Thermophilic Operation Research Articles

Optimization of separate hydrogen and methane production from cassava wastewater using two-stage upflow anaerobic sludge blanket reactor (UASB) system under thermophilic operation

The objective of this study was to investigate the separate hydrogen and methane productions from cassava wastewater by using a two-stage upflow anaerobic sludge blanket (UASB) system under thermophilic operation. Recycle ratio of the effluent from methane bioreactor-to-feed flow rate was fixed at 1:1 and pH of hydrogen UASB unit was maintained at 5.5. At optimum COD loading rate of 90kg/m3d based on the feed COD load and hydrogen UASB volume, the produced gas from the hydrogen UASB unit mainly contained H2 and CO2 which provided the maximum hydrogen yield (54.22mlH2/g COD applied) and specific hydrogen production rate (197.17ml/gMLVSSd). At the same optimum COD loading rate, the produced gas from the methane UASB unit mainly contained CH4 and CO2 without H2 which were also consistent with the maximum methane yield (164.87mlCH4/g COD applied) and specific methane production rate (356.31mlCH4/gMLVSSd). The recycling operation minimized the use of NaOH for pH control in hydrogen UASB unit.

Simultaneous Production of Hydrogen and Methane from Cassava Wastewater Using a Two Stage Upflow Anaerobic Sludge Blanket System Under Thermophilic Operation

The objective of this study was to investigate the hydrogen and methane production from cassava wastewater by using a two stage upflow anaerobic sludge blanket (UASB) system under thermophillic operation (55 °C). The recycle ratio of the effluent from the methane bioreactor-to-the feed flow rate was fixed at 1:1 and the solution pH in the hydrogen bioreactor was maintained at 5.5 by a pH controller while the solution pH of the methane bioreactor was not controlled. The liquid working volumes of the hydrogen and methane bioreactors were 4 and 24 L, respectively. The two stage UASB system was operated at different COD loading rates (30, 60, 90, 120, and 150 kg/m 3 d based on the hydrogen bioreactor volume). Under the optimum COD loading rate of 90 kg/m 3 d based on the hydrogen bioreactor volume corresponding to 15 kg/m 3 d based on the methane bioreactor volume for the hydrogen bioreactor, the produced gas contained 40 % H2, 52 % CO2 and 8 % CH4 and the system provided a maximum hydrogen yield and specific hydrogen production rate of 18 mL/g COD removed and 520 mL/L d, respectively. Under this optimum COD loading rate, the produced gas of the methane bioreactor contained 65 % CH4 and 35 % CO2 without hydrogen and the system provided a maximum methane yield and specific methane production rate of 115 mL/g COD removed and 650 mL/L d, respectively. The operation of recycling from methane bioreactor to hydrogen bioreactor optimized the use of NaOH for pH control in the hydrogen production step.

Hydrogen production from alcohol wastewater with added fermentation residue by an anaerobic sequencing batch reactor (ASBR) under thermophilic operation

The objective of this study was to investigate the enhancement of hydrogen production from alcohol wastewater by adding fermentation residue using an anaerobic sequencing batch reactor (ASBR) under thermophillic operation (55 °C) and at a constant pH of 5.5. The digestibility of the added fermentation residue was also evaluated. For a first set of previous experiments, the ASBR system was operated to obtain an optimum COD loading rate of 50.6 kg/m3 d of alcohol wastewater without added fermentation residue and the produced gas contained 31% H2 and 69% CO2. In this experiment, the effect of added fermentation residue (100–1200 mg/l) on hydrogen production performance was investigated under a COD loading rate of 50.6 kg/m3 d of the alcohol wastewater. At a fermentation residue concentration of 1000 mg/l, the produced gas contained 40% H2 and 60% CO2 without methane and the system gave the highest hydrogen yield and specific hydrogen production rate of 128 ml/g COD removed and 2880 ml/l d, respectively. Under thermophilic operation with a high total COD loading rate (51.8 kg/m3 d) and a short HRT (21 h) at pH 5.5, the ASBR system could only break down cellulose (41.6%) and hemicellulose (21.8%), not decompose lignin.

Comparison of mesophilic and thermophilic anaerobic digestion of sugar beet pulp: Performance, dewaterability and foam control

Digestion of sugar beet pulp was assessed in relation to biogas and methane production, foaming potential, and digestate dewaterability. Four 4-litre working volume digesters were operated mesophilically (37±0.5 °C) and four thermophilically (55±0.5 °C) over three hydraulic retention times. Digesters were operated in duplicate at organic loading rates (OLR) of 4 and 5 g volatile solids l(-1) day(-1) without water addition. Thermophilic digestion gave higher biogas and methane productivity than mesophilic and was able to operate at the higher OLR, where mesophilic digestion showed signs of instability. Digestate dewaterability was assessed using capillary suction time and frozen image centrifugation. The occurrence of, or potential for, stable foam formation was assessed using a foaming potential test. Thermophilic operation allowed higher loadings to be applied without loss of performance, and gave a digestate with superior dewatering characteristics and very little foaming potential.

Anaerobic digestion of wheat straw – Performance of continuous solid-state digestion

In this study the upflow anaerobic solid-state (UASS) reactor was operated at various conditions to optimize the process parameters for anaerobically digesting wheat straw in a continuous process. Additionally, particle size effects have been studied in the operation at 55 and 60°C. Moreover, the incremental effect of the organic loading rate (OLR) to the system was examined from 2.5 to 8 gVS L(-1) d(-1). It was found that the UASS operating at 60 °C with a small OLR yields highest methane production, but the advantage over thermophilic operation is negligible. The rise in OLR reduces the systems yields, as expected. From OLR=8 gVS L(-1) d(-1) a second stage is necessary to circumvent volatile fatty acids accumulation.

Co-digestion of bovine slaughterhouse wastes, cow manure, various crops and municipal solid waste at thermophilic conditions: a comparison with specific case running at mesophilic conditions

A co-digestion process was evaluated when mixing different ratios of agro-industrial residues, i.e. bovine slaughterhouse waste (SB); cow manure (M); various crop residues (VC); and municipal solid waste (MSW) by anaerobic batch digestion under thermophilic conditions (55 °C). A selected study case at mesophilic condition (37 °C) was also investigated. The performance of the co-digestion was evaluated by kinetics (k(0)). The best kinetic results were obtained under thermophilic operation when a mixture of 22% w/w SB, 22% w/w M, 45% w/w VC and 11% w/w MSW was co-digested, which showed a proper combination of high values in r(s)CH(4) and k(0) (0.066 Nm(3)CH(4)/kgVS*d, 0.336 d(-1)) during the anaerobic process. The effect of temperature on methane yield (Y(CH4)), specific methane rate (r(s)CH(4)) and k(0) was also analyzed for a specific study case; there a mixture of 25% w/w of SB, 37.5% w/w of M, 37.5% of VC and 0% of MSW was used. Response variables were severely affected by mesophilic conditions, diminishing to at least 45% of the thermophilic values obtained for a similar mixture. The effect of temperature suggested that thermophilic conditions are suitable to treat these residues.

음폐수의 중온 및 고온 산발효에서 초기 pH가 VFAs 생성에 미치는 영향

Abstract Batch cultivations were performed to evaluate the influences of the initial pH condition on mesophilic and thermophilic acidogenic fermentation with food waste recycling wastewater. In both conditions of mesophilic and thermophilic fermentation, TVFAs production rates were maximized at the initial pH 7 condition as 0.15 and 0.23 g TVFAs/L·hr,respectively. And pH was also maintained stably between 6 and 7 during 72hr acidogenic cultivation at both conditions. However, predominant VFA components were different according to reaction temperature conditions. In mesophilic condition, propionic acid which has low conversion efficiency to methane was accumulated up to 1,348 mg/L while acetic and butyric acid were predominant in thermophilic condition. Therefore, thermophilic acidogenic fermentation was superior for the effective VFAs production than mesophilic condition. From the DGGE analysis, the band patterns were different according to the initial pH conditions but the correlations of the each band were increased in similar pH conditions. These results mean that microbial communities were certainly affected by the initial pH condition. Consequently, the adjustment of the initial pH to neutral region and thermophilic operation are needed to enhance acidogenic fermentation of food waste recycling wastewater.

Hydrogen production from alcohol wastewater by an anaerobic sequencing batch reactor under thermophilic operation: Nitrogen and phosphorous uptakes and transformation

The objective of this study was to investigate hydrogen production from alcohol wastewater using an anaerobic sequencing batch reactor (ASBR) under thermophilic operation and at a constant pH of 5.5. Under the optimum COD loading rate of 68 kg/m3d, the produced gas contained 43% H2 without methane and the system provided a hydrogen yield and specific hydrogen production rate of 130 ml H2/g COD removed and 2100 ml H2/l d, respectively, which were much higher than those obtained under the mesophilic operation. Under thermophilic operation, both nitrogen and phosphate uptakes were minimal at the optimum COD loading rate for hydrogen production and most nitrogen uptake was derived from organic nitrogen. Under the thermophilic operation for hydrogen production, the nutrient requirement in terms of COD:N:P was found to be 100:6:0.5, which was much higher than that for the methenogenic step for methane production under both thermophilic and mesophilic operations and for the acidogenic step for hydrogen production under mesophilic operation.

Near‐infrared spectroscopic online monitoring of process stability in biogas plants

The conditions laid down in the Renewable Energy Source Act for production of electricity from biogas have led to an enormous expansion of new biogas plants in the recent years in Germany. Through near‐infrared reflection spectroscopy (NIRS) process stability of a biogas digester can be monitored online. This study presents the development of NIRS calibrations on acetic acid equivalents, acetic acid, and propionic acid concentrations in the digester substrate. Thereby, differences between thermophilic and mesophilic operations were measured and presented for the first time. Good calibration models were achieved by artificially increasing concentrations of the above‐mentioned acids in two 400 L experimental biogas digesters with mesophilic and thermophilic operation and applying support vector regression. The presented values demonstrate that calibration with NIRS is possible. In the thermophilic digester, a calibration model with a ratio of standard deviation and standard error of prediction (RPD) value of 3.21 was achieved for the parameter acetic acid and in the mesophilic digester a RPD of 4.91 for the same acid. For the parameter propionic acid, calibration models with RPD values of 4.23 and 4.78 were achieved for the thermophilic‐ and mesophilic‐operated digesters, respectively. The presented NIRS calibration can be used to develop an early warning system for process stability, which can be used for reliable optimization of biogas production to increase the methane yield.

Lab scale experiments using a submerged MBR under thermophilic aerobic conditions for the treatment of paper mill deinking wastewater

This paper describes the results of laboratory experiments using a thermophilic aerobic MBR (TMBR) at 50°C. An innovative use of submerged flat-sheet MBR modules to treat circuit wastewater from the paper industry was studied. Two experiments were conducted with a flux of 8–13L/m2/h without chemical membrane cleaning. COD and BOD5 elimination rates were 83% and 99%, respectively. Calcium was reduced from 110 to 180mg/L in the inflow to 35–60mg/L in the permeate. However, only negligible membrane scaling occurred. The observed sludge yield was very low and amounted to 0.07–0.29gMLSS/g CODeliminated. Consequently, the nutrient supply of ammonia and phosphate can be lower compared to a mesophilic process. Molecular-biological FISH analysis revealed a likewise high diversity of microorganisms in the TMBR compared to the mesophilic sludge used for start-up. Furthermore, ammonia-oxidising bacteria were detected at thermophilic operation.

Gas biofuels from solid substrate hydrogenogenic–methanogenic fermentation of the organic fraction of municipal solid waste

The objective of this work was to evaluate the performance of a two-stage hydrogenogenic–methanogenic (H–M) semi-continuous process in terms of mass retention time (MRT) for hydrogenogenic stage (H-stage), feed source for methanogenic stage (M-stage) and thermal regime (35 and 55°C) for both stages. The substrate was a model organic fraction of municipal solid wastes (OFMSW) at 35% total solids.In H-stage, mesophilic temperature had a positive significant effect on higher hydrogen productivities and lower amounts of hydrogen sinks compared to thermophilic operation. Calculations based on mass balances and biochemical stoichiometry confirmed that acid fermentation deviation was linked to low biohydrogen yields. The M-stage performance was influenced by both the temperature and feed source. Bioreactors in thermophilic regime performed better than mesophilic ones. Maximum methane productivity was 341NmL CH4/(kgwmrd) that corresponded to the thermophilic bioreactor fed with fermented solids from H stage at 14d MRT. The two-stage process showed higher gross energetic potential when compared to an only methanogenic process operated at equivalent MRT (control); this was due to a higher methane productivity in the M-stage of the series process. The main contribution of H-stage seemed to be associated to hydrolysis of the complex substrate thus generating metabolites for the M-stage rather than the hydrogen production itself.

Fermentative Hydrogen Production: Influence of Application of Mesophilic and Thermophilic Bacteria on Mass and Energy Balances

Fermentation of biomass residues and second generation biomasses is a possible way to enable a sustainable production of hydrogen. The HYVOLUTION-project investigates the production of hydrogen by a 2-stage fermentation process of biomass. It consists of a dark fermentation step of sugars to produce hydrogen, CO2 and organic acids followed by a photo-heterotrophic fermentation, in which all intermediates are converted to more hydrogen and CO2. This work compares the use of mesophilic and thermophilic bacteria in the dark fermentation step, analyzing the effects on the overall process. Based on experimental results, simulation models developed with Aspen Plus V7.1® are used to calculate the mass-and energy balances of the process. Results show that dark fermentation at mesophilic conditions requires a higher amount of feedstocks but almost no heat input as well as smaller equipment. However, better economic performance is assumed for the thermophilic operation of the dark fermentation step.

Evaluation of continuous mesophilic, thermophilic and temperature phased anaerobic digestion of microwaved activated sludge

The effects of microwave (MW) pretreatment, staging and digestion temperature on anaerobic digestion were investigated in a setup of ten reactors. A mesophilic reactor was used as a control. Its performance was compared to single-stage mesophilic and thermophilic reactors treating pretreated and non-pretreated sludge, temperature-phased (TPAD) thermophilic-mesophilic reactors treating pretreated and non-pretreated sludge and thermophilic–thermophilic reactors also treating pretreated and non-pretreated sludge. Four different sludge retention times (SRTs) (20, 15, 10 and 5 d) were tested for all reactors. Two-stage thermo–thermo reactors treating pretreated sludge produced more biogas than all other reactors and removed more volatile solids. Maximum volatile solids (VS) removal was 53.1% at an SRT of 15 d and maximum biogas increase relative to control was 106% at the shortest SRT tested. Both the maximum VS removal and biogas relative increase were measured for a system with thermophilic acidogenic reactor and thermophilic methanogenic reactor. All the two-stage systems treating microwaved sludge produced sludge free of pathogen indicator bacteria, at all tested conditions even at a total system SRT of only 5 d. MW pretreatment and staging reactors allowed the application of very short SRT (5 d) with no significant decrease in performance in terms of VS removal in comparison with the control reactor. MW pretreatment caused the solubilization of organic material in sludge but also allowed more extensive hydrolysis of organic material in downstream reactors. The association of MW pretreatment and thermophilic operation improves dewaterability of digested sludge.

Continuous hydrogen production from tofu processing waste using anaerobic mixed microflora under thermophilic conditions

We carried out continuous fermentative H 2 production from tofu (soybean curd)–processing waste (TPW) using anaerobic mixed microflora under thermophilic (60 °C) conditions and compared the rates and yields of H 2 production in a continuous stirred-tank reactor (CSTR) and a membrane bioreactor (MBR), wherein the membrane filtration unit was coupled to the CSTR. The TPW was diluted with tap water and then hydrolyzed by blending for 5 min in the presence of 0.5% HCl, and it was found that this protocol significantly increased the amount of soluble material in the mixture. The soluble chemical oxygen demand (SCOD)–to–total COD (TCOD) ratio jumped from 14% to 60%, and the soluble carbohydrate concentration was increased threefold, from 2.4 g/L to 7.2 g/L. Accordingly, H 2 production potential was increased 2.8-fold. In a CSTR operation using pretreated TPW as the substrate, a stable volumetric H 2 production rate (VHPR) of 8.17 ± 0.32 L H 2/L/d and a H 2 yield of 1.20 ± 0.05 mol H 2/mol hexose added at 8-h HRT were achieved. Substantial increases in the VHPR and H 2 yield over those obtained with the CSTR were observed in the MBR operation. The role of the MBR was to increase the retention time of the solid substrate and the concentration of microorganisms, thereby enhancing the substrate utilization rate for H 2 production. Acetic and butyric acids were the main liquid-state metabolites produced during the fermentation process, thus indicating that the thermophilic operation provided favorable conditions for H 2 production from TPW. A maximum H 2 yield of 1.87 mol H 2/mol hexose added was achieved at 8-h HRT and then gradually decreased to 1.00 mol H 2/mol hexose-equivalent at 2-h HRT. Meanwhile, the VHPR continuously increased to a maximum of 19.86 L H 2/L/d at 4-h HRT and then decreased with a high dilution rate as the HRT was lowered to 2 h (minimum). At 2-h HRT, the degradation of soluble carbohydrate was limited.

Anaerobic digestion as final step of a cellulosic ethanol biorefinery: Biogas production from fermentation effluent in a UASB reactor—pilot‐scale results

In order to lower the costs for second generation bioethanol from lignocellulosic biomass anaerobic digestion of the effluent from ethanol fermentation was implemented using an upflow anaerobic sludge blanket (UASB) reactor system in a pilot-scale biorefinery plant. Both thermophilic (53 degrees C) and mesophilic (38 degrees C) operation of the UASB reactor was investigated. At an OLR of 3.5 kg-VS/(m(3) day) a methane yield of 340 L/kg-VS was achieved for thermophilic operation (53 degrees C) while 270 L/kg-VS was obtained under mesophilic conditions (38 degrees C). For loading rates higher than 5 kg-VS/(m(3) day) the methane yields were, however, higher under mesophilic conditions compared to thermophilic conditions. The conversion of dissolved organic matter (VS(diss)) was between 68% and 91%. The effluent from the ethanol fermentation showed no signs of toxicity to the anaerobic microorganisms. However, a high content of suspended matter reduced the degradation efficiency. The retention time of the anaerobic system could be reduced from 70 to 7 h by additional removal of suspended matter by clarification. Implementation of the biogas production from the fermentation effluent accounted for about 30% higher carbon utilization in the biorefinery compared to a system with only bioethanol production.

A novel ecological role of the Firmicutes identified in thermophilic microbial fuel cells

Significant effort is currently focused on microbial fuel cells (MFCs) as a source of renewable energy. Most studies concentrate on operation at mesophilic temperatures. However, anaerobic digestion studies have reported on the superiority of thermophilic operation and demonstrated a net energy gain in terms of methane yield. As such, our studies focused on MFC operation and microbiology at 55 degrees C. Over a 100-day operation, these MFCs were stable and achieved a power density of 37 mW m(-2) with a coulombic efficiency of 89%. To infer activity and taxonomic identity of dominant members of the electricity-producing community, we performed phylogenetic microarray and clone library analysis with small subunit ribosomal RNA (16S rRNA) and ribosomal RNA gene (16S rDNA). The results illustrated the dominance (80% of clone library sequences) of the Firmicutes in electricity production. Similarly, rRNA sequences from Firmicutes accounted for 50% of those taxa that increased in relative abundance from current-producing MFCs, implying their functional role in current production. We complemented these analyses by isolating the first organisms from a thermophilic MFC. One of the isolates, a Firmicutes Thermincola sp. strain JR, not only produced more current than known organisms (0.42 mA) in an H-cell system but also represented the first demonstration of direct anode reduction by a member of this phylum. Our research illustrates the importance of using a variety of molecular and culture-based methods to reliably characterize bacterial communities. Consequently, we revealed a previously unidentified functional role for Gram-positive bacteria in MFC current generation.

Single-stage Sequencing Batch Process for Producing Class A Biosolids from AD Effluent at Terminal Island Treatment Plant

Terminal Island Treatment Plant converted its digesters to thermophilic operation with the objective to comply with the U.S. EPA Part 503 Biosolids Rule requirements for Class A biosolids. The following processes were tested: a) single-stage continuous; b) two-stage continuous; c) single-stage sequencing batch. Salmonella sp. were always non-detect in digester outflows (<3 MPN/4 g dry wt), whereas fecal coliform densities were usually below the Class A limit of 1000 MPN/g dry wt. However, the recurrence of fecal coliforms in post-digestion caused non-compliance with the Class A limit at the truck loading facility as the last point of plant control for compliance. After several design modifications of the post-digestion train, operation of the digesters as sequencing batch digesters according to the time-temperature requirement of Alternative 1 of the Part 503 Biosolids Rule achieved compliance for both Salmonella sp. and fecal coliforms at the last point of plant control (truck loading facility).

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.

Evaluation of thermophilic anaerobic digestion processes for full‐scale Class A biosolids disinfection at hyperion treatment plant

This paper describes 5 phases of full-scale testing at the City of Los Angeles Hyperion Treatment Plant (HTP) for producing Class A biosolids (U.S. EPA Part 503 Biosolids Rule) by thermophilic anaerobic digestion. Phases I and II were tests with a two-stage continuous-batch process in a thermophilic battery of six digesters and a designated post-digestion train that was isolated from mesophilic operations. These tests demonstrated that digester outflow biosolids met the Class A limits for fecal coliforms and Salmonella sp. However, fecal coliform densities sharply increased during post-digestion. The recurrence was possibly related to a combination of a large drop of the biosolids temperature after the dewatering centrifuges and contamination of thermophilically digested biosolids from mesophilic operations. Phase III was conducted after insulation and electrical heat-tracing of the post-digestion train to maintain a biosolids temperature throughout post-digestion at about the same level as in the digester outflow. Biosolids monitoring at the last points of plant control (silos at Truck Loading Facility and farm for land application) indicated that fecal coliform recurrence was prevented. After completing the conversion of HTP to thermophilic operation, certification tests of Phases IV and V demonstrated Class A compliance of a two-stage continuous-batch process under Alternatives 1 and 3 of the Part 503 Biosolids Rule, respectively. HTP received the permit for Class A (indeed exceptional quality) biosolids land application in Kern County, California, in December 2002 under Alternative 3. Since 2003, HTP has consistently complied with the federal and local standards for Class A biosolids, indicating that Class A limits can be met under conditions less stringent than defined by the Alternative 1 time-temperature requirement for batch treatment.

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.