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Aerobic digestion followed by dewatering is a widely applied method for sludge stabilization and reduction in decentralized wastewater treatment plants. It is important to enhance the sludge dewaterability of the aerobically digested sludge due to its considerable impact on cost of sludge disposal and management. In this study, an innovative technique is developed for improving the dewaterability of aerobically digested sludge by combined conditioning with persulfate (PS) and zero valent iron (ZVI). The results demonstrated that the dewaterability of aerobically digested sludge could be significantly enhanced with the PS and ZVI dosage in the range of 0–0.5 g/gTS and 0–0.4 g/gTS, respectively. The highest improvement was achieved at 0.05 g ZVI/g TS with 0.1 g PS/g TS, and the capillary suction time was reduced by ∼80%. The extracellular polymeric substances (EPS) characterization revealed that the combined PS-ZVI treatment could largely reduce proteins, polysaccharides and humic acids-like compounds in the tightly bounded EPS of the aerobically digested sludge, leading to bound water releasing from sludge flocs. The recovery of the ZVI particles could reach around 45%–80% after the treatment, further proved the sustainability of the approach. The proposed PS-ZVI conditioning would not have significant impact on the final choice of sludge disposal and the mainstream wastewater treatment. However, plant-scale test are still required for better assessing the proposed technique. Image 1 • Persulfate-ZVI conditioning enhanced the dewaterability of aerobically digested sludge. • Capillary suction time reduced by 80% at 0.05 g ZVI/g TS and 0.1 g persulfate/g TS. • Persulfate-ZVI treatment reduced the hydrophilic organic compounds in TB-EPS. • ZVI could be recovered and the cost for sludge disposal could be reduced. [ABSTRACT FROM AUTHOR]

Abstract: The quantification of nitrous oxide (N2O) emissions from open-surface wastewater treatment systems with surface aerators is difficult as emissions from the surface aerator zone cannot be easily captured by floating hoods. In this study, we propose and demonstrate a novel methodology to estimate N2O emissions from such systems through determination of the N2O transfer coefficient (k L a) induced by surface aerators based on oxygen balance for the entire system. The methodology is demonstrated through its application to a full-scale open oxidation ditch wastewater treatment plant with surface aerators. The estimated k L a profile based on a month-long measurement campaign for oxygen balance, intensive monitoring of dissolved N2O profiles along the oxidation ditch over a period of four days, together with mathematical modelling, enabled to determine the N2O emission factor from this treatment plant (0.52 ± 0.16%). Majority of the N2O emission was found to occur in the surface aerator zone, which would be missed if the gas hood method was applied alone. [Copyright &y& Elsevier]

Abstract: Soluble microbial products (SMP) are the major component of the residual organic fraction in biological wastewater treatment effluent. The impact of process parameters on SMP production by specific groups of bacteria is currently unknown. In this work, SMP production by activated sludge at different substrate concentrations, dissolved oxygen (DO) levels and temperatures, was evaluated by experimental and modeling approaches. The results showed that among the three parameters, SMP production was most sensitive to substrate concentration. Total SMP production was increased 70.5% by a threefold increase in substrate concentration, with SMP produced from heterotrophs, ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) increasing by 61.2%, 580.0% and 410.0%, respectively. The effect of temperature on SMP was less pronounced. Decreasing the temperature from 20 °C to 10 °C decreased total SMP by 17.2%, with SMP production from heterotrophs decreasing by 20.0%, and from the AOB and NOB increasing by 180.0% and 140.0%. DO concentration had nearly no effect on total and heterotrophic SMP production, while it did have a significant positive effect on autotrophic SMP production. SMP production from AOB and NOB decreased by 24.3% and 47.8%, respectively following a decrease in DO concentration from 8.7 to 1.5 mg/L. However, the net effect of DO on total SMP production was negligible. [Copyright &y& Elsevier]

Abstract: In this study, an integrated dynamic model was developed through combining a mechanistic model, a neural network (NN) model and a genetic algorithm approach, in order to simulate the performance of a full-scale municipal wastewater treatment plant (WWTP) with substantial influent fluctuations. As the base of the integrated model, the mechanistic model was initially established based on the activated sludge model 3 and the EAWAG bio-P module, and was used to generate the residuals for the NN model. The NN model was employed to build the relationship between the input and output variables. The network weights of the NN model were optimized with a genetic algorithm approach. The resulting integrated model was applied to simulate the 5-month performance of a full-scale WWTP with significant influent fluctuations, and the simulation results matched the measured ones of the WWTP well even under influent disturbance conditions. Compared with the individual mechanistic model and NN model, the integrated model was able to capture sufficient residual information to compensate for the inaccuracy of the mechanistic model and improve the extrapolative capability of the NN model. This model established in our work is demonstrated to be an effective and useful tool to simulate the performance of WWTPs. [Copyright &y& Elsevier]

• Fe(VI) treatment effectively removed ARGs from secondary wastewater effluent. • Fe(VI) induced microbial DNA damage and resulted in ARG elimination. • Coagulation initiated by Fe(VI) reduction products also contributed to ARG removal. • Relative abundances of viable potential ARG hosts decreased after Fe(VI) treatment. • Fe(VI) is promising for generating high-quality effluent with reduced ARGs risks. The widespread of antibiotic resistance genes (ARGs) in the environment can pose severe threats to public health. The wastewater treatment plant (WWTP) is regarded as an important hotspot of ARGs in the urban environment, but the removal of ARGs through conventional treatment techniques has been proven not sufficient. In this study, ferrate (Fe(VI)) was applied for the first time to remove intracellular ARGs from the secondary effluent of the WWTP. The results showed that Fe(VI) treatment could effectively remove 15 ARGs covering eight different types as well as intI1 , the most common integron important to ARGs horizontal transfer. The removal efficiencies of tested genes could reach 1.10-4.37 log at the Fe(VI) dosage of 10 mg-Fe/L, which is significantly higher than those achieved through traditional disinfection methods. The DNA gel electrophoresis suggested that Fe(VI) could induce microbial DNA damage and consequently resulted in ARGs elimination. The presence of ARGs in settled residues indicated that coagulation initiated by Fe(VI) reduction products also contributed to ARGs removal from wastewater. In addition, the viability and relative abundances of potential ARGs hosts in the wastewater were decreased after Fe(VI) treatment. This study suggested a promising prospect for applying Fe(VI) to efficiently remove ARGs from wastewater, and consequently to control their proliferation and transfer in the environment. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Wastewater treatment plants (WWTPs) have been identified as one of the reservoirs of antibiotics. Although nitrifying bacteria have been reported to be capable of degrading various antibiotics, there are very few studies investigating long-term effects of antibiotics on kinetic and microbial responses of nitrifying bacteria. In this study, cephalexin (CFX) and sulfadiazine (SDZ) were selected to assess chronic impacts on nitrifying sludge with stepwise increasing concentrations in two independent bioreactors. The results showed that CFX and SDZ at an initial concentration of 100 μ g/L could be efficiently removed by enriched nitrifying sludge, as evidenced by removal efficiencies of more than 88% and 85%, respectively. Ammonia-oxidizing bacteria (AOB) made a major contribution to the biodegradation of CFX and SDZ via cometabolism, compared to limited contributions from heterotrophic bacteria and nitrite-oxidizing bacteria. Chronic exposure to CFX (≥30 μ g/L) could stimulate ammonium oxidation activity in terms of a significant enhancement of ammonium oxidation rate (p < 0.01). In contrast, the ammonium oxidation activity was inhibited due to exposure to 30 μ g/L SDZ (p < 0.01), then it recovered after long-term adaption under exposure to 50 and 100 μ g/L SDZ. In addition, 16S rRNA gene amplicon sequencing revealed that the relative abundance of AOB decreased distinctly from 23.8% to 28.8% in the control phase (without CFX or SDZ) to 14.2% and 10.8% under exposure to 100 μ g/L CFX and SDZ, respectively. However, the expression level of amoA gene was up-regulated to overcome this adverse impact and maintain a stable and efficient removal of both ammonium and antibiotics. The findings in this study shed a light on chronic effects of antibiotic exposure on kinetic and microbial responses of enriched nitrifying sludge in WWTPs. Image 1 • Cephalexin (CFX) and sulfadiazine (SDZ) could be efficiently removed by enriched nitrifying sludge. • AOB made major a contribution to the biodegradation of CFX and SDZ via cometabolism. • CFX could stimulate ammonium oxidation rate (AOR), while SDZ would decrease AOR. • Chronic exposure to CFX and SDZ caused decrease in the abundance of AOB. • The amoA gene was up-regulated to maintain stable removal of ammonium and antibiotics. [ABSTRACT FROM AUTHOR]

Sulfur-containing substances in sewers frequently incur unpleasant odors, corrosion-related economic loss, and potential human health concerns. These observations are principally attributed to microbial reactions, particularly the involvement of sulfate-reducing bacteria (SRB) in sulfur reduction process. As a multivalent element, sulfur engages in complex bioreactions in both aerobic and anaerobic environments. Organic sulfides are also present in sewage, and these compounds possess the potential to undergo transformation and volatilization. In this paper, a comprehensive review was conducted on the present status regarding sulfur transformation, transportation, and remediation in sewers, including both inorganic and organic sulfur components. The review extensively addressed reactions occurring in the liquid and gas phase, as well as examined detection methods for various types of sulfur compounds and factors affecting sulfur transformation. Current remediation measures based on corresponding mechanisms were presented. Additionally, the impacts of measures implemented in sewers on the subsequent wastewater treatment plants were also discussed, aiming to attain better management of the entire wastewater system. Finally, challenges and prospects related to the issue of sulfur-containing substances in sewers were proposed to facilitate improved management and development of the urban water system. [Display omitted] • Transformation and emission of inorganic and organic sulfur components in sewers is reviewed. • Various strategies for mitigating sulfur-containing pollutants are critically discussed. • The measures imposed in sewers to control sulfur-containing pollutants inevitably impact WWTPs. • Current knowledge gaps and perspectives for the control and management of sulfur-containing substances are highlighted. [ABSTRACT FROM AUTHOR]

• Enrichment of comammox Nitrospira in a mainstream SBR in the step-feeding mode. • Candidatus Nitrospira nitrosa was the dominant (∼95 %) comammox species enriched. • Identification of K NH4 -N and K O2 for enriched comammox bacteria-dominant sludge. • Floccular sludge enriched with comammox Nitrospira possessed low N 2 O and NO yields. The discovery of complete ammonium oxidation (comammox) has subverted the traditional perception of two-step nitrification, which plays a key role in achieving biological nitrogen removal from wastewater. Floccular sludge-based treatment technologies are being applied at the majority of wastewater treatment plants in service where detection of various abundances and activities of comammox bacteria have been reported. However, limited efforts have been made to enrich and subsequently characterize comammox bacteria in floccular sludge. To this end, a lab-scale sequencing batch reactor (SBR) in the step-feeding mode was applied in this work to enrich comammox bacteria through controlling appropriate operational conditions (dissolved oxygen of 0.5 ± 0.1 g-O 2 /m3, influent ammonium of 40 g-N/m3 and uncontrolled longer sludge retention time). After 215-d operation, comammox bacteria gradually gained competitive advantages over counterparts in the SBR with a stable nitrification efficiency of 92.2 ± 2.2 %: the relative abundance of Nitrospira reached 42.9 ± 1.3 %, which was 13 times higher than that of Nitrosomonas , and the amoA gene level of comammox bacteria increased to 7.7 ± 2.1 × 106 copies/g-biomass, nearly 50 times higher than that of conventional ammonium-oxidizing bacteria. The enrichment of comammox bacteria, especially Clade A Candidatus Nitrospira nitrosa , in the floccular sludge led to (i) apparent affinity constants for ammonium and oxygen of 3.296 ± 0.989 g-N/m3 and 0.110 ± 0.004 g-O 2 /m3, respectively, and (ii) significantly low N 2 O and NO production, with emission factors being 0.136 ± 0.026 % and 0.023 ± 0.013 %, respectively. [Display omitted] [ABSTRACT FROM AUTHOR]

Engineering robust non-noble metal electrocatalysts towards efficient impure water (e.g., seawater, wastewater) oxidation is a prospective approach to achieve carbon neutrality via accelerating green hydrogen energy development. Herein, a NiCo layered double hydroxides (LDH)/NiFe LDH composite (NiCo-LDH/NiFe-LDH) was developed for oxygen evolution reaction (OER) via a hydrothermal process-electrodeposition method. The optimal NiCo-LDH/NiFe-LDH-30 composite only needed an overpotential (η) of 240 mV to drive 100 mA/cm2 in alkalized freshwater, with a low Tafel slope of 16.6 mV/dec and good stability for over 90 h. Further analyses suggested that the strong interface interaction between NiCo-LDH and NiFe-LDH accelerated the oxygen gas bubble evolution and boosted interfacial charge transfer, and the formed built-in electric field and higher oxidation state species (metal oxyhydroxides) contributed to the high intrinsic catalytic activity. The NiCo-LDH/NiFe-LDH-30 composite also held excellent OER activities in different impure water environments, including alkaline 0.5 M NaCl solution (η 100 = 333 mV), alkaline lake water (η 100 = 345 mV), and alkaline wastewater treatment plant (WWTP) effluent (η 100 = 320 mV). More importantly, the potential effects of Cl− and CO 3 2− in impure water were revealed during the OER process. This work elaborates on the role of built-in electric field and the strong coupling interaction in composite catalysts, which pave the way for the design of cost-effective catalysts with excellent adaptability in different water environments. [Display omitted] • NiCo-LDH/NiFe-LDH composite showed strong electronic coupling interaction. • Hybridizing NiCo-LDH with NiFe-LDH led to fast interfacial electron transfer. • NiCo-LDH/NiFe-LDH exhibited prominent OER performance in different water matrices. • Built-in electric field enhanced OER activity and stability of NiCo-LDH/NiFe-LDH. [ABSTRACT FROM AUTHOR]

The recovery of phosphorus (P) through vivianite crystallization offers a promising approach for resource utilization in wastewater treatment plants. However, this process encounters challenges in terms of small product size and low purity. The study aimed to assess the feasibility of using quartz sand as a seed material to enhance P recovery and vivianite crystal characteristics from anaerobic fermentation supernatant. Various factors, including seed dosage, seed size, Fe/P ratio, and pH, were systematically tested in batch experiments to assess their influence. Results demonstrated that the effect of seed enhancement on vivianite crystallization was more pronounced under higher seed dosages, smaller seed sizes, and lower pH or Fe/P ratio. The addition of seeds increased P recovery by 4.43% in the actual anaerobic fermentation supernatant and also augmented the average particle size of the recovered product from 19.57 to 39.28 μm. Moreover, introducing quartz sand as a seed material effectively reduced co-precipitation, leading to a notable 12.5% increase in the purity of the recovered vivianite compared to the non-seeded process. The formation of an ion adsorption layer on the surface of quartz sand facilitated crystal attachment and growth, significantly accelerating the vivianite crystallization rate and enhancing P recovery. The economic analysis focused on chemical costs further affirmed the economic viability of using quartz sand as a seed material for P recovery through vivianite crystallization, which provides valuable insights for future research and engineering applications. [Display omitted] • Quartz sand as seed material increased P recovery in anaerobic fermentation supernatant. • Seed-induced vivianite crystallization was enhanced at higher seed doses, smaller seed sizes, and lower pH or Fe/P ratios. • Quartz sand as seed material improved vivianite purity through reduced co-precipitation. • Quartz sand as seed material for vivianite crystallization was economically feasible. [ABSTRACT FROM AUTHOR]

[Display omitted] • The study firstly reviews MCFAs production from sewage sludge fermentation. • MCFAs synthesis can be suppressed by competitive metabolisms and sludge hydrolysis. • Competition and cooperation among microbes co-exist in sludge CE system. • Pretreatments and semi-/conductive materials are the main reinforcement strategies. • Pretreatments are efficient to impede byproducts formation and avoid BES dosing. Huge sludge volumes and increased energy demands are the main challenges faced by wastewater treatment plants (WWTPs) currently, while it stimulates organics transformation and carbon recovery from sludge treatment process. Due to higher energy density and greater hydrophobicity, medium-chain fatty acids (MCFAs) exhibit easier extraction and more widespread application than traditional short-chain fatty acids (SCFAs). Therefore, MCFAs synthesis from sewage sludge via chain elongation (CE) has recently attracted growing attention in anaerobic open culture fermentation. Nevertheless, a comprehensive and systematic review focused on MCFAs synthesis from sludge is not available so far. This study provides a critical review on sewage sludge-derived MCFAs synthesis. The metabolic pathways, potential influencing factors, and cooperative and competitive relationships among various microorganisms are first elucidated to understand the mechanisms, limitations, and microbial interactions involved in MCFAs production via open culture fermentation of sludge. Unfortunately, MCFAs synthesis from sewage sludge is mainly hampered by low organics accessibility, inferior electron transfer efficiency and presence of competitive microbes. To foster sludge degradation and high-value MCFAs production, various techniques based on pretreatment and facilitation of electron transfer have recently been proposed, which are comprehensively reviewed with the corresponding mechanisms meticulously discussed. Ultimately, knowledge gaps and new opportunities for sludge-derived MCFAs production are prospected. This review will offer theoretical and technological guidelines for the subsequent investigations and future engineering applications of MCFAs production from sewage sludge. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Sophorolipid enhanced the SCFA production from anaerobic fermentation of WAS. • Sophorolipid facilitated the WAS solubilization. • Sophorolipid enhanced release of biodegradable organics. • Inhibition of methanogens by sophorolipid led to accumulation of SCFA. • Sophorolipid increased the abundance of hydrolytic microbes and SCFA producers. Abstract It was found in this study that the presence of sophorolipid (SL) enhanced the production of short-chain fatty acid (SCFA) from anaerobic fermentation of waste activated sludge (WAS). Experimental results showed that with an increase of SL addition from 0 to 0.1 g/g TSS, the maximal SCFA yield increased from 50.5 ± 4.9 to 246.2 ± 7.5 mg COD/g VSS. The presence of SL reduced the surface tension between hydrophobic organics and fermentation liquid, which thereby accelerated the disintegration of WAS and improved the biodegradability of the released organics. SL promoted the carbon/nitrogen ratio of the fermentation system, enhancing the conversion of proteins in WAS. Moreover, SL suppressed severely the activities of methanogens, probably due to the drop of pH caused by SL addition. Amplicon sequencing analyses revealed that SL increased the abundance of hydrolytic microbes such as Bacteroides sp. and Macellibacteroides sp., and SCFA producers (e.g., Acinetobacter sp.). [ABSTRACT FROM AUTHOR]

Abstract Microplastics have aroused increasing concern as they pose threats to aquatic species as well as human beings. They do not only contribute to accumulation of plastics in the environment, but due to absorption they can also contribute to spreading of micropollutants in the environment. Studies indicated that wastewater treatment plants (WWTPs) play an important role in releasing microplastics to the environment. Therefore, effective detection of the microplastics and understanding their occurrence and fate in WWTPs are of great importance towards microplastics control. In this review, the up-to-date status on the detection, occurrence and removal of microplastics in WWTPs are comprehensively reviewed. Specifically, the different techniques used for collecting microplastics from both wastewater and sewage sludge, and their pretreatment and characterization methods are reviewed and analyzed. The key aspects regarding microplastics occurrence in WWTPs, such as concentrations, total discharges, materials, shapes and sizes are summarized and compared. Microplastics removal in different treatment stages and their retention in sewage sludge are explored. The development of potential microplastics-targeted treatment technologies is also presented. Although previous researches in microplastics have undoubtedly improved our level of understanding, it is clear that much remains to be learned about microplastics in WWTPs, as many unanswered questions and thereby concerns still remain; some of these important future research areas are outlined. The key challenges appear to be to harmonize detection methods as well as microplastics mitigation from wastewater and sewage sludge. Graphical abstract Image 1 Highlights • The detection, occurrence and behavior of microplastics in WWTPs are reviewed. • The collection, pretreatment and characterization of microplastics are analyzed. • The concentrations, materials and properties of microplastics are summarized. • The microplastics removal in WWTPs and retention in sewage sludge are explored. [ABSTRACT FROM AUTHOR]

Previous studies reported that free ammonia (FA) pretreatment could improve methane production from anaerobic digestion of waste activated sludge (WAS) effectively. However, details of how FA pretreatment improves methane production are poorly understood. This study therefore aims to reveal the underlying mechanisms of FA pretreatment affecting anaerobic digestion of WAS through a series of batch tests using either real sludge or synthetic media as the digestion substrates at different pH values. At pH 8.5 level, with an increase of FA level from 18.5 to 92.5 mg/L (i.e., NH+ 4-N: 100–500 mg/L; pH 8.5) the maximum methane yield varied between 194.0 ± 3.9 and 196.9 ± 7.7 mL/g of VSS (25 °C, 1 atm). At pH 9.5 or 10 level, however, with an increase of initial FA level from 103.2 to 516.2 mg/L, the maximal methane yield increased linearly. The mechanism studies revealed that FA pretreatment at high levels not only accelerated the disintegration of WAS but also enhanced the biodegradability of WAS. Although pH in the digesters was adjusted to 7.0 ± 0.1, the high levels of NH+ 4-N added or released led to substantial levels of residual FA ranging from 4.4 to 11.6 mg/L. It was found that this level of FA inhibited homoacetogenesis and methanogenesis significantly, though hydrolysis, acidogenesis, and acetogenesis processes were unaffected largely. Further analyses showed that the inhibition constant of FA to substrate degradation was in the sequence of dextran > glucose > hydrogen > acetate, indicating the methanogenesis process was more sensitive to FA. [ABSTRACT FROM AUTHOR]

The mainstream anaerobic ammonium oxidation (anammox) has attracted extensive attention recently, particularly due to its potential of transforming current wastewater treatment plants from energy consuming to energy neutral or positive. However, the presence of biodegradable chemical oxygen demanding (COD, 20–80 mg COD L −1 ) in the mainstream anammox reactor stimulates the growth of heterotrophic bacteria, which would compete for oxygen with ammonia-oxidizing bacteria (AOB) and for nitrite with anammox bacteria, thus interfering with the autotrophic nitrogen removal process. In the present work, with consideration of granule size distribution, a one-dimensional model describing the mainstream simultaneous partial nitrification, anammox and denitrification (SNAD) in a granule-based reactor was established, calibrated and validated, based on the long-term experimental results. Through applying the verified model, simulation studies were conducted and the results showed that the effluent total nitrogen concentration of <5 mg N L −1 could be achieved at C/N ratio of 0.2–0.6, DO concentration of 0.2–0.4 mg L −1 and granule radius of 300–600 μm. The combined effects indicated that the SNAD process with TN removal efficiency >90% was obtained at C/N ratio and DO concentration of 0.2–1.0 and 0.2–0.4 mg O 2 L −1 respectively. Finally, the various granule size distribution patterns were simulated, which confirmed that the size distribution needed to be incorporated in the model to accurately describe the granular anammox system, considering a model based on a uniform granule size does not reflect the real situations. These results provide guides to optimize the operation of mainstream granular autotrophic nitrogen removal process. [ABSTRACT FROM AUTHOR]

Biological nitrogen removal through the nitrite pathway (NH4+ → NO2- → N2) is favorable for wastewater treatment plants without sufficient carbon sources. This study demonstrates an innovative approach for attaining the nitrite pathway based on sludge treatment using free ammonia (FA, i.e., NH3). This approach is based on our innovative discovery in this study that FA at 210 mg NH3-N/L is far less biocidal to ammonium-oxidizing bacteria (AOB) than to nitrite-oxidizing bacteria (NOB). A total of 22% of the activated sludge from the sequencing batch reactor (SBR) receiving synthetic domestic wastewater was treated in an FA treatment unit at 210 mg NH3-N/L for 1 day. The FA-treated sludge was afterward recirculated back to the SBR. A nitrite accumulation ratio of above 90% was quickly achieved (in 40 days) and maintained stably in the SBR, indicating the establishment of the nitrite pathway. The NOB population and activity after implementing FA treatment was less than 5% of those without FA treatment, suggesting the washout of NOB. In contrast, the AOB population and activity in the SBR were not affected. The nitrogen-removal performance was significantly improved after incorporating the FA approach. The FA approach is a closed-loop approach and is economically and environmentally attractive. [ABSTRACT FROM AUTHOR]

[Display omitted] • Short-term FA and FNA treatment on FN sludge instantly reduce N 2 O at all DO levels. • The dependency of N 2 OR on DO was altered by exposing FN sludge to high FNA. • Short-term exposure of PN sludge to high FNA abolished N 2 O production. • FNA treatment on sludge led to 3.5% more cost, while FA treatment saved 2% cost. The potentials of using endogenous free ammonia (FA) and free nitrous acid (FNA) as nitrous oxide (N 2 O) mitigators were investigated in treatment of both mainstream and sidestream wastewater. Although the N 2 O emission factor of a sidestream partial-nitritation (PN) reactor (averaged 1.70 % ± 0.39 %, n = 30) was about 2.4 times higher than a mainstream full-nitrification (FN) reactor (averaged 0.72 % ± 0.24 %, n = 30) (P < 0.01), one-hour exposure of PN sludge to 1.5 mg HNO 2 -N/L FNA could virtually abolish N 2 O emission. As for FN sludge, both 45 mg NH 3 -N/L FA and 0.015 mg HNO 2 -N/L FNA successfully mitigated N 2 O production at varying dissolved oxygen (DO) levels (50 % vs 61 %), while 1.5 mg HNO 2 -N/L FNA not only reduced more N 2 O (92 %) but also altered the N 2 O dependency on DO. Both FNA and FA sludge treatment were effective N 2 O mitigation strategies with FNA toward the end of carbon neutrality and FA being more economically appealing (2 % cost saving). [ABSTRACT FROM AUTHOR]

Excess sludge management is a restrictive factor for the development of municipal wastewater treatment plants. The addition of metabolic uncouplers has been proven to be effective in sludge reduction. However, the long-term effect of metabolic uncoupler o-chlorophenol (oCP) on the biological wastewater treatment system operated in anaerobic-oxic mode is still unclear. To this end, two parallel reactors operated in anaerobic-oxic mode with and without 10 mg/L of oCP addition were investigated for 91 days. The results showed that 56.1 ± 2.3% of sludge reduction was achieved in the oCP-added system, and the nitrogen and phosphorus removal ability were negatively affected. Dosing oCP stimulated the formation of microbial products and increased the DNA concentration, but resulted in a decrease in the electronic transport activity of activated sludge. Microbial community analysis further demonstrated that a significant reduction of bacterial richness and diversity occurred after oCP dosing. However, after stopping oCP addition, the pollutant removal ability of activated sludge was gradually increased, but the sludge yield, as well as species richness and diversity, did not recover to the previous level. This study will provide insightful guidance on the long-term application of metabolic uncouplers in the activated sludge system. [Display omitted] • Sludge reduction of 56.1% was achieved in the o-chlorophenol (oCP)-added system. • oCP addition stimulated the microbial products formation of activated sludge. • The species richness and diversity were reduced with oCP addition. • The sludge yield, as well as species richness and diversity, did not recover after stopping oCP addition. [ABSTRACT FROM AUTHOR]

• Wastewater and sludge treatment changes MPs morphology, sizes and chemical properties. • Changes in the intrinsic properties of MPs can affect their transport and distribution patterns. • The physicochemical properties of MPs affect coexisting pollutants and leaching behavior, leading to changes in ecological risk. • Need for further field-studies to address physicochemical-related environmental risk. Microplastics (MPs) have been frequently detected in effluent wastewater and sludge in wastewater treatment plants (WWTPs), the discharge and agricultural application of which represent a primary source of environmental MPs contamination. As important as quantitative removal is, changes of physicochemical characteristics of MPs (e.g., shapes, sizes, density, crystallinity) in WWTPs are crucial to their environmental behaviors and risks and have not been put enough attention yet. This review is therefore to provide a current overview on the changes of physicochemical characteristics of MPs in WWTPs and their corresponding environmental risks. The changes of physicochemical characteristics as well as the underlying mechanisms of MPs in different successional wastewater and sludge treatment stages that mainly driven by mechanical (e.g., mixing, pumping, filtering), chemical (e.g., flocculation, advanced oxidation, ultraviolet radiation, thermal hydrolysis, incineration and lime stabilization), biological (e.g., activated sludge process, anaerobic digestion, composition) and their combination effects were first recapitulated. Then, the inevitable correlations between physicochemical characteristics of MPs and their environmental behaviors (e.g., migration, adsorption) and risks (e.g., animals, plants, microbes), are comprehensively discussed with particular emphasis on the leaching of additives and physicochemical characteristics that affect the co-exist pollutants behavior of MPs in WWTPs on environmental risks. Finally, knowing the summarized above, some relating unanswered questions and concerns that need to be unveiled in the future are prospected. The physicochemical properties of MPs change after passing through WWTP, leading to subsequent changes in co-contaminant adsorption, migration, and toxicity. This could threaten our ecosystems and human health and must be worth investigating. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: A full-scale Carrousel oxidation ditch wastewater treatment plant (WWTP) was simulated and optimized through integrating the activated sludge model 2d (ASM2d), support vector regression (SVR) and accelerating genetic algorithm (AGA). The ASM2d model after calibration and validation with the operating data was used to simulate the process. Operating parameters, including hydraulic retention times (HRTs) of anaerobic, anoxic and aerobic tanks, solids retention time (SRT) and internal recirculation ratio were subjected to optimization using SVR and AGA. The simulation results were normalized and SVR was employed to correlate the operating factors and the effluent quality. Then, the AGA approach was used to obtain the optimal operating conditions. The multiple-objective optimization with different weights indexes was adopted to achieve simultaneous nutrient removal. Compared with the present operating conditions, the HRT of the anoxic tank, the internal recirculation ratio and the SRT should be reduced, while the HRT of the aerobic tank should be prolonged to achieve better effluent quality. Such an integrated approach in this study offers an effective and useful tool to optimize the oxidation ditch process of WWTPs. [Copyright &y& Elsevier]

l RL promoted sludge solubilization and SCFA production in the acidogenic phase. l RL increases biochemical methane potential of sludge in the methanogenic phase. l The enhancement of the acidogenic phase by RL is a contributor to the promotion of methane yield. l RL could be applied to anaerobic digestion of waste activated sludge. In this work, a rhamnolipid (RL) pretreatment technology was proposed to promote methane production from two-phase anaerobic digestion of waste activated sludge. In the first phase (i.e., acidogenic phase), the WAS hydrolysis and acidogenesis were significantly enhanced after RL pretreatment for 4 day, under which the concentration of soluble protein and the short-chain fatty acids (SCFA) in the presence of RL at 0.04 g/g TSS was respectively 2.50 and 5.02 times higher than that without RL pretreatment. However, methane production was inhibited in the presence of RL. In the second phase (i.e., methanogenic phase), batch biochemical methane potential tests suggested that the addition of RL is effective in promoting anaerobic methane production. With an increase of RL dosage from 0 to 0.04 g/g TSS, the cumulative methane yield increased from 100.42 ± 3.01 to 168.90 ± 5.42 mL. Although the added RL could be utilized to produce methane, it was not the major contributor to the enhancement of methane yield. Further analysis revealed that total cumulative yield from the entire two-phase anaerobic digestion (sum of the yield of the acidogenic phase and methanogenic phase) increased from 113.42 ± 3.56 to 164.18 ± 5.20 mL when RL dosage increased from 0 to 0.03 g/g TSS, indicating that the addition of RL induced positive effect on the methane production of the entire two-phase anaerobic digestion. The enzyme activity analysis showed that although higher dosages of RL still inhibited the microorganisms related to methanogenesis to some extends in the methanogenic phase, the inhibitory effect was significantly weakened compared to the acidogenic phase. Microbial analysis revealed that RL reduced the abundance of Candidatus_Methanofastidiosum sp. while increased the abundance of Methanosaeta sp., causing the major methanogenesis pathway to change from hydrogenotrophic to aceticlastic. Moreover, the community of hydrolytic microbes and acidogens was shifted in the direction that is conducive to hydrolysis-acidogenesis. The findings reported not only expand the application field of RL, but also may provide supports for sustainable operation of wastewater treatment plants (WWTPs). [Display omitted] [ABSTRACT FROM AUTHOR]

Hydrogen production from waste activated sludge (WAS) was widely considered and intensively investigated as a promising technology to recover energy from wastewater treatment plants. To date, no efforts have been made on either systematic summarization or critical thinking of the application niche of hydrogen production from WAS treatment. It is therefore time to evaluate whether and how to recover hydrogen in a future paradigm of WAS treatment. In this critical review, the principles and potentials, microorganisms, possible technologies, and process parameters of hydrogen generation were analyzed. Microbial electrolysis cell shows high theoretical hydrogen yield and could utilize a variety of organic compounds as substrates, which is regarded as a prospective technology for hydrogen production. However, the poor organics utilization and rapid consumptions of produced hydrogen hindered hydrogen recovery from WAS. Based on the analysis of the current state of the literatures, the opportunities and challenges of hydrogen production from WAS are rethought, the detailed knowledge gaps and perspective of hydrogen production from WAS were discussed, and the probable solutions of hydrogen recovery from WAS treatment are figured out. To guide the application and development of hydrogen recovery, a more promising avenue through rational integration of the available technologies to form a hybrid process is finally proposed. The integrated operational paradigm of WWTPs could achieve substantial technical, environmental and economic benefits. In addition, how this hybrid process works is illustrated, the challenges of this hybrid process and future efforts to be made in the future are put forward. Image 1 • H 2 is mainly produced from the acidogenesis process of WAS dark fermentation. • Microbial electrolysis cell is a prospective technology for H 2 production. • The poor organics utilization and rapid H 2 consumption hinder hydrogen recovery. • A promising avenue through integration of the available technologies was proposed. • The hybrid process could achieve substantial technical and economic benefits. [ABSTRACT FROM AUTHOR]

• Underlying mechanisms of CaO 2 disintegrating EPS and cell membrane are reviewed. • Sludge dewatering and minimization can be achieved effectively by CaO 2. • Methane production from sludge anaerobic digestion can be enhanced significantly by CaO 2 -based treatment. • SCFA and hydrogen yield in sludge anaerobic fermentation can be improved prominently with CaO 2 addition. • CaO 2 can promote the removal of emerging contaminants via the generated free radicals oxidation and alkali. The treatment and disposal of waste activated sludge (WAS) has become one of the major challenges for the wastewater treatment plants (WWTPs) due to large output, high treatment costs and enriched substantial emerging contaminants (ECs). Therefore, reducing sludge volume, recovering energy and resource from WAS, and removing ECs and decreasing environmental risk have gained increasing attentions. Calcium peroxide (CaO 2), a versatile and safe peroxide, has been widely applied in terms of WAS treatment including sludge dewatering, anaerobic sludge digestion and anaerobic sludge fermentation due to its specific properties such as generating free radicals and alkali, etc., providing supports for sludge reduction, recycling, and risk mitigation. This review outlines comprehensively the recent progresses and breakthroughs of CaO 2 in the fields of sludge treatment. In particular, the relevant mechanisms of CaO 2 enhancing WAS dewaterability, methane production from anaerobic digestion, short-chain fatty acids (SCFA) and hydrogen production from anaerobic fermentation, and the removal of ECs in WAS and role of experiment parameters are systematically elucidated and discussed, respectively. Finally, the knowledge gaps and opportunities in CaO 2 -based sludge treatment technologies that need to be focused in the future are prospected. The review presented can supply a theoretical basis and technical reference for the application of CaO 2 for improving the treatment of WAS. Image, graphical abstract [ABSTRACT FROM AUTHOR]

• First reported N 2 O mitigation strategy permanently implemented at full-scale. • 35% N 2 O emission reduction was achieved with 20% less operational cost. • N 2 O mitigation did not affect nutrient removal performance. • Validated multi-pathway N 2 O model at full-scale to evaluate mitigation strategies. • Proposed a standard approach for mitigating N 2 O emissions at full-scale plants. Mitigation of nitrous oxide (N 2 O) emissions is of primary importance to meet the targets of reducing carbon footprints of wastewater treatment plants (WWTPs). Despite of a large amount of N 2 O mitigation studies conducted in laboratories, full-scale implementation of N 2 O mitigation is scarce, mainly due to uncertainties of mitigation effectiveness, validation of N 2 O mathematical model, risks to nutrient removal performance and additional costs. This study aims to address the uncertainties by investigating the quantification, development and implementation of N 2 O mitigation strategies at a full-scale sequencing batch reactor (SBR). To achieve this, N 2 O emission dynamics, nutrient removal performance and operation of the SBR were monitored to quantify N 2 O emissions, and identify the N 2 O generation mechanisms. N 2 O mitigation strategies centered on reducing dissolved oxygen (DO) levels were consequently proposed and evaluated using a multi-pathway N 2 O production mathematical model before implementation. The implemented mitigation strategy resulted in a 35% reduction in N 2 O emissions (from the emission factor of 0.89 ± 0.05 to 0.58 ± 0.06%), which was equivalent to annual reduction of 2.35 tonne of N 2 O from the studied WWTP. This could be mainly attributed to reductions in N 2 O generated via the NH 2 OH oxidation pathway due to the lowering of DO level. As the first reported mitigation strategy permanently implemented at a full scale WWTP, it showcased that the mitigation of N 2 O emissions at full-scale is feasible and that widely accepted N 2 O mitigation strategies developed in laboratory studies are also likely effective in full-scale plants. Furthermore, the close agreement between the validated and predicted N 2 O emission factors (0.58% vs 0.55%, respectively), showed that the N 2 O mathematical model is a useful tool to evaluate N 2 O mitigation strategies at full-scale. Importantly this work demonstrated that N 2 O mitigation does not necessarily require additional operational cost to meet reduction targets. In contrast, the N 2 O mitigation applied here reduced energy requirements for aeration by 20%. Equally important, long-term monitoring identified that N 2 O mitigation did not affect the nutrient removal performance of the plant. Finally, with the knowledge acquired in this study, a standard approach for mitigating N 2 O emissions from full-scale treatment plants was proposed. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Triclocarban (TCC) is a highly effective antibacterial agent, which is widely used in a variety of applications and present at significant levels (e.g., 760 μg/L) in wastewater worldwide. However, the interaction between TCC and nitrifiers, important microbial cultures in wastewater treatment plants, has not been documented. This work therefore aimed to evaluate the fate of TCC in a nitrifying culture and its impact on nitrifiers in four long-term nitrifiers-rich reactors, which received synthetic wastewater containing 0, 0.1, 1, or 5 mg/L TCC. Experimental results showed that 36.7%–50.7% of wastewater TCC was removed by nitrifying cultures in stable operation. Mass balance analysis revealed that the removal of TCC was mainly achieved through adsorption rather than biodegradation. Adsorption kinetic analysis indicated that inhomogeneous multilayer adsorption was responsible for the removal while fourier transform infrared spectroscopy indicated that several functional groups such as hydroxyl, amide and polysaccharide seemed to be the main adsorption sites. The adsorbed TCC significantly deteriorated settleability and performance of nitrifying cultures. With an increase of influent TCC from 0 to 5 mg/L, reactor volatile suspended solids and effluent nitrate decreased from 1200 ± 90 mg/L and 300.81 ± 7.52 mg/L to 880 ± 80 and 7.35 ± 4.62 mg/L while effluent ammonium and nitrite increased from 0.41 ± 0.03 and 0.45 ± 0.23 mg/L to104.65 ± 3.46 and 182.06 ± 7.54 mg/L, respectively. TCC increased the extracellular polymeric substances of nitrifying cultures, inhibited the specific activities of nitrifiers, and altered the abundance of nitrifiers especially Nitrospira sp.. In particular, TCC at environmentally relevant concentration (i.e., 0.1 mg/L) significantly inhibited NOB activity and reduced NOB population. Image 1 • Nitrifying cultures removed TCC mainly by inhomogeneous multilayer adsorption. • Hydroxyl, amide and polysaccharide seemed to be the main adsorption sites. • TCC significantly deteriorated settleability and performance of nitrifying cultures. • TCC inhibited nitrifiers especially Nitrospira sp. [ABSTRACT FROM AUTHOR]

• Freezing activated nitrite pretreatment (FN) enhanced anaerobic digestion from WAS. • FN benefited WAS disintegration and the biodegradable of organics released. • FN benefited the dewaterability or pathogens inactivation of digested sludge. • Freezing induced reactive derivatives from nitrite were the major contributors. • FN was economically and environmentally attractive. Nitrite-based pretreatment was demonstrated to effectively improve anaerobic digestion of waste activated sludge. It was found in this work that the freezing activated nitrite pretreatment could further enhance the performances. With the increase of nitrite addition from 0 to 600 mg NO 2 −-N/L during freezing process, the biochemical methane potential of pretreated-sludge gradually increased from 191.3 ± 8.0 to 233.2 ± 10.6 mL per gram volatile solid (VS), while only 178.6 ± 7.3 mL/g VS was obtained in the raw sludge. Mechanism explorations revealed that the freezing activated nitrite pretreatment remarkably facilitated the disintegration of sludge. Excitation emission matrix and fluorescence regional integration analyses further revealed that nitrite addition during freezing process promoted the proportion of biodegradable organics released, thereby providing more bio-available substrates for subsequent anaerobic digestion. Freezing condition induced reactive derivatives from nitrite (e.g., free nitrite acid, NO 2 , N 2 O 3) were assumed to be the major contributors to the enhanced sludge disintegration and recalcitrant organics (e.g., humic acid-like substances) degradation. It was also found that 600 mg NO 2 −-N/L addition activated by freezing pretreatment produced an anaerobically digested sludge with an improved dewaterability, as indicated by the decrease of the specific resistance to filterability and moisture content of dewatered cake. Moreover, 600 mg NO 2 −-N/L addition activated by freezing pretreatment and subsequent anaerobic digestion largely inactivated the pathogens to the levels below Class A biosolids requirements. Considering that nitrite can be in-situ produced in wastewater treatment plants through nitritation of the digestion liquid, this nitrite-based freezing process for sludge pretreatment was environmental-friendly and economically attractive. [ABSTRACT FROM AUTHOR]