Stepwise alkaline treatment coupled with ammonia stripping to enhance short-chain fatty acids production from waste activated sludge

A review on waste activated sludge pretreatment for improved volatile fatty acids production and their upcycling into polyhydroxyalkanoates

Waste activated sludge (WAS), a byproduct of wastewater treatment (WWTPs) facilities is challenging to manage because of its high organic content. Most of WAS is managed via anaerobic digestion (AD) to produce biogas, which is not deemed economically viable. The AD of WAS into volatile fatty acids (VFA) and their subsequent upcycling into polyhydroxyalkanoates (PHA) is gaining popularity due to their high value and uses. However, the fundamental issue with WAS is its low solubility, and pretreatment is required to increase it. Pretreatment disintegrates sludge floc and enhances its solubility, supports acetogens, and inhibits methanogens, leading to increased VFA synthesis in the AD process. The key factors influencing VFA yield include the size of the sludge granules, the mixing rate, and the presence of resistant organic components. Fermented broth containing VFA from AD can be utilized directly as a feedstock for microbial fermentation to produce PHA using both pure as well as mixed cultures. Utilisation of mixed cultures is useful since they are robust, able to consume a wide range of substrates, and do not require sterility. In addition, the VFA, which is made up of various organic acids, impacts the structure, productivity, characteristics, and type of PHA produced by microbial communities. Considering the importance of WAS management through VFA production and its integration with PHA production process this review article discusses the WAS pretreatment strategies, various factors that influence the AD process, trends in VFA to PHA production technologies with challenges, and possible solutions for integrated process development.

Performance of four thermophilic bacteria for primary sludge hydrolysis: Sludge disintegration and hydrolase activities

Thermophilic bacteria (TB) pretreatment is an efficient and environmentally friendly way for accelerating sludge hydrolysis. In this study, a complete comparison of the hydrolysis performance of Bacillus sp. AT07-1 (X1), Parageobacillus toebii X2 (X2), Geobacillus kaustophilus X3 (X3) and Parageobacillus toebii R-35642 (X4) was performed. Results indicated that pretreatment with four strains promoted the release of organic matter in extracellular polymeric substance and the disintegration of sludge structure, causing the increase of soluble substances. The total percent fluorescence response of tyrosine-like and soluble microbial by-products in dissolved organic matter increased to 64.8% after pretreatment with strain X4. Moreover, pretreatment with strain X4 resulted in the highest relative activities of α-glucosidase (1.4) and protease (2.0). Engineering implication and economic analysis verified that TB pretreatment has the potential for economic benefits and industrial applications. This study demonstrated that strain X4 exhibited the highest hydrolysis efficiency, providing a new strategy for accelerating primary sludge hydrolysis.

Role of phosphate in microalgal-bacterial symbiosis system treating wastewater containing heavy metals

Phosphorus is one of the important factors to successfully establish the microalgal-bacterial symbiosis (MABS) system. The migration and transformation of phosphorus can occur in various ways, and the effects of phosphate on the MABS system facing environmental impacts like heavy metal stress are often ignored. This study investigated the roles of phosphate on the response of the MABS system to zinc ion (Zn2+). The results showed that the pollutant removal effect in the MABS system was significantly reduced, and microbial growth and activity were inhibited with the presence of Zn2+. When phosphate and Zn2+ coexisted, the inhibition effects of pollutants removal and microbial growth rate were mitigated compared to that of only with the presence of Zn2+, with the increasing rates of 28.3% for total nitrogen removal, 48.9% for chemical oxygen demand removal, 78.3% for chlorophyll-a concentration, and 13.3% for volatile suspended solids concentration. When phosphate was subsequently supplemented in the MABS system after adding Zn2+, both pollutants removal efficiency and microbial growth and activity were not recovered. Thus, the inhibition effect of Zn2+ on the MABS system was irreversible. Further analysis showed that Zn2+ preferentially combined with phosphate could form chemical precipitate, which reduced the fixation of MABS system for Zn2+ through extracellular adsorption and intracellular uptake. Under Zn2+ stress, the succession of microbial communities occurred, and Parachlorella was more tolerant to Zn2+. This study revealed the comprehensive response mechanism of the co-effects of phosphate and Zn2+ on the MABS system, and provided some insights for the MABS system treating wastewater containing heavy metals, as well as migration and transformation of heavy metals in aquatic ecosystems.

Insights into the roles and mechanisms of a green-prepared magnetic biochar in anaerobic digestion of waste activated sludge

Methane is one of the most promising renewable energies to alleviate energy crisis or replace fossil fuels, which can be recovered from anaerobic digestion of bio-wastes. However, the engineering application of anaerobic digestion is always hindered by low methane yield and production rate. This study revealed the roles and mechanisms of a green-prepared magnetic biochar (MBC) in promoting methane production performance from waste activated sludge. Results showed that the methane yield reached 208.7 mL/g volatile suspended solids with MBC additive dosage of 1 g/L, increasing by 22.1 % compared to that in control. Mechanism analysis demonstrated that MBC could promote hydrolysis, acidification, and methanogenesis stages. This was because the properties of biochar were upgraded by loading nano-magnetite, such as specific surface area, surface active sites, and surface functional groups, which made MBC have greater potential to mediate electron transfer. Correspondingly, the activity of α-glucosidase and protease respectively increased by 41.7 % and 50.0 %, and then the hydrolysis performances of polysaccharides and proteins were improved. Also, MBC improved the secretion of electroactive substances like humic substances and cytochrome C, which could promote extracellular electron transfer. Furthermore, Clostridium and Methanosarcina, as well-known electroactive microbes, were selectively enriched. The direct interspecies electron transfer between them was established via MBC. This study provided some scientific evidences to comprehensively understand the roles of MBC in anaerobic digestion, with important implications for achieving resource recovery and sludge stabilization.

Insights into roles of triclosan in microalgal-bacterial symbiosis system treating wastewater

Triclosan (TCS) is an antimicrobial agent and frequently detected in wastewater or water body. This study investigated the role of TCS in microalgal-bacterial symbiosis (MABS) system treating wastewater. The results showed that the removal efficiencies of NH₄⁺-N, total nitrogen, and total phosphorus decreased under increased TCS stress, with decrease ratios of 32.0%, 28.9%, and 46.1%. The activities of microalgae were more affected than that of bacteria. The secretion of extracellular polymeric substances (EPSs) and activity of superoxide dismutase firstly increased and then decreased with aggravated TCS stress, while the accumulation of malondialdehyde increased, leading to increased permeability of cytomembrane and bioaccumulation of TCS. In addition, the aggregation properties of microalgae and bacteria were enhanced with TCS loading increasing, and the migration of TCS was affected by enhanced EPSs secretions and MABS aggregates. This work may provide some new insights into the roles of TCS in MABS system.

EDTA-enhanced alkaline anaerobic fermentation of landfill leachate-derived waste activated sludge for short-chain fatty acids production: Metals chelation and EPSs destruction

Alkaline anaerobic fermentation (AAF) of waste activated sludge (WAS) has been demonstrated to be promising for short-chain fatty acids (SCFAs) recovery. However, high-strength metals and EPSs in the landfill leachate-derived WAS (LL-WAS) would stabilize its structure, suppressing AAF performance. To improve sludge solubilization and SCFAs production, AAF was coupled with EDTA addition for LL-WAS treatment. The results show that sludge solubilization at AAF-EDTA was promoted by 62.8% than AAF, releasing 21.8% more soluble COD. The maximal SCFAs production of 477.4 mg COD/g VSS was thus achieved, i.e., 1.21 and 6.13 times those at AAF and the control, respectively. SCFAs composition was also improved with more acetic and propionic acids (80.8% versus 64.3%). Metals bridging EPSs were chelated by EDTA, which significantly dissolved metals from sludge matrix (e.g., 23.28 times higher soluble Ca than AAF). EPSs tightly bound with microbial cells were thus destructed (e.g., 4.72 times more protein release than alkaline treatment), causing an easier sludge disruption and subsequently a higher SCFAs production by hydroxide ions. These findings suggest an effective EDTA-supported AAF for metals and EPSs-rich WAS to recover carbon source.

Application and improvement methods of sludge alkaline fermentation liquid as a carbon source for biological nutrient removal: A review

Alkaline fermentation can reduce the amount of waste activated sludge and prepare sludge alkaline fermentation liquid (SAFL) rich in short-chain fatty acids (SCFAs), which can be used as a high-quality carbon source for the biological nutrient removal (BNR) process. This review compiles the production method of SAFL and the progress of its application as a BNR carbon source. Compared with traditional carbon sources, SAFL has the advantages of higher efficiency and economy, and different operating conditions can influence the yield and structure of SCFAs in SAFL. SAFL can significantly improve the nutrient removal efficiency of the BNR process. Taking SAFL as the internal carbon source of BNR can simultaneously solve the problem of carbon source shortage and sludge treatment difficulties in wastewater treatment plants, and further reduce the operating cost. However, the alkaline fermentation process results in many refractory organics, ammonia and phosphate in SAFL, which reduces the availability of SAFL as a carbon source. Purifying SCFAs by removing nitrogen and phosphorus, directly extracting SCFAs, or increasing the amount of SCFAs in SAFL by co-fermentation or combining with other pretreatment methods, etc., are effective measures to improve the availability of SAFL.

Role of extracellular polymeric substances in methane production from waste activated sludge induced by conductive materials

Conductive materials have been widely used to establish direct interspecies electron transfer (DIET) for enhancing methane production potential from anaerobic digestion of waste activated sludge (WAS). However, the roles of extracellular polymeric substances (EPSs) affected by conductive materials on anaerobic digestion have been rarely reported. This study selected four widely used conductive materials, i.e., granular active carbon (GAC), biochar (BC), zero-valent iron (ZVI), and magnetite (Mag), to reveal the roles of EPSs. Results showed that methane production potentials were increased by BC, ZVI and Mag compared to that of control, with increase ratios of 13.4 %, 22.2 %, and 12.2 %, while a decrease was observed by GAC. The contents, components and characteristics of EPSs were all affected by conductive materials. The contents of EPSs were increased by ZVI and Mag, while they were decreased by BC and GAC. The ratios between proteins and polysaccharides (PN/PS) in loosely bound EPSs (LB-EPSs) were reduced in all groups, while they were similar in tightly bound EPSs (TB-EPSs) of ZVI and Mag groups. In addition, the cytochrome C and redox properties were remarkably promoted in suspension rather than in LB- and TB-EPSs. It was found that the correlation relationships between the maximal methane production potential (Pmax) and PN/PS in EPSs were positive, as well as fluorescent substances, especially tyrosine-like and tryptophan-like substances, with R² of 0.96 and 0.98. Furthermore, the correlation relationships also existed between EPSs and microbial communities. Clostridium and Methanobacterium, potential DIET partners, presented significant positive correlation relationships (P < 0.05) with Pmax, PN/PS and fluorescent substances in EPSs. The findings may provide some new insights for mechanism investigation of anaerobic digestion induced by conductive materials.

Roles of zero-valent iron in anaerobic digestion: Mechanisms, advances and perspectives

With the rapid growth of population and urbanization, more and more bio-wastes have been produced. Considering organics contained in bio-wastes, to recover resource from bio-wastes is of great significance, which can not only achieve the resource recycle, but also protect the environment. Anaerobic digestion (AD) has been proved as one of the most promising strategies to recover bio-energy from bio-wastes, as well as to realize the reduction of bio-wastes. However, the conventional interspecies electron transfer is sensitive to environmental shocks, such as high ammonia, organic pollutants, metal ions, etc., which lead to instability or failure of AD. The recent findings have proved that the introduction of zero-valent iron (ZVI) in AD system can significantly enhance methane production from bio-wastes. This review systematically highlighted the recent advances on the roles of ZVI in AD, including underlying mechanisms of ZVI on AD, performance enhancement of AD contributed by ZVI, and impact factors of AD regulated by ZVI. Furthermore, current limitations and outlooks have been analyzed and concluded. The roles of ZVI on underlying mechanisms in AD include regulating reaction conditions, electron transfer mode and function of microbial communities. The addition of ZVI in AD can not only enhance bio-energy recovery and toxic contaminants removal from bio-wastes, but also have the potential to buffer adverse effect caused by inhibitors. Moreover, the electron transfer modes induced by ZVI include both interspecies hydrogen transfer and direct interspecies electron transfer pathways. How to comprehensively evaluate the effects of ZVI on AD and further improve the roles of ZVI in AD is urgently needed for practical application of ZVI in AD. This review aims to provide some references for the introduction of ZVI in AD for enhancing bio-energy recovery from bio-wastes.

Enhanced Methane Production from Sludge Anaerobic Digestion with the Addition of Potassium Permanganate

This work aims to reveal the effect of potassium permanganate (KMnO₄) on the sludge anaerobic digestion process, as well as the relevant mechanisms. Experimental data showed that the biomethane production was gradually increased from 159.3 ± 3.0 to 211.5 ± 5.1 mL/g VSS (volatile suspended solids) when the KMnO₄ content was increased from 0 to 0.08 g/g VSS, with an increasing rate of 32.8%. A further increase in the KMnO₄ dosage however resulted in the decline of the methane yield. First-order kinetic model analysis indicated that higher methane production potentials and hydrolysis rates were achieved in KMnO₄-added reactors than in the control. Mechanism analysis demonstrated that KMnO₄ not only efficiently disintegrated the sludge flocs, which resulted in the increased contents of dissolved organics, but also enhanced the proportion of biodegradable substances in the sludge liquor. Meanwhile, the biodegradabilities of recalcitrant humus and lignocellulose substances were found to be promoted by KMnO₄ treatment as higher methane yields were attained from KMnO₄-treated model substrates. 16S rRNA analysis illustrated that the functional microbes participated in anaerobic digestion were largely enriched in the KMnO₄-pretreated digestor. Furthermore, efficient inactivation of the fecal coliform was achieved by KMnO₄ pretreatment.

Stepwise freezing-thawing treatment promotes short-chain fatty acids production from waste activated sludge

Waste activated sludge (WAS), as the byproducts of wastewater treatment plants, has been greatly produced. With high cost and environmental risk of WAS disposal, to explore a low-cost and environment-friendly technology has been a great challenge. Considering that WAS is a collection of organic matters, anaerobic fermentation has been selected as a sustainable way to simultaneously recover resources and reduce environmental pollution. To recover short-chain fatty acids (SCFAs) has gained great concern because of the high value-added application and high-efficiency production process. Considering the temperature in some areas of the world can reach to below 0 °C, this study proposed an efficient strategy, i.e., stepwise freezing and thawing treatment, to promote SCFAs production. The maximal production of SCFAs, i.e., 246 mg COD/g volatile suspended solid, was obtained with the shortened retention time of five days. Mechanistic studies showed that the solubilization of both extracellular polymeric substances (EPSs) and microbial cells could be accelerated, with the EPSs removal of 58.3% for proteins and 59.0% for polysaccharides. Also, the hydrolysis process was promoted to provide more substrates for subsequent acidogenisis, and the functional microorganisms, such as Romboutsia, Paraclostridium, Macellibacteroides and Conexibacter, were greatly enriched, with a total abundance of 26.2%. Moreover, compared to control, methanogenesis was inhibited at a shortened sludge retention time (e.g., five days), which benefited to the accumulation of SCFAs, but the methane production was increased by 25.2% at a longer sludge retention time (e.g., ten days). Thus, these findings of this work may provide some new solutions for the enhanced resource recovery from WAS, and further for carbon-neutral operation of wastewater treatment plants.

Freezing-low temperature treatment facilitates short-chain fatty acids production from waste activated sludge with short-term fermentation

This study proposed a novel and high-efficiency strategy, i.e., freezing followed by low-temperature thermal treatment, to significantly promote short-chain fatty acids (SCFAs) production from waste activated sludge compared to traditional freezing/thawing treatment. The maximal production of SCFAs was 212 mg COD/g VSS with a shortened retention time of five days, and the potentially recovered carbon source, including SCFAs, soluble polysaccharides and proteins, reached 321 mg COD/g VSS, increased by 92.1 and 28.3% compared to sole freezing and thermal treatment. Both the solubilization and hydrolysis steps of WAS were accelerated, and the acid-producing microorganisms, such as Macellibacteroides, Romboutsia and Paraclostridium, were greatly enriched, with a total abundance of 13.9%, which was only 0.54% in control. Interestingly, the methane production was inhibited at a shortened retention time, resulting in SCFAs accumulation, whereas it was increased by 32.0% at a longer sludge retention time, providing a potential solution for energy recovery from WAS.