Enhancement of food waste digestion in the hybrid anaerobic solid-liquid system

The hybrid anaerobic solid-liquid (HASL) system is a modified two-phase anaerobic digester for food waste treatment. To enhance the performance of anaerobic digestion in the HASL system, thermal pre-treatment (heating at 150 degrees C for 1 h) and freezing/thawing (freezing for 24 h at-20 degrees C and then thawing for 12 h at 25 degrees C) were proposed for food waste pre-treatment before the anaerobic digestion. Both processes were able to alter the characteristics and structure of food waste favoring substance solubilization, and hence production of methane. However, there was no net energy gain when the energy required by the pre-treatment processes was taken into account.     

Anaerobic co-digestion of winery wastewater.

The operational performance of anaerobic batch reactors treating winery wastewater (WW) combined with waste activated sludge (WAS) in different proportions was investigated under mesophilic conditions. In these experiments it was shown that for anaerobic digestion of WW alone, methane production rate was lower than the rates achieved when WW and WAS were treated together. When WW was mixed with WAS at a concentration of 50% WW resulted in the highest methane production rates. A simplified anaerobic model was used to determine the main kinetic parameters; maximum COD reduction rate (q(DA)) and maximum methane generation rate (k(max)). The maximum values of q(DA) and k(max) were 16.50 kgCOD COD(-1) d(-1) and 14.34 kgCOD kgCOD(-1) d(-1), respectively.     

Submerged membrane bioreactor using fly ash filters: trials with distillery wastewater

This work presents preliminary results for distillery wastewater treatment in a MBR equipped with filters prepared from waste fly ash. The system was fabricated locally and employed submerged membranes in the 2-8 microm pore-size range. Distillery wastewater, after anaerobic digestion, was used as the feed and the bioreactor was inoculated with sludge obtained from a local distillery unit. The MBR was operated for around 250 days. The wastewater quality was monitored in terms of COD (chemical oxygen demand), colour, phenol, and MLSS (mixed liquor suspended solids) concentration. An average COD and phenol removal of 36% and 60% respectively was obtained. The maximum suspended solids retention by the ash filter was as high as 98%. The results were encouraging and further trials are currently underway to improve the performance.     

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.     

Winery and distillery wastewater treatment by anaerobic digestion.

Anaerobic digestion is widely used for wastewater treatment, especially in the food industries. Generally after the anaerobic treatment there is an aerobic post-treatment in order to return the treated water to nature. Several technologies are applied for winery wastewater treatment. They are using free cells or flocs (anaerobic contact digesters, anaerobic sequencing batch reactors and anaerobic lagoons), anaerobic granules (Upflow Anaerobic Sludge Blanket--UASB), or biofilms on fixed support (anaerobic filter) or on mobile support as with the fluidised bed. Some technologies include two strategies, e.g. a sludge bed with anaerobic filter as in the hybrid digester. With winery wastewaters (as for vinasses from distilleries) the removal yield for anaerobic digestion is very high, up to 90-95% COD removal. The organic loads are between 5 and 15 kgCOD/m3 of digester/day. The biogas production is between 400 and 600 L per kg COD removed with 60 to 70% methane content. For anaerobic and aerobic post-treatment of vinasses in the Cognac region, REVICO company has 99.7% COD removal and the cost is 0.52 Euro/m3 of vinasses.     

Anaerobic digestion of blackwater from vacuum toilets and kitchen refuse in a continuous stirred tank reactor (CSTR).

The objective of this research was mesophilic anaerobic digestion of blackwater from vacuum toilets (BW) and kitchen refuse (KR) in a CSTR within an ecological sanitation system. A detailed investigation of the BW characteristics was carried out. Research on anaerobic digestion was performed with CSTR of 101 volume at HRT of 10, 15 and 20 days. The digestion of BW at 20 days HRT showed stable performance without inhibition effects, in spite of relatively high ammonium concentrations. The removal of total and particulate COD was 61% and 53%, respectively, and the methane yield 10/CH4/cap/day. The addition of kitchen refuse (KR) improved the performance of the CSTR in terms of COD removal efficiency and methane yield. At 20 days HRT the removal of total and particulate COD increased up to 71% and 67%, respectively, and the methane yield to 27/CH4/cap/day. The results at 15 days HRT showed similar performance. At HRT of 10 days, the anaerobic treatment was limited but reached steady state conditions at higher VFA concentrations in the effluent, with a decrease of COD removal of 30 to 33% and of methane yields of 19 to 21%.     

Enhanced solubilisation of the residual fraction of municipal solid waste.

The Swedish ordinance banning the landfilling of organic material after 2005 has led to rapid developments towards waste incineration, while biological alternatives have been less studied. In this study, biological alternatives for enhanced methane production from residual municipal waste (the remaining waste fraction after source separation) are investigated. The strategies investigated were recirculation of leachate, recirculation of leachate with aeration, flooding of the solid bed, and enzyme addition after initial leaching in an anaerobic, batch, two-stage digestion process with recirculation of digested leachate. The degree of solubilisation of organic compounds achieved was higher for initial digestion in a two-stage anaerobic digestion system followed by the addition of cellulolytic enzymes than in the other strategies investigated. The overall net solubilisation achieved was 0.48 g COD/g VSadded corresponding to an increase of 34%. In addition, the digestion time was considerably reduced using this strategy. For the other strategies investigated the solubilisation yields obtained were similar, 0.31 g COD/g VSadded.     

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).     

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.     

A dispersion based model for anaerobic digestion of solid cattle wastes in a stratified thermophilic accumulation system.

A dynamic model has been developed to describe the anaerobic digestion of solid cattle waste in an accumulation system (AC). To calibrate the model an experiment was carried out at a lab-scale AC at 50 degrees C. The predicted methane production shows a very good agreement (i.e. R2 = 0.998) with the experimental data. However less agreement is evident for the intermediates. After model validation the model was applied to study the effect of different aspect ratios on the system performance. An optimum aspect ratio of 2-3 could be determined.     

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.     

Methane production and solids destruction in an anaerobic solid waste reactor due to post-reactor caustic and heat treatment.

This study was undertaken to determine the feasibility of caustic and heat treatment of sludge from a dry anaerobic reactor (DAR) with respect to increased methane production and solids destruction. The DAR was operated semi-continuously at 55 degrees C on sized-reduced municipal solid waste at a solids retention time of 15 days. A respirometer was employed to monitor the extent and rate of methane production from anaerobic biodegradation of DAR sludge that was treated with caustic and heat. Results indicate that caustic and heat treatment at 50 degrees C and 175 degrees C increased methane production by 22% and 52%, respectively. Also, volatile solids destruction increased from 46% to 58% and 83%, respectively. Based on these results, economic analysis for a full-scale 10(5) kg/d facility suggests that annual project revenue for 50 degrees C and 175 degrees C treatment is estimated at $21,000 and $445,000, respectively.     

Anaerobic co-digestion of sludge with other organic wastes and phosphorus reclamation in wastewater treatment plants for biological nutrients removal.

This paper deals with the performances obtained in full scale anaerobic digesters co-digesting waste activated sludge from biological nutrients removal wastewater treatment plants, together with different types of organic wastes (solid and liquid). Results showed that the biogas production can be increased from 4000 to some 18,000 m3 per month when treating some 3-5 tons per day of organic municipal solid waste together with waste activated sludge. On the other hand, the specific biogas production was improved, passing from 0.3 to 0.5 m3 per kgVS fed the reactor, when treating liquid effluents from cheese factories. The addition of the co-substrates gave minimal increases in the organic loading rate while the hydraulic retention time remained constant. Further, the potentiality of the struvite crystallisation process for treating anaerobic supernatant rich in nitrogen and phosphorus was studied: 80% removal of phosphorus was observed in all the tested conditions. In conclusion, a possible layout is proposed for designing or upgrading wastewater treatment plants for biological nutrients removal process.     

Pretreatment of silk-dyeing industrial wastewater by UASB reactor.

The objective of this study was to investigate the performance of the upflow anaerobic sludge blanket (UASB) reactor as the pretreatment system for silk-dyeing wastewater. Two laboratory-scale UASB reactors, with working volume of 15.59 I, were used during May 1998 to June 1999. The actual wastewater was diluted to reduce ammonium ion toxicity on anaerobic bacteria. The experiments were conducted at the organic loading rates (OLRs) of 0.52, 1.01, 1.04, 1.54 and 2.56 kgCOD/(m3 x d), treating only wastewater generated from the acid-dye process of mixed-species raw silk. It took approximately 4 1/2 months to reach the steady-state conditions. It was found that the COD removal was in the ranges of 74.1-85.3%, except at OLR 2.56 kgCOD/(m3 x d) where efficiency significantly dropped to 55.2%. The apparent color removal was in the similar trend as COD. During the study periods, wastewater input had various color shades while the effluent generally looked pale yellowish. The methane generation rates ranged from 0.18-0.31 m3/kg COD removed, with methane composition 81.0-88.1% in biogas. The average granule size in the sludge bed had slowly increased to 0.73 mm in the last experiment. It can be concluded that the UASB reactor is suitable as a pretreatment system for silk-dyeing wastewater. An OLR of 1 kgCOD/(m3 x d) and an influent concentration diluted to 2,600 mgCOD/l are suggested while COD and apparent color removal efficiency of 80% and 70%, respectively, can be expected.     

Comparison of the sensitivities of fish, Microtox and Daphnia-magna bioassays to amoxycillin in anaerobic/aerobic sequential reactor systems

In this study the anaerobic treatability of amoxycillin (AMX) was investigated in a laboratory-scale anaerobic multi-chamber bed reactor (AMCBR)/aerobic continuously stirred tank reactor (CSTR) system. The chemical oxygen demand (COD) and AMX removal efficiencies were around 94% in the AMCBR reactor at hydraulic retention times (HRTs) between 2.25 and 5.5 days. Decreasing the HRT appeared not to have a significant effect on the performance of the AMCBR up to a HRT of 1.13 days. The maximum methane production rate and methane percentage were around 1,100-1,200 mL/day and 55%, respectively, at HRTs between 2.25 and 5.5 days. The decrease in HRT to 1.5 days decreased slightly the gas productions (1,000 mL/day and 500 mL for total and methane gases) and methane percentage (45%). The AMCBR recovered back to its baseline performance within a couple of days. The acute toxicity of 150 mg/L AMX was monitored with Daphnia magna, Lepistes sp., and Vibrio fischeri acute toxicity tests. The acute toxicity removals were 98, 96 and 96% for V. fischeri, D. magna and Lepistes sp. in the effluent of the sequential system treating 150 mg/L AMX at HRTs of 2.25-5.5 days. Among the trophic organisms used in the acute toxicity tests the most sensitive organism was found to be bacteria (V. fischeri) while the most resistant organism was found to be fish (Lepistes sp.).     

Reliable method for assessing the COD mass balance of a submerged anaerobic membrane bioreactor (SAMBR) treating sulphate-rich municipal wastewater

The anaerobic treatment of sulphate-rich wastewater causes sulphate reducing bacteria (SRB) and methanogenic archaea (MA) to compete for the available substrate. The outcome is lower methane yield coefficient and, therefore, a reduction in the energy recovery potential of the anaerobic treatment. Moreover, in order to assess the overall chemical oxygen demand (COD) balance, it is necessary to determine how much dissolved CH(4) is lost in the effluent. The aim of this study is to develop a detailed and reliable method for assessing the COD mass balance and, thereby, to establish a more precise methane yield coefficient for anaerobic systems treating sulphate-rich wastewaters. A submerged anaerobic membrane bioreactor (SAMBR) treating sulphate-rich municipal wastewater was operated at 33 °C for an experimental period of 90 d, resulting in a high COD removal (approximately 84%) with a methane-enriched biogas of 54 ± 15% v/v. The novelty of the proposed methodology is to take into account the sulphide oxidation during COD determination, the COD removed only by MA and the dissolved CH(4) lost with the effluent. The obtained biomethanation yield (333 L CH(4) kg(-1) COD(REM MA)) is close to the theoretical value, which confirms the reliability of the proposed method.     

Enhancing waste decomposition and methane production in simulated landfill using combined anaerobic reactors.

The design of leachate recirculation to enhance municipal solid waste decomposition, methane production and microbial activities was taken into account in this study by using the combined anaerobic reactors of a simulated high organic content landfill reactor and a stabilized fixed film anaerobic reactor. The latter aimed to treat high strength leachate from the simulated landfill reactor before recirculation back to the former reactor. The results showed the possibility to apply this technique to high organic waste landfill where rapid and accumulative acids formed in the leachate were removed and treated by a fixed film anaerobic reactor. In addition, the treated leachate when circulated back to the simulated bioreactor landfill helps accelerate waste decomposition and methane production by providing buffer capacity as well as diluting organic content in the leachate to achieve the favourable conditions for acidogen and methanogen, respectively, to rapidly enter into the methanogenesis phase.     

Start-up and steady-state results of a full-scale UASB reactor treating malting wastewater.

An Imhoff tank was reconstructed into a 250 m3 UASB reactor in order to treat a malting plant wastewater. The UASB was inoculated with sludge from an anaerobic lagoon used for slaughterhouse wastewater treatment. After two months of operation the reactor achieved full load with an HRT of 17 h, a COD removal higher than 80% and a biogas production of 300 m3/day (77% average methane content), with an organic loading rate of 3.6 kgCOD/m3.d (0.24 kgCOD/kgVSS.d). A yield coefficient of 0.09 gVSS/gCODrem was found from a mass balance. The fat present in the inoculated sludge (48 mg/gSSV) did not affect the start up performance. Sludge from the inoculum with high content of fat (270 mg/gSSV), was separated by flotation in the first week of operation. The COD removal efficiency was scarcely influenced by the reactor operation temperature (17-25 degrees C).     

Thermal pre-treatment of primary and secondary sludge at 70 degrees C prior to anaerobic digestion.

In general, mesophilic anaerobic digestion of sewage sludge is more widely used compared to thermophilic digestion, mainly because of the lower energy requirements and higher stability of the process. However, the thermophilic anaerobic digestion process is usually characterised by accelerated biochemical reactions and higher growth rate of microorganisms resulting in an increased methanogenic potential at lower hydraulic retention times. Furthermore, thermal pre-treatment is suitable for the improvement of stabilization and could be realized at relatively low cost especially at low temperatures. The present study investigates the effect of the pre-treatment at 70 degrees C on thermophilic (55 degrees C) anaerobic digestion of primary and secondary sludge in continuously operated digesters. Thermal pre-treatment of primary and secondary sludge at 70 degrees C enhanced the removal of organic matter and the methane production during the subsequent anaerobic digestion step at 55 degrees C. It also greatly contributed to the destruction of pathogens present in primary sludge. Finally it results in enhanced microbial activities of the subsequent anaerobic step suggesting that the same efficiencies in organic matter removal and methane recovery could be obtained at lower HRTs.