Temperature-phased Anaerobic Digestion System Research Articles

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.

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.

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.

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.

Microbial population dynamics in temperature‐phased anaerobic digestion of municipal wastewater sludge

AbstractBACKGROUNDThe present study compared the performance and capability of thermophilic–mesophilic, temperature‐phased anaerobic digestion (TPAD) of wastewater sludge at 45 and 55 °C in terms of rate of hydrolysis and methanogenesis at two point interval times – December and January – with operational conditions kept constant (total solid content 6.5%, solids retention time 12.5 days).RESULTSThe reduction of volatile solids (VS) was 77% at 45 °C TPAD‐1 and 72% at 55 °C TPAD‐II. The accumulation of ammonia and volatile fatty acids (VFAs; propionic acid) were observed to be below the inhibitory range (>3000 mg L−1), whereas the methane (CH4) production (45 °C, 3.55 ± 0.47 L CH4 L−1 day−1; 35 °C, 1.44 ± 0.12 L CH4 L−1 day−1) remained considerably higher in TPAD‐I than TPAD‐II. Furthermore, the TPAD‐II system suffered from certain degree of instability due to high level of total VFAs (6087 ± 1578 mg L−1), low buffering capacity, increased level of total NH3 (2982 ± 219 mg L−1) and free NH3 (246 ± 25 mg L−1), and relatively reduced CH4 production (1.69 ± 0.1 L L−1 day). The bacterial and archaeal population of the TPADs investigated by 454 pyrosequencing and Illumina sequencing showed, in both TPAD systems, a bacterial community dominated by Firmicutes, followed by Bacteriodetes, Proteobacteria, Synergistetes and Actiniobacteria. The archaeal community was dominated by Methanimicrobia (74–84% Methanosarcina) and Methanobacteria (15–27% Methanobacterium). A progression from genus Clostridium to Coprothermobacter and Tepidanaerobacter, and Methanocarcina to Methanothermobacter and Methanobacterium was observed in TPAD‐II.CONCLUSIONSThis study proved the processes driving the dynamics of key microbial population and its correlation with hydrolytic functionality of TPAD systems. © 2019 Society of Chemical Industry

Temperature phased anaerobic digestion of municipal sewage sludge: a Bardenpho treatment plant study

The disposal of waste sludge generated from wastewater treatment plants (WWTPs) is a growing problem worldwide, and contributes to over 50% of the operating costs of current WWTPs. In this study, temperature-phased anaerobic digestion (TPAD) and conventional (single-stage) mesophilic anaerobic digestion were investigated in order to determine the most beneficial process for the intended digester facility to be constructed for the fermented primary and secondary sludge from a Bardenpho type biological nutrient removal plant. This was accomplished by considering several operational control parameters for three different sludge retention times. This study has shown that the TPAD system had significant improvement in biogas and methane production; solids and organic removal; pathogen reduction and dewaterability over the conventional digestion. In terms of overall volatile sulphur compounds formation normalized per volatile solids added, no significant effects were observed between TPAD and control digester.

Temperature-phased anaerobic digestion of the hydromechanically separated organic fraction of municipal solid waste with sewage sludge

This paper examines the temperature-phased anaerobic digestion (TPAD) process treating the mixture of municipal sewage sludge with the hydromechanically separated organic fraction of municipal solid waste (HS-OFMSW). The experiments showed that the performance of the TPAD process strongly depends on the conditions applied in the thermophilic stage. The TPAD system operated at a solids retention time (SRT) of 1 and 14 days in the first and second steps, respectively, achieved the overall methane yield of 333 l CH4 kgVS−1 and the volatile solids reduction of 52.1%, whereas the corresponding values reported for a control single-stage mesophilic process were 230 l CH4 kgVS−1 and 37.23%, respectively. However, when an SRT of the thermophilic reactor was extended to 2 days, the methane production in the subsequent mesophilic stage significantly decreased. It was therefore concluded that the prolonged exposure of feedstock to the thermophilic temperatures can lead to greater intensity of protein degradation. Consequently, higher amounts of ammonia are liberated to the liquid phase, which results in the inhibition of methanogenesis in the subsequent mesophilic step. Moreover, ammonia affects the release and distribution of volatile fatty acids, which also influences the performance of the whole TPAD system.

Volume ratios between the thermophilic and the mesophilic digesters of a temperature-phased anaerobic digestion system affect their performance and microbial communities

An experimental temperature-phased anaerobic digestion (TPAD) system, with the thermophilic digester operated at neutral pH and with a balanced acidogenesis and methanogenesis (referred to as NT-TPAD), was evaluated with respect to the microbial communities and population dynamics of methanogens when digesting dairy cattle manure at 15-day overall system hydraulic retention time (HRT). When fed a manure slurry of 10% total solid (TS), similar system performance, 36-38% volatile solid (VS) removal and 0.21-0.22 L methane g(-1) VS fed, was achieved between a 5-day and 7.5-day HRT for the thermophilic digester. However, the thermophilic digester achieved a greater volumetric biogas yield when operated at a 5-day RT than at a 7.5-day HRT (6.3 vs. 4.7 L/L/d), while the mesophilic digester had a stable volumetric biogas yield (about 1.0 L/L/d). Each of the digesters harbored distinct yet dynamic microbial populations, and some of the methanogens were significantly correlated with methane productions. Methanosarcina and Methanosaeta were the most important methanogenic genera in the thermophilic and the mesophilic digesters, respectively. The microbiological findings may help understand the metabolism that underpins the anaerobic processes within each of the two digesters of TPAD systems when fed dairy manure.

Effect of organic loading on the microbiota in a temperature-phased anaerobic digestion (TPAD) system co-digesting dairy manure and waste whey.

Temperature-phased anaerobic digestion (TPAD) has gained increasing attention because it provides the flexibility to operate digesters under conditions that enhance overall digester performance. However, research on impact of organic overloading rate (OLR) to microbiota of TPAD systems was limited. In this study, we investigated the composition and successions of the microbiota in both the thermophilic and the mesophilic digesters of a laboratory-scale TPAD system co-digesting dairy manure and waste whey before and during organic overloading. The thermophilic and the mesophilic digesters were operated at 50 and 35 °C, respectively, with a hydraulic retention time (HRT) of 10 days for each digester. High OLR (dairy manure with 5 % total solid and waste whey of ≥60.4 g chemical oxygen demand (COD)/l/day) resulted in decrease in pH and in biogas production and accumulation of volatile fatty acids (VFAs) in the thermophilic digester, while the mesophilic digester remained unchanged except a transient increase in biogas production. Both denaturant gradient gel electrophoresis (DGGE) and Illumina sequencing of 16S ribosomal RNA (rRNA) gene amplicons showed dramatic change in microbiota composition and profound successions of both bacterial and methanogenic communities. During the overloading, Thermotogae was replaced by Proteobacteria, while Methanobrevibacter and archaeon classified as WCHD3-02 grew in predominance at the expense of Methanoculleus in the thermophilic digester, whereas Methanosarcina dominated the methanogenic community, while Methanobacterium and Methanobrevibacter became less predominant in the mesophilic digester. Canonical correspondence analysis (CCA) revealed that digester temperature and pH were the most influential environmental factors that explained much of the variations of the microbiota in this TPAD system when it was overloaded.

Drivers of microbial community composition in mesophilic and thermophilic temperature-phased anaerobic digestion pre-treatment reactors

Temperature-phased anaerobic digestion (TPAD) is an emerging technology that facilitates improved performance and pathogen destruction in anaerobic sewage sludge digestion by optimising conditions for 1) hydrolytic and acidogenic organisms in a first-stage/pre-treatment reactor and then 2) methogenic populations in a second stage reactor. Pre-treatment reactors are typically operated at 55–65 °C and as such select for thermophilic bacterial communities. However, details of key microbial populations in hydrolytic communities and links to functionality are very limited. In this study, experimental thermophilic pre-treatment (TP) and control mesophilic pre-treatment (MP) reactors were operated as first-stages of TPAD systems treating activated sludge for 340 days. The TP system was operated sequentially at 50, 60 and 65 °C, while the MP rector was held at 35 °C for the entire period. The composition of microbial communities associated with the MP and TP pre-treatment reactors was characterised weekly using terminal-restriction fragment length polymorphism (T-RFLP) supported by clone library sequencing of 16S rRNA gene amplicons. The outcomes of this approach were confirmed using 454 pyrosequencing of gene amplicons and fluorescence in-situ hybridisation (FISH). TP associated bacterial communities were dominated by populations affiliated to the Firmicutes, Thermotogae, Proteobacteria and Chloroflexi. In particular there was a progression from Thermotogae to Lutispora and Coprothermobacter and diversity decreased as temperature and hydrolysis performance increased. While change in the composition of TP associated bacterial communities was attributable to temperature, that of MP associated bacterial communities was related to the composition of the incoming feed. This study determined processes driving the dynamics of key microbial populations that are correlated with an enhanced hydrolytic functionality of the TPAD system.

Evaluation of system performances and microbial communities of two temperature-phased anaerobic digestion systems treating dairy manure

Two temperature-phased anaerobic digestion (TPAD) systems, with the thermophilic digesters acidified by acidogenesis products (AT-TPAD) or operated at neutral pH and balanced hydrolysis/acidogenesis and methanogenesis (NT-TPAD), were evaluated to treat high-strength dairy cattle manure. Despite similar methane productions (about 0.22 L/g VS fed), the NT-TPAD system removed significantly more VS (36%) than the AT-TPAD system (31%) and needed no pH adjustments. The thermophilic digester of the NT-TPAD system dominated the system performance and performed significantly better than that of the AT-TPAD system. The opposite held true for the mesophilic digesters. Differences of the thermophilic digesters between two TPAD systems affected the microbial communities of both local and downstream digesters. Each digester harbored distinctive microbial populations, some of which were significantly correlated with system performance. Methanosarcina was the most important methanogenic genus in both TPAD systems, while Methanosaeta only in the NT-TPAD system. Their populations were inversely related to VFA concentrations.

Evaluation of system performance and microbial communities of a temperature-phased anaerobic digestion system treating dairy manure: Thermophilic digester operated at acidic pH

A temperature-phased anaerobic digestion system with the thermophilic digester acidified by acidogenesis products (referred to as AT-TPAD) was evaluated to treat high-strength dairy cattle manure at a 15-day retention time. Three temperatures (50, 55, and 60°C) were tested on the thermophilic digester, and 50°C was found to be the optimal temperature for overall performance of the AT-TPAD system, achieving 31% VS removal and 0.22 L methane/g VS fed. The mesophilic digester contributed significantly more to the overall system performance than the thermophilic digester. The thermophilic and the mesophilic digesters had different microbial communities under all conditions, and both microbial communities exhibited dynamic changes in response to different conditions. Certain microbial groups were found significantly correlated with the system performance. Methanosarcina was the most important methanogen genus of the AT-TPAD system and its population abundance was inversely correlated with high concentrations of volatile fatty acids (VFA).

Feedstocks Affect the Diversity and Distribution of Propionate CoA-Transferase Genes (pct) in Anaerobic Digesters

Anaerobic digestion (AD) is an attractive microbiological technology for both waste treatment and energy production. Syntrophic acetogenic bacteria are an important guild because they are essential for maintaining efficient and stable AD operation. However, this guild is poorly understood due to difficulties to culture them. In this study, we developed specific PCR assays targeting the propionate-CoA transferase genes (pct) to investigate their diversity and distribution in several mesophilic anaerobic digesters and a bench-scale temperature-phased AD (TPAD) system. Phylogenetic analysis of sequenced pct amplicons revealed the occurrence of Syntrophobacter fumaroxidans and six other clusters of putative pct genes. Principal coordinate analysis (PCoA) showed that pct diversity and abundance were largely correlated to the feedstocks of the digesters, while little difference was seen between the granular and the liquid fractions of each digester or between the two digesters of the TPAD system. Cluster-specific qPCR analysis revealed major impact of feedstocks and fractions on the abundance of pct genes. Readily fermentable substrates such as sugar- or starch-rich feedstocks selected for pct genes (Cluster I) related to Syntrophobacter, while manure feedstock selected for pct clusters related to pct of Clostridium spp. These results suggest that propionate metabolism can be affected by feedstocks and partition differently between solid and liquid phases in digesters. The PCR assays developed in this study may serve as a tool to investigate propionate-oxidizing bacteria in anaerobic digesters and other anaerobic environments.

Assessment of solid retention time of a temperature phased anaerobic digestion system on performance and final sludge characteristics

AbstractBACKGROUND: This paper describes the results obtained during the thermophilic/mesophilic temperature phased anaerobic digestion (TPAD) of sewage sludge on a pilot scale. The aim of this research study was not only to optimize the anaerobic digestion process, but also to obtain a digested sludge suitable for agricultural applications according to the legal requirements.RESULTS: Four TPAD assays were carried out: 5/15, 3/15, 3/12 and 3/9 (days/days of solid retention time) with a specific methane production (expressed as LCH4 g−1 VSdestroyed) of 0.77, 0.83, 0.66 and 0.20, respectively. TPAD 3/15 and 3/12 reached pathogen concentrations of less than 1000 MPN g−1 TS (faecal colifoms) and 3 MPN per 4 g TS (Salmonella spp.); therefore, these digested sludges can be considered Class A biosolids, according to the US Environmental Protection Agency. Concentrations of heavy metals rose after the anaerobic digestion of mixed sludge, but the final values were always below the limits required by legal regulations.CONCLUSION: TPAD 3/15 is the best option in terms of organic matter removal, CH4 generation, and process stability. TPAD 3/12 obtained the best final dewaterability and pathogen reduction and in general, showed much better results than those obtained by anaerobic mesophilic control (15 days of SRT). Copyright © 2012 Society of Chemical Industry

Temperature phased anaerobic digestion increases apparent hydrolysis rate for waste activated sludge

It is well established that waste activated sludge with an extended sludge age is inherently slow to degrade with a low extent of degradation. Pre-treatment methods can be used prior to anaerobic digestion to improve the efficiency of activated sludge digestion. Among these pre-treatment methods, temperature phased anaerobic digestion (TPAD) is one promising method with a relatively low energy input and capital cost. In this study, an experimental thermophilic (50–70 °C)–mesophilic system was compared against a control mesophilic–mesophilic system. The thermophilic–mesophilic system achieved 41% and 48% volatile solids (VS) destruction during pre-treatment of 60 °C and 65 °C (or 70 °C) respectively, compared to 37% in the mesophilic–mesophilic TPAD system. Solubilisation in the first stage was enhanced during thermophilic pre-treatment (15% at 50 °C and 27% at 60 °C, 65 °C and 70 °C) over mesophilic pre-treatment (7%) according to a COD balance. This was supported by ammonia–nitrogen measurements. Model based analysis indicated that the mechanism for increased performance was due to an increase in hydrolysis coefficient under thermophilic pre-treatment of 60 °C (0.5 ± 0.1 d −1), 65 °C (0.7 ± 0.2 d −1) and 70 °C (0.8 ± 0.2 d −1) over mesophilic pre-treatment (0.2 ± 0.1 d −1), and thermophilic pre-treatment at 50 °C (0.12 ± 0.06 d −1).

Putting microbes to work in sequence: Recent advances in temperature-phased anaerobic digestion processes

Methane biogas production through anaerobic digestion (or biomethanation) is one of the few technologies that both produce bioenergy and protect the environment. When the focus of anaerobic digestion (AD) is shifted from primarily wastewater treatment to bioenergy production, efficiency and process stability become critical to the economic viability of AD technologies. Temperature-phased anaerobic digestion (TPAD) is a promising process that can significantly enhance both digestion efficiency and process robustness. A TPAD system separates the conventional AD process into two phases, so both phases can be optimized according to their individual functional needs. In the first, thermophilic phase, the often rate-limiting hydrolysis step of polymeric feedstock is accelerated by elevated temperatures, while in the second, mesophilic phase, the fastidious syntrophic acetogens and methanogens are provided with permissive conditions where inhibitions to key guilds (e.g., syntrophic acetogens and methanogens) are attenuated. Although large-scale TPAD systems have not been applied widely, researchers have demonstrated the potential superiority of TPAD systems over single-stage digesters and other AD processes with enhanced VS (volatile solids) and pathogen removal; increased methane yield, process stability, OLR (organic loading rate); shorter HRT (hydraulic retention time); decreased foaming and short-chain fatty acids in effluent.

Temperature-phased anaerobic digestion (TPAD) to obtain class A biosolids: A semi-continuous study

Regulations regarding the land application of sewage sludge require the performance of highly efficient treatments for pathogen reduction, such as temperature-phased anaerobic digestion (TPAD). Several semi-continuous experiments with different solid residence times (SRTs) have been made to find the best combination - TPAD 15/15, TPAD 5/15, TPAD 3/15 or TPAD 3/12. TPAD systems showed better performance in terms of VS destruction, gas production, sludge dewaterability, and process stability at total SRT of 15 days, compared with single-stage mesophilic or thermophilic digestion at the same SRT. Average VS reductions (VSR) using the overall TPAD 5/15 (78%), TPAD 3/15 (87%), and TPAD 3/12 (87%) were significantly higher than mesophilic control operating with an SRT of 15 days (40% VSR). The best results were obtained for TPAD 3/15 with 0.62 LCH(4) g(-1) VS(r), faecal coliform 10(3) MPN/g TS, and Salmonella spp. 1 MPN/4 g TS. Although both TPAD 3/15 and TPAD 3/12 were efficient enough to obtain Class A biosolids.

Performance of temperature-phased anaerobic digestion (TPAD) system treating dairy cattle wastes

The performance of temperature-phased anaerobic digestion (TPAD) system in the stabilization of dairy cattle wastes at high solids concentrations has never been evaluated, though the process has been established as a feasible alternative to conventional mesophilic processes for the treatment of municipal wastewater sludges. In this study, the TPAD system operating at a retention time of 14 days was subjected to varying total solids (TS) concentrations (3.46–14.54%) of dairy cattle wastes. At TS concentrations lower than 12.20%, corresponding to system volatile solids (VS) loadings in the range of 1.87–5.82 g VS/L/day, the system achieved an average VS removal of 40.2%. The maximum VS destruction of 42.6% was achieved at a TS concentration of 10.35%. Methane recovery from the wastes was consistently within 0.21–0.22 L/g VS fed. There was a drop in the system performance with respect to VS removal and methane recovery at TS concentrations higher than 10.35%. volatile fatty acid/alkalinity ratios less than 0.35 in the thermophilic reactor and 0.10 in the mesophilic reactor were found favorable for stable operation of the system. For the entire range of TS concentrations, the indicator organism counts in the biosolids were within the limits specified by USEPA in 40 CFR Part 503 regulations for Class A designation. After digestion, nearly 80–85% of total phosphorus was associated with the biosolids.