, lineal models with the advantages of requiring less data, predicting future behavior and updating when a new set of data becomes available have been developed. The aim of this research was to contribute to the reduction of organic solid waste, generate biogas and develop a simple but accurate mathematical model to predict the behavior of the UASB reactor.

RESULTS

The system was maintained separate for 14 days during which hydrolytic and acetogenic bacteria broke down onion waste, produced and accumulated volatile fatty acids. On this day, two reactors were coupled and the system continued for 16 days more. The biogas and methane yields and volatile solid reduction were 0.6 ± 0.05 m3 (kg VSremoved)-1, 0.43 ± 0.06 m3 (kg VSremoved)-1 and 83.5 ± 9.8 %, respectively. The model application showed a good prediction of all process parameters defined; maximum error between experimental and predicted value was 1.84 % for alkalinity profile.

CONCLUSIONS

A linear predictive adaptive model for anaerobic digestion of onion waste in a two-stage process was determined under batch-fed condition. Organic load rate (OLR) was maintained constant for the entire operation, modifying effluent hydrolysis reactor feed to UASB reactor. This condition avoids intoxication of UASB reactor and also limits external buffer addition.

Leachate recirculation between alternating aged refuse bioreactors and its effect on refuse decomposition

In a sequencing batch bioreactor landfill system which combined a fresh and an aged refuse bioreactor, blockage occurred frequently in the aged refuse bioreactor during the treatment of leachate from the fresh refuse bioreactor. To overcome this problem, another aged refuse bioreactor was added, when blockage occurred, the two aged refuse bioreactor operated alternatively. A fresh refuse bioreactor F combined with two alternating aged refuse bioreactors A1 and A2 was called alternate recirculation process (ARP) in this study. The bioreactor system was operated in three stages, and the three bioreactors were exposed to air to facilitate surface re-aeration. The effect of the ARP on the accelerated degradation of fresh refuse was compared before and after blockage occurs in A1. The results indicated that ARP can improve the leachate production rate. The average daily net production rates of leachate in Stages 2 and 3 were approximately 2.1 and 1.6 mL (kgrefuse d)(-1), respectively, which exceeded that of Stage 1 (1.3 mL (kg refuse d)(-1)). The chemical oxygen demand and NH3-N concentrations of the leachate from Stage 1 are 1000 and 25mgL(-1) after 2.1 and 2.7 y, respectively. For Stages 2 and 3, these concentrations reach approximately after 0.877 and 1.3 y. Faster refuse settlement was observed in Stages 2 and 3, with an average daily settlement of approximately 0.11%, as compared with Stage 1 (approximately 0.099%). ARP can accelerate the biodegradation of the fresh refuse and overcome the problem of the blockage in the aged refuse reactor.

Feasibility study on anaerobic biodegradation of azo dye reactive orange 16

Higher colour removal (>90%) shows feasibility of azo dye degradation by anaerobic digestion. Hydrogenotrophic methanogens are the key methane producers. Long retention time is useful for degrading aromatic amines under anaerobic condition.

Anaerobic co-digestion of sewage sludge with shredded grass from public green spaces

Adding greenery from public spaces to the co-digestion process with sewage sludge was evaluated by shredding experiments and laboratory-scale batch and continuous mesophilic anaerobic fermentation experiments. The ratio of the shredded grass with 20mm or less in length by a commercially available shredder was 93%. The methane production was around 0.2NL/gVS-grass in the batch experiment. The continuous experiment fed with sewage sludge and shredded grass was stably operated for 81days. The average methane production was 0.09NL/gVS-grass when the TS ratio of the sewage sludge and the grass was 10:1. This value was smaller than those of other reports using grass silage, but the grass species in this study were not managed, and the collected grass was just shredded and not ensiled before feeding to the reactor for simple operation. The addition of grass to a digester can improve the carbon/nitrogen ratio, methane production and dewaterability.

Mesophilic anaerobic co-digestion of pulp and paper sludge and food waste for methane production in a fed-batch basis

Co-digestion of pulp and paper sludge (PPS) and food waste (FW) in a batch-fed digestion system was conducted on a laboratory scale. Three reactors named A1, A2, and A3 were tested. PPS and FW mixed at different mass ratios of 1:3, 1:1, and 3:1, respectively, were loaded in the reactors. Bioconversion at high efficiency was obtained in the system. The accumulative methane yield of each reactor was 144mLg(-1)VSfed (A1), 256 mL g(-1) VSfed (A2), and 123 mL g(-1)VSfed (A3). The soluble chemical oxygen demand (COD) removal efficiencies reached 73.2% (Al), 93.9% (A2), and 79.6% (A3). A pH in the range 5.8-8.4 was obtained in the three reactors without adjustment due to the high buffer capacity of the mixing feedstock. No toxicity inhibitions of volatile fatty acids and NH3-N occurred in reactor A2. This study showed that it was good for co-digestion of PPS and FW in a mass ratio of 1:1 for methane production, which resulted in higher methane yield, a greater buffer capacity, a higher organics removal efficiency, and a more stable process.

The effect of pH control and 'hydraulic flush' on hydrolysis and Volatile Fatty Acids (VFA) production and profile in anaerobic leach bed reactors digesting a high solids content substrate

The effect of hydraulic flush and pH control on hydrolysis, Volatile Fatty Acids (VFA) production and profile in anaerobic leach bed reactors was investigated for the first time. Six reactors were operated under different regimes for two consecutive batches of 28days each. Buffering at pH ∼6.5 improved hydrolysis (Volatile Solid (VS) degradation) and VFA production by ∼50%. Butyric and acetic acid were dominant when reactors were buffered, while only butyric acid was produced at low pH. Hydraulic flush enhanced VS degradation and VFA production by ∼15% and ∼32%, respectively. Most Probable Number (MPN) of cellulolytic microorganisms indicated a wash out when hydraulic flush was applied, but pH control helped to counteract this. The highest VS degradation (∼89%), VFA yield (0.84kgCODkg(-1)VS(added)) and theoretical methane potential (0.37m(3)CH(4)kg(-1)VS(added)) were obtained when pH control and hydraulic flush were applied, and therefore, these conditions are recommended.

Use of modeling to aid design of a two-phase grass digestion system

A sequentially fed leach bed system coupled with a leachate holding tank and an Upflow Anaerobic Sludge Blanket (UASB) was modeled based on 310d of grass silage digestion with the goal of generating specific design instruction. The model suggests the hydrolysis rate is proportional to the sprinkling rate and retention time. It suggests that raising the sprinkling rate by a third (from 600L/d to 800L/d) increases the volatile solids destruction from 70% to 80% for a retention time of 30d yielding 370L CH(4)/kg VS. The volume of the leachate holding tank has a minimal influence on methane production (reducing its volume by a factor of 2 reduces methane yield by 1%). The model suggests that for a constant sprinkling rate, shorter retention time increases daily methane production, but lowers specific methane yield (L CH(4)/kg VS). Longer retention time increases methane content in the biogas.

Methane production from anaerobic co-digestion of the separated solid fraction of pig manure with dried grass silage

Anaerobic co-digestion of the solid fraction of separated pig manure (SPM) with dried grass silage (DGS) was evaluated in three identical continuously stirred tank reactors (CSTRs) at 35±1 °C. The feedstock contained 20% DGS in CSTR1, 30% DGS in CSTR2 and 40% DGS in CSTR3 on a volatile solids (VS) basis. Organic loading rates (OLR) of 1.0, 1.5, 2.0 and 3.0 kg VS/m(3)/d were studied and it was found that the OLR affected the digester performance more than the DGS proportion in the feedstock. Tripling the OLR increased volumetric methane yields by 88% and decreased specific methane yields by 38%. At the OLR of 3 kg VS/m(3)/d, post-methane production potentials of digestates ranged from 38% to 41% of total methane production potentials of the feedstock. An energy yield estimation on a 654-sow pig unit showed that 268-371 MWh/a electricity and 383-530 MWh/a heat would be generated.

Optimizing the operation of a two-phase anaerobic digestion system digesting grass silage

This paper examines the optimization of an existing two-phase anaerobic digestion process using grass silage as a feedstock. The system comprises 6 leach beds connected to an upflow anaerobic sludge blanket (UASB). The existing system produced 305 L CH(4) kg(-1) VS added at an overall retention time of 42 days (6 leach beds emptied and fed sequentially every 7 days in series). The desired improvements were a reduction in retention time with increased methane production. It was noted in the existing system that biogas production and COD levels fell off in the last 2 days of each 7-day cycle. Thus the first change involved reduction in retention time to 30 days (6 leach beds fed sequentially every 5 days in series). This lead to a slight improvement in methane production (310 L CH(4) kg(-1) VS added). The second change was effected by separation of flows to the first stage (leach beds) and the second stage (UASB) through addition of an extra pump to optimize leaching. This led to an increase in CH(4) production (341 L CH(4) kg(-1) VS). The overall improvement from the existing system was an increase of 11.8% in methane production and a reduction in size or retention time of 40% (42 days decreased to 30 days retention time).

Effect of pig manure to grass silage ratio on methane production in batch anaerobic co-digestion of concentrated pig manure and grass silage

Anaerobic co-digestion of concentrated pig manure (PM) with grass silage (GS) at five different PM to GS volatile solid (VS) ratios of 1:0, 3:1, 1:1, 1:3 and 0:1 was evaluated by examining operation stability and methane (CH(4)) production potentials. The highest specific CH(4) yields were 304.2 and 302.8 ml CH(4)/g VS at PM to GS ratios of 3:1 and 1:1, respectively. The digestion systems failed at the ratio of 0:1. The lag phase lasted 29.5, 28.1, 24.6 and 21.3 days at the ratios of 1:0, 3:1, 1:1 and 1:3, respectively. The daily methane yield was linearly correlated with the acetic acid concentration, indicating methane production was probably associated with acetoclastic methanogenesis. The hydrolysis constant linearly decreased with increasing the fraction of GS in the feedstock. This study recommends applying the PM to GS ratio of 1:1 in practice due to a high specific methane yield and a short lag phase.

Biological pretreatment enhances biogas production in the anaerobic digestion of pulp and paper sludge

High efficient resource recovery from pulp and paper sludge (PPS) has been the focus of attention. The objective of this research was to develop a bio-pretreatment process prior to anaerobic digestion of PPS to improve the methane productivity. Active and inactive mushroom compost extracts (MCE) were used for pretreating PPS, followed by anaerobic digestion with monosodium glutamate waste liquor (MGWL). Laboratory-scale experiments were carried out in completely mixed bioreactors, 1-L capacity with 700 ml useful capacity. Optimal amount of active MCE for organics' solubilization in the step of pretreatment was 250 A.U./gVS( sludge). Under this condition, the PPS floc structure was well disrupted, resulting in void rate and fibre size diminishment after pretreatment. In addition, SCOD and VS removal were found to be 56% and 43.6%, respectively, after anaerobic digestion, being the peak value of VFA concentration determined as 1198 mg acetic acid L(-1). The anaerobic digestion efficiency of PPS with and without pretreatment was evaluated. The highest methane yield under optimal pretreatment conditions was 0.23 m(3) CH4/kgVS(add), being 134.2% of the control. The results indicated that MCE bio-pretreatment could be a cost-effective and environmentally sound method for producing methane from PPS.

Two-phase anaerobic co-digestion of olive mill wastes in semi-continuous digesters at mesophilic temperature

This study investigates for the first time, on laboratory scale, the possible exploitation of the advantages of two-phase anaerobic digestion for treating a mixture of olive mill wastewater (OMW) and olive mill solid waste (OMSW) using two sequencing semi-continuous digesters operated at mesophilic temperature (37+/-2 degrees C). The experiments were conducted at hydraulic retention times (HRTs) of 14 and 24 days corresponding to organic loading rates (OLRs) ranging from 5.54 to 14 g COD/L/day in the first stage (acidifier) and at HRTs of 18, 24 and 36 days corresponding to OLRs ranging from 2.28 to 9.17 g COD/L/day in the second stage (methanizer). The results indicated that volatile fatty acids (VFA) concentrations increased with the increase of either HRT or feed concentration and their high values were obtained with the most concentrated influent (196+/-5 g COD/L) digested at the longest HRT (24 days) corresponded to an OLR of 8.17 g COD/L/d. Furthermore, two-phase anaerobic digestion system has given the best performances concerning methane productivity, soluble COD (SCOD) and phenol removal efficiencies and effluent quality compared to those given by conventional one-phase anaerobic digestion (AD) reactors.

Comparison on the removal of phthalic acid diesters in a bioreactor landfill and a conventional landfill

The removal of phthalic acid diesters (PAEs) in municipal solid waste (MSW) from two simulated landfill reactors was compared. The results showed that the original concentrations of dimethyl phthalate (DMP), dibutyl phthalate (DBP) and dioctyl phthalate (DOP) in the refuse were 3.3 microg g(-1), 18.5 microg g(-1) and 0.8 microg g(-1), respectively. The concentrations of DMP and DBP in both leachate and refuse decreased greatly during decomposition of the waste in both reactors. The major loss of PAEs from the landfill occurred during an active methanogenic environment with a low concentration of volatile fatty acids (VFA) in the later period. In addition, strong correlations were found between the residual DMP, DBP concentrations and the biologically degradable material (BDM) of the refuse. Finally, PAEs degraded more rapidly in the landfill that was operated in conjunction with the methanogenic reactor when compared to the landfill with direct leachate discharge.

Effect of temperature on VFA's and biogas production in anaerobic solubilization of food waste

The effectiveness of methane fermentation treatment used in food waste processing is currently limited by solubilization and acidogenesis. In efforts to improve the treatment process, this study examined the effects of temperature on solubilization and acidogenesis. The solubilization rate of food waste, which was based on suspended solid removal, was 47.5%, 62.2%, 70.0%, 72.7%, 56.1% and 45.9% at 15 degrees C, 25 degrees C, 35 degrees C, 45 degrees C, 55 degrees C and 65 degrees C, respectively. Solubilization rate was accelerated from the middle to late experimental periods under mesophilic (35 degrees C and 45 degrees C) conditions. In contrast, overall solubilization rate was significantly lower under thermophilic (55 degrees C and 65 degrees C) conditions than under mesophilic conditions, although solubilization occurred rapidly in the early experimental period. The production of biogas was high under mesophilic conditions of 35 degrees C and 45 degrees C, at 64.7 and 62.7mL/g-VS, respectively, while it was scarce under thermophilic conditions. Solubilization of food waste was accelerated under both mesophilic and thermophilic conditions; however, solubilization rate was observed to be particularly high under mesophilic conditions, and a shortening of the hydraulic retention time is expected under thermophilic conditions.

Mono fermentation of grass silage by means of loop reactors

A loop reactor was operated for mono fermentation of grass silage without manure addition under mesophilic conditions (38 degrees C). An averaged specific biogas production of 0.50 m(N)(3) per kg volatile solids (VS) with a methane concentration of 52% at an organic loading rate of up to 3.5 kg(VS)/(m(3) d) was obtained. The retention time varied from 440 days at 1.0 kg(VS)/(m(3) d) to 50 days at 3.5 kg(VS)/(m(3) d). The degradation level was more than 60% based on VS and 75% based on COD. The first-order hydrolysis rate constant of the process was estimated to be 0.6 d(-1). Despite the relative high ammonium concentration of up to 4 g/l, the system worked stable for an operation period of 310 days. In particular the TS content in the fermenter was found to be a key parameter and should not exceed 12% in order to avoid instabilities.

Review of the integrated process for the production of grass biomethane

Production of grass biomethane is an integrated process which involves numerous stages with numerous permutations. The grass grown can be of numerous species, and it can involve numerous cuts. The lignocellulosic content of grass increases with maturity of grass; the first cut offers more methane potential than the later cuts. Water-soluble carbohydrates (WSC) are higher (and as such methane potential is higher) for grass cut in the afternoon as opposed to that cut in the morning. The method of ensiling has a significant effect on the dry solids content of the grass silage. Pit or clamp silage in southern Germany and Austria has a solids content of about 40%; warm dry summers allow wilting of the grass before ensiling. In temperate oceanic climates like Ireland, pit silage has a solids content of about 21% while bale silage has a solids content of 32%. Biogas production is related to mass of volatile solids rather than mass of silage; typically one ton of volatile solid produces 300 m(3) of methane. The dry solids content of the silage has a significant impact on the biodigester configuration. Silage with a high solids content would lend itself to a two-stage process; a leach bed where volatile solids are converted to a leachate high in chemical oxygen demand (COD), followed by an upflow anaerobic sludge blanket where the COD can be converted efficiently to CH(4). Alternative configurations include wet continuous processes such as the ubiquitous continuously stirred tank reactor; this necessitates significant dilution of the feedstock to effect a solids content of 12%. Various pretreatment methods may be employed especially if the hydrolytic step is separated from the methanogenic step. Size reduction, thermal, and enzymatic methodologies are used. Good digester design is to seek to emulate the cow, thus rumen fluid offers great potential for hydrolysis.

Effect of feed to inoculum ratios on biogas yields of food and green wastes

Biogas and methane yields of food and green wastes and their mixture were determined using batch anaerobic digesters at mesophilic (35+/-2 degrees C) and thermophilic (50+/-2 degrees C) temperatures. The mixture was composed of 50% food waste and 50% green waste, based on the volatile solids (VS) initially added to the reactors. The thermophilic digestion tests were performed with four different feed to inoculum (F/I) ratios (i.e., 1.6, 3.1, 4.0 and 5.0) and the mesophilic digestion was conducted at one F/I (3.1). The results showed that the F/I significantly affected the biogas production rate. At four F/Is tested, after 25 days of thermophilic digestion, the biogas yield was determined to be 778, 742, 784 and 396 mL/g VS for food waste, respectively; 631, 529, 524 and 407 mL/g VS for green waste, respectively; and 716, 613, 671 and 555 mL/g VS for the mixture, respectively. About 80% of the biogas production was obtained during the first 10 days of digestion. At the F/I of 3.1, the biogas and methane yields from mesophilic digestion of food waste, green waste and their mixture were lower than the yields obtained at thermophilic temperature. The biogas yields were 430, 372 and 358 mL/g VS, respectively, and the methane yields were 245, 206, and 185 mL/g VS, respectively.

The effect of enzyme addition on anaerobic digestion of JoseTall Wheat Grass

The effects of the addition of enzyme products containing cellulase, hemicellulase, and beta-glucosidase to anaerobic digestion systems were studied using JoseTall Wheat Grass (wheat grass) as a model substrate. Anaerobic digestion tests were performed using batch reactors operated at 50 degrees C. The application of enzyme products in three digestion configurations were simulated and investigated: (1) enzyme addition to a single-stage digester, (2) pre-treatment of wheat grass with enzymes followed by a single-stage anaerobic digestion, and (3) enzyme addition to the first stage (hydrolysis and acidification) of a two-stage digestion system. The enzyme products showed positive effects on the solubilization of wheat grass when used alone to treat the wheat grass. However, no significant differences in biogas and methane yields, and volatile solids reduction resulted when the enzyme products were tested in the anaerobic digestion systems. This reveals that the microorganisms present in the inoculum were effective in carrying out the digestion of wheat grass. The types of microorganisms present in the inoculum were identified using 16S rRNA sequence analysis. A comparison of the sequences between the different inocula revealed that the prevalent operational taxonomic units were similar, but that the acidified inoculum contained a higher percentage of the species Thermotogae.

Grass waste: a novel sorbent for the removal of basic dye from aqueous solution

The aim of the present work was to investigate the feasibility of grass waste (GW) for methylene blue (MB) adsorption. The adsorption of MB on GW material was studied as a function of GW dose (0.05-1.20 g), solution pH 3-10, contact time and initial concentration (70-380 mg/L). The influence of these parameters on the adsorption capacity was studied using the batch process. The experimental data were analyzed by the Langmuir and Freundlich isotherms. The adsorption isotherm was found to follow the Langmuir model. The monolayer adsorption capacity was found to be 457.640 mg/g. The kinetic data were fitted to the pseudo-first-order and pseudo-second-order models, and were found to follow closely the pseudo-second-order kinetic model. The results revealed that GW adsorbent is potentially low-cost adsorbent for adsorption of MB.

Influence of alkalinity on the stabilization of municipal solid waste in anaerobic simulated bioreactor

Four simulated landfill anaerobic bioreactors were performed to investigate the influence of alkalinity on the anaerobic treatment of municipal solid waste (MSW). Leachate was recirculated in all the four reactors. One reactor was operated without alkalinization. The other three were operated under alkaline conditions. Na(2)CO(3), NaHCO(3) and NaOH were added to leachate in the second, third and fourth reactor, respectively. Experimental results showed that CO(3)(2-) and HCO(3)(-) addition had a more pronounced effect on MSW stabilization while the effect of addition of OH(-) was weak. The concentration of COD, BOD(5), total nitrogen (TN), ammonium nitrogen (NH(4)(+)-N) and nitrate nitrogen (NO(3)(-)-N), etc. in leachate significantly reduced in four reactors. The removal efficiencies were 90.56%, 92.21%, 92.74% and 90.29% for COD, 66.45%, 72.38%, 68.62% and 68.44% for NO(3)(-)-N, and 96.5%, 98.75%, 97.75% and 98% for NO(2)(-)-N in the control, Na(2)CO(3), NaHCO(3) and OH(-) added reactors, respectively. The final BOD(5)/COD was 0.262, 0.104, 0.124, and 0.143, and pH was 7.13, 7.28, 7.42, and 7.24 for control, Na(2)CO(3) added, NaHCO(3) added, and OH(-) added reactor, respectively. Therefore, alkalinity addition had positive effect on the stabilization of MSW.

Micro-scale anaerobic digestion of point source components of organic fraction of municipal solid waste

The fermentation characteristics of six specific types of the organic fraction of municipal solid waste (OFMSW) were examined, with an emphasis on properties that are needed when designing plug-flow type anaerobic bioreactors. More specifically, the decomposition patterns of a vegetable (cabbage), fruits (banana and citrus peels), fresh leaf litter of bamboo and teak leaves, and paper (newsprint) waste streams as feedstocks were studied. Individual OFMSW components were placed into nylon mesh bags and subjected to various fermentation periods (solids retention time, SRT) within the inlet of a functioning plug-flow biogas fermentor. These were removed at periodic intervals, and their composition was analyzed to monitor decomposition rates and changes in chemical composition. Components like cabbage waste, banana peels, and orange peels fermented rapidly both in a plug-flow biogas reactor (PFBR) as well as under a biological methane potential (BMP) assay, while other OFMSW components (leaf litter from bamboo and teak leaves and newsprint) fermented slowly with poor process stability and moderate biodegradation. For fruit and vegetable wastes (FVW), a rapid and efficient removal of pectins is the main cause of rapid disintegration of these feedstocks, which left behind very little compost forming residues (2-5%). Teak and bamboo leaves and newsprint decomposed only to 25-50% in 30d. These results confirm the potential for volatile fatty acids accumulation in a PFBR's inlet and suggest a modification of the inlet zone or operation of a PFBR with the above feedstocks.

Microbiological characterization and specific methanogenic activity of anaerobe sludges used in urban solid waste treatment

This study presents the microbiological characterization of the anaerobic sludge used in a two-stage anaerobic reactor for the treatment of organic fraction of urban solid waste (OFUSW). This treatment is one alternative for reducing solid waste in landfills at the same time producing a biogas (CH(4) and CO(2)) and an effluent that can be used as biofertilizer. The system was inoculated with sludge from a wastewater treatment plant (WWTP) (Río Frío Plant in Bucaramanga-Colombia) and a methanogenic anaerobic digester for the treatment of pig manure (Mesa de los Santos in Santander). Bacterial populations were evaluated by counting groups related to oxygen sensitivity, while metabolic groups were determined by most probable number (MPN) technique. Specific methanogenic activity (SMA) for acetate, formate, methanol and ethanol substrates was also determined. In the acidogenic reactor (R1), volatile fatty acids (VFA) reached values of 25,000 mg L(-1) and a concentration of CO(2) of 90%. In this reactor, the fermentative population was predominant (10(5)-10(6)MPN mL(-1)). The acetogenic population was (10(5)MPN mL(-1)) and the sulphate-reducing population was (10(4)-10(5)MPN mL(-1)). In the methanogenic reactor (R2), levels of CH(4) (70%) were higher than CO(2) (25%), whereas the VFA values were lower than 4000 mg L(-1). Substrate competition between sulphate-reducing (10(4)-10(5)MPN mL(-1)) and methanogenic bacteria (10(5)MPN mL(-1)) was not detected. From the SMA results obtained, acetoclastic (2.39 g COD-CH(4)g(-1)VSS(-1)day(-1)) and hydrogenophilic (0.94 g COD-CH(4)g(-1)VSS(-1)day(-1)) transformations as possible metabolic pathways used by methanogenic bacteria is suggested from the SMA results obtained. Methanotrix sp., Methanosarcina sp., Methanoccocus sp. and Methanobacterium sp. were identified.

Anaerobic digestion of municipal solid waste and agricultural waste and the effect of co-digestion with dairy cow manure

Anaerobic digestion of dairy cow manure (CM), the organic fraction of municipal solid waste (OFMSW), and cotton gin waste (CGW) was investigated with a two-phase pilot-scale anaerobic digestion (AD) system. The OFMSW and CM were digested as single wastes and as combined wastes. The single waste digestion of CM resulted in 62m3 methane/ton of CM on dry weight basis. The single waste digestion of OFMSW produced 37m3 methane/ton of dry waste. Co-digestion of OFMSW and CM resulted in 172m3 methane/ton of dry waste. Co-digestion of CGW and CM produced 87m3 methane/ton of dry waste. Comparing the single waste digestions with co-digestion of combined wastes, it was shown that co-digestion resulted in higher methane gas yields. In addition, co-digestion of OFMSW and CM promotes synergistic effects resulting in higher mass conversion and lower weight and volume of digested residual.

A pilot plant two-phase anaerobic digestion system for bioenergy recovery from swine wastes and garbage

A pilot plant bioenergy recovery system from swine waste and garbage was constructed. A series of experiments was performed using swine feces (SF); a mixture of swine feces and urine (MSFU); a mixture of swine feces, urine and garbage (MSFUG); garbage and a mixture of urine and garbage (AUG). The system performed well for treating the source materials at a high organic loading rate (OLR) and short hydraulic retention time (HRT). In particular, the biogas production for the MSFUG was the highest, accounting for approximately 865-930 L kg(-1)-VS added at the OLR of 5.0-5.3 kg-VS m(-3) day(-1) and the HRT of 9 days. The removal of VS was 67-75%, and that of COD was 73-74%. Therefore, co-digestion is a promising method for the recovery of bioenergy from swine waste and garbage. Furthermore, the results obtained from this study provide fundamental information for scaling up a high-performance anaerobic system in the future.

Resource recovery potential from secondary components of segregated municipal solid wastes

Fermentable components of municipal solid wastes (MSW) such as fruit and vegetable wastes (FVW), leaf litter, paddy straw, cane bagasse, cane trash and paper are generated in large quantities at various pockets of the city. These form potential feedstocks for decentralized biogas plants to be operated in the vicinity. We characterized the fermentation potential of six of the above MSW fractions for their suitability to be converted to biogas and anaerobic compost using the solid-state stratified bed (SSB) process in a laboratory study. FVW and leaf litter (paper mulberry leaves) decomposed almost completely while paddy straw, sugarcane trash, sugarcane bagasse and photocopying paper decomposed to a lower extent. In the SSB process between 50-60% of the biological methane potential (BMP) could be realized. Observations revealed that the SSB process needs to be adapted differently for each of the feedstocks to obtain a higher gas recovery. Bagasse produced the largest fraction of anaerobic compost (fermentation residue) and has the potential for reuse in many ways.

Nitrogen removal in the bioreactor landfill system with intermittent aeration at the top of landfilled waste

High ammonia concentration of recycled landfill leachate makes it very difficult to treat. In this work, a vertical aerobic/anoxic/anaerobic lab-scale bioreactor landfill system, which was constructed by intermittent aeration at the top of landfilled waste, as a bioreactor for in situ nitrogen removal was investigated during waste stabilization. Intermittent aeration at the top of landfilled waste might stimulate the growth of nitrifying bacteria and denitrifying bacteria in the top and middle layers of waste. The nitrifying bacteria population for the landfill bioreactor with intermittent aeration system reached between 10(6) and 10(8) cells/dry g waste, although it decreased 2 orders of magnitude on day 30, due to the inhibitory effect of the acid environment and high organic matter in the landfilled waste. The denitrifying bacteria population increased by between 4 and 13 orders of magnitude compared with conventional anaerobic landfilled waste layers. Leachate NO(3)(-)-N concentration was very low in both two experimental landfill reactors. After 105 days operation, leachate NH(4)(+)-N and TN concentrations for the landfill reactor with intermittent aeration system dropped to 186 and 289 mg/l, respectively, while they were still kept above 1000 mg/l for the landfill reactor without intermittent aerobic system. In addition, there is an increase in the rate of waste stabilization as well as an increase of 12% in the total waste settlement for the landfill reactor with intermittent aeration system.