Volatile Solids Destruction Research Articles

Biogas production from water hyacinth (Eichhornia crassipes) harvested from River Wouri, Douala, Cameroon

Over the past three decades, growing global interest has been in replacing fossil fuels with clean energy from renewable resources. As a result, biogas generation from biodegradable organic waste has become increasingly popular, and water hyacinth is a menace wherever it exists. This study aimed to assess the effectiveness of water hyacinth (Eichhornia crassipes) from River Wouri as a biogas substrate. Equal volumes (700 mL) of water hyacinth and poultry-based inoculum were prepared, each with a duplicate. The 8 substrates were placed in 1-liter plastic and incubated in a water bath at 37± 2 °C for 106 days. The substrates were subsampled before and after incubation to analyse the dry matter, moisture, volatile solids, ash contents, and pH. The volume of biogas produced from each digester was collected in plastic bags and measured every three days; hence, the methane produced was calculated. Results indicated an increase in the moisture contents of the substrates, while pH levels remained unchanged after incubation. The rate of destruction of volatile solids increased with the level of added inoculum. Biogas production started almost immediately for all the samples and largely followed a dome shape over the incubation period. Cumulative methane produced were 10.23, 11.84, 5.88, and 0.71 LCH4/kgVS, respectively, for 100% water hyacinth, 90% water hyacinth, 80% water hyacinth and 0% water hyacinth. This study demonstrates the effectiveness of the weed in biogas production, which could be an important method to manage River Wouri while generating renewable energy.

Comprehensive hydrothermal pretreatment of municipal sewage sludge: A systematic approach

This study provides a comprehensive analysis of the potential impact of hydrothermal pretreatment (HTP) on municipal thickened waste-activated sludge (TWAS) and its integration with anaerobic digestion (AD). The research demonstrates that HTP conditions (170 °C, 3 bars for 30 min) can increase the solubilization of macromolecular organic compounds by 41%, which enhances biodegradability in semicontinuous bioreactors. This treatment also results in a 50% reduction in chemical oxygen demand (COD) and a 63% increase in the destruction of volatile solids (VS). The combination of HTP with AD significantly boosts methane yields by 51%, reaching 176 ml/g COD, and improves the digestate dewaterability, doubling the solid content in the dewatered cake. However, a higher polymer dose is required compared to conventional AD. Microbial community analysis correlates the observed performance and alterations; it indicates that HTP enhances resilience to stress conditions such as ammonia toxicity. This comprehensive study provides valuable insights into the transition from wastewater treatment plants (WWTPs) to resource recovery facilities (RRF) in line with circular economy principles.

Impact of volatile solids destruction on the shear and solid-liquid separation behaviour of anaerobic digested sludge

Systematic and comprehensive characterisation of shear and solid-liquid separation properties of sludge across a wide range of solids concentration and volatile solids destruction (VSD) is critical for design and optimization of the anaerobic digestion process. In addition, there is a need for studies at the psychrophilic temperature range as many unheated anaerobic digestion processes are operated under ambient conditions with minimal self-heating. In this study, two digesters were operated at different combinations of operating temperature (15–25 °C) and hydraulic retention time (16–32 d) to ensure a wide range of VSD in the range of 0.42–0.7 was obtained. For shear rheology, the viscosity increased 1.3 to 3.3 times with the increase of VSD from 43 % to 70 %, while other parameters (temperature, VS fraction) having a negligible impact. Analysis of a hypothetical digester indicated that there is an optimum VSD range 65–80 % where increase in viscosity due to the higher VSD is balanced by the decrease in solids concentration. For solid-liquid separation, a thickener model and a filtration model were used. No significant impact of VSD on the solids flux, underflow solids concentrations or specific solids throughput was observed in the thickener and filtration model. However, there was an increase in average cake solids concentration from 21 % to 31 % with increase of VSD from 55 % to 76 %, indicating better dewatering behaviour.

Achieving expanded sludge treatment capacity with additional benefits for an anaerobic digester using free ammonia pretreatment

Population growth rapidly increased waste activated sludge (WAS) production in wastewater treatment plants (WWTPs), making the expansion of sludge treatment capacity urgent. Free ammonia (FA) pretreatment is experimentally applied to expand the treatment capacity of an anaerobic digester through reducing sludge retention time (SRT) for the first time. Two semi-continuous flow mesophilic (37 °C) anaerobic digestion systems, control system with a uniform SRT of 12 d and the experimental systems with progressively reduced SRTs (from 12 d to 10 d and then 8 d), were operated for>7 months. The volatile solids (VS) destruction in the experimental system at a SRT of 8 d was comparable to the control system (30.0 ± 1.4 % vs 30.5 ± 1.7 %) but increased by 16.2 % (35.1 ± 1.5 % vs 30.2 ± 1.4 %) under an SRT of 10 d, which was supported by methane production and total chemical oxygen demand (COD) removal. The biomass-specific hydrolysis rate was significantly increased by up to 80 % (from 0.05 ± 0.01 g COD/g VS/d to 0.09 ± 0.01 g COD/g VS/d), which may contribute to the expanded capacity. The volatile fatty acids (VFAs)/alkalinity of systems maintained a reasonable range (0.01 – 0.06), suggesting the stability of digesters. FA pretreatment played a dominant role in the changes in the bacterial microbial community (52.80 % in PC1) and archaeal community (94.25 % in PC1). FA pretreatment improved the removal of pathogen by 1.3–2.0 log and antibiotic resistance genes by 34–86 %. This study first demonstrated that FA pretreatment expands the treatment capacity of an anaerobic digester by up to 50 % with economic and environmental benefits, promoting FA pretreatment to be a wider and pragmatic implementation for WWTPs.

Process Improvement of Biogas Production from Sewage Sludge Applying Iron Oxides-Based Additives

Iron additives are effective in the anaerobic sewage sludge digestion process, but the composition and dosage of these additives are not precisely defined. This research investigates the effects of three iron oxides-based additives on the destruction of volatile solids, the production and quality of biogas, as well as the quality of the supernatant. Additive No 1 contained >41.5% of FeO and >41.5% of Fe2O3, additive No 2 contained ≥86% of Fe3O4, and additive No 3 contained ≥98% of Fe3O4. The best results were obtained by applying an iron oxides-based additive with a higher content of divalent iron oxide. The increase in efficiency of the VSs destruction was not significant and on average 2.2%. The increase in biogas production was on average 20% while the average increase in the content of methane in the biogas was 6.3%. Applying the additive, the reduction in the concentration of ammonium nitrogen in the supernatant was up to 28%, as well as a reduction in the concentration of phosphate phosphorus in the supernatant by up to 3.1 times could be expected compared to the case when the additive was not applied. The dose of additive No 1 was between 7.5 g/kg of dry solids and 15 g/kg of dry solids in the lab-scale test. The dose was specified in the full-scale test, and the recommended dose of the additive was 10 g/kg of dry solids to improve biogas production.

Effect of Fe3O4 nanoparticles on anaerobic digestion of municipal wastewater sludge

Anaerobic digestion of waste activated sludge often achieves less than 50% destruction of volatile suspended solids and stabilization of chemical oxygen demand to methane gas. Although the impact of Fe3O4 nanoparticles on anaerobic digestion performance was investigated in past studies, this study focused directly on the impacts of the size and concentration of the Fe3O4 nanoparticles. Fe3O4 nanoparticles with two different ranges -- (12–18)-nm and (50–100)-nm -- and different concentrations were evaluated for their impact on anaerobic digestion of combined wastewater sludges (waste activated sludge + primary sludge). The volume of methane produced and the concentrations of volatile suspended solids, total chemical oxygen demand, and Fe2+ were measured during Biochemical Methane Potential (BMP) experiments. Adding Fe3O4 nanoparticles enhanced solids digestion and methanogenesis, and the impact was greater for methanogenesis. Adding 120 mg/L of the smaller nanoparticles had the greatest impact: a 1.7-fold increase in the methane yield, 26% increase in volatile suspended solids destruction, and 35% increase in total chemical oxygen demand removal, compared with the control. The smaller nanoparticles, which had higher impacts on anaerobic-digestion performance than the larger nanoparticles, led to greater release of soluble Fe2+, which may have decreased sulfide inhibition of methanogenesis, increased the rate of solids hydrolysis, or increased inter-species electron transport. A modified Gompertz model quantified how adding 120 mg/L of the (12–18)-nm nanoparticles increased the rate and extent of methanogenesis.

Novel free nitrous acid and ultrasonication pretreatment enhanced sludge biodegradability

ABSTRACT The main goal of this study was to investigate the novel combined Ultrasonication and Free Nitrous Acid (FNA) pretreatment on biodegradability and kinetics of thickened waste-activated sludge (TWAS). Partial factorial design with four levels of (0, 600, 1500, and 3000 KJ/Kg) for ultrasonication and 0, 0.7, 1.4, and 2.8 mg HNO2-N/L for FNA dose were examined creating 16 different combinations. Results revealed that combined pretreatment could significantly improve solubilization and solid destruction compared to solo pretreatments. The highest organic matter solubilization of 25.6% and volatile suspended solids destruction of 21.7% were observed when 2.8 mg HNO2-N/L and 1500 KJ/Kg were combined. Moreover, combining the pretreatments further enhanced biodegradability up to the highest percentage of 50.3% when pretreatment of 3000 KJ/Kg and 2.8 mg HNO2-N/L was applied. Also, the experimental data from a biochemical methane potential test was fitted well into First Order Kinetic and Modified Gompertz models, given that the coefficients of determination, R2, for models at all treatment levels were above 99%.

Enhanced mesophilic fermentation of waste activated sludge by integration with in-situ nitrate reduction

This study investigated the reduction of nitrate in a mesophilic waste activated sludge (WAS) fermentation system and determined the effect of nitrate reduction on the hydrolysis, acidogenesis and acetogenesis. Experimental results showed that the initial nitrate concentrations of 100, 200 and 400 mg/L were completely reduced in 1, 2 and 7 days, respectively. The destruction of volatile suspended solids was 1.2, 1.8 and 2.8 times, respectively, that without nitrate, demonstrating nitrate promoted the release of organic matter in sludge and enhanced the biodegradability of sludge organics. Moreover, batch tests using model substrates illustrated nitrate reduction promoted sludge hydrolysis and acetogenesis, but slightly inhibited acidogenesis. This study offers a feasible method to address two major problems currently faced by biological wastewater treatment plants, i.e. the overabundance of WAS and the lack of carbon sources for the denitrification process.

Improvement of biogas production and nitrogen recovery in anaerobic digestion of purple phototrophic bacteria by thermal hydrolysis

Purple phototrophic bacteria (PPB) are a novel driver to recover organics and nutrients from wastewater by assimilative growth. Depending on the source, assimilated resources from the PPB biomass can still be recovered after a releasing step. Anaerobic digestion (AD) releases carbonand nutrients, but the release is incomplete. Thermal hydrolysis (TH) as a pretreatment before AD improves the digestibility, release, and subsequent recovery potentials. This work determines the effects of TH in batch and continuous modes regarding methane potential, nutrients' release efficiencies, volatile solids destruction, degradability, and hydrolysis rates. Continuous runs over 165 days (d) confirmed enhanced recovery potentials, achieving up to 380 LCH4/kgVS (83 % solids destruction) and 73 % N release, respectively. The TH pretreatment is energy-intensive, but with appropriate heat recovery and increased methane production in the AD of the pretreated biomass, a combined configuration is energy positive.

Effect of nitrite on hydrolysis-acidification, biogas production and microbial community in semi-continuous two-phase anaerobic digestion of sewage sludge

Previous study found that the pre-treatment of sewage sludge with nitrite improves the biogas production during the mono/two-phase anaerobic digestion (AD) using batch biochemical methane potential tests. In this study, the effects of nitrite on hydrolysis-acidification, biogas production, volatile solids destruction and microbial composition in semi-continuous two-phase AD of sewage sludge were investigated. The addition of nitrite promotes sludge organic matter solubilization (+484%) and VFAs production (+98.9%), and causes an increase in the VS degradation rate during the AD process (+8.7%). The comparison of biogas production from the acidogenic and methanogenic reactors with or without the addition of nitrite implies that the nitrite has no significant effect on the overall biogas production of two-phase sludge AD process. High-throughput sequencing analysis shows that the microbial communities of bacteria and archaea in two-phase AD reactors significantly changes after the addition of nitrite. Vulcanibacillus (bacteria) and Candidatus Methanofastidiosum (archaea) become the dominant genera in the acidogenic and methanogenic reactors with the nitrite respectively. These findings provide new insights about using nitrite to promote the organic matter degradation of sewage sludge in a semi-continuous two-phase AD system.

Zero valent iron greatly improves sludge destruction and nitrogen removal in aerobic sludge digestion

Zero-valent iron (ZVI), a low-cost metallic material, has been previously applied in effectively enhancing sewage sludge anaerobic digestion. However, the potential role of ZVI on aerobic digestion of sludge, a completely different sludge treatment method from anaerobic digestion, is still unknown. Herein, the effects of ZVI on the performance of aerobic sludge digestion were systematically studied, focusing on the sludge degradation, nitrogen removal and sludge dewaterability. Results showed ZVI greatly increased the volatile solids (VS) destruction from 27.0 ± 1.3% to 50.0 ± 1.0% and significantly enhanced the TCOD removal from 26.0 ± 1.2% to 47.9 ± 0.9% in aerobic digesters with different ZVI levels (0–20 g/L). The metabolic intermediate transformation steps of solubilization, hydrolysis and catabolism processes in aerobic digestion were all revealed to be enhanced by ZVI. More importantly, the aerobic digesters with higher ZVI levels achieved higher inorganic nitrogen removal, even with higher sludge degradation for ammonium release, due to the occurrence of both chemical and biological denitrification induced by ZVI. Correspondingly, the microbial compositions in the digesters with ZVI shifted towards the direction that was conducive to sludge degradation and nitrogen removal (e.g., aerobic denitrification) compared to control. Further, the dewaterability of the aerobically digested sludge was also improved with ZVI addition, supported by the reducing capillary suction time (CST) and negative surface potential.

A mathematical model for the kinetics of concurrent volatile solids destruction and water evaporation during aerobic and anaerobic digestion of wastewater sludge under mesophilic and thermophilic temperatures

Experimental and computational approaches were employed to model kinetics of volatile solids destruction in the presence of water evaporation during biological digestion of wastewater sludge. Water evaporation occurred in both anaerobic and aerobic laboratory digesters with a much higher intensity in the latter. The water evaporation followed a linear trend over time. By inclusion of the linear equation for water evaporation rate, new kinetic models were developed to predict volatile solids (VS) and volatile suspended solids (VSS) concentrations of batch biological digesters. Modelled results using a first-order model were in good agreement with the measured VS concentrations of the anaerobic reactors operated at 35 °C and 55 °C (AnD.55) with coefficients of determination (R2) 0.92 and 0.84, respectively. VS degradation of AnD.55 was slightly more consistent with zero-order kinetics (R2 = 0.86). VS and VSS concentrations measured for batch aerobic digesters also agreed with the first-order model. R2 values were 0.91, 0.75 and 0.99 for reactors operated at 35 °C (AD.35), 45 °C and 55 °C, respectively. As for the progression in terms of VSS concentration, the highest R2 and lowest sum of squared error values belonged to AD.35. This is the first study that developed kinetic models for simultaneous VS degradation and water evaporation under both mesophilic and thermophilic conditions. The outcomes of this study indicate that water evaporation needs to be considered for accurate determination of the kinetic rate constant and solids reduction in laboratory and full-scale sludge digesters.

Calcium peroxide significantly enhances volatile solids destruction in aerobic sludge digestion through improving sludge biodegradability

This work put up a novel strategy of applying calcium peroxide (CaO2) in aerobic sludge digestion and provided insights into such system. The degradation percentage of sludge and total inorganic nitrogen production in the digesters with CaO2 at 0.02 g/g-VS-WAS increased by 25.8% and 18.8% of control. CaO2 addition allowed various key microbes related to organics degradation to accumulate in the system. Moreover, the modelling and chemical (i.e., excitation emission matrix (EEM) fluorescence and fourier transformation spectroscopy (FTIR)) analyses revealed that CaO2 addition enhanced sludge biodegradability with more release of biodegradable organics and increased degradation of recalcitrant organics, which can be transformed into biodegradable organics with the action of CaO2. Subsequent transformation test indicated that CaO2 enabled to promote hydrolysis and catabolism of biodegradable substrates in sludge. Further investigations on function mechanism suggested that CaO2 carried on positive action for sludge aerobic digestion mainly through the enhancement by ·OH.

Natural diatomite mediated continuous anaerobic sludge digestion: Performance, modelling and mechanisms

Anaerobic digestion is generally restricted by the low degradability of waste activated sludge (WAS) and slow hydrolysis rate. This work provided an innovative technique to promote continuous anaerobic digestion performance mediated by natural diatomite. In continuous tests over 125 days, 2 g/g-Total solids (TS) of natural diatomite addition increased daily methane production and volatile solids (VS) destruction by 23.3% and 23.5%, respectively, leading to a 5.6% decrease in digestate volume for disposal. Microbial community in the natural diatomite mediated digester changed toward a favorable direction for anaerobic sludge digestion. Process modelling indicated that the increased natural diatomite presence increased hydrolysis rate, biomethane potential and degradation extent of the rapidly biodegradable substrates in WAS, with the highest enhancement being approximately 78.9%, 44.5% and 45.0%, respectively, achieved at 2 g/g- TS diatomite. However, the slowly biodegradable substrates in WAS stayed relatively stable. The mechanism studies revealed that natural diatomite mitigated ammonia inhibition and enhanced electron transfer for WAS-to-methane conversion mediated by its strong adsorption capacity and conductivity. The versatility of the technology proposed is also verified by the diatomite mediated experiments using different natural diatomite from different sources. Compared to the most natural minerals, diatomite exhibited either a greater anaerobic digestion performance or a lower dosage, bringing strong benefits in economy, environment and technology.

Effect of a variable organic loading rate on process kinetics and volatile solids destruction in synthetic food waste-fed anaerobic digesters

With the increasing installation of weather-dependent renewable sources such as solar and wind power, the ability to produce electricity on demand to balance any shortfall in supply is becoming more important. Anaerobic digestion is a low-carbon energy source with the potential to be flexible to meet this need. An investigation was conducted into the response of two laboratory-scale anaerobic digesters at loading rate of 2.5 gVS L−1 day−1 over five months using a synthetic food waste as a substrate. One digester was consistently fed at the same rate, whereas the other digester was fed with periods of varying organic loading rate, from 0.1 to 7 gVS L−1 day−1, using a feed pattern derived from a record of restaurant food waste. The digester that had been fed at a variable rate showed a pronounced increase in biogas production after feed events and a 9.6% higher VS breakdown than the steady-feed digester (81% compared to 74%), with no effect on digester stability, volatile fatty acid concentration, overall biogas output or biogas quality. These findings support and encourage the use of variable-rate feeding to balance the electricity demand.

Semi-continuous anaerobic digestion of secondary sludge with free ammonia pretreatment: Focusing on volatile solids destruction, dewaterability, pathogen removal and its implications

Our previous work has reported the pretreatment of secondary sludge with free ammonia (NH3, FA) enhanced the methane production in batch biochemical methane potential tests. However, the batch biochemical methane potential test could only provide conservative results compared to continuous/semi-continuous anaerobic digestion. Also, the impacts of FA pretreatment on the key anaerobic digestion parameters, including volatile solids (VS) destruction, sludge dewaterability and pathogen removal, are still unknown. This study for the first time investigated these impacts using semi-continuous anaerobic digestion systems for 130 days. Pretreatment of secondary sludge for 24 h at an FA concentration of 560 mg NH3-N/L improved VS destruction by 26.4% (from 22.0 to 27.8%), supported by a similar increase of 28.6% in methane production (from 126.7 to 162.9 ml CH4/g VSfed). Model based analysis revealed that FA pretreatment improved the sludge degradability extent, which may be the reason for the enhanced VS destruction. Equally importantly, the dewaterability of the digested sludge with FA pretreatment was also enhanced by 9.2% (from 12.0 to 13.1% in solids content of the dewatered digested sludge), which could be partly attributed to the increased zeta potential from -16.7 to -14.5 mV. Anaerobic digestion with FA pretreatment enhanced the removals of Fecal Coliform and E. Coli by 1.3 and 1.4 log MPN/g TS (MPN: Most Probable Number; TS: Total Solids), indicating FA pretreatment was effective in enhancing pathogen removal. With inorganic solids representing 21% of the sludge used, the volume of dewatered sludge to be disposed of was reduced by 14.5% via FA pretreatment. This will substantially decrease the cost as evaluated by economic analysis. In brief, this study provides a promising strategy to enhance sludge reduction in anaerobic digestion and is of great significance in promoting the application of FA pretreatment strategy in the real world.

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.

Enhancing methane production from algae anaerobic digestion using diatomite

An innovative technology using diatomite to enhance methane production from anaerobic algae digestion was proposed in this study. The results of biochemical methane production (BMP) tests showed that 1, 2, 5 g/L of diatomite addition increased methane production by 22.5 ± 7.1%, 30.6 ± 5.1% and 32.8 ± 7.2% (p = 0.001, 0.0002, and 0.0002) compared to the control. In continuous operation experiments over 136 days, the parameters of digester containing volatile solid (VS) destruction and daily methane yield were increased by 31.2% and 32.1% with diatomite (5 g/L) addition. The addition of diatomite improved the solubilization, hydrolysis, acidification, and methanation processes in the anaerobic algae digestion, and the abundances of most key microorganisms (e.g., Methanosaeta sp. and Syntrophomonas sp.) involved in anaerobic digestion also increased correspondingly. The mechanism studies revealed that diatomite with strong conductivity could act as an electron shuttle, contributing to the increasing electron transfer efficiency for methane production. The novel strategy proposed in this study might bring significant economic and environmental benefits to algae treatment for energy recovery.

Comparative assessment of pre- and inter-stage hydrothermal treatment of municipal sludge for increased methane production.

Hydrothermal treatment (HT) is a promising technology to enhance anaerobic digestion (AD) of municipal sludge. However, the capacity of pre- and inter-stage HT (i.e., HT-AD and AD-HT-AD, respectively) to enhance the digestibility of municipal sludge has not been sufficiently explored. This study compared the efficacy of pre- and inter-stage HT performed from 90 to 185°C to enhance methane production from a mixture of primary sludge and waste activated sludge using mesophilic (35°C) biochemical methane potential tests. In both configurations, sludge solubilization increased with HT temperature. HT-AD, and to a greater extent AD-HT-AD, increased the release of ammonium nitrogen. Even though HT at 185°C dramatically increased sludge solubilization, the overall specific methane yield with HT at 185°C was lower than or comparable to that at lower HT temperatures in the HT-AD and AD-HT-AD configurations, respectively. Up to 155°C HT, the overall specific methane yield with the HT-AD configuration was higher by 4.9%-8.3% compared to the AD-HT-AD configuration. However, when the HT energy was considered, compared to the control (i.e., AD of sludge without HT), the net energy gain (ΔE) decreased as the HT temperature increased, becoming negative at an HT of 185°C. The AD-HT-AD configuration resulted in a higher overall volatile solids destruction (by 8.1 to 20.1%). In conclusion, for municipal sludge with a relatively high ultimate digestibility, as was the case in this study, HT-AD is preferable as it has a smaller footprint and is easier to operate than the AD-HT-AD configuration. However, given the significantly higher volatile solids destruction in the AD-HT-AD configuration, compared to the HT-AD configuration, AD-HT-AD may be more beneficial considering post-AD sludge handling processes. PRACTITIONER POINTS: Hydrothermal treatment (HT) increased the rate and extent of methane production from municipal sludge mixture. 155°C was the optimal temperature for either pre- or inter-stage HT to increase biogas production. Pre- and inter-stage HT resulted in comparable ultimate methane production. Pre-stage HT is preferable to inter-stage HT (smaller footprint, easier to operate). AD-HT-AD resulted in significantly higher volatile solids destruction compared to the HT-AD configuration.

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.