Two-phase Anaerobic Digestion Process Research Articles

Influence of lactic acid on the two-phase anaerobic digestion of kitchen wastes

To evaluate the influence of lactic acid on the methanogenesis, anaerobic digestion of kitchen wastes was firstly conducted in a two-phase anaerobic digestion process, and performance of two digesters fed with lactic acid and glucose was subsequently compared. The results showed that the lactic acid was the main fermentation products of hydrolysis-acidification stage in the two-phase anaerobic digestion process for kitchen wastes. The lactic acid concentration constituted approximately 50% of the chemical oxygen demand (COD) concentration in the hydrolysis-acidification liquid. The maximum organic loading rate was lower in the digester fed with lactic acid than that fed with glucose. Volatile fatty acids (VFAs) and COD removal were deteriorated in the methanogenic reactor fed with lactic acid compared to that fed with glucose. The specific methanogenic activity (SMA) declined to 0.343 g COD /(gVSS.d) when the COD loading were designated as 18.8 g/(L.d) in the digester fed with lactic acid. The propionic acid accumulation occurred due to the high concentration of lactic acid fed. It could be concluded that avoiding the presence of the lactic acid is necessary in the hydrolysis-acidification process for the improvement of the two-phase anaerobic digestion process of kitchen wastes.

Anaerobic membrane reactor with phase separation for the treatment of cheese whey

Two-phase anaerobic digestion of cheese whey was investigated in a system consisting of a stirred acidogenic reactor followed by a stirred methanogenic reactor, the latter being coupled to a membrane filtration system to enable removal of soluble effluent whilst retaining solids. The acidogenic reactor was operated at a hydraulic retention time (HRT) of one day, giving maximum acidification of 52.25% with up to 5 g/l volatile fatty acids, of which 63.7% was acetic acid and 24.7% was propionic acid. The methanogenic reactor received an organic load up to 19.78 g COD/l d, corresponding to a HRT of 4 days, at which 79% CODs and 83% BOD 5 removal efficiencies were obtained. Average removals of COD, BOD 5 and TSS in the two-phase anaerobic digestion process were 98.5%, 99% and 100%, respectively. The daily biogas production exceeded 10 times reactor volume and biogas methane content was greater than 70%.

UPGRADING AN EXISTING ANAEROBIC DIGESTION SYSTEM TO INCREASE CAPACITY AND PRODUCE CLASS A BIOSOLIDS

UPGRADING AN EXISTING ANAEROBIC DIGESTION SYSTEM TO INCREASE CAPACITY AND PRODUCE CLASS A BIOSOLIDSWhen the City of Lakeland, Florida, needed to expand the solids processing capacity at their Glendale WWTP, they opted to modify the existing anaerobic digestion system to produce Class A biosolids. The selected alternative was a proprietary two-phase anaerobic digestion process. A key feature of the process is the batch solids feed to and withdrawal from the first stage, thermophilic digester....Author(s)Michael R. LefflerH.J. CurtisSourceProceedings of the Water Environment FederationSubjectSession 11B: StabilizationDocument typeConference PaperPublisherWater Environment FederationPrint publication date Jan, 2006ISSN1938-6478SICI1938-6478(20060101)2006:2L.716;1-DOI10.2175/193864706783797393Volume / Issue2006 / 2Content sourceResiduals and Biosolids ConferenceFirst / last page(s)716 - 728Copyright2006Word count107

The Influence of pH on Hydrolysis and Acidogenesis of Kitchen Wastes in Two-phase Anaerobic Digestion

Batch and semi-continuous experiments were carried out to determine the effect of pH on hydrolysis and acidogenesis of kitchen wastes in the two-phase anaerobic digestion. The results of the batch experiment showed pH adjustment could improve both hydrolysis and acidogenesis rates of kitchen wastes. Compared with pH at 5, 9 and 11, pH 7 provided an optimum working condition for anaerobic digestion of kitchen wastes. At pH 7, about 86% of the total organic carbon (TOC) and 82% of chemical oxygen demand (COD) were solubilized and the maximum volatile fatty acid (VFA) concentration of 36g l−1 was achieved on the fourth day. The total VFA yield was 0.27 g (g total solid (TS))−1, which almost doubled the yield without pH adjustment (0.15 g (g TS)−1). The acidified products with lower concentration of lactic acid were more favorable to the subsequent methanogenesis. At pH 7, most of the protein was degraded into ammonia nitrogen (NH4 +-N), resulting in additional buffer of acidified liquid. According to this, a novel method of pH adjustment applying NH4 +-N buffer was established. The semi-continuous experiment indicated that adjusting pH to 7 in the acidogenic stage in the two-phase anaerobic digestion process would increase both TS loading rate (16g l−1 d−1) and recovery of biological energy (520 ml methane (CH4) (g TS)−1).

Survival of Salmonella spp. in a simulated acid-phase anaerobic digester treating sewage sludge.

The presence of pathogenic microorganisms in municipal waste sludge may create a serious outbreak of water borne diseases if the sludge is used for agricultural purpose. An attempt to decrease the number of pathogenic microorganisms, Salmonella spp. using a simulated acid-phase anaerobic digester was tested in a laboratory-scale batch experiment. Reduction of Salmonella spp. was demonstrated in a mixture of sludge and organic acid, simulating an acid digester of a two-phase anaerobic digestion process. A high concentration of organic acid at a pH value of 5.5-6.0 prevents a decrease in Salmonella spp. concentration. Almost complete destruction of Salmonella spp. is observed within two days if the pH value is maintained below 5.5.

Survival of Salmonella spp. in a simulated acid-phase anaerobic digester treating sewage sludge.

The presence of pathogenic microorganisms in municipal waste sludge may create a serious outbreak of water borne diseases if the sludge is used for agricultural purpose. An attempt to decrease the number of pathogenic microorganisms, Salmonella spp. using a simulated acid-phase anaerobic digester was tested in a laboratory-scale batch experiment. Reduction of Salmonella spp. was demonstrated in a mixture of sludge and organic acid, simulating an acid digester of a two-phase anaerobic digestion process. A high concentration of organic acid at a pH value of 5.5-6.0 prevents a decrease in Salmonella spp. concentration. Almost complete destruction of Salmonella spp. is observed within two days if the pH value is maintained below 5.5.

Bacterial Population of a Two-Phase Anaerobic Digestion Process Treating Effluent of Cassava Starch Factory

Different groups of microorganisms in a two-phase anaerobic system were enumerated to evaluate the prevalence of specific groups and species. Total and fecal coliforms showed similar values both in acidogenic and methanogenic reactors. The fecal streptococci were 4-fold higher in the acidogenic reactor, when compared with those of the methanogenic reactor. As expected, no methane forming or sulfate reducing bacteria were found in the acidogenic reactor. The populations of methanogenic bacteria were dominated by a mixed population of straight to curved rods and multicellular filaments which strongly resembled members of the genus Methanosaeta. Seven prevalent species of facultative anaerobic gram-negative bacteria were identified as Previdencia alcalifacienciens, Providencia rettgeri, Enterobacter cloaceae, Citrobacter freundii, Klebsiella oxytoca, Proteus pennri and Yersinia enterocolitica. The species Pseudomonas aeruginosa, Stenotrophomonas maltophila and Acinotobacter iwoffi, were the most frequently isolated glucose non-fermenting gram-negative bacilli. Among these species, only P. aeruginosa was present in high number in each sample.

Kraft-Mill Biosolids Treatment by Conventional and Biphasic Fermentation

Kraft primary and mixed primary-activated sludges were gasified by thermophilic (50 °C) high-rate and mesophilic two-phase digestion. Kraft sludges were deficient in nitrogen, phosphorus, and other unidentified nutrients. Municipal sludge proved to be a much better nutrient source than inorganic salts. Performance of the mesophilic (35°C), two-phase anaerobic digestion (TPAD) system was about the same as that of thermophilic single-stage, high-rate digestion at a 10-day HRT as judged in terms of methane yield (0.17 SCM/kg VS added) and VS reduction efficiency (43%). The mesophilic TPAD process yielded a higher methane-content (65%) fuel gas; it is capable of exhibiting higher net-energy recovery efficiency than thermophilic single-stage digestion.

Two-phase methanization of food wastes in pilot scale.

A 5 ton/d pilot scale two-phase anaerobic digester was constructed and tested to treat Korean food wastes in Anyang city near Seoul. The easily degradable presorted food waste was efficiently treated in the two-phase anaerobic digestion process. The waste contained in plastic bags was shredded and then screened for the removal of inert materials such as fabrics and plastics, and subsequently put into the two-stage reactors. Heavy and light inerts such as bones, shells, spoons, and plastic pieces were again removed by gravity differences. The residual organic component was effectively hydrolyzed and acidified in the first reactor with 5 d space time at pH of about 6.5. The second, methanization reactor converted the acids into methane with pH between 7.4 and 7.8. The space time for the second reactor was 15 d. The effluent from the second reactor was recycled to the first reactor to provide alkalinities. The process showed stable steady-state operation with the maximum organic loading rate of 7.9 kg volatile solid (VS)/m3/d and the volatile solid reduction efficiency of about 70%. The total of 3.6 tons presorted MSW containing 2.9 tons of food organic was treated to produce about 230 m3 of biogas with 70% (v/v) of methane and 80 kg of humus. This process is extended to full-scale treating 15 tons of food waste a day in Euiwang city and the produced biogas is utilized for the heating/cooling of adjacent buildings.

Kraft-mill biosolids treatment by conventional and biphasic fermentation

Kraft primary and mixed primary-activated sludges were gasified by thermophilic (50 °C) high-rate and mesophilic two-phase digestion. Kraft sludges were deficient in nitrogen, phosphorus, and other unidentified nutrients. Municipal sludge proved to be a much better nutrient source than inorganic salts. Performance of the mesophilic (35°C), two-phase anaerobic digestion (TPAD) system was about the same as that of thermophilic single-stage, high-rate digestion at a 10-day HRT as judged in terms of methane yield (0.17 SCM/kg VS added) and VS reduction efficiency (43%). The mesophilic TPAD process yielded a higher methane-content (65%) fuel gas; it is capable of exhibiting higher net-energy recovery efficiency than thermophilic single-stage digestion.

A two-phase anaerobic digestion (UASB-UASB) process: design criteria and optimal system loading capacity

This research evaluated the effectiveness and feasibility of two-phase (separation acid and methanogenic phases) anaerobic digestion of simulated sewage sludge using a UASB-UASB (upflow anaerobic sludge blanket) process. Predictive models of chemical oxygen demand (COD) (soluble) removal efficiency and CH4 gas production during loading maximization and the recovery (after failure) period were made. The optimum hydraulic retention time (HRT) for the A-UASB was about 1.0 d; for the M-UASB system, two different optimum HRTs were evident, depending on the operational mode. When the UASB-UASB system was run under the maximum loading possible, the best HRT in the M-UASB was about 2.0 d, to achieve high COD removal and concurrent optimum CH4 production; after deliberate overloading, to induce failure, and subsequent system recovery, the predicted optimum HRT in the methane unit was about 2.7 d, to achieve concurrent adequate CH4 production and COD removal. There also appeared to be a restructuring of the bacterial community inside the M-UASB, during the recovery period. For overall design purposes, optimum operating HRTs of 1 and 2 d, with an internal recycle rate of 1.6 and 2.5 times the influent flow rate, are recommended for A- and M-UASBs, respectively; a conservative organic loading rate of 19 kg COD (total)/(m3∙d) is suggested. Finally, the optimum HRTA-UASB/HRTM-UASB ratio was found to be about 0.63 and HRTA-UASB/HRTsystem was 0.38, which are within the boundaries of this research project. Key words: anaerobic sludge treatment, design criteria, optimum system loading capacity, two-phase anaerobic stabilization, upflow anaerobic sludge blanket.

A Two-Phase Anaerobic Digestion Process (UASB-UASB): Induced Failure and System Recovery Using a Step Loading Reduction Approach and Modified Recycle Ratio (RR)

This research demonstrated the feasibility of recovery of a UASB-UASB system treating simulated sewage sludge, after operating in a deliberate failure mode. The approach employed an application of step-loading reduction and a modification of the internal recycle ratio (RR). The system recovered exponentially (r2 = 0.99) and logarithmically (r2 = 0.81), with an increase in HRT (loading reduction), in so far as COD (sol) removal efficiency (92%) and CH4 gas production (100%), were concerned. The system appeared to return to its initial stage of operation, after 43 days of recovery period, by the application of a two-step increase in the HRTM-UASB at 1.5 and 5.5 times that of the failure HRTM-UASB (0.63 days); coincidentally, a dimensional time of 5.5 and 10 times the minimum HRT was employed for the overall system and the M-UASB, respectively.

Comparison of One-Phase and Two-Phase Anaerobic Digestion Processes in Characteristics of Substrate Degradation and Bacterial Population Levels

The comparison of one-phase and two-phase anaerobic digestion processes in the characteristics of substrate degradation and the bacterial population levels was investigated by using the chemostat-type reactors to which starch was fed as substrate when both processes were operated under the same experimental conditions. By decreasing the SRTs of both systems from 10.2 d to 5 d, 2.5 d and 1.75 d. it was found that the two-phase system was more stable to the change in pH than one-phase system. The CH4 recovery rates and COD removal rates in the two-phase system increased by 4 to 9% and 3 to 10%. respectively, although the CH4 recovery rate and the COD removal rate in the one-phase system were slightly higher than those in the two-phase system at the SRT of 10.2 d. The concentration of propionate in the effluent of the one-phase system was 30 to 50% higher than that in the two-phase system; while the concentrations of acetate and butyrate in the one-phase system were slightly lower than those in the two-phase one. The enumeration of the bacteria was performed by the MPN method. The population levels of acidogenic bacteria in both systems were in the same order (108 to 1010 MPN/ml). the population levels of hydrogenotrophs were also in the same order as the acidogenic bacteria in the two-phase system, while the population levels of hydrogenotrophs were 10 to 100 fold less than that of acidogenic bacteria in the one-phase system. The number of HAc-utilizing methanogens in the methanogenesis of the two-phase system were 2 to 10 times higher than that in the one-phase system. Therefore, the one-phase system cannot be regarded simply as the sum of acidogenesis and methanogenesis.

Pilot-Scale Demonstration of Two-Phase Anaerobic Digestion of Activated Sludge

An innovative pilot-scale two-phase anaerobic digestion process was developed to stabilize concentrated (7-5%) activated sludge at a 12-day SRT and a loading rate of 5 kg VS/m3 d. The pilot system exhibited an unusually high VS reduction of 73%, an aggregated carbohydrate-protein-lipid reduction of 71%, and a methane yield of 0.3 m3/kg VS added. Optimum acidogenic fermentation producing 9500 mg/l of organic acids was achieved at an HRT of 3 days and a loading rate of 16 kg VS/m3.d or higher. Enhanced acidogenic hydrolysis and prehydrolysis of polymerics and nocardial residues eliminated digester foaming. Sulfate and nitrate reductions, and syntrophic methane fermentation occurred during acidogenic conversions. Acetogenesis and aceticlastic methane fermentation were predominant in the methane digester, which generated 93% of the system methane production. Contrary to literature reports, there was no inhibition of acetogens or methanogens at a high ammonia-N concentration of 2500 mg/l and pH 7.7. The acid and methane digesters could be started quickly. They were resilient to temperature drops and loading fluctuations. Acid fermentation of 8% of the plant's WAS and addition of the resulting fermentation products (enzymes, organic acids, etc.) to the full-scale high-rate digester increased VS reduction by 46% and eliminated severe foaming. Full-scale two-phase digestion of WAS is scheduled to start in 1990.

Peat beneficiation by wet carbonization

Peat, as it exists in nature, contains about 90% water and has a heating value of about 19.8 MJ/kg (dry basis). The water in peat is very difficult to remove by mechaninal means. The current technology can dewater peat to a moisture content of only about 67 wt.%. Wet carbonization is a process whereby carbonaceous materials are heated under pressure for a period of time to produce a beneficiated product. In the case of peat, a low-sulfur solid fuel with enhanced heating value, improved dewaterability, and better handling characteristics is produced. A comprehensive study on the effects of operating conditions on the wet-carbonization characteristics of Minnesota peat was conducted. This study was jointly sponsored by the U.S. Department of Energy/Pittsburgh Energy Technology Center and Minnesota Power and Light Company. Tests were conducted in a 360-kg/h continuous process research unit. The operating conditions studied were temperatures ranging from 175 to 260°C; residence times of 1–60 min; atmosphere, steam and air; product filtrate recycle; and slurry concentration, 6.1–9.5 wt.%. The effluent process water was treated by IGT's two-phase anaerobic digestion process for energyrecovery. This process maximizes the production of methane and minimizes hydraulic retention times in the digesters. An integrated wet-carbonization process to produce a solid peat fuel with energy and by-product recoveries from the liquid effluent streams has been developed. This paper discusses the work accomplished in this study.

Evaluation of Effectiveness of Two-Phase Anaerobic Digestion Process Degrading Complex Substrate

The effectiveness of two-phase anaerobic digestion in degrading complex substrates was studied by using a continuous acidogenic reactor and batch experiments. When 4,600 mg COD/l of milk consisting of carbohydrates, proteins and lipids, was fed to the acidogenic reactor, carbohydrates were easily converted to acids although protein degradation was insufficient and lipids were not degraded. The condition which gave greater than 95% carbohydrate degradation was a pH of not less than 4.5 at a constant HRT of 18 hours, and HRT longer than 6 hours at a constant pH of 6.0. Low pH or short HRT within the optimal range brought about the production of more n-butyrate instead of propionate. Degradation of egg albumin in the two-phase system required a longer HRT (about 5 days) than the ordinary acidogenic reactor. Batch experiments using the mixed liquor from the acidogenic reactor suggest that phase separation is not very effective for the degradation of carbohydrates and proteins, but it can prevent the inhibition caused by lipids.

Upgrading of Anaerobic Digestion Processes for Night Soil

The upgrading of night soil anaerobic digestion processes was studied by field investigation of the performance of anaerobic digesters in existing night soil treatment plants and by serai-continuous experiments with bench-scale digesters. Both the average monthly amount of collected night soil fed to a digester and the concentration of volatile solids in night soil increased in winter. A good correlation was found to exist between gas production and digestion temperature. The COD removal rate in the first digester was markedly higher than that in the second digester. The stirring period in the first digester in one plant differs from that in the other plant. Stirring the first digester for too long a period reduces the rate of COD removal by the second digester in the two-stage anaerobic digestion process. The first digester should be stirred for more than one hour per day in order to promote gas production. Gas production and CODCr removal rate in the second digester were hardly affected by reductions in retention time ranging from 15 days to 5 days. As night soil contains a large amount of cellulose and other refractory organics, some kind of pretreatment of the night soil fed to the digester may be necessary for the promotion of acidogenesis in the two-phase anaerobic digestion process.

Influence of phase separation on the anaerobic digestion of glucose—II: Stability, and kinetic responses to shock loadings

A mineral medium, containing 1% (w/v) glucose as the main carbon source, was subjected to one-phase and to two-phase anaerobic digestion processes under comparable conditions. The one-phase system consisted of an anaerobic up-flow reactor containing both acidogenic as well as methanogenic populations. The two-phase system consisted of an acid reactor and a methane reactor connected in series allowing sequential acidogenesis and methanogenesis of the glucose. After maximum turnover rates of glucose had been attained in both systems, by gradually increasing feed supply rate, both systems were switched to the batch mode and subjected to shock loadings with glucose or fatty acids. Maximum specific turnover rates of fatty acids in the one-phase process averaged 0.39 g COD · g biomass −1 d −1 and 2.23 g g −1 d −1 for the methane reactor of the two-phase system. Charging the one-phase system with doses of glucose resulted mainly in an accumulation of propionate which was degraded relatively slowly. It was concluded that interspecies hydrogen transfer may become rate limiting at high loading rates, stimulating formation of propionate. Therefore a two-phase system, as compared with a one-phase digestion process for easily hydrolyzable carbohydrates, was characterized as being essentially the more stable.

Influence of phase separation on the anaerobic digestion of glucose—I maximum COD-turnover rate during continuous operation

A mineral medium containing 1% of glucose as the main carbon source was subjected to one-phase and to two-phase anaerobic digestion processes under comparable conditions. The one-phase system consisted of an anaerobic up-flow reactor containing both acidogenic as well as methanogenic populations allowing a complete conversion of the carbon source into gaseous end products and biomass. The two-phase system consisted of an acid reactor and a methane reactor connected in series allowing sequential acidogenesis and methanogenesis of the glucose. Performance of the one-phase system and of the methane reactor of the two-phase system is presented by carbon mass balances. By gradually increasing the feed supply to both systems, maximum turnover of COD was determined. Maximum specific sludge loadings of the methanogenic phase of the two-phase system was over 3 times higher than that of the one-phase system. In a second experiment both systems were subjected to overloading, resulting in the accumulation of volatile fatty acids (VFA). In the one-phase system propionate and acetate were formed in considerable amounts. Although acetate disappeared rapidly after cessation of the feed supply, no turnover of propionate was observed within one week. On overloading the methane reactor of the two-phase system accumulation of several fatty acids within the reactor was observed. Rapid conversion of all fatty acids took place immediately after interruption of feed supply. The eco-physiological significance of phase separation is discussed briefly.