Overview of anaerobic treatment: thermophilic and propionate implications.

Difficulties in achieving low propionate concentrations in anaerobically treated effluents are frequently reported in the literature (Ahring, 1994; Kugelman and Guida, 1989; Rimkus et al., 1982), especially at thermophilic temperatures, with concentrations as high as 1000 to 9600 mg/L sometimes produced. This paper will detail the effect of several variables on the performance of both mesophilic and thermophilic regimes. Studies concerning the effect of the following four important factors on performance are included: reactor configuration, inorganic nutrient supplementation, substrate characteristics, and the unique role of microbial consortia proximity in enhancing performance. Reactor configuration modifications, essential nutrient additions, and the importance of close microbial proximity were all found to contribute to improvement in thermophilic anaerobic digestion in all the studies. It was found that, in substrates that shunt significant amounts of the electron donor through propionate, performance was critically related to reactor optimization, with propionate removal efficiency considerably improved using intact upflow anaerobic sludge blanket granules, less so in a homogenized granule slurry blanket, and noticeably reduced even more when the completely stirred reactor configuration of homogenized granules was used. The critical importance of extremely close microbial consortia proximity in maintaining hydrogen intermediates at very low levels to efficiently convert propionate to hydrogen and acetate was demonstrated. Compared to mesophilic digestion, thermophilic digestion manifested elevated levels of propionate, except in the nonmixed reactors, which had close microbial consortia proximity. The reactor configuration with the best results was the anaerobic digestion elutriated phased treatment (ADEPT) scheme, in which the raw sludge was elutriated of its fermenting volatile fatty acids, as they are generated in a short 5- to 8-day solids retention time (SRT) in one reactor and the elutriate then metabolized by passing up through a methanogenic granule or slurry blanket (with its close microbial consortia proximity) in a separate reactor with a 20- to 50-day SRT. Loading rates and performance of the ADEPT reactor configuration were superior to the standard continuously stirred tank reactor, and ADEPT thermophilic temperatures allowed higher organic loading rates without high propionate concentrations in the effluent.     

Laboratory evaluation of thermophilic-anaerobic digestion to produce Class A biosolids. 1. Stabilization performance of a continuous-flow reactor at low residence time.

There is increasing interest in the United States in producing biosolids from municipal wastewater treatment that meet the criteria for Class A designation established by the U.S. Environmental Protection Agency. Class A biosolids are intended to be free of pathogens and also must meet requirements for reduction of the vector-attraction potential associated with untreated sludge. High-temperature processes are considered to produce Class A biosolids if the combination of operating temperature and treatment time exceeds minimum criteria, but this option is not applicable to mixed, continuous-flow reactors. Such reactors, or any combination of reactors that does not meet the holding time requirement at a specific temperature, must be demonstrated to inactivate pathogens to levels consistent with the Class A criteria. This study was designed to evaluate pathogen inactivation by thermophilic anaerobic digestion in a mixed, continuous-flow reactor followed by batch or plug-flow treatment. In this first of a two-part series, we describe the performance of a continuous-flow laboratory reactor with respect to physical and chemical operating parameters; microbial inactivation in the combined continuous-flow and batch treatment system is described in the second part. Sludges from three different sources were treated at 53 degrees C, while sludge from one of the sources was also treated at 55 and 51 degrees C. Relatively short hydraulic retention times (four to six days) were used to represent a conservative operating condition with respect to pathogen inactivation. Treatment of a fermented primary sludge led to an average volatile-solids (VS) destruction efficiency of 45%, while VS destruction for the other two sources was near or below 38%, the Class A criterion for vector attraction reduction. Consistent with other studies on thermophilic anaerobic digestion of sludges at short residence times, effluent concentrations of volatile fatty acids (VFAs) were relatively high. Also consistent with other studies, the most abundant VFA in the effluent was propionate. Gas production ranged from 0.3 to 0.5 m3/kg VS fed and from 0.8 to 1.3 m3/kg VS destroyed.     

Laboratory-scale comparison of anaerobic-digestion alternatives.

Laboratory-scale digesters were used to perform side-by-side evaluations of alternative digestion systems including thermophilic-mesophilic-phased digestion (TPAD), acid/methane-phased digestion, mesophilic digestion, and the performance of the thermophilic stage of TPAD alone. Total detention times in the systems evaluated varied from 8 to 20 days. Temperatures in the mesophilic stage of TPAD were varied from 35 to 43.3 degrees C and temperatures in the thermophilic stage were varied from 55 to 58.9 degrees C. The purpose of the study was to evaluate volatile solids (VS) reduction and thickening properties of the different systems. Parameters evaluated included solids, volatile acids, gas production and quality, various chemical constituents of the sludge, and thickening properties using polymer and ferric chloride plus polymer. The TPAD systems were found capable of higher VS destruction at total detention times of 10 and 12.5 days compared to single-stage mesophilic or thermophilic digestion at 15 to 20 days. A laboratory-scale thickening test was performed to evaluate thickening properties. Based on obtaining 95% capture in the test, the polymer demand of mesophilic and acid-methane-phased digestion was the lowest. The TPAD system required approximately 180% of the dosage of the mesophilic sludge and thermophilic sludge required approximately 310% of the dosage. Temperatures of 40.5 and 43.3 degrees C in the mesophilic stage of TPAD were found to increase the polymer requirement and decrease the VS reduction of the TPAD system. Higher temperatures (above 56.1 degrees C) and lower detention times (less than 15 days) in the thermophilic digesters seemed to result in problems with degradation of volatile acids, particularly propionic acid.     

青岛市菜市场垃圾的高温厌氧消化研究

在中试规模下进行菜市场垃圾高温厌氧消化试验,通过监测消化过程中的产气情况和消化液的pH、可溶解性COD(SCOD)、挥发性脂肪酸(VFA)、NH3-N、碱度等变化,确定菜市场垃圾厌氧消化的最大有机负荷,并分析对菜市场垃圾进行高温厌氧消化处理的可行性.结果表明,在中试规模下,菜市场垃圾高温厌氧消化的最高有机负荷可达6.2...     

Full-scale class A biosolids production by two-stage continuous-batch thermophilic anaerobic digestion at the hyperion treatment plant, Los Angeles, California.

The City of Los Angeles Hyperion Treatment Plant (HTP) (California) converted its anaerobic digesters to thermophilic operation to produce Class A biosolids. Phase IV tests demonstrated compliance of a two-stage, continuous-batch process with Alternative 1 of U.S. Environmental Protection Agency 40 CFR Part 503 (U.S. EPA, 1993), which defines the time-temperature requirement for batch treatment (T > or = 56.3 degrees C at 16-h holding). Fecal coliforms, Salmonella sp., viable helminth ova, and enteric viruses were not detected in biosolids in the postdigestion train, including the truck-loading facility and the farm for land application as the last points of plant control where compliance is to be demonstrated. The same results were achieved during Phase V tests, after lowering the second-stage holding temperature to 52.6 degrees C to reduce the elevated methyl mercaptan production that was observed during Phase IV. Hence, the Phase V process complied with Alternative 3 of 40 CFR Part 503. Currently, HTP operates its digesters under the same conditions as tested in Phase V. In 2003, monthly monitoring of the biosolids at the truck-loading facility and the farm for land application demonstrated consistent compliance with Alternative 3.     

Generation pattern of sulfur containing gases from anaerobically digested sludge cakes.

Eleven dewatered sludge cakes collected from anaerobic digesters at different treatment plants were evaluated for the amount, type, and pattern of odorous gas production. All but one of the sludge cakes were from mesophilic anaerobic digesters. One was from a thermophilic digester. The pattern and quantities of sulfur gases were found to be unique for each of the samples with regard to the products produced, magnitude, and subsequent decline. The main odor-causing chemicals were volatile sulfur compounds, which included hydrogen sulfide, methanethiol, and dimethyl sulfide. Volatile sulfur compound production peaked in 3 to 8 days and then declined. The decline was a result of conversion of organic sulfur compounds to sulfide. In one side-by-side test, a high-solids centrifuge cake generated more odorous compounds than the low-solids centrifuge cake. The data show that anaerobic digestion does not eliminate the odor potential of anaerobically digested dewatered cakes.     

Recurrence of fecal coliforms and Salmonella species in biosolids following thermophilic anaerobic digestion.

The U.S. Environmental Protection Agency (U.S. EPA) Part 503 Biosolids Rule requires the fecal coliform (indicator) or Salmonella species (pathogen) density requirements for Class A biosolids to be met at the last point of plant control (truck-loading facility and/or farm for land application). The three Southern Californian wastewater treatment plants in this study produced biosolids by thermophilic anaerobic digestion and all met the Class A limits for both fecal coliforms and Salmonella sp. in the digester outflow biosolids. At two plants, however, a recurrence of fecal coliforms was observed in postdigestion biosolids, which caused exceedance of the Class A limit for fecal coliforms at the truck-loading facility and farm for land application. Comparison of observations at the three plants and further laboratory tests indicated that the recurrence of fecal coliforms can possibly be related to the following combination of factors: (1) incomplete destruction of fecal coliforms during thermophilic anaerobic digestion, (2) contamination of Class A biosolids with fecal coliforms from external sources during postdigestion, (3) a large drop of the postdigestion biosolids temperature to below the maximum for fecal coliform growth, (4) an unknown effect of biosolids dewatering in centrifuges. At Hyperion Treatment Plant (City of Los Angeles, California), fecal coliform recurrence could be prevented by the following: (1) complete conversion to thermophilic operation to exclude contamination by mesophilically digested biosolids and (2) insulation and electrical heat-tracing of postdigestion train for maintaining a high biosolids temperature in postdigestion.     

Low-temperature inactivation of fecal coliforms in sludge digestion.

The goal of this research was to demonstrate the ability to achieve Class A pathogen standards in nonthermophilic acid digesters. It was proposed that the key mechanism responsible for fecal coliform inactivation was the presence of un-ionized volatile fatty acids. Lab-scale acid digesters were assembled and operated in a batch mode for 5 days at mesophilic (38 degrees C) and low-mesophilic (21 degrees C) temperatures and at different solids concentrations. The key factor recognized for successful pathogen inactivation was pH, which is also the main factor driving the shift in organic acids toward the un-ionized form. Compared to conventional mesophilic acid digestion, low-mesophilic acid digestion was effective in fecal coliform inactivation because the process maintained lower pH throughout the duration of the experiment, offered continuous release of organic acids, and showed higher concentrations of organic acids in un-ionized form, including acetate, propionate, butyrate, and valerate.     

Comparison of mechanical pretreatment methods for the enhancement of anaerobic digestion of pulp and paper waste activated sludge

The conventional anaerobic digestion process, requiring long solids retention times (SRTs) to digest solids, is currently viewed as impractical for the pulp and paper industry because of high capital costs associated with the construction of new digesters. Recent developments in sludge solubilization technology could be promising in reducing digester size, which also allows for the potential use of decommissioned tanks, both of which can reduce the capital cost. Three pretreatment technologies for use with anaerobic digestion were tested on laboratory-scale to investigate their feasibility. The SRTs in all three digesters systematically decreased from 20 to 3 days. The reference digester was fed waste activated sludge (WAS) to serve as the control at the same SRTs. The other digesters were fed WAS that had been preconditioned using mechanical shearing, sonication, or high-pressure homogenization technology. Anaerobic digestion with high-pressure homogenization produced as much methane at 3-day mean SRT as that from the reference digester operated at 20-day SRT. Therefore, a new digester can theoretically be 85% smaller than a conventional digester. An added benefit of WAS to methane conversion is the recovery of nutrients nitrogen and phosphorus.     

Anaerobic codigestion of municipal, farm, and industrial organic wastes: a survey of recent literature.

Codigestion of organic wastes is a technology that is increasingly being applied for simultaneous treatment of several solid and liquid organic wastes. The main advantages of this technology are improved methane yield because of the supply of additional nutrients from the codigestates and more efficient use of equipment and cost-sharing by processing multiple waste streams in a single facility. Many municipal wastewater treatment plants (WWTPs) in industrialized countries currently process wastewater sludge in large digesters. Codigestion of organic wastes with municipal wastewater sludge can increase digester gas production and provide savings in the overall energy costs of plant operations. Methane recovery also helps to reduce the emission of greenhouse gases to the atmosphere. The goal of this literature survey was to summarize the research conducted in the last four years on anaerobic codigestion to identify applications of codigestion at WWTPs. Because the solids content in municipal wastewater sludge is low, this survey only focuses on codigestion processes operated at relative low solids content (slurry mode). Semi-solid or solid codigestion processes were not included. Municipal wastewater sludge, the organic fraction of municipal solid waste, and cattle manure (CAM) are the main wastes most often used in codigestion processes. Wastes that are codigested with these main wastes are wood wastes, industrial organic wastes, and farm wastes. These are referred to in this survey as codigestates. The literature provides many laboratory studies (batch assays and bench-scale digesters) that assess the digestibility of codigestates and evaluate the performance and monitoring of codigestion, inhibition of digestion by codigestates, the design of the process (e.g., single-stage or two-stage processes), and the operation temperature (e.g., mesophilic or thermophilic). Only a few reports on pilot- and full-scale studies were found. These evaluate general process performance and pretreatment of codigestates, energy production, and treatment costs.     

Extended acid digestion for inactivation of fecal coliforms.

The objective of this research was to establish a correlation between inactivation of fecal coliforms caused by organic acids in their unionized form in batch acid digesters and semicontinuously fed acid digesters at both mesophilic (38 degrees C) and low-mesophilic (24 degrees C) temperatures. Batch acid digesters achieved a U.S. Environmental Protection Agency Class A level of fecal coliforms within 6 to 7 days of digestion at both temperatures. Semicontinuously fed, staged, acid-digestion systems achieved Class A standards on average only at mesophilic temperature at a solids retention time of 11 days. Systems operated at low-mesophilic temperatures did not achieve Class A standards.     

Combined anaerobic/aerobic digestion: effect of aerobic retention time on nitrogen and solids removal

A combined anaerobic/aerobic sludge digestion system was studied to determine the effect of aerobic solids retention time (SRT) on its solids and nitrogen removal efficiencies. After the anaerobic digester reached steady state, effluent from the anaerobic digester was fed to aerobic digesters that were operated at 2- to 5-day SRTs. The anaerobic system was fed with a mixture of primary and secondary sludge from a local municipal wastewater treatment plant. Both systems were fed once per a day. The aerobic reactor was continuously aerated with ambient air, maintaining dissolved oxygen level at 1.1 +/- 0.3 mg/L. At a 4-day or longer SRT, more than 11% additional volatile solids and 90% or greater ammonia were removed in the aerobic digester, while 32.8 mg-N/L or more nitrite/nitrate also was measured. Most total Kjeldahl nitrogen removal was via ammonia removal, while little organic nitrogen was removed in the aerobic digester.     

青岛市餐厨垃圾与菜市场垃圾混合高温厌氧消化研究

在中试规模下,研究青岛市餐厨垃圾与菜市场垃圾混合(质量比1∶1)高温厌氧消化实验,通过监测厌氧消化过程中产气量、气体组成等产气情况和消化液中pH值、SCOD、NH3-H、VFAs含量和组分等化学指标变化,确定混合厌氧消化的最大有机负荷,并分析混合高温厌氧消化技术的可行性,结果表明,(1)青岛市餐厨垃圾与菜市场垃圾混合高温厌氧消化产甲烷具有技术可行性;(2)混合厌氧消化的最大有机负荷可达4.069 kg VS/(m3.d);(3)当系统最大有机负荷时,每天每千克VS最高可产生甲烷量0.346 m3;(4)混合厌氧消化可削减氨氮对餐厨垃圾单独厌氧消化产沼气的影响。     

餐厨垃圾的高温厌氧消化处理研究

在55℃条件下,研究了餐厨垃圾高温厌氧消化过程中pH、VFA、总磷、产气量以及COD、TS和VS的变化,结果表明：在消化过程中pH先下降后上升,总VFA浓度增大,而其中游离态VFA浓度先增大后减少。消化过程总产气量为43 211 mL,整个过程消耗TS的平均产气率为158.98 mL/g TS,COD去除率为38.76％,总磷去除率达到98.87％,底物TS和VS去除率分别31.71％和50.24％。经过厌氧消化处理,沼渣中不含对人体和许多动物有害的沙门氏菌、痢疾杆菌、大肠杆菌及粪大肠杆菌,而富含有机质、氮、磷和钾等的营养物质和芽孢杆菌、防线菌等有益菌种。     

High-solids anaerobic digestion of municipal sludge pretreated by thermal hydrolysis

High-solids anaerobic digestion can consistently achieve 55 to 60% volatile solids destruction after thermal hydrolysis pretreatment, which reduces its viscosity and increases the fraction of soluble organic matter. For feed sludge with total solids concentrations between 6.8 and 8.2%, the process is stable at hydraulic retention times of 9 to 12 days, significantly increasing the treatment capacity of existing digesters or, in treatment plants without spare capacity, helping to postpone, reduce, or even avoid costly infrastructure investments. Process stability is related to the high concentration of soluble organic matter in the digesters. High-solids temperature-phased digestion appears to be superior to high-solids mesophilic digestion, with respect to process flexibility and stability, biosolids stabilization, and biogas generation, although ammonia inhibition may have occurred. Implementation of high-solids digestion could significantly reduce operation and maintenance costs of solids-handling operations.     

Biodegradation of polylactide in aerobic and anaerobic thermophilic conditions

Biodegradable polymers are designed to resist a number of environmental factors during use, but to be biodegradable under disposal conditions. The biodegradation of polylactide (PLLA) was studied at different elevated temperatures in both aerobic and anaerobic, aquatic and solid state conditions. In the aerobic aquatic headspace test the mineralisation of PLLA was very slow at room temperature, but faster under thermophilic conditions. The clear effect of temperature on the biodegradability of PLLA in the aquatic tests indicates that its polymer structure has to be hydrolysed before microorganisms can utilise it as a nutrient source. At similar elevated temperatures, the biodegradation of PLLA was much faster in anaerobic solid state conditions than in aerobic aquatic conditions. The behaviour of PLLA in the natural composting process was similar to that in the aquatic biodegradation tests, biodegradation starting only after the beginning of the thermophilic phase. These results indicate that PLLA can be considered as a compostable material, being stable during use at mesophilic temperatures, but degrading rapidly during waste disposal in compost or anaerobic treatment facilities.     

A kinetic study of anaerobic digestion of olive mill wastewater at mesophilic and thermophilic temperatures

The kinetics of the anaerobic digestion of olive mill wastewater (OMW) was studied in the mesophilic and thermophilic ranges of temperature. Two completely mixed continuous flow bioreactors operating at 35 degrees C and 55 degrees C and with an average biomass concentration of 5.45 g VSS litre(-1) were used. The thermophilic reactor worked satisfactorily between hydraulic retention times (HRT) of 10 to 40 days, removing between 94.6 and 84.4% of the initial chemical oxygen demand (COD). In contrast, the mesophilic reactor showed a marked decrease in substrate utilization and methane production at a HRT of 10 days. TVFA levels and the TVFA/alkalinity ratio were higher and close to the suggested limits for digester failure. The yield coefficient for methane production (1 CH(4) STP g(-1) COD(added)) was 28% higher in the thermophilic process than in the mesophilic one. Macroenergetic parameters, calculated using Guiot's kinetic model, gave yield coefficients for the biomass (Y) of 0.18 (mesophilic) and 0.06 g VSS g(-1) COD (thermophilic) and specific rates of substrate uptake for cell maintenance (m) of 0.12 (mesophilic) and 0.27 g COD g(-1) VSS.day(-1) (thermophilic). The experimental results showed the rate of substrate uptake (R(s); g COD g(-1) VSS.day(-1)), correlated with the concentration of biodegradable substrate (S(b); g COD litre(-1)), through an equation of the Michaelis-Menten type for the two temperatures used.     

Retooling the ethanol industry: thermophilic anaerobic digestion of thin stillage for methane production and pollution prevention.

Anaerobic digestion of corn ethanol thin stillage was tested at thermophilic temperature (55 degrees C) with two completely stirred tank reactors. The thin stillage wastestream was organically concentrated with 100 g/L total chemical oxygen demand and 60 g/L volatiles solids and a low pH of approximately 4.0. Steady-state was achieved at 30-, 20-, and 15-day hydraulic retention times (HRTs) and digester failure at a 12-day HRT. Significant reduction of volatile solids was achieved, with a maximum reduction (89.8%) at the 20-day HRT. Methane yield ranged from 0.6 to 0.7 L methane/g volatile solids removed during steady-state operation. Effluent volatile fatty acids below 200 mg/L as acetic acid were achieved at 20- and 30-day HRTs. Ultrasonic pretreatment was used for one digester, although no significant improvement was observed. Ethanol plant natural gas consumption could be reduced 43 to 59% with the methane produced, while saving an estimated $7 to $17 million ($10 million likely) for a facility producing 360 million L ethanol/y.     

The digestibility of waste activated sludges.

Laboratory digestion studies using waste activated sludges (WAS) were conducted to compare the digestion performance between anaerobic and aerobic processes. Nine samples of WAS from seven wastewater treatment plants were collected and batch-digested under both anaerobic and aerobic conditions for 30 days at 25 degrees C. The cation content of wastewater (both floc and solution phases) and solution biopolymer (protein and polysaccharide) was measured before and after digestion and compared with volatile solids destruction data. The study revealed that each digestion process was associated with a distinct biopolymer fraction, which accounted for differences in volatile solids reduction under anaerobic and aerobic conditions. The anaerobic digestion data showed strong correlations between soluble protein generation, ammonium production, percent volatile solids reduction, and floc iron (Fe). These data suggest that the amount of volatile solids destroyed by anaerobic digestion depends on the Fe content of floc. In aerobic digestion, polysaccharide accumulated in solution along with calcium and magnesium. For aerobic digestion, correlations between divalent cation release and the production of inorganic nitrogen were found. This implies that divalent cation-bound biopolymer, thought to be lectin-like protein, was the primary organic fraction degraded under aerobic conditions. The results of the study show that the cation content in wastewater is an important indicator of the material that will digest under anaerobic or aerobic conditions and that some of the volatile solids will digest only under either anaerobic or aerobic conditions.     

Optimal loading rates and economic analyses for anaerobic digestion of poultry waste

Four combinations of litter and carcasses from broiler chickens were examined utilizing a thermophilic, stirred-tank digester of demonstration size of approximately 10,000 gal. Under computed optimal loading rates, litter with paper bedding had the highest daily production of methane over an 8-day retention period. The greatest methane production per lb of volatile solids was achieved over 10 days with litter and paper bedding combined with carcasses. This research found that sufficient poultry litter is generated within 20 mi (32 km) of Moorefield, WV, to support a commercial-sized digester operation. However, anaerobic digestion of poultry waste cannot be financially supported by methane production alone. To be financially viable, anaerobic digestion requires a disposal fee for poultry waste and/or the sale of the digested solid effluent as an organic fertilizer to retail markets.