Co-digestion of intermediate landfill leachate and sewage sludge as a method of leachate utilization

This study examines the co-digestion of intermediate landfill leachate and sewage sludge from a municipal wastewater treatment plant. Application of leachate as a co-fermentation component increased the concentrations of soluble organic compounds (expressed as total organic carbon), ammonium nitrogen, and alkalinity in the digester influents.The biogas yield obtained from the co-fermentation of a 20:1 sewage sludge: intermediate leachate mixture was 1.30 m³ per kg of removed volatile solids (VS), while that from a 10:1 mixture was 1.24 m³ per kg of removed VS. These values exceeded the biogas yield for the sludge alone by 13% and 8%, respectively. The leachate addition influenced the proportion of methane to a minor extent. Increased methane yields of 16.9% and 6.2% per kg of removed VS were found for the two sewage sluge:intermediate leachate mixtures, respectively.     

Biogas production from different substrates in an experimental Continuously Stirred Tank Reactor anaerobic digester

Different mixtures were digested in a single-stage, batch, mixed, laboratory scale mesophilic anaerobic digester at the Biomass Research Centre Laboratory (University of Perugia). The yield and the composition of biogas from the different substrates were evaluated and the cumulative curves were estimated. Two experimental campaigns were carried out, the first on three mixtures (chicken, pig and bovine manures), the second on animal and vegetal biomasses (chicken and cow manure, olive husk) with different inocula (rumen fluid and digested sludge). In the first campaign pig manure mixture showed the maximum biogas production (0.35 Nm³/kg) and energy content (1.35 kWh/kg VS); in the second one the differences in produced biogas from the different inocula were analyzed: olive husk with piggery manure anaerobically digested as inoculum showed the higher biogas (0.28 Nm³/kg VS) and methane yield (0.11 Nm³/kg VS), corresponding to an energetic content of 1.07 kWh/kg VS. All data obtained from the laboratory scale anaerobic digester are comparable to the values in literature for several biomass and in particular for olive husk, dairy manure and chicken manure.     

Dual anaerobic co-digestion of sewage sludge and confectionery waste

Three configurations for a dual digestion system were examined. The units were based on three 5 l completely stirred tank reactors (CSTR). A first-stage thermophilic digester was used to provide the feed to each of the two second-stage mesophilic (35°C) digesters. Using a mixture of sewage sludge and strong confectionery waste, the thermophilic digester was operated at 55°C with a hydraulic retention time of 4 h. The mesophilic digesters were operated at hydraulic retention times of 8, 12 and 15 days. In terms of the reduction of volatile solids (VS), the three dual digestion configurations were similar but were more effective than the single-stage reactor which was used as a control. However, based on the specific methane yield (m³ CH₄/kg VS removed), the configuration with a first stage operating at 55°C and a secondary digester at 35°C with a hydraulic retention time of 12 days was the most effective. This configuration also maintained a more stable pH, irrespective of the quality of the feed sludge.     

Novel membrane bioreactor: Able to cope with fluctuating loads, poorly water soluble VOCs, and biomass accumulation

We determined the effect of different mixing intensities on the performance, methanogenic population dynamics, and juxtaposition of syntrophic microbes in anaerobic digesters treating cow manure from a dairy farm. Computer automated radioactive particle tracking in conjunction with computational fluid dynamics was performed to quantify the shear levels locally. Four continuously stirred anaerobic digesters were operated at different mixing intensities of 1,500, 500, 250, and 50 revolutions per min (RPM) over a 260-day period at a temperature of 34 +/- 1 degrees C. Animal manure at a volatile solids (VS) concentration of 50 g/L was fed into the digesters daily at five different organic loading rates between 0.6 and 3.5 g VS/L day. The different mixing intensities had no effect on the biogas production rates and yields at steady-state conditions. A methane yield of 0.241 +/- 0.007 L CH(4)/g VS fed was obtained by pooling the data of all four digesters during steady-state periods. However, digester performance was affected negatively by mixing intensity during startup of the digesters, with lower biogas production rates and higher volatile fatty acids concentrations observed for the 1,500-RPM digester. Despite similar methane production yields and rates, the acetoclastic methanogenic populations were different for the high- and low-intensity mixed digesters with Methanosarcina spp. and Methanosaeta concilii as the predominant methanogens, respectively. For all four digesters, epifluorescence microscopy revealed decreasing microbial floc sizes beginning at week 4 and continuing through week 26 after which no microbial flocs remained. This decrease in size, and subsequent loss of microbial flocs did not, however, produce any long-term upsets in digester performance.     

Quantifying electricity generation and waste transformations in a low-cost, plug-flow anaerobic digestion system

Methane production, electricity production, and wastewater transformations were quantified for a digestion system that combines biogas from a swine digester and dairy digester in Costa Rica. The low-cost, plug-flow digesters were not heated and were operated in the lower portion of the mesophilic range (25–27 °C).The dairy digester produced 27.5 m³/day of biogas with 62.6% methane and reduced organic matter (COD) by 86%. The swine digester produced 6.0 m³/day of biogas with 76.4% methane and reduced COD by 92%. Combining biogas from a swine and dairy digester, increased electricity production due to the higher biogas production rate of the dairy farm and the higher quality biogas obtained from the swine farm. The farm’s 2-h peak electricity demand (12.9 kW/day) was 81.8% met. The electricity was produced using manure equivalent to the quantity excreted by 5 dairy cows and 40 pigs remaining in corrals 100% of the time.The $21,000 capital cost of the digester project will be recovered in 10.1 years through electricity savings and reductions in wastewater fines. If the generator were more appropriately sized for the farm, the capital recovery time would have been 7.6 years.     

Development of mixed inoculum for methane enriched biogas production

Inocula were collected from four different sources such as Jajmau tannery waste treatment plant (ITW), Jajmau municipal waste treatment (IMW), Unnao distillery (IDW) and a batch reactor, in which the sludge of a field scale biogas reactor was added to cow dung slurry to develop inoculum (IBS). A combination of these mixed inocula were used for biogas production at 35°C in laboratory scale reactor (10 L capacity) and the average yield of biogas (0.547 Lg^?1 volatile solid (VS)) and methane (0.323 Lg^?1VS) in 41 d was higher in case of mixed inoculum IMW ₁ (IMW+IBS), with maximum methane content in biogas (68% during 27–30 d), as compared to other mixed inocula as well as control i.e. ITW ₁ (ITW+IBS), IDW₁ (IDW+IBS) and IBS. The corresponding yields of gas were biogas (0.505, 0.536 and 0.456 Lg^?1VS), methane (0.288, 0.305, and 0.245 Lg^?1VS) where as, the corresponding maximum methane content in biogas was 62% during 29–33d, 64% during 29–33 d and 62% during 27–29 d in ITW₁, IDW₁ and IBS.     

Anaerobic digestion of secondary residuals from an anaerobic bioreactor at a brewery to enhance bioenergy generation

Many beer breweries use high-rate anaerobic digestion (AD) systems to treat their soluble high-strength wastewater. Biogas from these AD systems is used to offset nonrenewable energy utilization in the brewery. With increasing nonrenewable energy costs, interest has mounted to also digest secondary residuals from the high-rate digester effluent, which consists of yeast cells, bacteria, methanogens, and small (hemi)cellulosic particles. Mesophilic (37 °C) and thermophilic (55 °C) lab-scale, low-rate continuously-stirred anaerobic digestion (CSAD) bioreactors were operated for 258 days by feeding secondary residuals at a volatile solids (VS) concentration of ∼40 g l⁻¹. At a hydraulic retention time (HRT) of 15 days and a VS loading rate of 2.7 g VS l⁻¹ day⁻¹, the mesophilic bioreactor showed an average specific volumetric biogas production rate of 0.88 l CH₄ l⁻¹ day⁻¹ and an effluent VS concentration of 22.2 g VS l⁻¹ (43.0% VS removal efficiency) while the thermophilic bioreactor displayed similar performances. The overall methane yield for both systems was 0.21 l CH₄ g⁻¹ VS fed and 0.47–0.48 l CH₄ g⁻¹ VS removed. A primary limitation of thermophilic digestion of this protein-rich waste is the inhibition of methanogens due to higher nondissociated (free) ammonia (NH₃) concentrations under similar total ammonium (NH₄ ⁺) concentrations at equilibrium. Since thermophilic AD did not result in advantageous methane production rates or yields, mesophilic AD was, therefore, superior in treating secondary residuals from high-rate AD effluent. An additional digester to convert secondary residuals to methane may increase the total biogas generation at the brewery by 8% compared to just conventional high-rate digestion of brewery wastewater alone. JIMB-2008: BioEnergy—Special issue.     

A kinetic study of the anaerobic digestion of piggery waste using down-flow stationary fixed film reactors

Summary In this paper the behaviour of the down-flow stationary fixed film digesters is studied at laboratory and bench scale. Several organic loading rates are applied to the reactors in order to examine the support surface behaviour. Specific support surfaces of about 50 m²/m³ void volume seems to be optimal. A set of experiments carried out in a continuous stirred reactor is used to fit the kinetic constants of the Chen and Hashimoto's model. The model is then used to assess its applicability to the DSFF digesters. The results show that its application, is possible as a first approximation.Nomenclature B₀ Ultimate methane yield (m³ CH₄/Kg VS) - B Specific methane production (m³ CH₄/Kg VS) - CSTR Continuous stirred tank reactor - DSFF Down-flow Stationary Fixed Film - HRT Hydraulic retention time (days) - K Kinetic constant of the Chen and Hashimoto model (dimensionless) - S Biodegradable substrate concentration (g/l) - SLR Superficial loading rate (Kg VS/m²·d) - SSS Specific support surface (m² support surface/m³ digester void volume) - S₀ Initial substrate concentration (g/l) - VS Volatile solids (g/l) - VFA Volatile Fatty Acids (mg/dm³) - Microorganisms specific growth (day^-1) - _m Kinetic constant of Chen and Hashimoto's model (day^-1) - Retention time (days) - _m Minimum retention time to avoid microorganisms washout (days)     

The effects of digestion temperature and temperature shock on the biogas yields from the mesophilic anaerobic digestion of swine manure

In order to obtain basic design criteria for anaerobic digesters of swine manure, the effects of different digesting temperatures, temperature shocks and feed loads, on the biogas yields and methane content were evaluated. The digester temperatures were set at 25, 30 and 35 degrees C, with four feed loads of 5%, 10%, 20% and 40% (feed volume/digester volume). At a temperature of 30 degrees C, the methane yield was reduced by only 3% compared to 35 degrees C, while a 17.4% reduction was observed when the digestion was performed at 25 degrees C. Ultimate methane yields of 327, 389 and 403 mL CH(4)/g VS(added) were obtained at 25, 30 and 35 degrees C, respectively; with moderate feed loads from 5% to 20% (V/V). From the elemental analysis of swine manure, the theoretical biogas and methane yields at standard temperature and pressure were 1.12L biogas/g VS(destroyed) and 0.724 L CH(4)/g VS(destroyed), respectively. Also, the methane content increased with increasing digestion temperatures, but only to a small degree. Temperature shocks from 35 to 30 degrees C and again from 30 to 32 degrees C led to a decrease in the biogas production rate, but it rapidly resumed the value of the control reactor. In addition, no lasting damage was observed for the digestion performance, once it had recovered.     

Anaerobic digestion of source-segregated domestic food waste: Performance assessment by mass and energy balance

An anaerobic digester receiving food waste collected mainly from domestic kitchens was monitored over a period of 426 days. During this time information was gathered on the waste input material, the biogas production, and the digestate characteristics. A mass balance accounted for over 90% of the material entering the plant leaving as gaseous or digestate products. A comprehensive energy balance for the same period showed that for each tonne of input material the potential recoverable energy was 405 kWh. Biogas production in the digester was stable at 642 m³ tonne⁻¹ VS added with a methane content of around 62%. The nitrogen in the food waste input was on average 8.9 kg tonne⁻¹. This led to a high ammonia concentration in the digester which may have been responsible for the accumulation of volatile fatty acids that was also observed.     

Batch and continuous biogas production from grass silage liquor

Herein batch and continuous mesophilic anaerobic digestion of grass silage liquor was studied. The continuous process was carried out in Armfield digesters with an OLR ranging from 0.851 to 1.77 kg COD m⁻³ day⁻¹. The effect of recirculation of effluent from the digester was investigated using different OLRs of grass silage liquor feed. These results showed that as the OLR increased, the methane yield decreased for the reactor with no recycle and increased for the reactor with recycle. However, the COD removal for both digesters was nearly the same at the same OLR. Overall these studies show that grass silage liquor can produce a high quality methane steam between 70% and 80% and achieve methane yields of 0.385 m³ kg⁻¹ COD.     

Anaerobic digestion of sugar beet pulps

Summary Anaerobic digestion of sugar beet pulps was studied in a 70 l digestor with sequential feeding, after enzymatic hydrolysis by Trichoderma harzianum cellulases. During the 130 days feeding, 3.6 m³ of biogas were produced with an average content of 58% CH₄ from 270 l of hydrolysed pulps at 20 g VS/l. Average yield and production rate were respectively 0.67 m³/kg VS and 0.4 m³/ kg VS and 0.4 m³/m³ of digestor per day.     

Multi stage high rate biomethanation of poultry litter with self mixed anaerobic digester

A multi stage high rate biomethanation process with novel self mixed anaerobic digester (SMAD) was developed in the present study to reduce the hydraulic residence time (HRT), increase the volatile solids (VS) loading rate, improve the VS destruction efficiency and enhance the methane yield. Specific design features of SMAD were useful in mixing the digester contents without consuming power and de-alienated the problem of scum formation. In the first phase, poultry litter having 10% total solids (TS) was subjected to high rate biomethanation in multi stage configuration (SMAD-I and II in series with UASB reactor). It was observed that gross VS reduction of 58%, gross methane yield of 0.16 m³ kg⁻¹ (VS reduced) and VS loading rate of 3.5 kg VS m⁻³ day⁻¹ at HRT of 13 days was obtained. In the second phase SMAD-II was bypassed from the process scheme keeping the other parameters same as in the first phase. The results obtained were not as encouraging as in the first phase. The study showed that multi stage configuration with SMAD design improved the anaerobic digestion process efficiency of poultry litter.     

Anaerobic co-digestion of food waste and piggery wastewater: focusing on the role of trace elements

The objective of this study was to evaluate the feasibility of anaerobic co-digestion of food waste and piggery wastewater, and to identify the key factors governing the co-digestion performance. The analytical results indicated that the food waste contained higher energy potential and lower concentrations of trace elements than the piggery wastewater. Anaerobic co-digestion showed a significantly improved biogas productivity and process stability. The results of co-digestion of the food waste with the different fractions of the piggery wastewater suggested that trace element might be the reason for enhancing the co-digestion performance. By supplementing the trace elements, a long-term anaerobic digestion of the food waste only resulted in a high methane yield of 0.396 m³/kg VS_added and 75.6% of VS destruction with no significant volatile fatty acid accumulation. These results suggested that the typical Korean food waste was deficient with some trace elements required for anaerobic digestion.     

An Analysis of the Energy Potential of Anaerobic Digestion of Agricultural By-Products and Organic Waste

Anaerobic digestion is a promising option for recycling agricultural by-products and some organic wastes. While both agricultural by-products and wastes have no direct commercial value, their management is both complicated and costly. One option to simplify by-product management and reduce the costs associated with biogas plant feedstock is to substitute dedicated crops with vegetal by-products. Given that the chemical composition of some of these by-products can differ considerably from more typical biogas plant feedstock (such as maize silage), more complete knowledge of these alternatives to produce environmentally friendly energy is warranted. To this end, batch trials under mesophilic conditions were conducted to evaluate the potential biogas yield of many agricultural by-products: maize stalks, rice chaff, wheat straw, kiwi fruit, onions, and two expired organic waste products (dairy and dry bread) from the retail mass-market. Among the considered biomasses, the highest methane producer was the expired dairy product mixture, which yielded 554 l_NCH₄ kg⁻¹ volatile solids (VS). Maize stalks and wheat straw produced the lowest yields of 214 and 285 l_NCH₄ kg⁻¹VS, respectively. An assessment of the biogas and methane yields of each biomass was also undertaken to account for the specific chemical composition of each biomass as it can affect the anaerobic digestion operating system. Finally, the total Italian green energy production that might be derived from feeding all these biomasses to a biogas digester was estimated, in order to understand its potential impact.     

Anaerobic digestion of animal waste: waste strength versus impact of mixing

We studied the effect of mode of mixing (biogas recirculation, impeller mixing, and slurry recirculation) and waste strength on the performance of laboratory scale digesters. The digesters were fed with 5% and 10% manure slurry, at a constant energy supply per unit volume (8 W/m3). The experiments were conducted in eight laboratory scale digesters, each having a working volume of 3.73 L, at a controlled temperature of 35+/-2 degrees C. Hydraulic retention time (HRT) was kept constant at 16.2 days, resulting in a total solids (TS) loading rate of 3.08 g/Ld and 6.2 g/Ld for 5% and 10% manure slurry feeds, respectively. Results showed that the unmixed and mixed digesters performed quite similarly when fed with 5% manure slurry and produced biogas at a rate of 0.84-0.94 L/Ld with a methane yield of 0.26-0.31 L CH4/g volatile solids (VS) loaded. This was possibly because of the low solids concentration in the case of 5% manure slurry, where mixing created by the naturally produced gas might be sufficient to provide adequate mixing. However, the effect of mixing and the mode of mixing became prominent in the case of the digesters fed with thicker manure slurry (10%). Digesters fed with 10% manure slurry and mixed by slurry recirculation, impeller, and biogas recirculation produced approximately 29%, 22% and 15% more biogas than unmixed digester, respectively. Deposition of solids inside the digesters was not observed in the case of 5% manure slurry, but it became significant in the case of 10% manure slurry. Therefore, mixing issue becomes more critical with thicker manure slurry.     

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

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

Digestion of sand-laden manure slurry in an upflow anaerobic solids removal (UASR) digester

Studies on the performance of a laboratory scale upflow anaerobic solids removal (UASR) digester were carried out using sand-laden cow manure slurries having total solids (TS) concentration as 50 and 100 g/l. Hydraulic retention time (HRT) was maintained as 32.4 days, which resulted in the volatile solids (VS) loading rates of 1 and 1.64 g/l d. The UASR system was designed to remove sand from the manure slurry, while anaerobically digesting biodegradable solids inside a single reactor. To enhance the contact of microorganisms and substrate, the liquor from the top of the digester was recirculated through the bed of settled solids at its bottom. Volatile solids reduction through this process was observed to be 62% and 68% in the case of feed slurries having TS concentration as 50 and 100 g/l (referred in the text as 5% and 10% feed slurries), respectively. The methane production rates were observed to be 0.22 and 0.38 l/l d, while methane yield was 0.21 and 0.27 l CH₄/g VS loaded, for 5% and 10% feed slurries, respectively. This indicates that the increase in the VS loading had a positive impact on methane production rate and methane yield. It would be of interest to study the performance of a UASR digester at higher solids loadings and with longer solids retention times. Nonetheless, the presented study showed that sand-laden manure slurries can be successfully digested in a UASR digester producing methane energy equivalent to 4 kW h per m³ of digester volume per day.     

Psychrophilic anaerobic digestion of guinea pig manure in low-cost tubular digesters at high altitude

Guinea pig is one of the most common livestock in rural communities of the Andes. The aim of this research was to study the anaerobic digestion of guinea pig manure in low-cost unheated tubular digesters at high altitude. To this end, the performance of two pilot digesters was monitored during 7 months; and two greenhouse designs were compared. In the dome roof digester the temperature and biogas production were significantly higher than in the shed roof digester. However, the biogas production rate was low (0.04 m(biogas)(3)m(digester)(-3) d(-1)), which is attributed to the low organic loading rate (0.6 kg(VS)m(digester)(-3)d(-1)) and temperature (23°C) of the system, among other factors. In a preliminary fertilization study, the potato yield per hectare was increased by 100% using the effluent as biofertilizer. Improving manure management techniques, increasing the organic loading rate and co digesting other substrates may be considered to enhance the process.     

Influence of lignin on the methanization of lignocellulosic wastes

The effect on anaerobic digestion of reducing the lignin content of vine shoots to 1% (w/w), by treatment with sodium chlorite in an acid medium at 80°C, is reported. The yields of methane obtained were 240 ml of CH₄ g⁻¹ of VS (volatile solids) fed for untreated vine shoots, and 370 ml of CH₄ g⁻¹ of VS fed for treated vine shoots. A mathematical model was used to calculate the kinetic parameters H and μ, and the increased biodegradability of the substrate in which lignin had been removed was confirmed. A study of the mass balances of the process under optimum conditions (temperature = 35°C; loading rate of 1 g litre⁻¹ digester day⁻¹) enabled the percentage of degraded cellulose to be calculated (35·5% for untreated vine shoots, 81·5% for the treated vine shoots), as were the volumes of biogas and methane produced per gram of VS introduced (VS₁) and degraded. The blocking effect of lignin on the methanization process was confirmed.