Thermophilic Digestion Research Articles

Development of an efficient anaerobic co-digestion process for garbage, excreta, and septic tank sludge to create a resource recycling-oriented society

In order to develop a resource recycling-oriented society, an efficient anaerobic co-digestion process for garbage, excreta and septic tank sludge was studied based on the quantity of each biomass waste type discharged in Ooki machi, Japan. The anaerobic digestion characteristics of garbage, excreta and 5-fold condensed septic tank sludge (hereafter called condensed sludge) were determined separately. In single-stage mesophilic digestion, the excreta with lower C/N ratios yielded lower biogas volumes and accumulated higher volumes of volatile fatty acid (VFA). On the other hand, garbage allowed for a significantly larger volatile total solid (VTS) digestion efficiency as well as biogas yield by thermophilic digestion. Thus, a two-stage anaerobic co-digestion process consisting of thermophilic liquefaction and mesophilic digestion phases was proposed. In the thermophilic liquefaction of mixed condensed sludge and household garbage (wet mass ratio of 2.2:1), a maximum VTS loading rate of 24g/L/d was achieved. In the mesophilic digestion of mixed liquefied material and excreta (wet mass ratio of 1:1), biogas yield reached approximately 570ml/g-VTS fed with a methane content of 55% at a VTS loading rate of 1.0g/L/d. The performance of the two-stage process was evaluated by comparing it with a single-stage process in which biomass wastes were treated separately. Biogas production by the two-stage process was found to increase by approximately 22.9%. These results demonstrate the effectiveness of a two-stage anaerobic co-digestion process in enhancement of biogas production.

Structure and activity of thermophilic methanogenic microbial communities exposed to quaternary ammonium sanitizer

Food processing facilities often use antimicrobial quaternary ammonium compound (QAC) sanitizers to maintain cleanliness. These QACs can end up in wastewaters used as feedstock for anaerobic digestion. The aim of this study was to measure the effect of QAC contamination on biogas production and structure of microbial communities in thermophilic digester sludge. Methane production and biogas quality data were analyzed in batch anaerobic digesters containing QAC at 0, 15, 50, 100 and 150mg/L. Increasing sanitizer concentration in the bioreactors negatively impacted methane production rate and biogas quality. Microbial community composition data was obtained through 16S rRNA gene sequencing from the QAC-contaminated sludges. Sequencing data showed no significant restructuring of the bacterial communities. However, significant restructuring was observed within the archaeal communities as QAC concentration increased. Further studies to confirm these effects on a larger scale and with a longer retention time are necessary.

Nitrous oxide and methane emissions during storage of dewatered digested sewage sludge

This study investigated the effect on greenhouse gas emissions during storage of digested sewage sludge by using a cover during storage or applying sanitisation measures such as thermophilic digestion or ammonia addition. In a pilot-scale storage facility, nitrous oxide and methane emissions were measured on average twice monthly for a year, using a closed chamber technique. The thermophilically digested sewage sludge (TC) had the highest cumulative emissions of nitrous oxide (1.30% of initial total N) followed by mesophilically digested sewage sludge stored without a cover (M) (0.34%) and mesophilically digested sewage sludge stored with a cover (MC) (0.19%). The mesophilically digested sewage sludge sanitised with ammonia and stored with a cover (MAC) showed negligible cumulative emissions of nitrous oxide. Emissions of methane were much lower from TC and MAC than from M and MC. These results indicate that sanitisation by ammonia treatment eliminates the production of nitrous oxide and reduces methane emissions from stored sewage sludge, and that thermophilic digestion has the potential to reduce the production of methane during storage compared with mesophilic digestion. The results also indicate a tendency for lower emissions of nitrous oxide and higher emissions of methane from covered sewage sludge compared with non-covered.

Role of temperature on microbial community profiles in an anaerobic bioreactor for treating PTA wastewater

Abstract Thermophilic digestion usually has higher specific gas output and faster reaction rate, the temperature fluctuations of purified terephthalate (PTA) manufacturing wastewater from 45 °C to 90 °C makes PTA wastewater very suitable and favorable for thermophilic treatment. However, the startup way of thermophilic digester is very critical as unsuitable startup ways may lead to inefficiency of the system. In this study, the experimental temperatures were set at 37 °C, 43 °C and 52 °C respectively by step increase strategy in order to culture thermophilic microbial organisms involved in anaerobic TA degradation. The performance and microbial community profiles of PTA wastewater treatment system were investigated under different operational temperatures using 454-pyrosequencing approach. During the step temperature increase process, 37 °C was proved to be the most favorable operational temperature with a highest COD removal of 96.10(±1.58)%, but with undergoing 43 °C, COD removal decreased to 90.42(±2.65)%, and 52 °C seemed to be the worst as it achieved only 70.08(±5.46)% COD reduction. Step-wise temperature increase is tested not very appropriate to culture thermophilic microorganism for TA degradation as prospective. Furthermore, during startup stage, the predominant bacteria family shifted from Syntrophorhabdaceae (belonging to δ-proteobacteria) at 37 °C to Comamonadaceae (belonging to β-proteobacteria) at 43 °C and OPB95 (belonging to OP8) at 52 °C. For archaeal community, hydrogenotrophic methanogen Methanobacteriaceae was predominant at all three temperatures, and it was proved to be most abundant at 37 °C compared with the other two temperatures.

Enhanced biogas production from rice straw by selective micronutrients under solid state anaerobic digestion

Biomethanation of rice straw (RS) was studied in a batch mode at high total solid content (TSC) of 25% in outdoor pilot scale digesters. Performance was monitored for over six months by supplementing Nickel and Cobalt 15 and 10mgkg−1 RS to each of mesophilic and thermophilic digesters for 35 and 21days retention time (RT), respectively. The average biogas production from mesophilic and thermophilic digesters were found varying 310 and 396Lkg−1TS, respectively. The corresponding figures for the control digesters were 225 and 270Lkg−1TS. Around 37 and 46% higher biogas production was recorded by supplementing the micronutrients in mesophilic and thermophilic digesters, respectively. Methane content in biogas was 57–59%. Matured compost had nitrogen, phosphorus and potassium contents of 1.0–1.2, 1.3–2.2, and 1.2–2.1%, respectively. The results demonstrated that the present process is faster, requires less than 85% water and produces green energy in addition to manure in less time compared to conventional process.

Rapid start-up of one-stage deammonification MBBR without addition of external inoculum.

In recent years, the anammox process has emerged as a useful method for robust and efficient nitrogen removal in wastewater treatment plants (WWTPs). This paper evaluates a one-stage deammonification (nitritation and anammox) start-up using carrier material without using anammox inoculum. A continuous laboratory-scale process was followed by full-scale operation with reject water from the digesters at Bekkelaget WWTP in Oslo, Norway. A third laboratory reactor was run in operational mode to verify the suitability of reject water from thermophilic digestion for the deammonification process. The two start-ups presented were run with indigenous bacterial populations, intermittent aeration and dilution, to favour growth of the anammox bacterial branches. Evaluation was done by chemical and fluorescence in situ hybridization analyses. The results demonstrate that anammox culture can be set up in a one-stage process only using indigenous anammox bacteria and that a full-scale start-up process can be completed in less than 120 days.

Comparison of effects of ferric nitrate additions in thermophilic, mesophilic and psychrophilic aerobic digestion for sewage sludge

The effects of ferric nitrate additions in thermophilic, mesophilic and psychrophilic conditions were investigated in aerobic digestion for sewage sludge. The results showed that the addition of chemical could significantly enhance the volatile solid (VS) removal by shortening 7 days of stabilization time, and increasing 5.52% and 2.12% after 21 days of digestion at 55 °C, 35 °C and 15 °C comparing to the control, respectively. Higher efficiency of removing excessive volatile fatty acid (VFA) was achieved at higher temperature of digestion with ferric nitrate, which resulted in the different orders and changing tendencies of individual VFA. Thus, greater rate of biodegradation of organic substances and higher level of stabilization were obtained for sludge with ferric nitrate at higher temperature, with lower total chemical oxygen demand and soluble chemical oxygen demand. Acceleration of protein disintegration with higher pH, and lower oxidation reduction potential but lower total phosphorus was also acquired by addition of ferric nitrate. VS concentration with chemical is well fitted by an exponential decay function over a temperature range of 15–55 °C, which represented that the improvement of stabilization of sludge with ferric nitrate was almost independent of temperature, but the chemical itself.

Metaproteomic analysis of the microbial community present in a thermophilic swine manure digester to allow functional characterization: A case study

Anaerobic digestion is a widely accepted approach to the treatment of organic waste and uses a complex consortium of microorganisms. In this study, a metaproteomic approach was used to analyze the proteins expressed in an anaerobic digester that ran on swine manure at 55 °C. The extracted proteins were separated by sodium dodecylsulfate polyacrylamide gel electrophoresis, and identified by LC-ESI-MS/MS. The study identified proteins with functions that are related to hydrolysis, acidogenesis and methanogenesis. The majority of the identified proteins (42%) were involved in the translation, ribosomal structure and biogenesis, which were followed by proteins associated with energy production and conversion (28%). The majority of the above proteins were eubacterial, with about 87% being Proteobacteria. Among the methanogenic archaeal proteins present, acetyl-CoA decarbonylase from Methanosarcina spp., which are able to produce methane from both acetate and CO2, were identified. The present study is the first to investigate the metaproteome of a thermophilic anaerobic digester and to demonstrate the presence in the system of a high bacterial diversity where Proteobacteria are dominant. The study also provides evidence of types of microbial activity taking place during thermophilic anaerobic digestion when swine manure is used as the sole feedstock to produce biogas.

Mechanism and Effect of Temperature on Variations in Antibiotic Resistance Genes during Anaerobic Digestion of Dairy Manure.

Animal manure comprises an important reservoir for antibiotic resistance genes (ARGs), but the variation in ARGs during anaerobic digestion at various temperatures and its underlying mechanism remain unclear. Thus, we performed anaerobic digestion using dairy manure at three temperature levels (moderate: 20 °C, mesophilic: 35 °C, and thermophilic: 55 °C), to analyze the dynamics of ARGs and bacterial communities by quantitative PCR and 16S rRNA gene sequencing. We found that 8/10 detected ARGs declined and 5/10 decreased more than 1.0 log during thermophilic digestion, whereas only four and five ARGs decreased during moderate and mesophilic digestion, respectively. The changes in ARGs and bacterial communities were similar under the moderate and mesophilic treatments, but distinct from those in the thermophilic system. Potential pathogens such as Bacteroidetes, Proteobacteria, and Corynebacterium were removed by thermophilic digestion but not by moderate and mesophilic digestion. The bacterial community succession was the dominant mechanism that influenced the variation in ARGs and integrons during anaerobic digestion. Thermophilic digestion decreased the amount of mesophilic bacteria (Bacteroidetes and Proteobacteria) carrying ARGs. Anaerobic digestion generally decreased the abundance of integrons by eliminating the aerobic hosts of integrons (Actinomycetales and Bacilli). Thermophilic anaerobic digestion is recommended for the treatment and reuse of animal manure.

Effects of geographic area, feedstock, temperature, and operating time on microbial communities of six full-scale biogas plants

The objective of this study was to investigate the effect of different animal feedings operated in two distinct PDO (protected designation of origin) cheese production areas (Parmigiano Reggiano and Grana Padano) on the microbiome of six full-scale biogas plants, by means of Illumina sequencing and qPCR techniques. The effects of feedstock (cattle slurry manure, energy crops, agro-industrial by-products), temperature (mesophilic/thermophilic), and operating time were also examined, as were the relationships between the predominant bacterial and archaeal taxa and process parameters. The different feedstocks and temperatures strongly affected the microbiomes. A more biodiverse archaeal population was highlighted in Parmigiano Reggiano area plants, suggesting an influence of the different animal feedings. Methanosarcina and Methanosaeta showed an opposite distribution among anaerobic plants, with the former found to be related to ammonium concentration. The Methanoculleus genus was more abundant in the thermophilic digester whereas representation of the Thermotogales order correlated with hydraulic retention time.

Thermophilic and mesophilic temperature phase anaerobic co-digestion (TPAcD) compared with single-stage co-digestion of sewage sludge and sugar beet pulp lixiviation

The performance of temperature phase anaerobic co-digestion (TPAcD) for sewage sludge and sugar beet pulp lixiviation (using the process of exchanging the digesting substrate between spatially separated thermophilic and mesophilic digesters) was tested and compared to both single-stage mesophilic and thermophilic anaerobic co-digestion. Two Hydraulic Retention Times (HRT) were studied in the thermophilic stage of anaerobic digestion in two temperature phases, maintaining the optimum time of the mesophilic stage at 10 days, obtained as such in single-stage anaerobic co-digestion. In this way, we obtained the advantages of both temperature regimes.Volatile solids removal efficiency from the TPAcD system depended on the sludge exchange rate, but fell within the 72.6–64.6% range. This was higher than the value of 46.8% obtained with single-stage thermophilic digestion and that of 40.5% obtained with mesophilic digestion. The specific methane yield was 424–468 ml CH4 per gram of volatile solids removed, similar to that of single-stage mesophilic anaerobic digestion. The increase in microbial activity inside the reactor was directly proportional to the organic loading rate (OLR) (or inversely proportional to the HRT) and inversely proportional to the size of the microbial population in single-stage anaerobic co-digestion systems.

Use of real-time PCR with propidium monoazide for enumeration of viable Escherichia coli in anaerobic digestion

A combination of propidium monoazide (PMA) with real-time quantitative polymerase chain reaction (PMA-qPCR) was optimized to enumerate only viable Escherichia coli in anaerobic digestion processes. Repeating the PMA treatment twice and a final concentration of 100 μM resulted in an effective exclusion of DNA from heat-treated E. coli cells. In three anaerobic digestion processes, real-time PCR, PMA-qPCR, and the most probable number method (MPN) were used to estimate the numbers of total, viable, and culturable E. coli cells, respectively. Culturable concentrations of fecal coliforms were also measured by the membrane filter method. For thermophilic digestion, the reductions in total and viable E. coli cells from the digester influent to the effluent were significantly lower than those in culturable cells and fecal coliforms by two to four orders of magnitude. For mesophilic digestion, the differences in the reductions in E. coli and fecal coliforms counts were less than two orders of magnitude. Based on the measurements of viable E. coli determined by the PMA-qPCR method, the microbial quality of digester effluents was discussed for agricultural application, and pasteurization after anaerobic digestion was suggested for the destruction of viable pathogens.

Thermophilic versus Mesophilic Anaerobic Digestion of Sewage Sludge: A Comparative Review.

During advanced biological wastewater treatment, a huge amount of sludge is produced as a by-product of the treatment process. Hence, reuse and recovery of resources and energy from the sludge is a big technological challenge. The processing of sludge produced by Wastewater Treatment Plants (WWTPs) is massive, which takes up a big part of the overall operational costs. In this regard, anaerobic digestion (AD) of sewage sludge continues to be an attractive option to produce biogas that could contribute to the wastewater management cost reduction and foster the sustainability of those WWTPs. At the same time, AD reduces sludge amounts and that again contributes to the reduction of the sludge disposal costs. However, sludge volume minimization remains, a challenge thus improvement of dewatering efficiency is an inevitable part of WWTP operation. As a result, AD parameters could have significant impact on sludge properties. One of the most important operational parameters influencing the AD process is temperature. Consequently, the thermophilic and the mesophilic modes of sludge AD are compared for their pros and cons by many researchers. However, most comparisons are more focused on biogas yield, process speed and stability. Regarding the biogas yield, thermophilic sludge AD is preferred over the mesophilic one because of its faster biochemical reaction rate. Equally important but not studied sufficiently until now was the influence of temperature on the digestate quality, which is expressed mainly by the sludge dewateringability, and the reject water quality (chemical oxygen demand, ammonia nitrogen, and pH). In the field of comparison of thermophilic and mesophilic digestion process, few and often inconclusive research, unfortunately, has been published so far. Hence, recommendations for optimized technologies have not yet been done. The review presented provides a comparison of existing sludge AD technologies and the gaps that need to be filled so as to optimize the connection between the two systems. In addition, many other relevant AD process parameters, including sludge rheology, which need to be addressed, are also reviewed and presented.

Ammonia as an In Situ Sanitizer: Influence of Virus Genome Type on Inactivation.

Treatment of human excreta and animal manure (HEAM) is key in controlling the spread of persistent enteric pathogens, such as viruses. The extent of virus inactivation during HEAM storage and treatment appears to vary with virus genome type, although the reasons for this variability are not clear. Here, we investigated the inactivation of viruses of different genome types under conditions representative of HEAM storage or mesophilic digestion. The goals were to characterize the influence of HEAM solution conditions on inactivation and to determine the potential mechanisms involved. Specifically, eight viruses representing the four viral genome types (single-stranded RNA [ssRNA], double-stranded RNA [dsRNA], single-stranded DNA [ssDNA], and double-stranded DNA [dsDNA]) were exposed to synthetic solutions with well-controlled temperature (20 to 35°C), pH (8 to 9), and ammonia (NH3) concentrations (0 to 40 mmol liter(-1)). DNA and dsRNA viruses were considerably more resistant than ssRNA viruses, resulting in up to 1,000-fold-longer treatment times to reach a 4-log inactivation. The apparently slower inactivation of DNA viruses was rationalized by the higher stability of DNA than that of ssRNA in HEAM. Pushing the system toward harsher pH (>9) and temperature (>35°C) conditions, such as those encountered in thermophilic digestion and alkaline treatments, led to more consistent inactivation kinetics among ssRNA and other viruses. This suggests that the dependence of inactivation on genome type disappeared in favor of protein-mediated inactivation mechanisms common to all viruses. Finally, we recommend the use of MS2 as a conservative indicator to assess the inactivation of ssRNA viruses and the stable ΦX174 or dsDNA phages as indicators for persistent viruses. Viruses are among the most environmentally persistent pathogens. They can be present in high concentrations in human excreta and animal manure (HEAM). Therefore, appropriate treatment of HEAM is important prior to its reuse or discharge into the environment. Here, we investigated the factors that determine the persistence of viruses in HEAM, and we determined the main mechanisms that lead to their inactivation. Unlike other organisms, viruses can have four different genome types (double- or single-stranded RNA or DNA), and the viruses studied herein represent all four types. Genome type appeared to be the major determinant for persistence. Single-stranded RNA viruses are the most labile, because this genome type is susceptible to degradation in HEAM. In contrast, the other genome types are more stable; therefore, inactivation is slower and mainly driven by the degradation of viral proteins. Overall, this study allows us to better understand the behavior of viruses in HEAM.

Increased loading rates and specific methane yields facilitated by digesting grass silage at thermophilic rather than mesophilic temperatures

This study was conducted to advance the understanding of thermophilic grass digestion. Late harvested grass silage was fermented at thermophilic conditions at increasing organic loading rates (OLR). Stable digestion took place at an OLR between 3 and 4gVSL−1d−1. This enabled specific methane yields (SMY) as high as 405LCH4kgVS−1. An accumulation of volatile fatty acids (VFA), accompanied by a gradual deterioration of pH, FOS/TAC (ratio of VFA to alkalinity) arose at an OLR between 5 and 7gVSL−1d−1, yet inhibition did not occur. SMY decreased with reduced retention time ranging between 336 and 358LCH4kgVS−1 at OLR 7 and 5gVSL−1d−1 respectively. The biomethane efficiencies remained high (92–103%) at corresponding retention times. Comparative results indicated a superior performance with respect to higher loading and SMY as compared with mesophilic conditions.

Efficiently Treated Sewage Sludge Supplemented with Nitrogen and Potassium Is a Good Fertilizer for Cereals

AbstractSewage sludge contains phosphorus (P), nitrogen (N), potassium (K) and organic matter in both soluble and particulate forms. However, agricultural use of sludge is limited because of possible problems due to hygiene, heavy metals, inner quality variation and non‐balanced ratios of nutrients. We tested sustainable fertilization of cereals with (i) urea‐spiked granulated sludge, (ii) composted anaerobic sludge digestate, (iii) dried (dewatered), anaerobic thermophilic digestate sludge mixture and (iv) dried (dewatered) sludge. The sludge products were applied to barley and wheat with variable combinations of N and K supplements in Nordic climate. Sludge products supplemented with N and K and mineral NPK fertilizers containing soluble 70 kgN ha−1 achieved barley dry yields above 3500 kg ha−1. The grain N content was 22·3 g kg−1 in barley fertilized with granulated sludge fortified with NK (soluble N 83 kg ha−1), while grain N content was 20 g kg−1 when fertilized with 90 kgN ha−1 NPK. The wheat dry yields were 2890 and 2850 kg ha−1 fertilized with granulated sludge fortified with NK when soluble N was 49 kg ha−1 and NPK with 110 kgN ha−1 of these grains containing N 29·2 and 29·0 g kg−1. The results mean that correctly treated and formulated sludge products can be used. The risks caused by enteric microorganisms or heavy metals could be managed. The residual nitrate concentrations in soil were low. Sludge products could be developed by optimizing nutrient ratios and their structure. Sludge producers should provide farmers with detailed physicochemical data about each sludge portion. Copyright © 2016 John Wiley & Sons, Ltd.

Complete reduction of highly concentrated contaminants in piggery waste by a novel process scheme with an algal-bacterial symbiotic photobioreactor

The complete reduction of highly concentrated contaminants in piggery waste was achieved with an innovative process scheme consecutively combining autothermal thermophilic aerobic digestion (ATAD), an expanded granular sludge bed (EGSB) and a microalgal-bacterial symbiotic vertical photobioreactor (VPBR), followed by biomass recycling for effluent polishing. Contaminants in piggery waste, such as high organic and inorganic matter, total nitrogen (TN), and total phosphorus (TP) contents, were successfully reduced in the newly implemented system. The concentrations of volatile solids (VS) and the chemical oxygen demand (COD) for organic matter in the feed were reduced by approximately 99.3% and 99.7%, respectively, in the innovative system. The overall reduction efficiencies in TN, ammoniacal nitrogen, and TP were 98.8, 98.4, and 93.5%, respectively, through ammonia gas emission, coagulated sludge disposal, and the algal-bacterial symbiotic polishing process. Fecal coliform density was decreased to <1.7 × 104 CFU g−1 total solids. Biogas and CH4 in the EGSB were generated in the range of 0.36–0.79 and 0.18–0.44 L g−1 [VS removed], respectively, and contained 245 ± 19 ppm (v/v) [H2S].

Anaerobic Codigestion of Sludge: Addition of Butcher's Fat Waste as a Cosubstrate for Increasing Biogas Production.

Fat waste discarded from butcheries was used as a cosubstrate in the anaerobic codigestion of sewage sludge (SS). The process was evaluated under mesophilic and thermophilic conditions. The codigestion was successfully attained despite some inhibitory stages initially present that had their origin in the accumulation of volatile fatty acids (VFA) and adsorption of long-chain fatty acids (LCFA). The addition of a fat waste improved digestion stability and increased biogas yields thanks to the higher organic loading rate (OLR) applied to the reactors. However, thermophilic digestion was characterized by an effluent of poor quality and high VFA content. Results from spectroscopic analysis suggested the adsorption of lipid components onto the anaerobic biomass, thus disturbing the complete degradation of substrate during the treatment. The formation of fatty aggregates in the thermophilic reactor prevented process failure by avoiding the exposure of biomass to the toxic effect of high LCFA concentrations.

Anaerobic digestion of food waste – Effect of recirculation and temperature on performance and microbiology

Recirculation of digestate was investigated as a strategy to dilute the food waste before feeding to anaerobic digesters, and its effects on microbial community structure and performance were studied. Two anaerobic digesters with digestate recirculation were operated at 37 °C (MD + R) and 55 °C (TD + R) and compared to two additional digesters without digestate recirculation operated at the same temperatures (MD and TD). The MD + R digester demonstrated quite stable and similar performance to the MD digester in terms of the methane yield (around 480 mL CH4 per gVSadded). In both MD and MD + R Methanosaeta was the dominant archaea. However, the bacterial community structure was significantly different in the two digesters. Firmicutes dominated in the MD + R, while Chloroflexi was the dominant phylum in the MD. Regarding the thermophilic digesters, the TD + R showed the lowest methane yield (401 mL CH4 per gVSadded) and accumulation of VFAs. In contrast to the mesophilic digesters, the microbial communities in the thermophilic digesters were rather similar, consisting mainly of the phyla Firmicutes, Thermotoga, Synergistetes and the hydrogenotrophic methanogen Methanothermobacter. The impact of ammonia inhibition was different depending on the digesters configurations and operating temperatures.

Thermophilic digestion of waste-activated sludge coupled with solar pond

Thermophilic anaerobic digestion (AD) is an efficient treatment process for waste activated sludge with enhanced hydrolysis and digestion rates. However, the costs associated with maintaining high temperature for thermophilic digester should be minimized and the thermal stability of the reactor should be maximized for its practical use. This study tested an integrated system consisting of a solar pond and an AD reactor for digestion of waste activated sludge. The integrated system could be stably operated at 51.6 ± 1.5 °C in sunny days (and nights) and maintained digestion performance over at least three days with cloud and rain. On the contrary, the control reactor without solar pond experienced significant temperature fluctuation and poor digestion performance. After 29d, the integrated system thermophilic reactor removed 65.0 ± 4.2% of total chemical oxygen demands and produced 79% excess biogas. The concentrations of soluble chemical oxygen demand, protein and saccharides in digester with solar pond were higher than those in that without the pond. Also, the presence of solar pond enhanced hydrolysis and degradation of soluble microbial byproduct-like compounds with carboxylic groups and amide-2 groups in sludge.