Wastewater Treatment Capacity Research Articles

One-year operation of a 20-L submerged anaerobic membrane bioreactor for real domestic wastewater treatment at room temperature: Pursuing the optimal HRT and sustainable flux

A 20 L hollow-fiber submerged anaerobic membrane bioreactor (SAnMBR) was used to treat real domestic wastewater at 25 °C with hydraulic retention times (HRTs) ranging from 4 to 12 h. The process performance was evaluated by organic removal efficiency, biogas production, sludge yield, and filtration behaviors during one-year's operation. For HRTs ranging between 6 and 12 h, the AnMBR showed good organic removal efficiency with chemical oxygen demand (COD) and biochemical oxygen demand (BOD) removal efficiencies of about 89% and 93%, respectively. The biogas yield was 0.26 L-gas/g-CODfed, with approximately 80% methane content, and the sludge yield was 0.07–0.11 g-VSS/g-CODrem. While at an HRT of 4 h, with the higher wastewater treatment capacity and organic loading rate (OLR), the biogas production was lower (0.17 L-gas/g-CODfed), and the sludge production was higher (0.22 g-VSS/g-CODrem). The organic removal performance (COD 84% and BOD 89%) at HRT of 4 h was acceptable due to the effective separation effect of the membrane filtration process. According to COD balance analysis, the low biogas yield and high sludge yield at HRT of 4 h were due to insufficient biodegradation under an OLR of 2.05 g-COD/L-reactor/d. Theoretical calculations based on Henry's law indicate that the ideal methane content in the biogas should be 82–85% when the operational temperature was 25 °C. To achieve a high flux and sustainable AnMBR operation, the impact of mixed liquor suspended solid (MLSS) and gas sparging velocity (GSV) on the filtration performance was analyzed. The critical flux increased with increase in the GSV from 24.2 to 174.3 m/h, but decreased with increase in the MLSS concentration from 8.2 to 20.2 g/L. Therefore, decreasing fouling rate to 0.8–1.2 kPa/d by efficiently controlling GSV and MLSS, sustainable operation could be achieved at a flux of 0.34 m/d.

ABACO: A New Model of Microalgae-Bacteria Consortia for Biological Treatment of Wastewaters

Microalgae-bacteria consortia have been proposed as alternatives to conventional biological processes to treat different types of wastewaters, including animal slurry. In this work, a microalgae-bacteria consortia (ABACO) model for wastewater treatment is proposed, it being calibrated and validated using pig slurry. The model includes the most relevant features of microalgae, such as light dependence, endogenous respiration, and growth and nutrient consumption as a function of nutrient availability (especially inorganic carbon), in addition to the already reported features of heterotrophic and nitrifying bacteria. The interrelation between the different populations is also included in the model, in addition to the simultaneous release and consumption of the most relevant compounds, such as oxygen and carbon dioxide. The implementation of the model has been performed in MATLAB software; the calibration of model parameters was carried out using genetic algorithms. The ABACO model allows one to simulate the dynamics of different components in the system, and the relative proportions of microalgae, heterotrophic bacteria, and nitrifying bacteria. The percentage of each microbial population obtained with the model was confirmed by respirometric techniques. The proposed model is a powerful tool for the development of microalgae-related wastewater treatment processes, both to maximize the production of microalgal biomass and to optimize the wastewater treatment capacity.

Functions of various bacteria for specific pollutants degradation and their application in wastewater treatment: a review

Nowadays, wastewater pollution has become a serious deterioration problem for biotic and abiotic environs due to tenacity, and bioremediation plays a significant role in restoring pollutions. Treatment strategies, such as physicochemical approaches, are inadequate to clean pollutions due to its sustainability. Bacterial remediation is an environmentally friendly, economically affordable, and acceptable alternative; thus, scholars have made progress using bioremediation as a successful treatment strategy. Different bacteria species with biodegradation potential have been isolated, characterized, and their performance has been evaluated depending on the contaminants contained in the wastewater. But, all species studied do not have a capacity for complete degradations of the targeted contaminant and are not effective in situations with wastes containing a variety of pollutants. In this review, different bacteria species with different remediation capacities based on types of contaminants (heavy metals, dyes, pesticides, and oils) were presented. Also, the basics of bacterial remediation, its mechanisms, principles, factors affecting the remediation process were discussed. By applying bacterial community analysis in the wastewater, important preliminary evidence can be obtained. The evidence could tell which species more effective in bioremediation of the polluted water. The review also explains the existing advances with extremophile, particularly alkaliphilic bacteria, and their uses in effluent treatment, and their prospects are provided. Several alkaliphilic bacterial species have been isolated for the potential decolorization efficiency, the Halomonas and Marinobacter adopted in the saline environment having a capacity for wastewater treatment.

From pea soup to water factories: wastewater paradigms in India and the Netherlands

Freshwater scarcity has increased in the cities of the global South due to rapid urban agglomeration and changing climate. Alternative water resources such as treated wastewater can play a significant role to reduce the water supply-demand gap. In the recent past, wastewater has been used solely for irrigation and other allied agriculture purposes, with limited focus on reuse for other purposes within the cities. Despite the progress in wastewater treatment technology and various policy frameworks, in low income and lower-middle-income countries, limited progress has been made. Through this article, we compare three aspects, representing the wastewater paradigms in India and the Netherlands. The three elements are 1) framing, 2) policy goals, and 3) technical and financial instruments. Using policy document analysis and interviews, we compare water and related policies prepared in India and the Netherlands. We found that the wastewater paradigms have evolved in the two countries. In India, the wastewater paradigms have realized paradigm changes from ‘water resource to meet a basic human need’ to ‘water as an engine of economic growth’ and then to ‘water scarcity and beautification of cities’. In the case of the Netherlands, the wastewater paradigms have changed from an emphasis on ‘public health and environmental concerns’ to the ‘circular economy of wastewater’. Although the Netherlands has to still meet the water quality targets of the European Water Directive Framework with regards to micropollutants, the country has made significant progress towards wastewater treatment and reuse in the last four decades. On the contrary, the Indian wastewater policy domain has room for improvement in terms of designing appropriate financial instruments and governance strategies to increase the urban wastewater treatment capacity and reuse. This article concludes that the use of the concept of wastewater paradigm is useful to show the progress and challenges in the two countries.

Upgrading fluidized bed bioelectrochemical reactors for treating brewery wastewater by using a fluid-like electrode

Anaerobic digestion has historically shown critical operational limitations for treating industrial wastewater. Our work aims to evaluate the resilience capacity of a novel concept so-called microbial electrochemical fluidized bed reactor (ME-FBR) for treating real brewery wastewater under continuous operation mode over one year period. All assays were run in parallel using a conventional anaerobic fluidized bed reactor (AFBR). The resilience tests were designed attending to the most typical operational problems showed by the AFBR technology in real brewery wastewater treatment plants. Four different stress situations were tested: i) pollutants overloading (as high as 51.2 kgCOD/m3 d), ii) presence of an active biocide in the fed stream (5% Vbiocide/Vreactor), iii) operation of the reactors after long starvation periods (16 days) and iv) operation at low temperature (<25 °C). Our pre-pilot scale ME-FBR outperformed traditional AFBR for wastewater treatment capacity under all stress test regarding COD removal rate, total nitrogen (TN) removal rate and bioenergy recovery (bioelectrochemical-assisted hydrogen generation). Among all stress test, low temperatures and long starvation periods deeply decrease the robustness of both technologies.

Enhancing oxygen reduction reaction in air-cathode microbial fuel cells treating wastewater with cobalt and nitrogen co-doped ordered mesoporous carbon as cathode catalysts

The sluggish oxygen reduction reaction (ORR) on the cathode severely limits the energy conversion efficiency of microbial fuel cells (MFCs). In this study, cobalt and nitrogen co-doped ordered mesoporous carbon (Cox-N-OMC) was prepared by heat-treating a mixture of cobalt nitrate, melamine and ordered mesoporous carbon (OMC). The addition of cobalt nitrate remarkably improved the ORR reactivity, compared to the nitrogen-doped OMC catalyst. By optimizing the dosage of cobalt nitrate (x = 0.6, 0.8 and 1.0 g), the Co0.8-N-OMC catalyst displayed excellent ORR catalytic performances in neutral media with the onset potential of 0.79 V (vs. RHE), half-wave potential of 0.59 V and limiting current density of 5.43 mA/cm2, which was comparable to the commercial Pt/C catalyst (0.86 V, 0.60 V and 4.76 mA/cm2). The high activity of Co0.8-N-OMC catalyst was attributed to the high active surface area, higher total nitrogen amount, and higher relative distribution of graphitic nitrogen and pyrrolic nitrogen species. Furthermore, single chamber microbial fuel cell (SCMFC) with Co0.8-N-OMC cathode exhibited the highest power density of 389 ± 24 mW/m2, chemical oxygen demand (COD) removal of 81.1 ± 2.2% and coulombic efficiency (CE) of 17.2 ± 2.5%. On the other hand, in the Co1.0-N-OMC catalyst, increasing the cobalt dosage from 0.8 to 1.0 g resulted in more oxidized-N species, and the reduced power generation in SCMFC (360 ± 8 mW/m2). The power generated by these catalysts and results of electrochemical evaluation were strongly correlated with the total nitrogen contents on the catalyst surface. This study demonstrated the feasibility of optimizing the dosage of metal to enhance wastewater treatment capacity.

Are the Water Quality Improvement Measures of China's South-to-North Water Diversion Project Effective? A Case Study of Xuzhou Section in the East Route.

Xuzhou is the hub city of the east route of China’s South-to-North Water Diversion (SNWD) project and implemented dozens of measures to ensure the water quality security of the water transmission line. In order to detect the effectiveness of water quality improvement measures, the monthly water quality data of five water quality parameters from 2005 to 2015 of six state-controlled monitoring sites in Xuzhou section were selected for analysis. The results showed that the water quality improved from 2.95 in 2005 to 2.74 in 2015, as assessed by the comprehensive water quality identification index (CWQII), and basically reached the Class III standards of China’s Environmental Quality Standard for Surface Water (GB3838-2002) from 2011 to 2015. The trend analysis showed that the decline of ammonia nitrogen (NH3-N) was the most obvious among the five water quality parameters. However, the concentrations of phosphorus (TP) showed significant upward trends at three sites. The positive abrupt change of time series of water quality occurred in 2009–2011. The identification of influencing factors of water quality changes by multivariate statistical methods found that the urbanization factor accompanied by a decrease in agricultural nonpoint source pollution emissions and the enhancement of wastewater treatment capacity, the closure of factories with substandard emissions and precipitation were the major influencing factors of most water quality parameters, which confirmed the effectiveness of measures for water quality improvement in Xuzhou.

Optimization of the wastewater treatment capacity of a short rotation willow coppice vegetation filter

The objective of this study was to determine the conditions to optimize the wastewater treatment efficiency of a short rotation willow coppice (SRWC) plantation (Salix miyabeana ‘SX67’) used as a vegetation filter to treat small municipal primary effluents (with less than 800 population equivalent). With the aim of maximizing the annual amount of wastewater treated, the effect of adjusting the hydraulic loading rate (HLR) according to the estimated evapotranspiration was tested at demonstration scale under humid continental climate conditions. We proposed a new method to calculate the evapotranspiration rate from plant physiological data, introducing an α factor based on direct transpiration measurements. This method increased the accuracy of the water balance, with a prediction of the crop coefficient (kc) based on either an seasonal approach (R2 of 0.88) or a monthly approach (R2 of 0.94). This led to a more precise estimation of the pollutant loading reaching the groundwater and could be used after plantation establishment as a fine-tuning tool. Adjusting the HLR to that of evapotranspiration between May and October led to an annual increase of 2 mm/d (around 0.35 m3/m2 per growing season) in HLR, while maintaining a pollutant loading removal efficiency of at least 96% for organic matter, 99% for total phosphorus and 93% for total nitrogen. A high HLR at the end of the season caused nitrogen leaching into groundwater, indicating that the HLR should be decreased in October, when willow growth is greatly reduced.

Effects of Different Biological Carriers in Microbial Fuel Cells.

In this study, to investigate their effects on battery power generation performance and wastewater treatment capacity, coal semicoke granular-activated carbon, granular graphite, and walnut shell-activated carbon were added to the anode compartment of a microbial fuel cell. As revealed from the experimental results, adding activated carbon and graphite can significantly decrease the startup time of microbial fuel cells as well as provide the shortest startup time of coal semicoke-activated carbon fluidized bed microbial fuel cells (MGAC-MFCs). The activated carbon particle diameter did not increase from 0.275 to 0.55 mm, and the voltage changed the chemical oxygen demand (COD) degradation efficiency. However, the 0.275 mm activated carbon exhibited a maximum open-circuit voltage of 935 mV as well as a COD degradation efficiency of 95%, and the operation cycle was shortened. After running a cycle, the COD removals of different systems were 85, 93, and 89%, and the maximum value was obtained by the MGAC-MFC system. After the activated carbon and graphite particles were added, the electrical performance and production capacity of the sewage treatment microbial fuel cells were significantly enhanced.

Water balance of municipal wastewater irrigation in a coastal forested watershed

AbstractIn the southeastern United States, coastal communities face challenges for water resources and wastewater treatment capacity. In North Carolina, 51 municipalities irrigate forests with municipal wastewater to absorb nutrients, reduce direct effluent discharge to surface waters, and recharge groundwater. Most facilities have land‐applied wastewater for decades, but there are no quantitative studies on the hydrologic impacts of this practice. This study developed a simulated water balance for the largest forest land‐application system in North Carolina which treats wastewater daily by irrigating 30 km2 of a mixed hardwood‐loblolly pine forest. A distributed hydrological model (MIKE SHE) was adapted to simulate 20 years of watershed evapotranspiration (ET) and water table depth (WTD) under irrigated and nonirrigated conditions. We found that irrigation impact to annual and monthly WTD was negligible in years with average and above average rainfall. For wet years, drainage increased with irrigation while ET and WTD remained similar to nonirrigated conditions. In dry years, ET was 31 to 39 mm higher in irrigated forest than nonirrigated forest though the change in groundwater storage remained close to zero annually. Our simulation study suggested that the drivers of on‐site drainage were predominantly rainfall and irrigation, and the annual watershed drainage increased in volumes equal to 93%–100% of the added annual irrigation input. This study offers insights to water balance dynamics in irrigated forests and coastal forest resiliency to variable wastewater hydraulic loading.

Decrease of both river flow and quality aggravates water crisis in North China: a typical example of the upper Yongding River watershed.

Due to unevenly distributed water resources, semi-arid regions are particularly prone to severe water shortage and quality degradation. In this study, based on long-term hydrological database (1935-2015), and the latest available water quality data sets (2011-2016), we analyzed the water crisis and its driving forces in the upper Yongding River watershed, a typical water shortage area in North China. The results showed that human induced excessive water consumption is responsible for the significantly decreased river flow over the past eight decades. Although the capacity of the watershed wastewater treatment has improved, current water quality does not meet the requirements of the national water management goals, because of the excessive nitrogen and CODCr (chemical oxygen demand), which mainly come from the wastewater and feedlots discharge. Due to the decreased river flow, current Yongding River is unable to dilute and assimilate pollutions. The analysis of river pollutant load illustrated that more than 60 % of the nitrogen in the river water system is diverted for reservoir storage, and more than 50 % of the CODCr and TP are diverted for irrigation, thereby, increasing the risk of reservoirs eutrophication and threatening food safety. Besides, the high Cl- (388.2 ± 322.5 mg/L) and SO42- (470.6 ± 357.7 mg/L) imply that the upper river water are not suitable for drinking and irrigation purposes, and a potential risk of salinization if the river flow continues to decrease. We conclude that water resources over extraction and quality degradation are the main driving factors of the Yongding River water crisis.

Effects of an external magnetic field on microbial functional genes and metabolism of activated sludge based on metagenomic sequencing

This study explored the effect of 70-mT magnetic field on wastewater treatment capacity for activated sludge in long-term laboratory-scale experiments. Metagenomic sequencing were conducted based on Illumina HiSeq 2000 platform after DNA extraction of the activated sludge. Then the effect of the magnetic field on the microbial unigene and metabolic pathways in activated sludge was investigated. As a result, higher pollutant removal was observed at 70 mT, with which the elimination of total nitrogen (TN) was the most effective. Functional genes annotated based on eggNOG database showed that unigenes related to information storage and processing were enhanced by the magnetic field. For CAZy classification, category such as glycosyl transferases was more abundant in the reactor with magnetic field, which has been shown to promote the entire energy supply pathway. Additionally, in the KEGG categories, unigenes related to signaling molecules and interaction were significantly inhibited. Through the enrichment analysis of the nitrogen metabolism pathway, the magnetic field inhibited anabolic nitrate reduction by significantly inhibiting enzymes such as [EC:1.7.7.2], [EC:1.7.7.1], [EC:3.5.5.1], [EC:1.4.1.2] and [EC:4.2.1.1], which are related to the improvement of the denitrification ability. This study can provide insight for future research on the response mechanism of activated sludge to magnetic fields.

Urban water security of China’s municipalities: Comparison, features and challenges

Current urban water security studies failed to incorporate sustainability concerns and the objectives of urban water management practices. China’s municipalities, namely Shanghai, Beijing, Tianjin and Chongqing, have faced great pressure on water security, however, literature has not yet investigated their features and concerns of urban water security. To address these challenges, this study suggested a water security assessment framework which incorporates the features of urban water management transitions and sustainability aspects to analyze the urban water security condition of the four municipalities during 2011–2017. In addition, the Criteria Importance Through Intercriteria Correlation (CRITIC) method was employed to detect the weights of indicators without incorporating personal value judgements to enhance the results robustness. The results point out that Beijing has had the best water security condition, followed by Tianjin, Shanghai and Chongqing. In addition, Shanghai has had the largest improvement of water security condition among the municipalities during the investigated period. The condition of water use efficiency, river quality, pollutant discharge, wastewater treatment capacity, green covered area and crop area affected by droughts and floods has showed a great variation between the four municipalities. River quality improvement, and mitigation of droughts and floods damages have remained top priority in Tianjin and Beijing. The Shanghai government should lay a particular concern on water use reduction and the regulation of wastewater and pollutant discharges. The Chongqing government should reinforce the coverage of sewage and drainage infrastructure, advance water use efficiency and balance water intensity of economic activities to avoid long term damages on water security. The assessment framework and approach are conducive to investigate and communicate water security condition of cities and can assist tailoring policy measurements to cities in different water security contexts.

Customized thin and loose cake layer to mitigate membrane fouling in an electro-assisted anaerobic forward osmosis membrane bioreactor (AnOMEBR)

Anaerobic forward osmosis membrane bioreactor (AnOMBR) is a potential wastewater treatment technology, due to its low energy consumption and high effluent quality. However, membrane fouling is still a considerable problem which causes dwindling of water flux and shortening the membrane lifetime. In this study, electro-assisted anaerobic forward osmosis membrane bioreactor (AnOMEBR) was developed to treat wastewater and mitigate membrane fouling, in which the conductive FO membrane was used both as the separation unit and cathode. The formation, development and alleviation of membrane fouling in AnOMEBR were investigated. The results showed that the soluble microbial products (SMP) content and the proteins/polysaccharides (PN/PS) value in AnOMEBR were 26% and 15% lower than that in AnOMBR, respectively. The absolute value of Zeta of sludge mixture in AnOMEBR was 1.2 times that of the AnOMBR. The increase in the interaction between the membrane surface and the negatively charged foulants could inhibit the adsorption of foulants on membrane surface in the initial stage of membrane fouling. The strong interaction among foulants further affected the composition, structure and thickness of the cake layer on the FO membrane surface. AnOMEBR with a shorter hydraulic retention time, a higher organic loading rate and a lower osmotic pressure difference, could still obtain a lower flux decline rate of 0.063 LMH/h, which was 35.7% lower than AnOMBR. The wastewater treatment capacity of AnOMEBR was nearly 1.5 times that of the AnOMBR. This work provides an efficient strategy for mitigating membrane fouling and improving wastewater treatment capacity.

Oily wastewater treatment by a continuous flow microbial fuel cell and packages of cells with serial and parallel flow connections

This study reports the results of the application of microbial fuel cells (MFC) in refinery wastewater (RW) treatment. In this research, the effect of hydraulic retention time (HRT), and scale-up on the performance of a novel expandable modular design of single-chamber MFC (SCMFC) has been investigated. In the first part of the paper, the effect of HRT on chemical oxygen demand (COD) removal and electricity generation was examined. The generated steady open-circuit voltage (OCV) was 785 mV at HRT 90 h, and the provided maximum power density (PD) was 113 mW/m2 at HRT 15 h. At HRT of 45 h, COD removal increased up to 87% via an increase in the HRT. In the second part, the scale-up of SCMFC was investigated by serial (SFC) or parallel (PFC) connecting the outlets and inlets of fluid flows. The average produced OCV was 760 mV in PFC mode, and average produced PD in PFC and SFC modes were 97 and 75.6 mW/m2, respectively. COD removal in SFC and PFC modes were reported to be 89 and 42%, respectively. Compared to PFC mode, SFC mode was more efficient in terms of COD removal and coulombic efficiency. However, it produced lower PD compared to PFC mode. It is possible to control the quality and capacity of wastewater treatment by using combining the SFC and PFC mode connections in packages of MFCs.

Assessing the Ability to Treat industrial Wastewater by Constructed Wetland Model Using the Brachiaria mutica

The speed of urbanization—industrialization in Vietnam is increasing, which helps to boost the economy and at the same time create great environmental pressure. Industrial wastewater from production processes is always abundant and discharged continuously every day with diverse pollution components. Wastewater treatment of by simple and low-cost methods has been concerned. In this study, constructed wetland (CW) model using the Brachiaria mutica has been researched to evaluate the performance of Horizontal subsurface flow (HSSF) CW in treating industrial effluents. The model built with dimensions is 1.2 m × 0.4 m × 0.6 m and operated under 4 research organic load rates respectively 40; 60; 80; 100 kgCOD/ha.day. Average treatment efficiency of pollution parameters through study is COD 93.86%, T–N 97.46%, T–P 91.12%, TSS 83.07%, BOD5 97.72%. The output concentration values of the pollution parameters are smaller than the allowable values in column A QCVN 40: 2011/ BTNMT. The results of this study will be the premise to find solutions for industrial wastewater treatment with easy to operate and environmentally friendly. The process of industrial wastewater treatment using CW planning B. mutica is simple and efficient. The success of this work is possible to expand research on wastewater treatment capacity of CW planning B. mutica with different kinds of wastewater such as domestic wastewater, livestock wastewater, medical wastewater.

Externalidades ambientales y resiliencia lacustre en la Ciénega de Chignahuapan, México

Este estudio analiza los conflictos que producen los modos de apropiación de los recursos naturales y las externalidades ambientales que alteran las funciones ecosistémicas de la Ciénega de Chignahuapan o Atarasquillo. Se utilizó literatura, documentos oficiales, estadísticas y cartografía, la observación in situ y la información oral de los actores sustenta la base empírica de la investigación. Las externalidades regionales que amenazan la estabilidad del cuerpo lacustre expresan cambios de uso de suelo, disminución de los servicios hídricos del bosque, contaminación de afluentes, construcción de vialidades regionales, actividad industrial, re densificación demográfica, urbanización y sub urbanización. Las externalidades locales manifiestan alteración del régimen hidrológico, quemas de tule, introducción de peces exóticos, contaminación por descargas residuales crudas, proliferación de malezas terrestre y acuática, transferencia de los derechos de la actividad cinegética y aprovechamiento de subsistencia de los recursos lacustres. La presión exhibe un escenario desalentador que anticipa descenso de la capacidad natural del ecosistema lentico para superar las externalidades negativas. La visión sostenible prescribe estrategias de colaboración, autogestión y regulación, con el objeto de que las industrias acaten la responsabilidad ambiental para tratar y conservar el agua; los gobiernos incrementen la capacidad de tratamiento de aguas residuales y los usuarios mejoren el uso y cuidado del recurso. En las unidades de manejo monitorear el cumplimiento de la conservación, aplicar la ley en materia cinegética y prácticas ilegales, regular el arrendamiento cinegético por terceros, recuperar el conocimiento tradicional y desarrollar proyectos alternativos que moderen el uso de los recursos lacustres.Palabras clave: Externalidades, resiliencia, Ciénega, apropiación, recursos naturales.

Visible-Light Photocatalytic Ozonation Using Graphitic C3N4 Catalysts: A Hydroxyl Radical Manufacturer for Wastewater Treatment.

Photocatalytic ozonation (light/O3/photocatalyst), an independent advanced oxidation process (AOP) proposed in 1996, has demonstrated over the past two decades its robust oxidation capacity and potential for practical wastewater treatment using sunlight and air (source of ozone). However, its development is restricted by two main issues: (i) a lack of breakthrough catalysts working under visible light (42-43% of sunlight in energy) as well as ambiguous property-activity relationships and (ii) unclear fundamental reasons underlying its high yield of hydroxyl radicals (•OH). In this Account, we summarize our substantial contributions to solving these issues, including (i) new-generation graphitic carbon nitride (g-C3N4) catalysts with excellent performance for photocatalytic ozonation under visible light, (ii) mechanisms of charge carrier transfer and reactive oxygen species (ROS) evolution, (iii) property-activity relationships, and (iv) chemical and working stabilities of g-C3N4 catalysts. On this basis, the principles/directions for future catalyst design/optimization are discussed, and a new concept of integrating solar photocatalytic ozonation with catalytic ozonation in one plant for continuous treatment of wastewater regardless of sunlight availability is proposed.The story starts from our finding that bulk/nanosheet/nanoporous g-C3N4 triggers a strong synergy between visible light (vis) and ozone, causing efficient mineralization of a wide variety of organic pollutants. Taking bulk g-C3N4 as an example, photocatalytic ozonation (vis/O3/g-C3N4) causes the mineralization of oxalic acid (a model pollutant) at a rate 95.8 times higher than the sum of photocatalytic oxidation (vis/O2/g-C3N4) and ozonation. To unravel this synergism, we developed a method based on in situ electron paramagnetic resonance (EPR) spectroscopy coupled with an online spin trapping technique for monitoring under realistic aqueous conditions the generation and transfer of photoinduced charge carriers and their reaction with dissolved O3/O2 to form ROS. The presence of only 2.1 mol % O3 in the inlet O2 gas stream can trap 1-2 times more conduction band electrons than pure O2 and shifts the reaction pathway from inefficient three-electron reduction of O2 (O2 → •O2- → HO2• → H2O2 → •OH) to more efficient one-electron reduction of O3 (O3 → •O3- → HO3• → •OH), thereby increasing the yield of •OH by a factor of 17. Next, we confirmed band structure as a decisive factor for catalytic performance and established a new concept for resolving this relationship, involving "the number of reactive charge carriers". An optimum balance between the number and reducing ability of photoinduced electrons, which depends on the interplay between the band gap and the conduction band edge potential, is a key property for highly active g-C3N4 catalysts. Furthermore, we demonstrated that g-C3N4 is chemically stable toward O3 and •O2- but that •OH can tear and oxidize its heptazine units to form cyameluric acid and further release nitrates into the aqueous environment. Fortunately, •OH usually attacks organic pollutants in wastewater in preference to g-C3N4, thus preserving the working stability of g-C3N4 and the steady operation of photocatalytic ozonation. This AOP, which serves as an in situ •OH manufacturer, would be of interest to a broad chemistry world since •OH radicals are active species not only for environmental applications but also for organic synthesis, polymerization, zeolite synthesis, and protein footprinting.

UU-type parallel mini-hydrocyclone group for oil-water separation in methanol-to-olefin industrial wastewater

Methanol to olefin (MTO) process is a chemical technology for producing low-carbon olefins from coal-based or natural gas-based methanol. It has changed the pattern of low-carbon olefins relying on petroleum cracking, and opened a new process for producing low-carbon olefins. However, the wastewater generated in the MTO process is difficult to handle. Numbering-up of mini-hydrocyclone is often used to enhance the processing capacity of wastewater treatment when mini-hydrocyclone is used for oil-water separation in methanol-to-olefin process. In this paper, the efficiency and pressure drop distribution of a single mini-hydrocyclone were studied in the laboratory. In addition, according to the UU parallel configuration model, 150 mini-hydrocyclones arranged in parallel were established and used for the oil-water separation process of methanol-to-olefin reaction wastewater. The industrial operation results show that the average flow rate of long-period operation is 282 m3/h, the average pressure drop was 0.31 MPa, and the average separation efficiency of the mini-hydrocyclone group was 90 %. Especially for heavy components such as alkanes and aromatics, it had a good separation effect. Thus, it is shown that the mini-hydrocyclone group has good applicability in oil-water separation.

Seasonal characterization of municipal wastewater and performance evaluation of a constructed wetland system in Addis Ababa, Ethiopia

Rapid urbanization and industrialization are increasing fresh water consumption and lead to a corresponding rise in wastewater generation. Because of insufficient capacity of wastewater treatment, many of Addis Ababa’s residents are forced to dispose domestic waste into the environment. The purpose of this study was to characterize wastewater and evaluate the performance of constructed wetland. The physicochemical and bacteriological parameters of municipal wastewater was analysed in dry and wet seasons. Three parallel pilot horizontal subsurface flow constructed wetland was constructed, lined with polyethylene and filled with gravel. The two HSSFCW cells were planted with Vetiveria zizanioides and Phragmites karka while one cell was unplanted and used for control. The wastewater was loaded with 0.177 m3/day flow rate at hydraulic loading rate of 0.05 m/day with a hydraulic retention time of 3 days. Standing nutrient accumulation and aboveground biomass of plants were determined. The findings revealed a significant difference in concentration of COD, BOD5, NH4+ and temperature of the municipal wastewater between dry and wet seasons. The HSSFCW cell planted with V. zizanioides removed more TSS (96.7%), BOD5 (92%), COD (90.8%), TP (88.5%), TC (4.7 log units) and E. coli (4.1 log units) than that planted with P. karka. The maximum above-ground dry biomass and the maximum standing nutrient accumulation were achieved by P. karka than V. zizanioides at the end of experiment. It was concluded that the HSSFCW cells planted with V. zizanioides and P. karka have a potential of removing nutrients, organic matter and pathogen efficiently and was not affected by seasonal variation.