Temperature-phased Anaerobic Digestion Research Articles

Temperature-phased anaerobic sludge digestion effectively removes antibiotic resistance genes in a full-scale wastewater treatment plant

Sludge is a major by-product and the final reservoir of antibiotic resistance genes (ARGs) in wastewater treatment plants (WWTPs). Temperature-phased anaerobic digestion (TPAD), consisting of thermophilic anaerobic digestion (AD) (55 °C) and mesophilic AD processes (37 °C), has been implemented in WWTPs for sludge reduction while improving the biomethane production. However, the impact of TPAD on the ARGs' fate is still undiscovered in lab-scale experiments and full-scale WWTPs. This study, for the first time, investigated the fate of ARGs during the TPAD process across three seasons in a full-size WWTP. Ten typical ARGs and one integrase gene of class 1 integron (intI1) involving ARGs horizontal gene transfer were examined in sludge before and after each step of the TPAD process. TPAD reduced aac(6′)-Ib-cr, blaTEM, drfA1, sul1, sul2, ermb, mefA, tetA, tetB and tetX by 87.3–100.0 %. TPAD reduced the overall average absolute abundance of targeted ARGs and intI1 by 92.39 % and 92.50 %, respectively. The abundance of targeted ARGs in sludge was higher in winter than in summer and autumn before and after TPAD. During the TPAD processes, thermophilic AD played a major role in the removal of ARGs, contributing to >60 % removal of ARGs, while the subsequent mesophilic AD contributed to a further 31 % removal of ARGs. The microbial community analysis revealed that thermophilic AD reduced the absolute abundance of ARGs hosts, antibiotic resistant bacteria. In addition, thermophilic AD reduced the abundance of the intI1, while the intI1 did not reproduce during the mesophilic AD, also contributing to a decline in the absolute abundance of ARGs in TPAD. This study demonstrates that TPAD can effectively reduce the abundance of ARGs in sludge, which will suppress the transmission of ARGs from sludge into the natural environment and deliver environmental and health benefits to our society.

Determination of operational parameters for the first stage of continuous temperature-phased anaerobic digestion of oily food waste: Influent concentration, hydraulic retention time, pH control and temperature

Anaerobic digestion is promising in both food waste reduction and energy upgrading. However, the intermediates in anaerobic digestion of such an oily food waste, long chain fatty acids, not only have low bioavailability but also significantly threaten operation stability. Temperature-phased anaerobic digestion is an emerging technology in recent years, expected to overcome those issues. Given significance of the first stage of the two-stage process, continuous thermophilic digestion of oily food waste was conducted, with mesophilic digestion operated in parallel as control, to determine practical operational parameters from operation performance and stability. The feeding concentration at approximately 4.50 % of volatile solids was identified to be applicative for a relatively stable operation. Due to slow β-oxidation for long chain fatty acids, hydraulic retention time for the first stage needed longer than 10 d to maintain anaerobes’ survival so that they were not inhibited. In addition to the risk from inhibition, readily acidified risk still existed, and instant pH control was demonstrated to be efficient in maintaining the habitat of microbial metabolism. Compared with no viable bacteria detected under uncontrolled pH, 36.7 % live cells could still be detected under controlled pH. Overall, thermophilic digestion obviously achieved advantages in higher substance degradation and methane production with lower stability risk. The genera Methanosarcina and Methanothermobacter played a significant role, with the ratio at about 2:1. While bacterial viability was not reduced remarkably in preliminary period of inhibition, long-term operation under high lipid loading resulted in its loss.

Anaerobic co-digestion of sewage sludge and wine vinasse mixtures in single-stage and sequential-temperature processes

The present work consisted of the energetic valorization of sludge from wastewater treatment plants and discharges from the wine sector by means of anaerobic co-digestion. The valorization was approached through the application of two different processes: a) single-stage mesophilic anaerobic digestion process (35 °C) and, b) Temperature - Phased Anaerobic Digestion (TPAD) with separation of microorganisms: thermophilic acidogenic (55 °C) and mesophilic methanogenic (35 °C) phases.Mesophilic methanogenic biodegradability tests (BMP) were developed to determine the biogas (methane) production yield in single-stage process and sequential thermophilic hydrogen (BHP) and BMP were developed to study the biogas (hydrogen and methane) production yield in anaerobic co-digestion processes in TPAD process.The results obtained showed that the single-stage mesophilic anaerobic co-digestion process was more effective for methane generation with yields of 55.39 mL CH4/g VSadded for the sludge and vinasse mixture, compared to 45.79 mL CH4/g VSadded for the TPAD process.

Principles and potential of thermal hydrolysis of sewage sludge to enhance anaerobic digestion

Sewage sludge is rich source of carbon, nutrients, and trace elements and can be subjected to proper treatment before disposal to fulfill government legislation and protect receiving environments. Anaerobic digestion (AD) is a well-adopted technology for stabilizing sewage sludge and recovering energy-rich biogas and nutrient-rich digestate. However, a slow hydrolysis rate limits the biodegradability of sludge. In the present study we have attempted to explain the potential of thermal hydrolysis to enhance anaerobic digestion of sewage sludge. Thermal pretreatment improves biodegradability and recycling of the sludge as an excellent energy and nutrients recovery source at reasonable capital (CAPEX) and operational (OPEX) costs. Other pretreatments like conventional (below/above 100 °C), temperature-phased anaerobic digestion (TPAD), microwave and chemically mediated thermal pretreatment have also been accounted. This review provides a holistic overview of sludge's characterization and value-added properties, various techniques used for sludge pretreatment for resource recovery, emphasizing conventional and advanced thermal pretreatment, challenges in scale-up of these technologies, and successful commercialization of thermal pretreatment techniques.

Digested Sludge Quality in Mesophilic, Thermophilic and Temperature-Phased Anaerobic Digestion Systems

Anaerobic digestion (AD) technology is commonly used to treat sewage sludge from activated sludge systems, meanwhile alleviating the energy demand (and costs) for wastewater treatment. Most often, anaerobic digestion is run in single-stage systems under mesophilic conditions, as this temperature regime is considered to be more stable than the thermophilic one. However, it is known that thermophilic conditions are advantageous over mesophilic ones in terms of methane production and digestate hygienisation, while it is unclear which one is better concerning the digestate dewaterability. Temperature-phased anaerobic digestion (TPAD) is a double-stage AD process that combines the above-mentioned temperature regimes, by operating a thermophilic digester followed by a mesophilic one. The aim of this study is to compare the digestate quality of single-stage mesophilic and thermophilic AD and TPAD systems, in terms of the dewaterability, pathogenic safety and lower calorific value (LCV) and, based on the comparison, consider digested sludge final disposal alternatives. The research is conducted in lab-scale reactors treating waste-activated sludge. The dewaterability is tested by two methods, namely, centrifugation and mechanical pressing. The experimental results show that the TPAD system is the most beneficial in terms of organic matter degradation efficiency (32.4% against 27.2 for TAD and 26.0 for MAD), producing a digestate with a high dewaterability (8.1–9.8% worse than for TAD and 6.2–12.0% better than for MAD) and pathogenic safety (coliforms and Escherichia coli were not detected, and Clostridium perfringens were counted up to 4.8–4.9 × 103, when for TAD it was only 1.4–2.5 × 103, and for MAD it was 1.3–1.8 × 104), with the lowest LCV (19.2% against 15.4% and 15.8% under thermophilic and mesophilic conditions, respectively). Regarding the final disposal, the digested sludge after TAD can be applied directly in agriculture; after TPAD, it can be used as a fertilizer only in the case where the fermenter HRT assures the pathogenic safety. The MAD digestate is the best for being used as a fuel preserving a higher portion of organic matter, not transforming into biogas during AD.

Temperature Phased Anaerobic Digestion at the Intermediate Zone of 45 °C: Performances, Stability and Pathogen Deactivation

Temperature phased anaerobic digestion (TPAD) systems with conventional sequences (first stage of 55 ℃ and second stage of 35 ℃) have been widely studied. However, very limited studies were available on TPAD system with the first stage operated at the intermediate zone of 45 °C, mainly due to the notion that limited microbial activity occurs within this zone. The objective of this research was to evaluate the performance, stability and the capability of 45 °C TPAD in producing class A biosolids, in comparison to a conventional TPAD. Four combinations of TPAD systems were studied, 45 ℃ TPAD 2.5/10 (1st stage solids retention time (SRT) 2.5 days/2nd stage SRT 10 days), 45 ℃ TPAD 7.5/10, 55 ℃ TPAD 2.5/10 and 55 ℃ TPAD 7.5/10. Among all, 45 ℃ TPAD 7.5/10 was found to have the best performances, attributed to its high volatile solids (VS) destruction (58%), minimal acetate accumulation (127 mg/L), high methane yield (0.58 m3 CH4/kg VS removed), high COD destruction solid COD (sCOD; 74% and total COD (tCOD) 54%) and minimal free NH3 content (67.5 mg/L). As for stability, stable pH distribution, high alkalinity content and low VFA to alkalinity ratio, indicated a well-buffered system. Additionally, the system had also able to produce class A biosolids. Therefore, proved that TPAD system operated at the intermediate zone of 45 ℃ can perform better than the conventional TPAD, hence, highlighting its economic advantage.

Co-Digestion of Rice Straw with Cow Manure in an Innovative Temperature Phased Anaerobic Digestion Technology: Performance Evaluation and Trace Elements

Rice straw is an agricultural residue produced in abundant quantities. Open burning and plowing back the straw to the fields are common practices for its disposal. In-situ incorporation and burning cause emissions of greenhouse gas and particulate matter. Additionally, the energy potential of rice straw is lost. Anaerobic digestion is a technology that can be potentially used to utilize the surplus rice straw, provide renewable energy, circulate nutrients available in the digestate, and reduce greenhouse gas emissions from rice paddies. An innovative temperature phased anaerobic digestion technology was developed and carried out in a continuous circulating mode of mesophilic and hyperthermophilic conditions in a loop digester (F1). The performance of the newly developed digester was compared with the reference digester (F2) working at mesophilic conditions. Co-digestion of rice straw was carried out with cow manure to optimize the carbon to nitrogen ratio and to provide the essential trace elements required by microorganisms in the biochemistry of methane formation. F1 produced a higher specific methane yield (189 ± 37 L/kg volatile solids) from rice straw compared to F2 (148 ± 36 L/kg volatile solids). Anaerobic digestion efficiency was about 90 ± 20% in F1 and 70 ± 20% in F2. Mass fractions of Fe, Ni, Co, Mo, Cu, and Zn were analyzed over time. The mass fractions of Co, Mo, Cu, and Zn were stable in both digesters. While mass fractions of Fe and Ni were reduced at the end of the digestion period. However, no direct relationship between specific methane yield and reduced mass fraction of Fe and Ni was found. Co-digestion of rice straw with cow manure seems to be a good approach to provide trace elements except for Se.

Temperature phased anaerobic digestion (TPAD) of organic fraction of municipal solid waste (OFMSW) and digested sludge (DS): Effect of different hydrolysis conditions

Hydrolysis is the most critical stage in high solids Temperature Phased Anaerobic Digestion (TPAD). In this paper two different Organic Fraction of Municipal Solid Waste (OFMSW) types were tested in co-digestion with Digested Sludge (DS) at different temperatures: 37, 55 and 65 °C. Volatile fatty acids (VFAs), soluble chemical oxygen demand (CODs) and Biochemical Methane Production (BMP) were measured and calculated after 0, 24, 48 and 72 h hydrolysis. The results showed that both the BMP and the methane production rate improved. A Solids Retention Time (SRT) of 72 h at a temperature of 55 °C gave the best results: the reaction rate constant k was 0.34 d-1 and the BMP was 250 mLCH4/gMV, which were 47% and 19% higher compared to the reference (0 h hydrolysis). The CODs and VFAs profiles during hydrolysis showed how OFMSW initial characteristics can affect the performance of temperature phased anaerobic digestion.

Effects of biological pretreatments of microalgae on hydrolysis, biomethane potential and microbial community

Parameters of temperature-phased anaerobic digestion (TPAD) were varied to study their effects on hydrolysis, biomethane potential (BMP), and microbial diversity of microalgae biodegradation. Anaerobic pretreatments at 85 °C demonstrated the release of soluble carbohydrate and protein molecules under low microbial metabolic activity. However, at 55 °C, anaerobic pretreatments showed superior performance in methane yield, nutrient release, and volatile fatty acids (VFAs) production due to dominant Clostridium. Furthermore, the highest destruction of volatile solids (VS) was observed during aerobic pretreatments at 55 °C under the influence of various quantities of these genera – Luteimonas, Symbiobacterium, Soehngenia, Thermobacillus, and Ureibacillus. Statistical analysis revealed that hydrolysis and BMP were not correlated. However, soluble nitrogen and phosphorous showed strong correlation with methane (r = 0.623 and 0.948, respectively) under thermo-anaerobic pretreatment, while VS removal and concentrations of acetic and butyric acids and lipids were positively correlated with each other under thermo-aerobic pretreatment.

Assessment of hyper-thermophilic/mesophilic temperature-phased anaerobic co-digestion of crude glycerine and sewage sludge

Temperature-phased anaerobic digestion is considered to be a valuable alternative to achieve new challenges in Wastewater Treatment Plants (WWTP) and biomethane production. This system makes it possible to remove/separate gas toxicants (mainly H2S) in the first phase reactor gas flow (hydrolysis reactor) and improve the profitability of the upgraded biogas flow obtained in the second phase reactor (methanogenic) (i.e. injection into the natural gas grid, vehicle fuel, biogas fuel cell or solid oxide fuel cell quality). The purpose of this study is to investigate the temperature-phased (hyper-thermophilic/mesophilic) anaerobic co-digestion of crude glycerine (CG) and sewage sludge (SS) in semi-continuous reactors. Specifically, to determine the effect of temperature-phased in: i) the quantity and quality of biogas production and ii) the effect on dewatering properties of the digestate obtained in comparison with conventional mesophilic systems. Results show that with a feed composed of 95% SS and 5% CG, the hyper-thermophilic/mesophilic system presents an increase in VSrem (Volatile solids removed) of 5.6% in comparison to the conventional mesophilic system. In reference to biogas quality, the hyper-thermophilic/mesophilic system presents a H2S content decrease of >67% in comparison to the control mesophilic reactor. Moreover, an energy balance shows that the proposed system reaches up to 68% extra heat production in comparison to the conventional mesophilic anaerobic system.

Life Cycle Assessment of the Mesophilic, Thermophilic, and Temperature-Phased Anaerobic Digestion of Sewage Sludge

In this study the environmental impact of the anaerobic digestion (AD) of sewage sludge within an activated sludge wastewater treatment plant (WWTP) was investigated. Three alternative AD systems (mesophilic, thermophilic, and temperature-phased anaerobic digestion (TPAD)) were compared to determine which system may have the best environmental performance. Two life cycle assessments (LCA) were performed considering: (i) the whole WWTP (for a functional unit (FU) of 1 m3 of treated wastewater), and (ii) the sludge line (SL) alone (for FU of 1 m3 of produced methane). The data for the LCA were obtained from previous laboratory experimental work in combination with full-scale WWTP and literature. According to the results, the WWTP with TPAD outperforms those with mesophilic and thermophilic AD in most analyzed impact categories (i.e., Human toxicity, Ionizing radiation, Metal and Fossil depletion, Agricultural land occupation, Terrestrial acidification, Freshwater eutrophication, and Ozone depletion), except for Climate change where the WWTP with mesophilic AD performed better than with TPAD by 7%. In the case of the SL alone, the production of heat and electricity (here accounted for as avoided environmental impacts) led to credits in most of the analyzed impact categories except for Human toxicity where credits did not balance out the impacts caused by the wastewater treatment system. The best AD alternative was thermophilic concerning all environmental impact categories, besides Climate change and Human toxicity. Differences between both LCA results may be attributed to the FU.

Comparative analysis between a conventional and a temperature-phased anaerobic digestion system: Monitoring of the process, resources transformation and energy balance

This study was carried out with the principal aim of obtaining reliable outcomes for the future implementation of a temperature-phased anaerobic digestion (TPAD) process in a large (2 M population equivalent, p.e.) WWTP. With the aid of pilot-scale (10 L) reactors fed by pure primary sludge (PS), a TPAD process, where the first and the second reactor were operated at 50 °C and 38 °C, respectively, was compared with a conventional mesophilic (38 °C) anaerobic digestion (AD) process. The initial hydraulic retention time (HRT) of the first, acidogenic, reactor of the TPAD was reduced from 3 to 2 days in the second part of the test.The results demonstrated that the TPAD system had been stable for all the duration of the test (approx. 100 days), as testified by the steady values of pH and tVFAs/TA ratio, notwithstanding the decrease in the HRT. The TPAD proved to be more efficient in volatile solid (VS) reduction and methane generation, compared to the conventional mesophilic AD process. In fact, the VS reduction increased from 42% to approx. 55% and the specific methane potential (SMP) from 280 to 332 NL/kg VS added. An excellent phase separation was observed between the two acidogenic and methanogenic reactors, as demonstrated by the low SMP (only 3% of the overall production) recorded from the first reactor of the TPAD system.However, the energy analysis demonstrated that the higher SMP obtained in the TPAD was not sufficient to compensate the higher amounts of heat required for sludge heating and heat loss compensation. Only a process of heat recovery could make the TPAD system really profitable, thus increasing the aliquot of energy in the form of methane, available for users external to the WWTP, by 20%. This result represents a step in the evolution of traditional WWTPs towards more energy efficient and sustainable facilities.

Sustainable management of lignocellulosic wastes: Temperature strategies for anaerobic digestion of artichoke

Lignocellulosic residues are a widely available energy resource, but their conversion into biogas through biomethanation may be hindered by a chemical structure with strong bonds. A strategy based on temperature-phased anaerobic digestion (TPAD) is here applied to optimize anaerobic digestion of artichoke as a representative lignocellulosic waste. The experiments consist of a first thermophilic stage conducted at 55 °C (7 and 5 days), followed by a second mesophilic stage at 35 °C. In addition, for comparison, single-stage temperature trials are carried out within the most common conditions for biomethanation (mesophilic at 35 °C, and thermophilic at 55 °C), as well as at the intermediate range (42 °C). In general, biomethanation of artichoke residues under TPAD configuration provides better performance than single-stage systems. Particularly, the best performance is observed for the TPAD arrangement using a thermophilic phase of 7 days. Thus, at this optimal configuration, volatile solids (VS) removal of 54%, and biogas and methane yields of 442 mL/g-VS and 375 mL-CH4/g-VS were reached, respectively. Moreover, a kinetic model coupling biogas generation and substrate consumption has been applied. Good agreement is found for both variables in each system tested. The calculated non-biodegradable substrate (S∞) is lower in TPAD systems (mean 49.6%) compared with that of single-stages (average 53.9%). The maximum specific growth rate of biomass (μM) is higher for TPAD digesters (mean 0.0625 d−1), although the thermophilic single-stage reactor reaches a close value (0.0615 d−1). The biogas yield (α) is similar for all conditions (0.358 L/g-VS), but with a maximum value of 0.428 L/g-VS for the TPAD digester with the 7-days thermophilic period. Therefore, temperature-phased anaerobic digestion has proved to be an efficient strategy for sustainable management of artichoke wastes.

Synergistic Co-Digestion of Microalgae and Primary Sludge to Enhance Methane Yield from Temperature-Phased Anaerobic Digestion

A two-stage temperature-phased mesophilic anaerobic digestion assay was carried out to study the interaction between various biological pretreatment conditions and the possible synergistic co-digestion of microalgae and primary sludge. The study of growth kinetics of the biochemical methane potential test revealed that a maximum of 36% increase in methane yield was observed from co-digestion of a substrate pretreated by thermophilic aerobic conditions (55 °C and HRT = 2 days) and an 8.3% increase was obtained from the anaerobic pretreated substrate (55 °C and HRT = 3 days). Moreover, no synergistic effects on methane yields were observed in co-digesting the substrate pretreated with high temperature (85 °C). The study also identified specific conditions in which interaction between biological pretreatment and co-digestion might substantially reduce methane yield. Careful optimization of operating conditions, both aerobic and anaerobic pretreatment at moderate thermophilic conditions, can be used as a biological pretreatment to enhance methane yield from the co-digestion of microalgae and primary sludge.

Methods for anaerobic digestion model fitting—comparison between heuristic and automatic approach

The article demonstrates if automatic optimization can be better than manual adjustment. The subject of optimization was the temperature-phased anaerobic digestion (TPAD) model. A selection of 3 parameters per each reactor in the process chain was appointed—reaction rate for propionate conversion, acetate conversion, and hydrolysis. Overall, both methods provided very convergent results. However, the total summary error (TSE) for the automatic algorithm was always moderately lower than for manual—the difference varied between 16.16 and 57.05 percentage points. Although the manual method has significant advantages—adjustment was more homogenous and gave more uniform fitting. Finally, cross-validation was performed to unify the values between the experimental series. The result was a total number of 4 values for each optimized constant—for two temperature points in each of two methods. Due to inconclusive information about the accuracy, averaged values were calculated to use in further researches. The recommendation from this article is to connect the best aspect of both methods to achieve the most accurate results.

Temperature-phased anaerobic co-digestion of food waste and paper waste with and without recirculation: Biogas production and microbial structure

Two temperature-phased anaerobic digestion (TPAD) systems (55 °C in the first reactor and 35 °C in the second reactor) with and without recirculation were operated in parallel for the co-digestion of food waste and paper waste. A long-term experiment was carried out for these two systems with the paper waste ratios elevated from 0 to 50%. The removal efficiencies of COD, TS, VS, carbohydrate and protein in the recirculated TPAD system were higher than those of the non-recirculated system. The successful acclimation of thermophilic cellulose-degrading bacteria in the first reactor (RT1), partly due to recirculation, ensured the effective degradation of cellulose when the paper waste ratio was higher than 40%, resulting in the production of large amounts of hydrogen in reactor RT1. In the absence of recirculation, the main substance produced in the first reactor of the non-recirculated system (T1) was lactic acid. This gradually led to over-acidification and a low degradation efficiency and no methane or hydrogen was produced in T1. Recirculation helped to establish a stable bacterial community capable of producing bio-hydrogen in reactor RT1. The relatively low pH of 5.5 in the RT1 inhibited the activity of hydrogenotrophic archaea without consuming hydrogen, facilitating high hydrogen production levels.

Dissecting methanogenesis for temperature-phased anaerobic digestion: Impact of temperature on community structure, correlation, and fate of methanogens

This study investigated the relationship between the temperature (35, 42, and 55 °C) used in temperature-phased anaerobic digestion (TPAD) and fate of methanogens between the two anaerobic digestion (AD) phases. Methanogens were profiled by using next generation sequencing (NGS) and droplet digital PCR approaches. The results showed that optimal combined temperatures for methane production were 55 °C during biological hydrolysis (BH) and 35 or 42 °C during AD. BH exhibited much lower archaeal population and was more susceptible to changes in temperature, compared to the AD phase. Additionally, we demonstrated, for the first time, that the BH step could affect the subsequent AD phase by altering AD methanogen composition and improve the stability of the process by enriching the rapidly growing Methanosarcina in the BH-AD process. These results are significant for understanding the mechanisms and stability of methane production in TPAD systems.

Repurposing pre-pasteurization as an in situ thermal hydrolysis pretreatment process for enhancing anaerobic digestion of municipal sludge: a horizontal comparison between temperature-phased and standalone thermophilic or mesophilic anaerobic digestion

This study evaluated the effect of pre-pasteurization (70 °C for 45 min) on the performance of temperature-phased anaerobic digestion (TPAD) in horizontal comparison with standalone mesophilic and thermophilic anaerobic digestion (AD).

A novel acetogenic bacteria isolated from waste activated sludge and its potential application for enhancing anaerobic digestion performance

The development of anaerobic digestion (AD) for volatile fatty acids (VFAs) production from waste activated sludge (WAS) is arrested due to low hydrolysis and acidification efficiency. This study proposed to enhance WAS reduction and VFAs accumulation during AD process via bioaugmentation of acetate-producing bacteria. Four acetogens were firstly isolated from a temperature-phased anaerobic digestion (TPAD) system. The acetate production efficiency of different isolates ranged from 15.8 to 73.7 mg acetate/g TOC, in which the bacterial strain NJUST19 was found to be the most effective strain. The results of morphological, biochemical characteristics as well as phylogenetic analysis showed that the isolate NJUST19 was Gram-positive and rod-shaped, catalase-negative, nitrate reduction-positive, methyl red-negative and capable of starch and gelatin hydrolysis, for which the name of Clostridium sp. NJUST19 was proposed. The optimal culture conditions (i.e. initial pH and temperature) were evaluated for their effects on microbe growth of selected NJUST19, and the maximum acetate production was observed at pH 9.0 and temperature of 40 °C. In the case of modified TPAD system inoculated with Clostridium sp. NJUST19, total suspended solids (TSS) removal rate and maximum VFAs accumulation increasing to 35.3% and 4200 mg/L, respectively, which was much higher than that of control (21.9% and 2894 mg/L). These results indicated that Clostridium sp. NJUST 19 is capable of enhancing digestion efficiency with a great benefit for VFAs production, offering potential prospects for bioaugmentation of WAS anaerobic digestion.

Strategy of adjusting recirculation ratio for biohythane production via recirculated temperature-phased anaerobic digestion of food waste

The stable H2 production is the important link in the anaerobic biohythane fermentation. In order to address the key parameters for the stable H2 production in the recirculated temperature-phased anaerobic digestion (R-TPAD) system, a critical strategy was proposed to adjust the recirculation ratio (R). The adjusting strategy was examined on a lab-scale R-TPAD system. The R-TPAD system was started up and operated with R = 1. Another system without recirculation (R = 0) was also operated as the control system. By narrowing the probable range from 0 < R < 1, the value of R = 0.4 was obtained for stable biohythane production from R-TPAD system. The harvested R-TPAD process achieved the volatile solids removal of 84.8% and the biohythane yield of 475 L/kg-VSfed with the H2 content of 10.5%, where the H2 yield efficiency was 248.0 mL-H2/CODremoved. The success of operating R-TPAD was attributed to the proposed strategy, which used the tendency of existing CH4 and the pH above 4.0 as the critical indicators for the stable H2 production.