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Aerobic granular sludge (AGS) in continuous-flow reactors (CFRs) has attracted significant interest, with notable progress in research and application over the past two decades. Cumulative studies have shown that AGS-CFRs exhibit comparable morphology, settleability, and pollutant removal efficiency to AGS cultivated in sequencing batch reactors, despite their smaller particle sizes. Shear force and selection pressure are the primary drivers of granulation. While not mandatory for granulation, feast/famine conditions play a crucial role in ensuring long-term stability and nutrient removal. Additionally, bioaugmentation can facilitate the granulation process. Furthermore, this paper comprehensively assesses the application of AGS-CFRs in full-scale wastewater treatment plants (WWTPs). Currently, AGS-CFRs have been implemented in nine WWTPs, encompassing two distinct processes. Hydrocyclone-based densified activated sludge significantly enhances sludge density, settleability, and biological phosphorus removal efficiency, thus increasing treatment capacity. The microaerobic–aerobic configuration with internal separators can induce granulation, ensuring long-term stability, eliminating the need for external clarifiers, and reducing land and energy requirements. This review demonstrates the high potential of AGS-CFRs for intensifying existing WWTPs with minimal retrofitting needs. However, further research is required in granulation mechanisms, long-term stability, and nutrient removal to promote the widespread adoption of AGS.

Conventional methods to clean wastewater actually lead to incomplete treatments, calling for advanced technologies to degrade recalcitrant pollutants. Herein we review solar photo-oxidation to degrade the recalcitrant contaminants in industrial wastewater, with focus on photocatalysts, reactor design and the photo-Fenton process. We discuss limitations due to low visible-light absorption, catalyst collection and reusability, and production of toxic by-products. Photodegradation of refractory organics by solar light is controlled by pH, photocatalyst composition and bandgap, pollutant properties and concentration, irradiation type and intensity, catalyst loading, and the water matrix.

The down-flow hanging sponge (DHS) process has been rigorously studied as a significant post-treatment of anaerobically pretreated sewage and later for broader applications in industrial wastewater and pre-settled sewage treatment. The DHS has several advantages over conventional wastewater treatment technologies, i.e., simple process control, less workforce requirement, less footprint, low construction, operation, and maintenance costs due to simple design and less mechanical parts. DHS technology requires no external aeration, which saves energy use and minimizes the associated costs. Low excess sludge yield in the DHS process adds to economic efficiency. DHS achieved high organics removal efficiency up to 99% with considerable nutrient and pathogen removals at extremely short HRT. Up-to-date, six DHS configurations have been developed and extensively tested at pilot scale applications under different operational conditions, i.e., HRTs, OLRs, and SRTs. Accordingly, the DHS module has successfully been translated into full-scale demonstration plants in Japan, India, Egypt, and Thailand. This paper comprehensively reviews the mechanism and development history of the DHS system. The process performance for municipal and industrial wastewaters treatment covers a broad range of parameters, i.e., organics, nutrients, pathogens (bacteria, viruses, parasites) has been summarized. The unique feature of sludge reduction efficiency of DHS is comprehensively discussed. Moreover, the process microbiology, energy efficiency, emerging application of DHS for direct sewage treatment, pilot and full-scale experiences in the last 20 years are also reviewed. DHS technology has not yet being fully explored, mainly for nutrients removal, process modelling, optimization of wastewater and air flow configurations in DHS reactor, and biomass growth, which could help for further advancement in the process for better performance and wider applications.

Over the last decades, ultrafiltration (UF) has been and will continue to play an important role in the Chinese drinking water industry. The hybrid membrane processes have been emerging as promising alternatives to traditional treatment processes. This review paper offers a thorough overview of the current status of UF technology in the drinking water industry in China, while also evaluating the landscape of different hybrid membrane processes. The paper conducts statistical analysis on the projects operational between 2004 and 2022; and those currently under construction. This analysis accentuates the evolution of scale and capacity, geographical distribution characteristics, and the driving forces. Furthermore, the characteristics and application scenarios of several hybrid membrane processes are emphatically described, including direct UF, gravity-driven membrane process, coagulation-UF, activated carbon-UF, medium-flow process, long-flow process, and double-membrane process. A granular dissection of UF membrane market distribution follows, including an incisive comparison between submerged and pressurized UF membrane systems. Finally, the potential trajectory of UF in the Chinese drinking water industry is prospected. UF applications have grown significantly in China's drinking water industry, with the dominance of medium- and long-flow membrane processes. UF technology will contribute to future decentralized water supply.

Nitrogen removal in treatment wetlands is influenced by many factors, and the presence of electron donors (biodegradable organic matter) and electron acceptors (nitrate ions) is the main limiting one; for obtaining these conditions, multistage treatment wetlands (MTWs) are required, where an extensive nitrification can be obtained in the first stages under aerobic conditions leaving then to the following anoxic/anaerobic stages the duty of the denitrification. Most of the biodegradable organic matter is however oxidised in the first stages, and therefore, the inlet to the denitrification beds is usually poor of easily degradable carbon sources. This study is comparing the long-term performances obtained at several MTWs operating in Europe (North and South) and North Africa in order to understand if there is a significant avail in making use of the influent chemical oxygen demand (COD)/N ratio during the design phase for ensuring proper performances in terms of N overall removal. The statistic analysis performed in this study have shown that MTWs are capable to ensure sufficient removal of both organic and nutrients even in unfavourable proportions of macronutrients (C and N). The usual assumptions for conventional biological treatment systems concerning adequate C/N ratios seem to be dubious in case of wastewater treatment in MTWs.

With the unique cell compartmentalization and the ability to simultaneously oxidize ammonium and reduce nitrite into nitrogen gas, anaerobic ammonium-oxidizing (anammox) bacteria have challenged our recognitions of microorganism. The research conducted on these bacteria has been extended from bench-scale tryouts to full-scale reactor systems. This review addresses the recently discovered versatile properties of anammox bacteria and the applications and obstacles of implementing the anammox process in ammonia-rich wastewater treatment. We also discuss the merits and drawbacks of traditional and anammox-based processes for nitrogen removal and suggest areas for improvement.[PUBLICATION ABSTRACT]

Carwashes are highly water-consuming processes that require wastewater treatment before discharge into a sewer system due to the complex composition of leachate. Anionic surfactants (AS) are the main constituents of this wastewater because of their cleaning and solubilization properties; they can be potentially dangerous for the environment if not adequately treated. Constructed wetlands (CWs) are low-cost systems increasingly used to treat different types of wastewater; however, there are few studies on their use for the treatment of carwash wastewater. In this study, an innovative constructed wetland arranged in a “cascade” to simulate a wall system (WCCW) was experimented in 2010 and 2011 to treat AS. Three plant species were tested at different AS inlet concentrations (10, 50, and 100 mg L −1 ) with two hydraulic retention times (HRTs; 3 and 6 days): ribbon grass ( Typhoides arundinacea (L.) Moench (syn. Phalaris arundinacea L.) var. picta; Ta), water mint ( Mentha aquatica L.; Ma), and divided sedge ( Carex divisa Hudson; Cd). All plant species grew constantly over the experimental period, showing a capacity to tolerate even the highest AS concentration. Using the HRT of 6 days, raising the inlet concentration increased the AS outlet concentration, with similar values for the treatments (median values of 0.13–0.15, 0.47–0.78, and 1.19–1.46 mg L −1 at inlet concentrations in the order 10, 50, and 100 mg L −1 ). The shorter HRT led to significant differences among treatments in the reduction of outlet concentration, the best result being given by the tanks vegetated with Ma ( A  = 97.7 % with outlet concentration 0.35 mg L −1 ). After treatments of the WCCW, the AS content was reduced almost completely, with removal in the ranges 0.07–10.2 g m −2  day −1 for tanks planted with Ta, 0.10–9.1 g m −2  day −1 for Ma tanks, and 0.11–9.5 g m −2  day −1 for Cd tanks depending on the inlet concentration.

Clogging often leads to a decrease of the treatment performance of wetlands. The aims of this study were to compare the impact of different design and operational variables on the treatment efficiency and clogging processes and to model suspended solid (SS) accumulation within the saturated wetland zone using the Wang-Scholz model. Different vertical-flow constructed wetlands were operated from June 2011 until April 2014. Four treatment periods were assessed: set-up, first year after set-up period, second year after set-up period and diesel spill (for selected filters only). The filter with the highest chemical oxygen demand (COD) loading but no diesel contamination performed the best in terms of COD and biochemical oxygen demand (BOD) removal for the fourth and final treatment period. Filters contaminated by diesel performed worse in terms of COD and BOD but considerably better regarding nitrate-nitrogen removal. Serious clogging phenomena impacting negatively on the treatment performance and the hydraulic conductivity were not observed. Modelling results were generally poor for the set-up period, adequate for the first 2 years after the set-up period and variable after the diesel spill. The Wang-Scholz model performed well for less complex operations.

The effect on the environment of the operation of sludge treatment in reed beds (STRB) system is seen as quite limited compared to traditional sludge treatment systems such as mechanical dewatering, drying and incineration with their accompanying use of chemicals and energy consumption. There are several STRB systems in Denmark receiving sludge from urban wastewater treatment plants. Stabilization and mineralization of the sludge in the STRB systems occur during a period between 10 and 20 years, where after the basins are emptied and the sludge residue typically is spread on agricultural land. In the present study, the sludge residue quality after treatment periods of 10–20 years from four Danish STRBs is presented. After reduction, dewatering and mineralization of the feed sludge (dry solid content of 0.5–3 %) in the STRB systems, the sludge residue achieved up to 26 % dry solid, depending on the sludge quality and dimensioning of the STRB system. The concentration of heavy metals and hazardous organic compounds in the sludge residue that are listed in the Danish and EU legislation for farmland application of sludge was below the limit values. The nitrogen and phosphorus concentrations as an average in the sludge residue were 28 and 36 g/kg dry solid (DS), respectively. In addition, mineralization on average across the four STRB systems removed up to 27 % of the organic solids in the sludge. The investigation showed that the sludge residue qualities of the four STRBs after a full treatment period all complied with the Danish and European Union legal limits for agricultural land disposal.

The aim of this paper is to present a survey on the current status of wastewater treatment systems in 11 central and eastern European (CEE) countries, with a focus on rural areas and on small treatment wetlands for settlements of below 2,000 people. The results indicate that CEE countries have insufficient sanitation systems with different performance efficiencies. These differences stem from the different historical, political and economic developments as well as legislation in the previous five to six decades. CEE settlements with less than 2,000 inhabitants represent almost 30.0 % of the overall number of persons living in CEE countries. These data show that CEE countries have a mainly rural character, but this is slowly decreasing. Over 42 million inhabitants are waiting for proper sanitation systems. This requirement will be ever-growing in the next future, and it is important to know and to prepare appropriate steps for progressive realisation.