Sludge Production Research Articles

Challenges in using Electrocoagulation Process in Removal of Nickel Metal in Wastewater: a Literature Review

In recent years, the surge in nickel production, driven by the growing demand for electric vehicle batteries, has raised concerns regarding environmental consequences. The nickel mining and processing industries contribute to increased nickel levels in wastewater, presenting a serious threat to aquatic ecosystems and human health. This article emphasizes the urgency of developing effective technologies for treating nickel-contaminated wastewater. Electrocoagulation emerges as a promising method, providing high efficiency, minimal sludge production, and cost-effectiveness. The article critically and systematically reviews the potential of the electrocoagulation process in nickel removal from wastewater. In the review, we identify and analyze nearly 32 studies published from 2013 to 2023. We discuss contaminant removal mechanisms and analyze trends in the use of operational parameters. This article identifies the most commonly applied conditions: aluminum electrodes, inter-electrode spacing ≥ 1 cm, current density ≤ 10 mA/cm², initial pH 6 ≤ pH < 11, electrolysis time < 60 min, batch operation, and initial nickel concentration > 50 mg/L. This comprehensive review serves as a foundational resource for advancing electrocoagulation technology in the removal of heavy metals from nickel wastewater.

Iron-retrofitted anaerobic baffled reactor system for rural wastewater treatment: Stable performance of nutrients removal with phosphorus recovery and minimal sludge production

An iron-retrofitted anaerobic baffled reactor (ABR) system was developed for the effective treatment of rural wastewater with reduced maintenance demand and aeration costs. Average removal efficiencies of chemical oxygen demand, total nitrogen and total phosphorus of 99.4%, 62.7% and 92.6% were achieved respectively, when the ABR system was operating at steady state. With effective bioreduction of FeIII in the anaerobic chambers, phosphorus was immobilized in the sludge as vivianite, the sole phosphorus-carrying mineral. The FeIII in the recirculated sludge induced Feammox in the ABR reactor, contributing 14.8% to total nitrogen removal. Biophase separation and enrichment of microorganisms associated with iron and nitrogen transformations were observed in the system after Fe dosing, which enhanced the removal of pollutants. The coupling of Feammox and vivianite crystallization to remove nitrogen and phosphorus in an iron-retrofitted ABR would appear to be a promising technology for rural wastewater treatment.

Impact of legacy and unintentionally produced polychlorinated biphenyls (PCBs) in effluents from two wastewater treatment plants in rivers near Barcelona, Spain

Polychlorinated biphenyls (PCBs) are a family of 209 congeners listed as Persistent Organic Pollutants in the Stockholm Convention. Although there has been a lot of focus on those congeners present in the Aroclor or Clophen technical mixtures commercialized in the past (legacy PCBs), other industrial processes such as paint and pigment production can generate other congeners as byproducts (Unintentionally Produced PCBs or UP-PCBs). The present study focuses on the analysis of 72 PCB congeners (including 42 UP-PCBs) in the two major rivers surrounding the city of Barcelona -Llobregat and Besós rivers-, and their levels in two wastewater treatment plants during the production of effluents and reclaimed water. It was observed that WWTP can efficiently remove PCBs from untreated water during sludge production where concentrations are six orders of magnitude higher than in water (in the ng g−1 and pg L−1 ranges, respectively). Although PCB levels in the effluent and reclaimed water replenishing the rivers are not negligible, these do not significantly increase the concentrations already found in the studied rivers, and in most cases PCB concentrations in river water are reduced after merging with the reclaimed water due to dilution effect. The presence of UP-PCB-11 (not present in the Aroclor technical mixtures) in the analyzed water and sludge samples is significant (ranging from 22 to 25 % of the total PCB amount in the Besós river), being often one of the most abundant PCB congeners.

Optimization of the Photo-Fenton process for the effective removal of chemical oxygen demand and phenols in portable toilet wastewater: A treatment study under real world conditions

Wastewater from portable toilets (WWPT) is characterized by a high content of organic matter and a variety of chemical compounds that retain bad odors, especially phenols, a type of pollutant that is difficult to degrade by conventional treatments; in addition, it is persistent, toxic, and accumulates in the aquatic environment. Although different successful experiences with the use of Photo-Fenton are reported in the scientific domain, its application in WWPT is scarce and warrants study due to the wide use of portable toilets. The objective of this study was to evaluate the Photo-Fenton oxidation process in the removal of organic matter expressed as COD in a WWPT, as well as the reduction of phenols and BOD5. The experimental runs were carried out in a 0.50 L batch reactor to evaluate the effect of the factors (H2O2: 0.019, 25.56, 40.67, 87.24, 148.91, 174.45 g L−1 and pH: 2.80, 3.00, 3.27, 4.40, 5.53, 6.00 UNT) on COD removal and sludge production. It was found that the optimum operating conditions of pH 4.72 and H2O2 dosage of 174.45 g L−1 reduced the concentration of phenols by 97.83 % and 95.49 % of COD. In addition, 98.01 % of BOD5 was reduced, resulting in a biodegradability ratio (BOD5/COD) of 0.23 compared to the untreated wastewater of 0.53. From a cost perspective, the use of Photo-Fenton to treat wastewater under these conditions would be US$ 1.15 per liter.

Carbon footprint analysis of wastewater treatment processes coupled with sludge in situ reduction

The goal of this study was to assess the impacts or benefits of sludge in situ reduction (SIR) within wastewater treatment processes with relation to global warming potential in wastewater treatment plants, with a comprehensive consideration of wastewater and sludge treatment. The anaerobic side-stream reactor (ASSR) and the sludge process reduction activated sludge (SPRAS), two typical SIR technologies, were used to compare the carbon footprint analysis results with the conventional anaerobic - anoxic - oxic (AAO) process. Compared to the AAO, the ASSR with a typical sludge reduction efficiency (SRE) of 30 % increased greenhouse gas (GHG) emissions by 1.1 - 1.7 %, while the SPRAS with a SRE of 74 % reduced GHG emissions by 12.3 - 17.6 %. Electricity consumption (0.025 - 0.027 kg CO2-eq/m3), CO2 emissions (0.016 - 0.059 kg CO2-eq/m3), and N2O emissions (0.009 - 0.023 kg CO2-eq/m3) for the removal of secondary substrates released from sludge decay in the SIR processes were the major contributor to the increased GHG emissions from the wastewater treatment system. By lowering sludge production and the organic matter content in the sludge, the SIR processes significantly decreased the carbon footprints associated with sludge treatment and disposal. The threshold SREs of the ASSR for GHG reduction were 27.7 % and 34.6 % for the advanced dewatering - sanitary landfill and conventional dewatering - drying-incinerating routes, respectively. Overall, the SPRAS process could be considered as a cost-effective and sustainable low-carbon SIR technology for wastewater treatment.

Biochar from Co-Pyrolyzed Municipal Sewage Sludge (MSS): Part 1: Evaluating Types of Co-Substrates and Co-Pyrolysis Conditions.

With the increasing production of municipal sewage sludge (MSS) worldwide, the development of efficient and sustainable strategies for its management is crucial. Pyrolysis of MSS offers several benefits, including volume reduction, pathogen elimination, and energy recovery through the production of biochar, syngas, and bio-oil. However, the process can be limited by the composition of the MSS, which can affect the quality of the biochar. Co-pyrolysis has emerged as a promising solution for the sustainable management of MSS, reducing the toxicity of biochar and improving its physical and chemical properties to expand its potential applications. This review discusses the status of MSS as a feedstock for biochar production. It describes the types and properties of various co-substrates grouped according to European biochar certification requirements, including those from forestry and wood processing, agriculture, food processing residues, recycling, anaerobic digestion, and other sources. In addition, the review addresses the optimization of co-pyrolysis conditions, including the type of furnace, mixing ratio of MSS and co-substrate, co-pyrolysis temperature, residence time, heating rate, type of inert gas, and flow rate. This overview shows the potential of different biomass types for the upgrading of MSS biochar and provides a basis for research into new co-substrates. This approach not only mitigates the environmental impact of MSS but also contributes to the wider goal of achieving a circular economy in MSS management.

Mercury Discharge Inventory Based on Sewage Treatment Process in China

Mercury pollution is a serious public health problem. China’s extensive use and reliance on mercury has led to water pollution, particularly the presence of methylmercury in water. Estimating total mercury emissions from wastewater in China is challenging due to the large amount and wide range of emissions. An estimation model for total mercury content in sewage in China was established by establishing a relationship between sewage treatment volume, mercury content in effluent after sewage treatment, and the data of sludge production and mercury content in the sewage treatment plant. It was determined that only 3% of mercury entered the air during sewage treatment, 27.5% of mercury entered the effluent, and about 69.5% of mercury entered the sludge, based on the treatment of existing wastewater treatment plants in China. From 2002 to 2021, the average annual sewage mercury emission in China was 32.07 Mg, and the emissions were higher in densely populated and economically developed provinces such as Beijing, Shandong, Hebei, and Guangdong. By 2025, China’s mercury emissions are projected to reach 55.41 Mg. By 2035, China’s mercury emissions are projected to reach 49.3 Mg.

Investigating the efficiency of electrocoagulation using similar/dissimilar electrodes for the detoxification of Coralene Rubine dye: a cost effective approach.

Efficient treatment of textile dyeing wastewater can be achieved through electrocoagulation (EC) with minimal sludge production; however, the selection of the appropriate electrode is essential in lowering overall costs. Also, the reuse of the treated aqueous azo dye solution from this process has not been explored in detail. With these objectives, this study aims to treat synthetic azo dye solutions and achieve high colour removal efficiency (CRE%) using similar (Ti-Ti) and dissimilar (Ti-Cu) metal electrodes through EC with an attempt to reduce the cost. The aqueous Coralene Rubine GFL azo dye was used to examine the efficiency and cost of the EC process. X-Ray Photoelectron Spectroscopy was used to study the EC mechanism, while High Performance Liquid Chromatography was used to analyse the degradation of the dye and the formation of intermediate compounds. The concentration of metal ions in the treated dye solution was quantified using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES), with Ti-Ti treated solution having 14.20mg/L concentration of Ti and Ti-Cu treated solution having 0.078mg/L of Ti and 0.001mg/L of Cu, respectively. Colour removal efficiency of 99.49% was obtained for both electrode sets, with a lower operating time and voltage for dissimilar metal combination. Ecotoxicity studies showed negligible toxicity of Ti-Cu treated dye samples compared to untreated solutions. Survival rate, protein estimation, and catalase activity was used to validate the treatment method's efficacy. The study found that the dissimilar electrode material exhibited reduced toxicity due to the presence of heavy metals below the permissible limit.

Enhanced wastewater treatment with an AnF-AAO system for improved internal carbon source utilization

The main challenge in removing nutrients from municipal wastewater in China is the lack of available carbon sources. While hydrolysis acidification tanks can improve wastewater biodegradability by effectively utilizing internal carbon sources, high sludge concentrations are difficult to control in traditional tank variants. In this study, an innovative anaerobic filter (AnF) hydrolysis acidification reactor composed of a continuously stirred tank reactor (CSTR) and cloth media filter was designed to regulate and maintain high sludge concentrations in the hydrolysis acidifier. The reactor was used as a pretreatment unit for the anaerobic/anoxic/oxic (AAO) units and combined into an AnF-AAO system to explore the effectiveness of internal carbon source utilization in wastewater. The results indicate that as the sludge concentration in the hydrolysis acidifier increased, the hydrolysis and acidification processes became more efficient. The optimal sludge concentration was 40 g/L, which significantly increased the production of soluble chemical oxygen demand and volatile fatty acids. Above this concentration, the efficiency decreased. Compared to traditional AAO processes, the AnF-AAO system achieved superior total nitrogen and phosphorus removal with shorter hydraulic retention times and reduced sludge production by a significant amount of 35%. Due to its capacity for enhancing internal carbon source utilization, the AnF-AAO system constitutes a promising approach for sustainable urban wastewater treatment.

Systematic Review of Poultry Slaughterhouse Wastewater Treatment: Unveiling the Potential of Nanobubble Technology

Aeration is crucial for the biological decomposition of organic compounds in wastewater treatment. However, it is a highly energy-intensive process in traditional activated sludge systems, accounting for 50% to 75% of a plant’s electricity consumption and making it a major cost driver for wastewater treatment plants. Nanobubbles (NBs), characterized by their tiny size with diameters less than 200 nm, have emerged as a potential alternative to the low efficiency of aeration and high sludge production in aeration systems. NBs proved effective in removing COD and other pollutants from wastewater. For example, when applied in flotation, aeration, and advanced oxidation, NBs achieved up to 95%, 85%, and 92.5% COD removal, respectively. Considering the recent advancements in wastewater treatment, a compelling need arises for a thorough investigation of the effectiveness and mechanisms of nanobubbles in this field. This systematic review summarizes recent advancements in understanding nanobubbles (NBs) and their unique properties that enhance physical, chemical, and biological water and wastewater treatment processes. Moreover, this study reviews various methods for generating NBs and provides an in-depth review of their applications in wastewater treatment, with a particular focus on poultry slaughterhouse wastewater (PSW) treatment.

Unravelling the impact of oxic-settling-anaerobic cycle implementation and solid retention time on sludge generation, membrane operation, and contaminant removal in membrane bioreactors

Sewage sludge production represents a critical issue nowadays. The oxic-settling-anaerobic (OSA) system is a promising strategy for reducing sludge production during biological wastewater treatment. However, the distinct effect of OSA process and solid retention time (SRT) on sludge production has not been clarified yet. Therefore, during this study, the impacts of OSA system implementation at different SRT on sludge generation, membrane operation, and contaminant removal were elucidated in two lab-scale membrane bioreactors working at 20 d (MBR20) and 60 d (MBR60). After OSA implementation, the MBR-OSA20 and MBR-OSA60 were operated at a hydraulic retention time in the sludge holding tank (HRTSHT) of 6–12 h and of 6–24 h, respectively. The estimated observed sludge yield (Yobs) showed a reduction of 38–48 % for the MBR-OSA20, but no beneficial effect was observed for MBR-OSA60 at 6 h of HRTSHT. At the HRTSHT of 24 h, Yobs decreased by 17 % due to hydrolysis and acidogenic biomass fermentation. OSA implementation and long SRT had no negative effects on organic carbon and ammonium removal (96(±3) % and 100 %, respectively) and improved total nitrogen (TN) reduction by 19–41 %. Membrane fouling and sludge filterability were not significantly affected, whereas sludge settleability worsened after the implementation of the OSA cycle. This activity provided additional insights about the distinct impact of the OSA process and SRT on sludge production and characteristics, confirming the OSA process as a valid solution for reducing sludge production under a moderate SRT, but revealing a negligible effect on Yobs under a prolonged SRT and a short HRTSHT.

Significant in situ sludge yield reduction in an acidic activated sludge system

Minimizing sludge generation in activated sludge systems is critical to reducing the operational cost of wastewater treatment plants (WWTPs), particularly for small plants where bioenergy is not recovered. This study introduces a novel acidic activated sludge technology for in situ sludge yield reduction, leveraging acid-tolerant ammonia-oxidizing bacteria (Candidatus Nitrosoglobus). The observed sludge yield (Yobs) was calculated based on the cumulative sludge generation and COD removal during 400 d long-term operation. The acidic process achieved a low Yobs of 0.106 ± 0.004 gMLSS/gCOD at pH 4.6 to 4.8 and in situ free nitrous acid (FNA) of 1 to 3 mg/L, reducing sludge production by 58 % compared to the conventional neutral-pH system (Yobs of 0.250 ± 0.003 gMLSS/gCOD). The acidic system also maintained effective sludge settling and organic matter removal over long-term operation. Mechanism studies revealed that the acidic sludge displayed higher endogenous respiration, sludge hydrolysis rates, and higher soluble microbial products and loosely-bounded extracellular polymer substances, compared to the neutral sludge. It also selectively enriched several hydrolytic genera (e.g., Chryseobacterium, Acidovorax, and Ottowia). Those results indicate that the acidic pH and in situ FNA enhanced sludge disintegration, hydrolysis, and cryptic growth. Besides, a lower intracellular ATP content was observed for acidic sludge than neutral sludge, suggesting potential decoupling of catabolism and anabolism in the acidic sludge. These findings collectively demonstrate that the acidic activated sludge technology could significantly reduce sludge yield, contributing to more cost- and space-effective wastewater management.

Raw material design, sintering temperature optimization and development mechanism investigation of self-foaming porous bricks with high solid waste addition

In recent years, the contradiction between the huge production of municipal sludge and industrial residue and the shortage of landfill space has driven the exploration of novel effective methods to valorize these solid wastes. In this research, blast furnace slag (BFS), municipal sewage sludge (MSS) and kaolin are combined innovatively as precursor materials to manufacture self-foaming porous bricks, and the sintering behavior of bricks under different BFS doses and sintering temperatures is deeply studied to reveal the development mechanism. The results indicate that the formation of Fe2O3-hematite during sintering leads to a reddish-brown appearance of the bricks, while the generation of CaAl2Si2O8-anorthite with lower melting points helps to enhance the vitrification process. Besides, the increase in BFS dose or sintering temperature has significant impacts on the evolution of the brick physicochemical properties. This occurs because changing the dose of BFS or sintering temperature can obviously regulate the internal morphological characteristics of bricks, such as particle consolidation, pore size and structural compactness. Among them, large-sized pores are considered as the weak areas of mechanical strength, which is also the responsibility for the worst compressive strength of bricks sintered at 1300 °C. Finally, under the optimal sintering conditions of 15 % BFS dose and 1250 °C sintering temperature, the compressive strength, water absorption, apparent porosity, bulk density and linear shrinkage of the resulting brick are 65 MPa, 0.45 %, 2.50 %, 2.34 g/cm3 and 19.56 %, respectively. Overall results demonstrate that using recycled sludge and waste slag in brick manufacturing can produce products with desired performance, providing preferable solutions for solid waste valorization and sustainable brick production.

Application of Bimetallic Hydroxide/Graphene Composites in Wastewater Treatment.

The increasing discharge of antibiotic wastewater leads to increasing water pollution. Most of these antibiotic wastewaters are persistent, strongly carcinogenic, easy to bioaccumulate, and have other similar characteristics, seriously jeopardizing human health and the ecological environment. As a commonly used wastewater treatment technology, non-homogeneous electro-Fenton technology avoids the hazards of H2O2 storage and transportation as well as the loss of desorption and reabsorption. It also facilitates electron transfer on the electrodes and the reduction of Fe3+ on the catalysts, thereby reducing sludge production. However, the low selectivity and poor activity of electro-synthesized H2O2, along with the low concentration of its products, combined with the insufficient activity of electrically activated H2O2, results in a low ∙OH yield. To address the above problems, composites of layered bimetallic hydroxides and carbon materials were designed and prepared in this paper to enhance the performance of electro-synthesized H2O2 and non-homogeneous electro-Fenton by changing the composite mode of the materials. Three composites, NiFe layered double hydroxides (LDHs)/reduced graphene oxide (rGO), NiMn LDHs/rGO, and NiMnFe LDHs/rGO, were constructed by the electrostatic self-assembly of exfoliated LDHs with few-layer graphene. The LDHs/rGO was loaded on carbon mats to construct the electro-Fenton cathode materials, and the non-homogeneous electro-Fenton oxidative degradation of organic pollutants was realized by the in situ electrocatalytic reduction of O2 to ∙OH. Meanwhile, the effects of solution pH, applied voltage, and initial concentration on the performance of non-homogeneous electro-Fenton were investigated with ceftazidime as the target pollutant, which proved that the cathode materials have an excellent electro-Fenton degradation effect.

Comprehensive strategies for effective fecal sludge management in Humanitarian Settings: A case study of Cox’s Bazar Rohingya Response

The influx of Forcibly Displaced Myanmar Nationals (FDMN) into Bangladesh has necessitated the development of robust Fecal Sludge Management (FSM) strategies in Cox’s Bazar to ensure safe and sustainable sanitation solutions. Effective fecal sludge management (FSM) is crucial in humanitarian settings for public health, environmental protection, and dignity. This study investigates comprehensive FSM strategies in Cox’s Bazar Rohingya refugee camps, one of the largest and most complex humanitarian crises. Using a mixed-methods approach, it combines quantitative data from surveys and field observations with qualitative insights from interviews and document analysis to examine the FSM value chain, including sludge production, containment, collection/emptying, transport, treatment, and disposal/reuse. Findings show high sludge production rates require frequent desludging, with challenges in containment due to structural weaknesses and privacy concerns. Collection and emptying face logistical and health issues, while transport is hindered by difficult terrain and limited vehicle availability. Treatment facilities are often inadequate, leading to operational challenges and environmental risks. Despite these issues, disposal and reuse practices in agriculture show promise but need safety and community acceptance improvements. The study recommends increasing desludging capacity, enhancing containment designs, improving transport infrastructure, expanding treatment facilities, and promoting safe disposal and reuse practices. Emphasizing community engagement, technological innovations, and coordinated efforts among stakeholders is crucial for sustainable FSM systems. Future research should focus on innovative technologies, long-term sustainability, and scalability of FSM strategies across contexts. This research provides valuable insights and practical recommendations for improving sanitation in similar emergency contexts worldwide.

Factorial experimental design for removal of Indigo Carmine and Brilliant Yellow dyes from solutions by coagulation

Textile and food industries produce huge amounts of wastewaters containing dye residues. When these wastewaters are discharged to receiving surface waters like as lakes and rivers, aesthetically unpleasant situations form. Therefore, these wastewaters should be treated. Wastewater treatment is sometimes an expensive operation and cheap methods should be developed. The removal of Indigo Carmine (I.C., Acid dye) and Brilliant Yellow (B.Y., Azo dye) from synthetically prepared solutions was studied by coagulation using iron chloride salt in a batch reactor at room temperature. As an experimental approach, two leveled factorial design with three factors was applied as a function of pH (4-12), iron chloride amount (0.1-0.4 g/500 mL) and dye concentration (100-200 mg/L). Low pHs supported to removal of these two dyes. The results showed that 100% I.C. dye removal and 90.5% B.Y. dye removal were achieved. The all parameters were statistically insignificant for both the dyes. Indigo Carmine and Brilliant Yellow dyes were removed from solutions successfully. The applied treatment method was evaluated as promising due to low sludge production, low cost, low coagulation duration and high performance. A time span of 5 minutes was found as enough for removals of both of the dyes. After treatment of I.C. and B.Y. dyes by coagulation, the coagulated dyes were determined as unreusable due to iron complex by these dyes. Flocculation was found to be ineffective. A continuous flow reactor was successfully adopted for these dyes.

Recent Advanced Biological Wastewater Treatment Technologies

Conventional wastewater treatment approaches such as activated sludge processes often rely on energy-intensive and resource-demanding processes. However, emerging technologies offer promising solutions to improve the sustainability of wastewater management. This review examines three innovative technologies, including anaerobic membrane bioreactor (AnMBR), partial nitritation/anammox (PN/A), and microalgae-based processes, and their potential advantages over traditional methods. AnMBR combine membrane filtration with anaerobic biological treatment, enabling high-quality effluent, reduced sludge production, and a compact footprint. The PN/A is a two-step biological process that can achieve efficient nitrogen removal with lower aeration requirements. Microalgae-based systems leverage photosynthetic organisms to remove nutrients and organic matter while generating biomass for potential resource recovery. Each alternative technology has strengths and limitations that must be carefully evaluated based on the site-specific factors, such as wastewater characteristics, treatment objectives, and local conditions. Continued research and technological development is necessary to address the remaining technical and economic barriers impeding their wider adoption. Integrating multiple alternative technologies in a treatment train can help optimize overall performance by leveraging the complementary advantages of different processes. This holistic approach can advance the sustainable wastewater management, promoting resource efficiency, energy savings, and environmental protection.

Highly efficient phosphorus removal by a novel prolonged stirring pelleting coagulation for sludge water treatment

Sludge water with high total phosphorus (TP) and suspended solids (SS) easily causes the effluent quality to be substandard in wastewater treatment plants. Although the conventional coagulation process can achieve TP removal, higher sludge production occurred due to loose flocs generation. The prolonged stirring pelleting coagulation (PSPC) process, based on one-by-one agglomeration and mechanical syneresis theories, was proposed to achieve compact flocs and reduced sludge production. Compared with the conventional coagulation process, the removal efficiency of TP was improved by >20 %, settling volume was reduced by about 30 %, and the dosages of polyaluminum chloride (PAC) and polyacrylamide (PAM) were saved by 50 % and 30 %, respectively in the PSPC process. The PSPC process produced pellet-like flocs with the d50 of 6.27 mm, settling velocity of 21.07 mm/s, effective density of 0.1411 g/cm3, and three-dimensional fractal dimension (D3) of 2.094 under the optimal conditions of 50 mg/L PAC and 10 mg/L PAM. With the d50 decreased from 38.38 μm to 21.81 μm, fragmentation of primary particles was realized by high-intensity stirring. Therefore, the mechanism of the PSPC process involves reducing voids in primary particles, achieving regular combination of microflocs under appropriate PAC, PAM, and hydraulic control. The PSPC process shows promising prospects for treating high SS wastewater and may have practical implications for enhancing coagulation.

Mechanisms and Strategies of Advanced Oxidation Processes for Membrane Fouling Control in MBRs: Membrane-Foulant Removal versus Mixed-Liquor Improvement.

Membrane bioreactors (MBRs) are well-established and widely utilized technologies with substantial large-scale plants around the world for municipal and industrial wastewater treatment. Despite their widespread adoption, membrane fouling presents a significant impediment to the broader application of MBRs, necessitating ongoing research and development of effective antifouling strategies. As highly promising, efficient, and environmentally friendly chemical methods for water and wastewater treatment, advanced oxidation processes (AOPs) have demonstrated exceptional competence in the degradation of pollutants and inactivation of bacteria in aqueous environments, exhibiting considerable potential in controlling membrane fouling in MBRs through direct membrane foulant removal (MFR) and indirect mixed-liquor improvement (MLI). Recent proliferation of research on AOPs-based antifouling technologies has catalyzed revolutionary advancements in traditional antifouling methods in MBRs, shedding new light on antifouling mechanisms. To keep pace with the rapid evolution of MBRs, there is an urgent need for a comprehensive summary and discussion of the antifouling advances of AOPs in MBRs, particularly with a focus on understanding the realizing pathways of MFR and MLI. In this critical review, we emphasize the superiority and feasibility of implementing AOPs-based antifouling technologies in MBRs. Moreover, we systematically overview antifouling mechanisms and strategies, such as membrane modification and cleaning for MFR, as well as pretreatment and in-situ treatment for MLI, based on specific AOPs including electrochemical oxidation, photocatalysis, Fenton, and ozonation. Furthermore, we provide recommendations for selecting antifouling strategies (MFR or MLI) in MBRs, along with proposed regulatory measures for specific AOPs-based technologies according to the operational conditions and energy consumption of MBRs. Finally, we highlight future research prospects rooted in the existing application challenges of AOPs in MBRs, including low antifouling efficiency, elevated additional costs, production of metal sludge, and potential damage to polymeric membranes. The fundamental insights presented in this review aim to elevate research interest and ignite innovative thinking regarding the design, improvement, and deployment of AOPs-based antifouling approaches in MBRs, thereby advancing the extensive utilization of membrane-separation technology in the field of wastewater treatment.

Approaching the circular economy: Biological, physicochemical, and electrochemical methods to valorize agro-industrial residues, wastewater, and industrial wastes

The circular economy policy enables the use of waste as an input to generate value repeatedly or at least to have the opportunity to be used a second time to extract valuable substances such as metals, dyes, hydrogen, phosphorus, nitrogen, nutrients, and fertilizers. For this reason, the present paper discusses approaches to valorizing wastewater, communal production, and agricultural wastes. In particular, techniques are considered to valorize biomass, lignin-containing residues, wastewater with a high organic content, sewage sludge, oily sludge, mine tailings, fly ashes, and iron production wastes. The procedures evaluated include biological processes, microbial fuel cells, electrokinetics, electro-demulsification, electrodialysis, and the Fenton process; their rational application promotes the reuse of different chemical and biological compounds to eventually supply some economic sectors with raw materials in a sustainable manner, solve the problem of pollution in the environment, and generate power at the same time. Of particular and emerging interest is the valorization of iron wastes, which serves to obtain low-cost catalysts for wastewater treatment processes. Additionally, the correct characterization of the chemical composition of wastes and determination of their final use allows better results and less time to valorize any wastes considered.