Long solid retention time (SRT) has minor role in promoting methane production in a 65°C single-stage anaerobic sludge digester

The mechanisms of pH regulation on promoting volatile fatty acids production from kitchen waste

The anaerobic acid production experiments were conducted with the pretreated kitchen waste under pH adjustment. The results showed that pH 8 was considered to be the most suitable condition for acid production, especially for the formation of acetic acid and propionic acid. The average value of total volatile fatty acid at pH 8 was 8814 mg COD/L, 1.5 times of that under blank condition. The average yield of acetic acid and propionic acid was 3302 mg COD/L and 2891 mg COD/L, respectively. The activities of key functional enzymes such as phosphotransacetylase, acetokinase, oxaloacetate transcarboxylase and succinyl-coA transferase were all enhanced. To further explore the regulatory mechanisms within the system, the distribution of microorganisms at different levels in the fermentation system was obtained by microbial sequencing, results indicating that the relative abundances of Clostridiales, Bacteroidales, Chloroflexi, Clostridium, Bacteroidetes and Propionibacteriales, which were great contributors for the hydrolysis and acidification, increased rapidly at pH 8 compared with the blank group. Besides, the proportion of genes encoding key enzymes was generally increased, which further verified the mechanism of hydrolytic acidification and acetic acid production of organic matter under pH regulation.

Characterizing properties and environmental behaviors of organic matter in sludge using liquid chromatography organic carbon detection and organic nitrogen detection: A mini-review

The presence of organic matter in sludge plays a significant role in sludge dewatering, anaerobic sludge digestion, resource (i.e., protein) recovery and pollutants removal (i.e., heavy metals) from sludge, as well as post-application of sludge liquid and solid digestate. This study summarized the current knowledge on using liquid chromatography organic carbon detection and organic nitrogen detection (LC-OCD-OND) for characterization and quantification of organic matter in sludge samples related with sludge treatment processes by fractionating organic matter into biopolymers, building blocks, humic substances, low molecular weight (LMW) acids, low LMW neutrals, and inorganic colloids. In addition, the fate, interaction, removal, and degradation of these fractions in different sludge treatment processes were summarized. A standardized extraction procedure for organic components in different extracellular polymeric substances (EPS) layers prior to the LC-OCD-OND analysis is highly recommended for future studies. The analysis of humic substances using the LC-OCD-OND analysis in sludge samples should be carefully conducted. In conclusion, this study not only provides a theoretical foundation and technical guidance for future experiments and practices in characterizing sludge organic matter using LC-OCD-OND, but also serves as a valuable resource for consulting engineers and other professionals involved in sludge treatment.

Insights into the management of food waste in developing countries: with special reference to India

Up to one third of the food that is purposely grown for human sustenance is wasted and never consumed, with adverse consequences for the environment and socio-economic aspects. In India, managing food waste is a significant environmental concern. Food waste output is increasing in Indian cities and towns as a result of the country's urban expansion, modernization, and population growth. Poor management of food waste can have negative consequences for the environment and pose a risk to the public's health issues. This review focuses on the current challenges, management strategies, and future perspectives of food waste management in India. The efficient management of food waste involves a comprehensive study regarding the characterization of food waste and improved waste management methods. In addition, the government policies and rules for managing food waste that is in effect in India are covered in this review.

Enhanced methanogenic degradation and membrane fouling associated with protein-EPS by extending sludge retention time in a high-solid anaerobic membrane bioreactor treating concentrated organic sludge

The improvement of organic sludge destruction efficiency and methanogenic performance is a key concern during anaerobic digestion toward maximum energy recovery. In this study, a high-solid anaerobic membrane bioreactor (AnMBR) was operated continuously for the treatment of organic sludge from Japanese small-scale collective wastewater treatment facility (Johkasou), and digestion efficiency was enhanced by the optimizing solid retention time (SRT). Degradation efficiency of the substrate improved from 36 % to 52 % and the biogas yield was enhanced from 0.37 to 0.51 L/g-VSfed when the SRT was extended from 30 to 60 d. The net energy yield of AnMBR at SRT 60 days was 9.83 kJ/g-VSfed, and the corresponding energy sufficiency ratio was 181 %, indicating that SRT extension could enhance substrate destruction with significant energy recovery potential. However, a long SRT is characterized by high mixed liquor total solids (MLTS), small particle size, high extracellular polymeric substances content, and poor filterability, which exert detrimental effects on membrane operation. Membrane fouling was effectively controlled by regulating the flux at a sustainable rate. The low fouling region and transition region of operating flux were determined as 0.21-4.6 L/m2/h (LMH) and 1.5-5.7 LMH, respectively, when MLTS was 25-50 g/L, and the main contributors to membrane fouling were high protein fractions and small sludge flocs. The current study proposes a promising method to promote digestion efficiency and provided adequate guidance for membrane operation at super-high MLTS by presenting practical engineering applications of AnMBRs in solid waste treatment.

Promoting short-chain fatty acids production from sewage sludge via acidogenic fermentation: Optimized operation factors and iron-based persulfate activation system

Promoting short-chain fatty acids (SCFAs) production and ensuring the stability of SCFAs-producing process are becoming the two major issues for popularizing the acidogenic fermentation (AF). The key controlling operating and influencing factors during anaerobic fermentation process were thoroughly reviewed to facilitate better process performance prediction and to optimize the process control of SCFAs promotion. The wide utilization of iron salt flocculants during wastewater treatment could result in iron accumulating in sewage sludge which influenced AF performance. Additionally, appropriate ferric chloride (FC) could promote the SCFAs accumulation, while poly ferric sulfate (PFS) inhibited the bioprocess. Iron/persulfate (PS) system was proved to effectively enhance the SCFAs production while mechanism analysis revealed that the strong oxidizing radicals remarkably enhanced the solubilization and hydrolysis. Moreover, the changes of oxidation-reduction potential (ORP) and pH caused by iron/PS system exhibited more negative effects on the methanogens, comparing to the acidogenic bacteria. Furthermore, performance and mechanisms of different iron species-activating PS, organic chelating agents and iron-rich biochar derived from sewage sludge were also elucidated to extend and strengthen understanding of the iron/PS system for enhancing SCFAs production. Considering the large amount of generated Fe-sludge and the multiple benefits of iron activating PS system, carbon neutral wastewater treatment plants (WWTPs) were proposed with Fe-sludge as a promising recycling composite to improve AF performance. It is expected that this review can deepen the knowledge of optimizing AF process and improving the iron/PS system for enhancing SCFAs production and provide useful insights to researchers in this field.

The r/K selection theory and its application in biological wastewater treatment processes

Understanding the characteristics of functional organisms is the key to managing and updating biological processes for wastewater treatment. This review, for the first time, systematically characterized two typical types of strategists in wastewater treatment ecosystems via the r/K selection theory and provided novel strategies for selectively enriching microbial community. Functional organisms involved in nitrification (e.g., Nitrosomonas and Nitrosococcus), anammox (Candidatus Brocadia), and methanogenesis (Methanosarcinaceae) are identified as r-strategists with fast growth capacities and low substrate affinities. These r-strategists can achieve high pollutant removal loading rates. On the other hand, other organisms such as Nitrosospira spp., Candidatus Kuenenia, and Methanosaetaceae, are characterized as K-strategists with slow growth rates but high substrate affinities, which can decrease the pollutant concentration to low levels. More importantly, K-strategists may play crucial roles in the biodegradation of recalcitrant organic pollutants. The food-to-microorganism ratio, mass transfer, cell size, and biomass morphology are the key factors determining the selection of r-/K-strategists. These factors can be related with operating parameters (e.g., solids and hydraulic retention time), biomass morphology (biofilm or granules), and operating modes (continuous-flow or sequencing batch), etc., to achieve the efficient acclimation of targeted r-/K-strategists. For practical applications, the concept of substrate flux was put forward to further benefit the selective enrichment of r-/K-strategists, fulfilling effective management and improvement of engineered pollution control bioprocesses. Finally, the future perspectives regarding the development of the r/K selection theory in wastewater treatment processes were discussed.

Calcium hypochlorite pretreatment improves thermophilic digestion of waste activated sludge in an upflow anaerobic sludge blanket reactor

Anaerobic wasted activated sludge (WAS) digestion has been widely applied to reduce sludge volume and generate bioenergy in the form of methane. However, anaerobic WAS digestion performance is often challenged with poor hydrolysis of biomass cellular structures. In the present study, the feasibility of using calcium hypochlorite (Ca(ClO)₂) to improve the thermophilic digestion of WAS was studied. Two thermophilic upflow anaerobic sludge bed (UASB) reactors (one with and one without Ca(ClO)₂ pretreatment) were operated for 120 days under low and high organic loading rate (OLR) conditions, corresponding hydraulic retention time (HRT) of 10 days and 6 days, respectively. Both reactors achieved satisfied performance during the studied period. Under the low OLR condition, Ca(ClO)₂ pretreatment significantly improved WAS total volatile solids (VS) removal efficiency (from 48.06 ± 2.63% to 57.34 ± 3.54%) and methane yield (from 289.2 ± 27.6 to 362.2 ± 36.7 N mL/g VS). However, no significant improvement was observed under the high OLR condition. g_S1 and g_Fervidobacterium were predominant bacteria in the thermophilic UASB reactor fed with Ca(ClO)₂ pretreated WAS. Methanosarcina was dominant archaea in both reactors. The treatment mechanism and application potential of using Ca(ClO)₂ to enhance the WAS digestibility were further discussed.

Distribution patterns of functional microbial community in anaerobic digesters under different operational circumstances: A review

Anaerobic digestion (AD) processes are promising to effectively recover resources from organic wastes or wastewater. As a microbial-driven process, the functional anaerobic species played critical roles in AD. However, the lack of effective understanding of the correlations of varying microbial communities with different operational factors hinders the microbial regulation to improve the AD performance. In this paper, the main anaerobic functional microorganisms involved in different stages of AD processes were first demonstrated. Then, the response of anaerobic microbial community to different operating parameters, exogenous interfering substances and digestion substrates, as well as the digestion efficiency, were discussed. Finally, the research gaps and future directions on the understanding of functional microorganisms in AD were proposed. This review provides insightful knowledge of distribution patterns of functional microbial community in anaerobic digesters, and gives critical guidance to regulate and enrich specific functional microorganisms to accumulate certain AD products.

Predicting the performance of anaerobic digestion using machine learning algorithms and genomic data

Modeling of anaerobic digestion (AD) is crucial to better understand the process dynamics and to improve the digester performance. This is an essential yet difficult task due to the complex and unknown interactions within the system. The application of well-developed data mining technologies, such as machine learning (ML) and microbial gene sequencing techniques are promising in overcoming these challenges. In this study, we investigated the feasibility of 6 ML algorithms using genomic data and their corresponding operational parameters from 8 research groups to predict methane yield. For classification models, random forest (RF) achieved accuracies of 0.77 using operational parameters alone and 0.78 using genomic data at the bacterial phylum level alone. The combination of operational parameters and genomic data improved the prediction accuracy to 0.82 (p<0.05). For regression models, a low root mean square error of 0.04 (relative root mean square error =8.6%) was acquired by neural network using genomic data at the bacterial phylum level alone. Feature importance analysis by RF suggested that Chloroflexi, Actinobacteria, Proteobacteria, Fibrobacteres, and Spirochaeta were the top 5 most important phyla although their relative abundances were ranging only from 0.1% to 3.1%. The important features identified could provide guidance for early warning and proactive management of microbial communities. This study demonstrated the promising application of ML techniques for predicting and controlling AD performance.

An anaerobic dynamic membrane bioreactor for enhancing sludge digestion: Impact of solids retention time on digestion efficacy

An anaerobic dynamic membrane bioreactor (AnDMBR), which enabled the decoupling of hydraulic retention time (HRT) and solids retention time (SRT), was used for enhancing sludge digestion, with the associated mechanisms elucidated. With the increase of SRT, the biogas production and sludge reduction rate were both enhanced. The specific biogas production and volatile solids (VS) reduction rate were improved to 0.79 L/g VS and 55.9% under SRT 50 d, respectively. Microbial community analysis revealed that Chloroflexi, which is capable of degrading metabolites and dead cells, was enriched at longer SRT. Further analysis showed that both acetoclastic and hydrogenotrophic methanogenesis contributed to the enhanced biogas production under higher SRT compared to the dominance of acetoclastic methanogenesis under lower SRT. The enhanced utilization of organic matter and acetate at longer SRT further confirmed the mechanisms. The results highlighted the potential of AnDMBR for high-efficient sludge digestion.

Decentralized energy from portable biogas digesters using domestic kitchen waste: A review

Anaerobic digestion is one of the main waste-to-energy technologies in reducing the volume of biodegradable waste into energy-rich biogas. Recent studies have revealed that kitchen wastes as a feedstock possess great potential in energy production and anaerobic digestion proved to be a promising technology among different kitchen waste management techniques such as incineration, pyrolysis, gasification, landfills, composting, etc. To anaerobically treat feedstock, an airtight enclosed container commonly known as biodigester will be employed. To suffice the energy requirement for cooking in the rural areas and recently even in the urban areas, a small-scale biogas unit commonly referred to as portable type biodigester is blooming as an attractive alternative for the production of biogas domestically. Hence, this review emphasizes on anaerobic digestion of kitchen wastes and the design of portable type biodigester. The present review provides an overview of different kitchen waste management techniques. The paper also discusses the different types of biomass feedstock and provides a generalized procedure for the design of a portable biogas unit. This study confirms that the systematic design of biogas units and proper feeding of kitchen waste offers an advantage of effective utilization of wastes in the production of decentralized energy.

Two-Stage anaerobic digestion in agroindustrial waste treatment: A review

The anaerobic digestion is a process widely recognized as an interesting alternative for the treatment and stabilization of residual organic substrates. However, several technical limitations were observed based on the characteristics of the organic matter submitted to the process, such as the presence of high concentrations of soluble sugars or fats. The technology of anaerobic digestion in multiple stages is described as a viable option in the control of variables, optimizing the environmental conditions of the main microorganisms involved in the process, assuring high solid removal and methane production, besides allowing a higher energy yield through the generation of molecular fuel hydrogen. Several studies reviewed the process of anaerobic digestion in multiple stages in the treatment of food waste, although few report its use applied directly to agroindustrial residues. Thus, the present work aims to review the literature evaluating the scenario and viability of the multi-stage anaerobic digestion process applied to agroindustrial effluents. Effluents such as manipueira, vinasse, and dairy wastewater are substrates that present high yields when treated by AD processes with stage separation. The high concentration of easily fermentable sugars results in a high production of molecular hydrogen (co-product of the production of volatile acids in the acid phase) and methane (methanogenic phase). The great challenges related to the development of the sector are focused on the stability of the composition and yield of hydrogen in the acid phase, besides the problems resulting from the treatment of complex residues. Thus, the present study suggests that future works should focus on the technologies of new microorganisms and optimization of process parameters, providing maturation and scale-up of the two-stage anaerobic digestion technique.

Deep insights into the network of acetate metabolism in anaerobic digestion: focusing on syntrophic acetate oxidation and homoacetogenesis

Acetate is a pivotal intermediate product during anaerobic decomposition of organic matter. Its generation and consumption network is quite complex, which almost covers the most steps in anaerobic digestion (AD) process. Besides acidogenesis, acetogenesis and methanogenesis, syntrophic acetate oxidation (SAO) replaced acetoclastic methanogenesis to release the inhibition of AD at some special conditions, and the importance of considering homoacetogenesis had also been proved when analysing anaerobic fermentations. Syntrophic acetate-oxidizing bacteria (SAOB), with function of SAO, can survive under high temperature and ammonia/ volatile fatty acids (VFAs) concentrations, while, homoacetogens, performed homoacetogenesis, are more active under acidic, alkaline and low temperature (10°C-20°C) conditions, This review summarized the roles of SAO and homoacetogenesis in AD process, which contains the biochemical reactions, metabolism pathways, physiological characteristics and energy conservation of functional bacteria. The specific roles of these two processes in the subprocess of AD (i.e., acidogenesis, acetogenesis and methanogenesis) were also analyzed in detail. A two phases anaerobic digester is proposed for protein-rich waste(water) treatment by enhancing the functions of homoacetogens and SAOB compared to the traditional two-phases anaerobic digesters, in which the first phase is fermentation phase including acidogens and homoacetogens for acetate production, and second phase is a mixed culture coupling syntrophic fatty acids bacteria, SAOB and hydrogenotrophic methanogens for methane production. This review provides a new insight into the network on production and consumption of acetate in AD process.

A novel strategy for improving volatile fatty acid purity, phosphorus removal efficiency, and fermented sludge dewaterability during waste activated sludge fermentation

Volatile fatty acids (VFAs) from waste activated sludge (WAS) via alkaline fermentation have been shown to provide an effective alternative carbon source for biological nutrient removal in wastewater treatment plants (WWTPs) that promotes the subsequent release of phosphorus (P) and refractory dissolved organic matter. The dewatering ability of fermented sludge is known to decrease during alkaline fermentation. Here, a novel strategy of initiating fermentation at a pH of 10 was developed to improve VFA purity, P removal efficiency, and fermented sludge dewaterability during WAS fermentation. Although VFAs concentration was lower (1.69 ± 0.09 g COD/L) when the pH was only initially adjusted to pH 10 (R_IA) relative to when the pH was maintained at 10 on a daily basis (R_DC), the purity of VFAs in the fermented liquid was improved (58.48%). Furthermore, the release of total phosphorous (TP) in R_IA was 5.90 times lower than that in R_DC (139.37 mg/L). The normalized capillary suction time and specific resistance to filtration in R_IA decreased to 42.23% and 40.70%, respectively, suggesting that the dewaterability of fermented sludge also improved. The amount of alkali needed was 17.44 kg for each ton of total solid (TS) in R_IA, which was 5.49 times lower than that in R_DC. Thus, approximately 45.44 USD was saved in operational costs for each ton of TS processed in R_IA. These results indicated that VFAs production via initial pH 10 fermentation was a robust and cost-efficient way for providing carbon resources in WWTPs.

Changes in the characteristics of dissolved organic matter during sludge treatment: A critical review

Dissolved organic matter (DOM) of sludge is a heterogeneous mixture of high to low molecular weight organic substances which is including proteinaceous compounds, carbohydrates, humic substances, lipids, lignins, organic acids, organic micropollutants and other biological derived substances generated during wastewater treatment. This paper reviews definition, composition, quantification, and transformation of DOM during different sludge treatments, and the complex interplay of DOM with microbial communities. In anaerobic digestion, anaerobic digestion-refractory organic matter, particularly compounds showing polycyclic steroid-like, alkane and aromatic structures can be generated after pretreatment. During dewatering, the DOM fraction of low molecular weight proteins (< 20,000 Dalton) is the key parameter deteriorating sludge dewaterability. During composting, decomposition and polymerization of DOM occur, followed by the formation of humic substances. During landfill treatment, the composition of DOM, particularly humic substances, are related with leachate quality. Finally, suggestions are proposed for a better understanding of the transformation and degradation of DOM during sludge treatment. Future work in sludge studies needs the establishment and implementation of definitions for sample handling and the standardization of DOM methods for analysis, including sample preparation and fractionation, and data integration. A more detailed knowledge of DOM in sludge facilitates the operation and optimization of sludge treatment technologies.

Optimization of Bioslurry-Available Plant Nutrients Using T. brownii and Acanthaceae spp. Biocatalysts

The plant extracts of T. brownii and Acanthaceae spp. have been used as biocatalysts by several communities in Kenya to hasten anaerobic digestion. This study aimed at assessing the viability of these two extracts in hastening the availability of plant nutrients from bioslurry at ambient conditions. A controlled research design was followed using uncooked kitchen waste as the substrate for 28 retention days. Changes in bioslurry physicochemical properties and available plant nutrients were monitored every 7 days using wet chemistry and spectroscopic methods. The findings indicated that the two extracts significantly impacted the levels of available plant nutrients in the bioslurry compared to the control samples. T. brownii additives significantly increased the levels of lime content, total Kjeldahl nitrogen, total phosphorus, phosphoric acid, sulfur, and soluble silicic acid. On the contrary, Acanthaceae spp. additives significantly increased the levels of calcium, potassium, nitrates, total ammoniacal nitrogen, sulfates, and phosphates in the bioslurry samples. The use of these plant extracts thus reduces the time taken while increasing the concentration of available plant nutrients from bioslurry.

The Challenges of a Biodiesel Implementation Program in Malaysia

The palm biodiesel industry is facing many challenges implementing biodiesel program in Malaysia. This paper addresses the importance of the B10 blend (10% biodiesel, 90% petroleum diesel), global challenges of palm oil import and export, and protective measures for continuous positive growth of the palm oil sector. Palm oil is the backbone of Malaysia’s economy, covering more than 5% of its gross domestic product (GDP). The key steps taken by the Malaysian government for the successful implementation of the B10 program are discussed in this review study. Till now, B5 and B7 biodiesel programs have been successfully implemented in Malaysia. The B10 biodiesel program is attractive because of the developed local palm oil sector. The B10 biodiesel program will increase the use of renewable energy sources, and is expected to increase the productivity of palm oil and biodiesel implementation in the country. Despite successful B5 and B7 programs, Malaysia is facing challenges for the implementation of biodiesel due to fluctuation in crude palm oil prices, low domestic usage of palm oil, and vehicle warranty. The improvement of palm oil and promotion of B10 through targeted agencies in the central region of Malaysia will help to implement the biodiesel program successfully.

Genome-centric microbiome analysis reveals solid retention time (SRT)-shaped species interactions and niche differentiation in food waste and sludge co-digesters

Co-digestion of food waste with sewage sludge is widely applied for waste stabilization and energy recovery around the world. However, the effect of solid retention time (SRT) on the microbial population dynamics, metabolism and interspecies interaction have not been fully elucidated. Here, the influence of SRTs (5-25 days) on the performance of the co-digestion system was investigated and state-of-the-art genome-centric metagenomic analysis was employed to uncover the dynamics and metabolic network of the key players underlying the well-functioned and poorly-functioned co-digestion microbial communities. The results of the microbial analyses indicated that SRT largely shaped microbial community structure by enriching the syntrophic specialist Syntrophomonas and CO₂/H₂ ( formate)-using methanogen Methanocorpusculum in the well-functioned co-digester operated at SRT of 25 days, while selecting acid-tolerant populations Lactobacillus at SRT of 5 days. The metagenome assembled genomes (MAGs) of key players, such as Syntrophomonadaceae, Methanocorpusculum, and Mesotoga, were retrieved, additionally, the syntrophic acetate oxidation plus hydrogenotrophic methanogenesis (SAO-HM) were proposed as the dominant pathway for methane production. The metabolic interaction in the co-digestion microbial consortia was profiled by assigning MAGs into functional guilds. Functional redundancy was found in the bacterial groups in hydrolysis step, and the members in these groups reduced the direct competition by niche differentiation.

The performance evaluation and kinetics response of advanced anaerobic digestion for sewage sludge under different SRT during semi-continuous operation

Sludge retention time (SRT) is vital for advanced anaerobic digestion (AD) to realize energy self-sufficient. However, the criteria on reasonable SRT has not been fully understood. This study investigated the performance and kinetics response of AD under different SRT in semi-continuous AD with microwave (MW) pretreatment, according to the long-term operation and methane production during one feeding interval. Results showed that modified Gompertz model better described the kinetics than first-order model. At short SRT (15 d), pretreatment coupled with two-stage AD preserved methane production with the high attainable methane potential (B₀) of 257.98 mL/g VS and hydrolysis rate constant (k_hyd) of 0.075 h^-1. But the acceptable decrease of methane production rate seems to be unavoidable, which was possibly derived from the evolution of methanogenesis pathway. This study emphasized the importance of improved methane production rate in semi-continuous AD under short SRT rather than methane production potential obtained from batch experiment.

Enhancement of syntrophic acetate oxidation pathway via single walled carbon nanotubes addition under high acetate concentration and thermophilic condition

The effect of single walled carbon nanotubes (SWCNT) on methane production under high acetate concentration and thermophilic condition was evaluated. An isotope labeling experiment verified that >85% of methane was generated from syntrophic acetate oxidation (SAO) at 50, 100 and 150 mM acetate and almost 100% at 200 mM. SWCNT addition had little effect on the methanogenesis pathway, whereas it accelerated methane production via decreasing lag phase times and increasing maximum methane production rates. Electrochemical impedance spectroscopy (EIS) results revealed the electrical resistivity of sludge in groups of SWCNT was distinctly smaller than CK groups, indicating higher sludge conductivity was achieved. Further, the results of communities described that Coprothermobacter and Thermacetogenium played the most important role in SAO under all conditions. Meanwhile, the enriched Thermacetogenium and direct interspecies electron transfer (DIET) pathway in SAO consortia contributed to the acceleration of methane production via SWCNT addition.

Effects of individual volatile fatty acids (VFAs) on phosphorus recovery by magnesium ammonium phosphate

Volatile fatty acids (VFAs) are a major component of dissolved organic matter in alkaline fermentation supernatants. In this study, effects of different VFAs (acetate, propionate, and butyrate) on phosphorus recovery, as magnesium ammonium phosphate (MgNH₄PO₄·6H₂O, or MAP), were studied. Results showed that optimal pH was 9.5 and MAP purity was ∼70% in VFA-free wastewater. With VFA addition, MAP purities of precipitates were higher, reaching 75%-85%. The crystalline characterization of precipitates suggested that VFAs had a weak complexation ability with Mg²⁺ and NH₄⁺. Further, pH changes during the MAP crystallization process were monitored and results indicated VFAs only contributed to the alkalinity condition, which, in turn, improved the MAP crystallization process. These data provide for a better understanding of P recovery by MAP precipitates from VFA-rich fermented supernatants.

Performance and microbial community of hydrogenotrophic methanogenesis under thermophilic and extreme-thermophilic conditions

In this study, hydrogenotrophic methanogenesis with respect to methanogenic activity and microbial structures under extreme-thermophilic conditions were examined, and compared with the conventional thermophilic condition. The hydrogenotrophic methanogens were successfully acclimated to the temperatures of 55, 65 and 70 °C. Although acclimation was slower at 65 and 70 °C, hydrogenotrophic methanogenesis remained fairly stable. High-throughput sequencing using 16S rRNA analysis showed that the higher temperatures resulted in single archaea community dominated by hydrogenotrophic Methanothermobacter. Moreover, the syntrophic bacteria changed from Coprothermobacter and Thermodesulfovibrio at 55 °C to Thermodesulfovibrio at 70 °C. Specific hydrogenotrophic methanogenic rate at 70 °C was 98.6 ± 4.2 Nml CH₄/g VS/hr, which was over 4-folds higher than that 8at 55 °C. The lag phase under extreme-thermophilic conditions was longer than thermophilic condition, which was probably due to the archaeal structure with low diversity. Extreme-thermophilic condition resulted in a shift in methanogenesis pathway from acetoclastic methanogenesis to hydrogenotrophic methanogenesis with the enrichment of Methanothermobacter thermautotrophicus.

Hydrogen production from a thermophilic alkaline waste activated sludge fermenter: Effects of solid retention time (SRT)

This study aims to investigate the effects of solid retention times (SRTs) on hydrogen production via thermophilic alkaline fermentation of waste activated sludge. The reactor was subjected to a SRT from 10 to 6 days during approximately 82 days of operation. The results revealed that SRT had minor influence on hydrolysis and hydrolysis efficiency in different phases were from 48.11% to 50.55%. Nevertheless, the efficiency of acidogenesis process was highly related to SRT and longer SRT could enhance the acidogenesis. On the other hand, acidogenesis efficiency was also related to H₂ partial pressure and high H₂ partial pressure negatively affected the acidogenesis. Thus, the maximum acidification was achieved in phase 1 (21.29%) resulting in the maximum H₂ yield in phase 1 (95.94 mL/g VSS; SRT = 10 days; H₂ partial pressure = 0-18%). Phyla Actinobacteria and Proteobacteria, who are highly related to hydrolytic microbial population, were abundant in all phases that resulted in high hydrolysis extent. H₂ production was attributed to the relative high abundance of Clostridia. Thus, this study suggested that longer SRT and lower H₂ partial pressure was necessary to improve the H₂ yield under alkaline pH condition.