Optimization of municipal sludge and grease co-digestion using disintegration technologies

Many drivers tend to foster the development of renewable energy production in wastewater treatment plants as many expectations rely upon energy recovery from sewage sludge, for example through biogas use. This paper is focused on the assessment of grease waste (GW) as an adequate substrate for co-digestion with municipal sludge, as it has a methane potential of 479-710 LCH(4)/kg VS, as well as the evaluation of disintegration technologies as a method to optimize the co-digestion process. With this objective three different pre-treatments have been selected for evaluation: thermal hydrolysis, ultrasound and enzymatic treatment. Results have shown that co-digestion processes without pre-treatment had a maximum increment of 128% of the volumetric methane productivity when GW addition was 23% inlet (at 20 days of HRT and with an OLR of 3.0 kg COD/m(3)d), compared with conventional digestion of sewage sludge alone. Concerning the application of the selected disintegration technologies, all pre-treatments showed improvements in terms of methane yield (51.8, 89.5 and 57.6% more for thermal hydrolysis, ultrasound and enzymatic treatment, respectively, compared with non-pretreated wastes), thermal hydrolysis of GW and secondary sludge being the best configuration as it improved the solubilization of the organic matter and the hydrodynamic characteristics of digestates.     

Simultaneous treatment of sewage sludge and food waste by the unified high-rate anaerobic digestion system.

This study aimed to evaluate the performance of the unified high-rate anaerobic digestion (UHAD) system treating co-substrate of sewage sludge and food waste. A 24-hr operating sequence consisted of four steps including fill, react, settle, and draw. The effects of co-substrate and organic loading rate (OLR) on the performance were investigated to verify the system applicability. In each OLR, the UHAD system showed higher CH4 recovery (> 70%), CH4 yield (0.3 L CH4/g VSadded) and CH4 production rate (0.6 L CH4/L/d) than the control system. In the specific methanogenic activity (SMA) tests on thermophilic biomass of the UHAD system, the average SMA of acetate (102 mL CH4/gVSS/d) was much higher than those of butyrate (85 mL CH4/g SS/d) and propionate (42 mL CH4/gVSS/d). It was demonstrated that the UHAD system for co-digestion resulted in higher methane yield and methane production rate due to sequencing batch operation, thermophilic digestion, and co-digestion. The enhanced performance could be attributed to longer retention time of active biomass, faster hydrolysis, higher CH4 conversion rate, and balanced nutrient conditions of co-substrate in the UHAD system. Consequently, this optimized unification could be a viable option for the simultaneous treatment of two types of OFMSW with high stability.     

Comparison of methane production by co-digesting fruit and vegetable waste with first stage and second stage anaerobic digester sludge from a two stage digester

Fruit and vegetable waste (FVW) was co-digested with first stage (FSS) and second stage anaerobic digester sludge (SSS) separately, over the course of 10 days, in batch reactors. Addition of FVW significantly increased the methane production in both sludges. After 10 days of digestion FSS + FVW produced 514 ± 57 L CH(4) kg VS(-1)(added) compared with 392 ± 16 L CH(4) for the SSS + FVW. The increased methane yield was most likely due to the higher inoculum substrate ratio of the FSS. The final VS and COD contents of the sewer sludge and FVW mixtures were not significantly different from the control values suggesting that all of the FVW added was degraded within 10 days. It is recommended that FVW be added to the first stage of the anaerobic digester in order to maximize methane generation.     

Anaerobic monodigestion of poultry manure: determination of operational parameters for CSTR

In this work the anaerobic monodigestion for the treatment of turkey manure was evaluated, without its codigestion with another substrate. The effect of the organic loading rate (OLR) and the substrate concentration (high total solids (TS) concentration) or product concentration (high volatile fatty acids (VFA) and/or ammonia (NH(3)-N) concentrations) was studied. The results show that for a continuous stirred tank reactor (CSTR) operation, a maximum of 40 g/L of TS and 4.0 g/L of ammonium (NH(4)(+)) was required. In addition, the maximum organic loading rate (OLR) will not exceed 1.5 kg VS/m(3)d. Higher TS and NH(4)(+) concentrations and OLR lead to a reduction on the methane productivity and volatile solids (VS) removal. During the CSTR operation, a high alkalinity concentration (above 10 g/L CaCO(3)) was found; this situation allowed maintaining a constant and appropriate pH (close to 7.8), despite the VFA accumulation. In this sense, the alkalinity ratio (α) is a more appropriate control and monitoring parameter of the reactor operation compared to pH. Additionally, with this parameter a VS removal of 80% with a methane productivity of 0.50 m(3)(CH4)/m(3)(R)d is achieved.     

ADM1 performance using SS-OFMSW with non-acclimated inoculums

This paper assesses the anaerobic digestion (AD) of the source-sorted organic fraction of municipal solid waste (SS-OFMSW). For this purpose, an experimental programme was implemented involving the operation and monitoring of two bench-scale anaerobic digesters, continuously fed with SS-OFMSW. The mathematical model (ADM1) was then applied to simulate the process of AD of SS-OFMSW. While start-up of the digesters was relatively slow, re-inoculation with cattle manure with effluent dilution reduced the acclimation period and achieved better stability, accommodating a feeding rate at an OLR = 2.39 kg TVS m(-3) day(-1). The high recorded methane gas production rate, reaching (0.1-2.5 m(3) CH(4)/m(3) reactor day), confirms the excellent biodegradability of the type of waste used (SS-OFMSW) and its suitability for AD. Satisfactory simulations of soluble chemical oxygen demand (COD), pH, and methane composition of biogas were obtained, whereas volatile fatty acid (VFA) concentrations in both reactors were over-predicted albeit capturing its general trend.     

The effect of temperature on the performance and stability of thermophilic anaerobic digestion.

Sustainable operation of an anaerobic sewage sludge digester requires the effective shuttling of carbon from complex organic material to methane gas. The accumulation of intermediates and metabolic products such as volatile fatty acids and hydrogen gas not only reveal inefficiency within the digestion process, but can be detrimental to reactor operation at sufficiently high levels. Eight anaerobic digesters (1 mesophilic and 7 thermophilic) were operated in order to determine the effect of steady-state digestion temperature on the operational stability and performance of the digestion process. Replicate reactors operated at 57.5 degrees C, the highest temperature studied, were prone to accumulation of volatile fatty acids (4052 and 3411 mg/L as acetate) and gaseous hydrogen. Reactors operated at or below 55 degrees C showed no such accumulation of intermediate metabolites. Overall methanogenesis was also greatly reduced at 57.5 degrees C (0.09 L CH4/g VS fed) versus optimal methane formation at 53 degrees C (0.40 L CH4/g VS fed). Microbial community assessment and free energy calculations suggest that the accumulation of fatty acids and hydrogen, and relatively poor methanogenic performance at 57.5 degrees C are likely due to temperature limitations of thermophilic aceticlastic methanogens.     

Effect of biological additives and micronutrients on the anaerobic digestion of physicochemical sludge.

Two additives (lyophilized bacilli and enzymes) and a solution of micronutrients (Fe, Co, Ni and Mo) were tried separately and in combination, in order to assess their effect on the anaerobic digestion of waste sludge from an enhanced primary treatment (EPT) of municipal wastewater. Three batch tests were carried out in serological bottles. In the first test, addition of bacilli increased production of methane from day 11 and at day 1 7 the production was 95% greater than the control. In that experiment, the concentration of volatile fatty acids (VFAs) was 1,391 mg/L, 40% lower than the control. In the second test, the combination of micronutrients with bacilli, reached from the first days a better methane production than the control, 167% higher in day 17. At the end of the experiment, this combination achieved a lower concentration of VFAs and a greater percentage of volatile solid removal than the rest of the treatments. The third test was based on an experimental design in order to statistically determine the best doses of bacilli additive and micronutrients. The anaerobic thermophilic digestion of sludge from aluminium sulfate EPT will be improved with the addition of Fe: 4.5 mg/g VS, Ni: 0.75 mg/g VS, Co: 0.45 mg/g VS, Mo: 0.09 mg/g VS and bacilli additive: 12 mg/g VS.     

Anaerobic digestion of corn ethanol thin stillage in batch and by high-rate down-flow fixed film reactors

Thin stillage (CTS) from a dry-grind corn ethanol plant was evaluated as a carbon source for anaerobic digestion (AD) by batch and high rate semi-continuous down-flow stationary fixed film (DSFF) reactors. Biochemical methane potential (BMP) assays were carried out with CTS concentrations ranging from approximately 2,460-27,172 mg total chemical oxygen demand (TCOD) per litre, achieved by diluting CTS with clean water or a combination of clean water and treated effluent. High TCOD, SCOD and volatile solids (VS) removal efficiencies of 85 ± 2, 94 ± 0 and 82 ± 1% were achieved for CTS diluted with only clean water at an organic concentration of 21,177 mg TCOD per litre, with a methane yield of 0.30 L methane per gram TCOD(removed) at standard temperature and pressure (STP, 0 °C and 1 atmosphere). Batch studies investigating the use of treated effluent for dilution showed promising results. Continuous studies employed two mesophilic DSFF anaerobic digesters treating thin stillage, operated at hydraulic retention times (HRT) of 20, 14.3, 8.7, 6.3, 5 and 4.2 d. Successful digestion was achieved up to an organic loading rate (OLR) of approximately 7.4 g TCOD L(-1)d(-1) at a 5 d HRT with a yield of 2.05 LCH(4) L(-1)d(-1) (at STP) and TCOD and VS removal efficiencies of 89 ± 3 and 85 ± 3%, respectively.     

Evaluating limiting steps of anaerobic degradation of food waste based on methane production tests.

This research adapted a batch test for biochemical methane production (BMP) to follow the degradation of complex compounds such as proteins and vegetable oils. The test measured the transformation of albumin and olive oil into methane under mesophilic and thermophilic conditions and assess limiting step in the overall degradation process. The thermophilic sludge used for the BMP tests was adapted during ten month from mesophilic sludge while being fed food waste. As compared to acetic acid, the specific rate of transformation of albumin and olive oil into methane reached 22 and 51%, respectively, under mesophilic conditions. Acetoclastic methanogenesis was not the limiting step in the presence of albumin or olive oil (and its monomer-like molecules such as amino acids, glycerol and oleic acid). Rather, the degradation of albumin was restricted by the presence of proteins. The thermophilically adapted sludge showed good proteolytic activity, but its acetoclastic methanogens were unable to degrade olive oil, because of the inhibitory effect of oleic acid.     

Temperature effects on the trophic stages of perennial rye grass anaerobic digestion

Continuous Stirred Tank Reactors (CSTRs), operated in batch mode, were used to evaluate the feasibility of psychrophilic (low temperature) digestion of perennial rye grass in a long term experiment (150 days) for the first time. The reactors were operated in parallel at 3 different temperatures, 10, 15 and 37 degrees C. Hydrolysis, acidification and methanogenesis were assessed by VS degradation, by soluble chemical oxygen demand (SCOD) and volatile fatty acids (VFA) production, and by methane production, respectively. Hydrolysis was the rate-limiting step at all temperatures and the rates and extent of hydrolysis were considerably lower at 15 and 10 degrees C, than at 37 degrees C. The total VS degradation was 53%, 34% and 19% at 37, 15 and 10 degrees C, respectively. Acidification was not affected by temperature and VFA production and consumption was balanced in all cases, except at 10 degrees C. Methane yields were 0.215 m3 CH4 kg(-1) VS(-1) added, 0.160 m3 CH4 kg(-1) VS(-1) added and 0.125 m3 CH4 kg(-1) VS(-1) added at 37, 15 and 10 degrees C, respectively. Methanogenesis was not strongly affected at 15 C but it became rate-limiting at 10 degrees C. Overall, the solid degradation and methane production performance under psychrophilic conditions was encouraging and greater than previously reported. Considering the non-acclimated, mesophilic nature of the inoculum, there are grounds to believe that low-temperature anaerobic digestion of grass could be feasible if coupled to efficient hydrolysis of the biomass.     

Control of C/N ratio for butyric acid production from textile wastewater sludge by anaerobic digestion

Increasing textile wastewaters and their biotreatment byproduct-waste activated sludge are serious pollution problems. Butyric acid production from textile wastewater sludge by anaerobic digestion at different C/N ratios was investigated. Adding starch to textile wastewater sludge with a C/N ratio of 30 increased the butyric acid concentration and percentage accounting for total volatile fatty acids (TVFAs) to 21.42 g/L and 81.5%, respectively, as compared with 21.42 g/L and 10.6% of textile wastewater sludge alone. The maximum butyric acid yield (0.45 g/g VS), conversion rate (0.74 g/g VS(digest)) and production rate (2.25 g/L d) was achieved at a C/N ratio of 30. The biological toxicity of textile wastewater sludge also significantly decreased after the anaerobic digestion. The study indicated that the anaerobic co-digestion of textile wastewater sludge and carbohydrate-rich waste with appropriate C/N ratio is possible for butyric acid production.     

Dry anaerobic digestion of differently sorted organic municipal solid waste: a full-scale experience.

This paper presents a comparison of dry anaerobic digestion reactors fed with differently sorted municipal organic solid wastes. One reactor was fed with source sorted organic wastes and a second reactor was fed with mixed organic wastes consisting of grey wastes, mechanically selected municipal solid wastes and sludge. The two reactors utilised the same process (Valorga) and operational conditions at full scale. The results of the study emphasise the influence of the kind of treated material on the process performances, especially in terms of biogas and methane production, thus, energy reclamation. The reactor treating the source sorted organic waste and the reactor treating the mixed organic wastes generated some 200 m3 and 60 m3 of biogas per ton of waste treated, respectively, while the specific methane production was some 0.40 and 0.13 m3CH4/kgTVS, respectively. The mass balance and the final fate of the digested material from the two reactors were also clearly different. As for the costs, these were some 29 Euro per ton of treated waste (50% for personnel) and 53 Euro/ton for disposing of the rejected materials. Incomes were some 100 Euro/ton (on average) and an other 15 Euro/ton came from green certificates. The initial investment was 16 million Euros.     

Thermal pretreatment of the solid fraction of manure: impact on the biogas reactor performance and microbial community.

Application of thermal treatment at 100-140 degrees C as a pretreatment method prior to anaerobic digestion of a mixture of cattle and swine manure was investigated. In a batch test, biogasification of manure with thermally pretreated solid fraction proceeded faster and resulted in the increase of methane yield. The performances of two thermophilic continuously stirred tank reactors (CSTR) treating manure with solid fraction pretreated for 40 minutes at 140 degrees C and non-treated manure were compared. The digester fed with the thermally pretreated manure had a higher methane productivity and an improved removal of the volatile solids (VS). The properties of microbial communities of both reactors were analysed. The specific methanogenic activity (SMA) test showed that both biomasses had significant activity towards hydrogen and formate, while the activity with the VFA - acetate, propionate and butyrate - was low. The kinetic parameters of the VFA conversion revealed a reduced affinity of the microbial community from the CSTR fed with thermally pre-treated manure for acetate, propionate and butyrate. The bacterial and archaeal populations identified by t-RLFP analysis of 16S rRNA genes were found to be identical in both systems. However, a change in the abundance of the species present was detected.     

Biochemical methane potential (BMP) of agro-food wastes from the Cider Region (Spain)

An inventory of agro-food industry organic waste streams with a high potential for biogas transformation was studied in a logistically viable area (Cider Region, Asturias, Spain). Three industries were selected as the most viable ones: livestock, dairy and beverage. The potential for methane production from six wastes (beverage waste, BW; milled apple waste, MA; milk waste, MK; yogurt waste, YG; fats and oils from dairy wastewater treatment, F&O and cattle manure, CM) at five different substrate:inoculum ratios (0.25, 0.50, 0.75, 1.00 and 1.50) was evaluated in laboratory batch assays. Obtained methane yields ranged from 202-549 mL STP CH(4)·g VS waste(-1), and the methane content in biogas ranged from 58-76%. The ultimate practical biochemical methane potentials were slightly affected by the substrate:inoculum ratio. The estimation of the regional fluxes of waste and methane potentials suggests anaerobic digestion as a sustainable solution for the valorization of the organic wastes generated in this Region.     

Anaerobic digestion elutriated phased treatment of piggery waste.

The performance of a novel high-rate anaerobic process, the anaerobic digestion elutriated phased treatment (ADEPT) process, for treating a slurry-type piggery waste (55 g COD/L and 37 g TS/L) was investigated. The ADEPT process consists of an acid elutriation slurry reactor for hydrolysis and acidification, followed by an upflow anaerobic sludge bed reactor for methanification. This process provides stable and high system performance with short HRT (7.4 d) and better effluent quality (2 g SCOD/L and 0.68 g VSS/L) due to the alkaline pH condition for hydrolysis/acidification phase, high refractory solids removal and ammonia toxicity reduction. The optimum pH and HRT for hydrolysis/acidogenesis of the piggery waste were 9 and 5 days at both 35 degrees C and 55 degrees C conditions. The hydrolysis and acidification rate in the mesophilic reactor were 0.05 d(-1) and 0.11 d(-1), meaning that hydrolysis was a limiting step. SCOD production by the hydrolysis was about 0.26 g SCOD/g VS(fed) (3.6 g SCOD/g VS reduction). Methane production and content in the system were 0.3 L CH4/g VS(fed) (0.67 L CH4/g VS destroyed) and 80%, respectively, corresponding to 0.23 L CH4/g COD removal (@STP).     

Towards new indicators for the prediction of solid waste anaerobic digestion properties.

The biochemical composition can be seen as a good indicator of both the biodegradability and the methane potential of a given waste. The work presented here is an attempt to elaborate a typology of wastes and to compare it to the anaerobic degradation characteristics. The first data indicate that there is a link between the ligno-cellulosic content of the waste and the biodegradability. When dealing with application to anaerobic digestion processes, having a tool to predict the ability of the waste to be degraded could be of the greatest interest for preventing failures, estimating biogas production, methane content, or for the management of co-digestion processes.     

A critical review of experimental and predicted methane generation from anaerobic codigestion

Anaerobic digestion is increasingly being considered as a treatment option for an extensive range of waste biomass, due to the potential for energy recovery, in the form of methane production, and lower sludge volumes relative to aerobic treatment processes. Furthermore, when two substrates are codigested (i.e. digested together), added benefits are foreseeable, such as increased methane production and detoxification of toxic compounds via cometabolic degradation pathways. The objectives of this study were to compare experimental and predicted methane production from codigestion literature studies in order to objectively evaluate digester performance. Two predictive methods were used, both assuming methane yields are additive: literature values for digestion of single substrates and a stoichiometric method using model substrates to represent different substrates. Waste sources included in the analysis were primary sewage sludge, waste activated sludge, cow manure, waste paper, grease trap sludge, fat oil and grease and algal sludge. It was found that methane production could approximately be predicted using both methods, with literature methane yields from the same study being the most accurate predictor. One important finding from this study was that the assumption that methane yields are additive is a reasonable one. Furthermore, both predictive methods may be usefully employed as a screening tool to compare methane yields between different types and blends of substrates.     

Anaerobic digestion of space mission wastes.

The technical feasibility of applying leachbed high-solids anaerobic digestion for reduction and stabilization of the organic fraction of solid wastes generated during space missions was investigated. This process has the advantages of not requiring oxygen or high temperature and pressure while producing methane, carbon dioxide, nutrients, and compost as valuable products. Anaerobic biochemical methane potential assays run on several waste feedstocks expected during space missions resulted in ultimate methane yields ranging from 0.23 to 0.30 L g-1 VS added. Modifications for operation of a leachbed anaerobic digestion process in space environments were incorporated into a new design, which included; (1) flooded operation to force leachate through densified feedstock beds; and (2) separation of biogas from leachate in a gas collection reservoir. This mode of operation resulted in stable performance with 85% conversion of a typical space solid waste blend, and a methane yield of 0.3 Lg per g VS added after a retention time of 15 days. These results were reproduced in a full-scale prototype system. A detailed analysis of this process was conducted to design the system sized for a space mission with a six-person crew. Anaerobic digestion compared favorably with other technologies for solid waste stabilization.     

Continuous hydrogen and methane production in a two-stage cheese whey fermentation system

The feasibility of integrating biological hydrogen and methane production in a two-stage process using mixed cultures and cheese whey powder (CWP) as substrate was studied. The effect of operational parameters such as hydraulic retention time (HRT) and organic loading rate (OLR) on the volumetric hydrogen (VHPR) and methane (VMPR) production rates was assessed. The highest VHPR was 28 L H2/L/d, obtained during stable operation in a CSTR at HRT and OLR of 6 h and 142 g lactose/ L/d, respectively. Moreover, hydrogen (13 L/L/d) was produced even at HRT as low as 3.5 h and OLR of 163 g lactose/L/d, nonetheless, the reactor operation was not stable. Regarding methane production in an UASB reactor, the acidified effluent from the hydrogen-producing bioreactor was efficiently treated obtaining COD removals above 90% at OLR and HRT of 20 g COD/L/d and 6 h, respectively. The two-stage process for continuous production of hydrogen and methane recovered over 70% of the energy present in the substrate. This study demonstrated that hydrogen production can be efficiently coupled to methane production in a two-stage system and that CWP is an adequate substrate for energy production.     

Optimised anaerobic treatment of house-sorted biodegradable waste and slaughterhouse waste in a high loaded half technical scale digester.

Anaerobic co-digestion of organic wastes from households, slaughterhouses and meat processing industries was optimised in a half technical scale plant. The plant was operated for 130 days using two different substrates under organic loading rates of 10 and 12 kgCOD.m(-3).d(-1). Since the substrates were rich in fat and protein components (TKN: 12 g.kg(-1) the treatment was challenging. The process was monitored on-line and in the laboratory. It was demonstrated that an intensive and stable co-digestion of partly hydrolysed organic waste and protein rich slaughterhouse waste can be achieved in the balance of inconsistent pH and buffering NH4-N. In the first experimental period the reduction of the substrate COD was almost complete in an overall stable process (COD reduction >82%). In the second period methane productivity increased, but certain intermediate products accumulated constantly. Process design options for a second digestion phase for advanced degradation were investigated. Potential causes for slow and reduced propionic and valeric acid degradation were assessed. Recommendations for full-scale process implementation can be made from the experimental results reported. The highly loaded and stable codigestion of these substrates may be a good technical and economic treatment alternative.