A review on treatment of petroleum refinery and petrochemical plant wastewater: A special emphasis on constructed wetlands

An overview of recent advances in treatment of complex dye-containing wastewater and its techno-economic assessment

Industries such as textiles, polymers, pharmaceuticals, papers, and tanneries are the key contributors to the global economy. These industries utilize various types of synthetic dyes in their processes, leading to discharge of dyes-contaminated wastewater. The wastewater generally contains various types of dyes (such as methyl orange, congo red, malachite green, etc.), which have a detrimental impact on the ecosystem and human health due to their toxic, carcinogenic, and mutagenic nature. As the result, it is crucial to treat the dyes-contaminated wastewater to protect the environment and render it suitable for reuse, mitigating the escalating global demand for clean water. This review provides a comprehensive overview of dyes and their treatment technologies (i.e., physical, chemical, and biological treatment). Among various treatment methods, the biological treatment is widely employed due to its energy efficiency and eco-friendliness. However, biological treatment faces challenges such as slow processing rates and limited effectiveness in handling low-biodegradability pollutants (BOD5/COD <0.2). This review also highlighted recent advancements in treatment technologies and explored the emerging integrated treatment method that aims to achieve higher removal efficiency for a low biodegradability index dye-contaminated wastewater. Additionally, a techno-economic assessment is presented, analyzing the cost-effectiveness of the emerging technologies in real-world applications. Further, the critical research gaps and future outlooks are also discussed. Overall, the review aims to contribute to the ongoing efforts to improve wastewater treatment processes and promote sustainable water management practices.

Investigating the effect of hydraulic residence time, artificial aeration and plants presence on different constructed wetland designs treating oil industry effluent

Constructed Wetlands (CW) have gained popularity over the last decades due to their cost-effectiveness, easy and simple operation and environmental compatibility in wastewater treatment. This ecological engineering technology appears particularly ideal for low-income regions. In this study, three widely used CW types (horizontal flow, vertical flow, and hybrid CW) were constructed and evaluated for their effectiveness in removing various pollution parameters (BOD5, COD, TSS, NH4-N, NO3-N, and TP) from an industrial effluent. Different configurations were tested such as CW type, hydraulic residence time, plants presence, and artificial aeration. Results showed that the hybrid CW configuration (i.e., vertical flow CW followed by horizontal subsurface flow CW) achieved the highest removal rates of all pollutants, i.e., more than 90% of BOD5, COD, TSS, and NH4-N. The single horizontal flow and vertical flow CW designs showed variations in the removal of NO3-N and TP (less than 30%), which were significantly improved (50% and 70%, respectively) by using the hybrid CW system. Artificial aeration significantly improves the performance of the CW system, especially for ammonia nitrogen and organic matter removal, while plants presence is also beneficial in the treatment performance. An 8-days HRT seems to be adequate for high removal rates in passive CW designs, though in aerated wetlands a lower HRT of 4 days seems sufficient. These findings suggest that the hybrid CW system could be a promising option for efficient wastewater treatment in developing regions.

Modeling and simulation for the sustainable recovery of aromatics (BTX) from petrochemical industrial wastewater

Petrochemical wastewater is a major industrial source of pollution that produces a variety of toxic organic and inorganic pollutants, naturally present or added during the process. These pollutants are a serious threat to the soil, water, environment, and human being due to their complex and hazardous nature. Glycols such as monoethylene glycol (MEG), diethylene glycol (DEG), triethylene glycol (TEG), and aromatics (BTX-benzene, toluene, and xylene) are the most common organic impurities present in petrochemical wastewater. The objective of this paper is to recover aromatics and water from petrochemical industrial wastewater. The reclamation process is used to remove inorganic impurities such as heavy metals Fe, Zn, Pb, Mn, Al, Ni, As, Cr, Cu, Cd, and K and salts. In the present work, 1% sodium bi-carbonate (NaHCO3) is used to precipitate the inorganic impurities present in the wastewater at 40 °C atmospherically. Aspen Hysys simulation software is used for modeling and simulation for the treatment process using NRTL (non-random-two-liquid) thermodynamic model. The process generated from Aspen Hysys is validated with lab experiments. To support global sustainable development, this study is focused on reducing, reusing, and recycling separation techniques such as centrifuge separation and vacuum distillation have been used. The characterization of regenerated water was performed using ICP-OES (inductively coupled plasma-optical emission spectroscopy) to determine the reduction in heavy metals. It was found that > 99.5% of heavy metals were removed. The regeneration of these aromatics is necessary for economic and environmental reasons so that it can be reused to avoid its disposal in and contamination of natural environments.

A review on hazards and treatment methods of released antibiotics in hospitals wastewater during the COVID-19 pandemic

Drugs and related goods are widely used in order to promote public health and the quality of life. One of the most serious environmental challenges affecting public health is the ongoing presence of antibiotics in the effluents generated by pharmaceutical industries and hospitals. Antibiotics cannot be entirely removed from wastewater using the traditional wastewater treatment methods. Unmetabolized antibiotics generated by humans can be found in urban and livestock effluent. The antibiotic present in effluent contributes to issues with resistance to antibiotics and the creation of superbugs. Over the recent 2 years, the coronavirus disease 2019 pandemic has substantially boosted hospital waste volume. In this situation, a detailed literature review was conducted to highlight the harmful effects of untreated hospital waste and outline the best approaches to manage it. Approximately 50 to 70% of the emerging contaminants prevalent in the hospital wastewater can be removed using traditional treatment strategies. This paper emphasizes the numerous treatment approaches for effectively eliminating emerging contaminants and antibiotics from hospital wastewater and provides an overview of global hospital wastewater legislation and guidelines on hospital wastewater administration. Around 90% of ECs might be eliminated by biological or physical treatment techniques when used in conjunction with modern oxidation techniques. According to this research, hybrid methods are the best approach for removing antibiotics and ECs from hospital wastewater. The document outlines the many features of effective hospital waste management and might be helpful during and after the coronavirus disease 2019 outbreak, when waste creation on all hospitals throughout the globe has considerably increased.

Efficient treatment of secondary treated refinery wastewater using sand biofiltration: Removal of hazardous organic pollutants

This study explored the potential of sand biofiltration for tertiary treatment of real refinery wastewater. The biofilter (2 cm (I.D.) x 15 cm (L)) operated on secondary treated refinery wastewater at flow rate of 1 mL/min had empty bed contact time (EBCT) of 47.12 min for one circulation. Maximum reduction in COD after 4, 8 and 12 times recirculation was 25 %, 52 % and 56 %; while the TOC reduction was 33 %, 43 % and 51 %, respectively, after biofilm development over 30 days. Quantification using two dimensional gas chromatography - time of flight mass spectrometry (GCxGC-TOF MS) revealed that several of the identified target compounds could not be detected in the wastewater after 12 recirculations. After 8 times recirculation, most of the compounds showed very high removal efficiency. For biofiltration over the flow rate range 2-10 mL/min, the reduction in COD and NH4+-N ranged from 62-73 % and 78-86 %, respectively, after 8 times recirculation. The nitrite concentration first increased and subsequently decreased, while the nitrate concentration continuously increased with increase in the number of recirculations. Solid phase micro-extraction (SPME) analysis of the aqueous phase using GCxGC-TOF MS and a semi-quantitative approach indicated that the removal of predominant classes of compounds was greater than 95 % after 8 times recirculation, with maximum reduction occurring in the first pass through the biofilter. Assimilable organic carbon (AOC) reduction was 98 % after 8 times recirculation. Metagenomic analysis revealed that Proteobacteria was the most dominant phylum in the biofilter. Many known polynuclear aromatic hydrocarbon (PAH) degraders, such as Sphingomonadales, Burkholderiales, Rhodobacterales and Rhodospirillales, were found in the biofilter leading to high removal efficiency of hazardous organic pollutants.

Photocatalytic removal of ammonia nitrogen from water: investigations and challenges for enhanced activity

Ammonia nitrogen (NH3-N/NH4+-N) serves as a crucial chemical in biochemistry and fertilizer synthesis. However, it is also a toxic compound, posing risks from eutrophication to direct threats to human health. Ammonia nitrogen pollution pervades water sources, presenting a significant challenge. While several water treatment technologies exist, biological treatment, though widely used, has its limitations. Hence, green and efficient photocatalytic technology emerges as a promising solution. However, current monolithic semiconductor photocatalysts prove inadequate in controlling ammonia nitrogen pollution. Therefore, this review focuses on enhancing semiconductor photocatalysts' efficiency through modification, discussing four mechanisms: (1) mono-ionic modification; (2) metallic and non-metallic modification; (3) construct heterojunctions; and (4) enhancement of synergistic effects of multiple technologies. The influencing factors of photocatalytic ammonia nitrogen removal efficiency are also explored. Moreover, the review outlines the limitations of current photocatalytic pollution treatment and discusses future development trends and research challenges. Currently, the main products of ammonia nitrogen removal include NO3-, NO2-, and N2. To mitigate secondary pollution, the green process of converting ammonia nitrogen to N2 using photocatalysis emerges as a fundamental approach for future treatment. Overall, this review aims to deepen understanding of photocatalysis in ammonia nitrogen treatment and guide researchers toward widespread implementation of this endeavor.

The impact of ozone treatment on the removal effectiveness of various refractory compounds in wastewater from petroleum refineries

Large volumes of wastewater are generated during petroleum refining processes. Petroleum refinery wastewater (PRW) can contain highly toxic compounds that can harm the environment. These toxic compounds can be a challenge in biological treatment technologies due to the effects of these compounds on microorganisms. These challenges can be overcome by using ozone (O3) as a standalone or as a pretreatment to the biological treatment. Ozone was used in this study to degrade the organic pollutants in the heavily contaminated PRW from a refinery in Mpumalanga province of South Africa. The objective was achieved by treating the raw PRW using ozone at different ozone treatment times (15, 30, 45, and 60 min) at a fixed ozone concentration of 3.53 mg/dm3. The ozone treatment was carried out in a 2-liter custom-designed plexiglass cylindrical reactor. Ozone was generated from an Eco-Lab-24 corona discharge ozone generator using clean, dry air from the Afrox air cylinder as feed. The chemical oxygen demand, gas chromatograph characterization, and pH analysis were performed on the pretreated and post-treated PRW samples to ascertain the impact of the ozone treatment. The ozone treatment was effective in reducing the benzene, toluene, ethylbenzene, and xylenes (BTEX) compounds in the PRW. The 60-min ozone treatment of different BTEX pollutants in the PRW resulted in the following percentage reduction: benzene 95%, toluene 77%, m + p-xylene 70%, ethylbenzene 69%, and o-xylene 65%. This study has shown the success of using ozone in reducing the toxic BTEX compounds in a heavily contaminated PRW.

Using structural equation modeling to assess the genotoxic and mutagenic effects of heavy metal contamination in the freshwater ecosystems: A study involving Oreochromis niloticus in an urban river

Chemical pollutants represent a leading problem for aquatic ecosystems, as they can induce genetic, biochemical, and physiological changes in the species of these ecosystems, thus compromising their adaptability and survival. The Capibaribe River runs through the state of Pernambuco, located in Northeastern Brazil, and passes through areas of agricultural cultivation, densely populated cities, and industrial centers, primarily textiles. Despite its importance, few ecotoxicological studies have been conducted on its environment, and knowledge about pollution patterns and their effects on its biota is still being determined. The objective of this study was to evaluate the water quality and the damage supposed to be caused by pollutants on the DNA specimens of Nile tilapia (Oreochromis niloticus) obtained from seven strategic points of Capibaribe. Tilapia specimens and water were collected during the rainy and dry seasons from 2015 to 2017. The following characteristics were analyzed: physicochemical (six), metal concentration (seven), local pluviosity, micronuclei, and comet assay. The physicochemical and heavy metal analyses were exploratory, whereas the ecotoxicological analyses were hypothetical. To verify this hypothesis, we compared the groups of fish collected to the results of the micronuclei test and comet assay. We created a Structural Equation Model (SEM) to determine how each metal's micronuclei variables, damage index, pluviosity, and concentration were related. Our results demonstrated that the highest values for markers of genetic damage were detected at points with the highest heavy metal concentrations, especially iron, zinc, manganese, chromium, and cadmium. The SEM demonstrated that metals could explain the findings of the genotoxicity markers. Moreover, other pollutants, such as pesticides, should be considered, mainly where the river passes through rural areas. The results presented here demonstrate that the Capibaribe River has different degrees of contamination and confirm our hypothesis.

Petroleum wastewater: Environmental protection, treatment, and safe reuse: An overview

Oil petroleum production consumes about 1.0-7.2 bbl. The needed water for such production ranges between 0.47 and 7.2 L water to 1.0 L crude. Between 80 and 90% of the consumed water is disposed of as wasted effluents. Consequently, there is an important connection between petroleum production and the contamination of the environment and surface water in addition to their ecotoxicological effects. The objective of the present review is to through light on the hazardous impact of petroleum wastewater on the environment and water ways. The present study presents several wastewater treatment technologies in handling the petroleum produced water (PPW) and reducing the hazardous impact to the environment. Safe reuse is also presented including simple, advanced, and environmentally friendly techniques. The reported treatment technologies are divided into five main categories: membrane technologies, biological treatment processes, electro-chemical coagulation, physical/chemical treatment processes (dissolved air flotation (DAF)/air flotation (IAF), adsorption, and chemical flocculation), and catalytic oxidation including chemicals such as advanced and Fenton oxidation processes (AOPs). The analysis and observation of each treatment process are also presented. Implementing of these processes in sequential and/or in combined to avoid the drawbacks of any poor treatment are discussed. The present review discusses; also, in detail each of these treatment technologies and their efficiency including the observation and conclusions of each one. The study shows; also; how the final treated effluent can be reused for non-potable purposes as an additional water resource according to the degree of decontamination. An additional advantage of treatment is protection of both the environment and the water ways by avoiding any discharge of such hazardous wastewater.

Neuro-fuzzy modelling of a continuous stirred tank bioreactor with ceramic membrane technology for treating petroleum refinery effluent: a case study from Assam, India

A continuous stirred tank bioreactor (CSTB) with cell recycling combined with ceramic membrane technology and inoculated with Rhodococcus opacus PD630 was employed to treat petroleum refinery wastewater for simultaneous chemical oxygen demand (COD) removal and lipid production from the retentate obtained during wastewater treatment. In the present study, the COD removal efficiency (CODRE) (%) and lipid concentration (g/L) were predicted using two artificial intelligence models, i.e., an artificial neural network (ANN) and a neuro-fuzzy neural network (NF-NN) with a network topology of 6-25-2 being the best for NF-NN. The results revealed the superiority of NF-NN over ANN in terms of determination coefficient (R2), root mean square error (RMSE), and mean absolute percentage error (MAPE). Three learning algorithms were tested with NF-NN; among them, the Bayesian regularization backpropagation (BR-BP) outperformed others. The sensitivity analysis revealed that, if solid retention time and biomass concentrations were maintained between 35 and 75 h and 3.0 g/L and 3.5 g/L, respectively, high CODRE (93%) and lipid concentration (2.8 g/L) could be obtained consistently.

Industrial wastewater treatment using floating wetlands: a review

Industrial wastewater generated from various production processes is often associated with elevated pollutant concentrations and environmental hazards, necessitating efficient treatment. Floating wetlands (FWs) have emerged as a promising and eco-friendly solution for industrial wastewater treatment, with numerous successful field applications. This article comprehensively reviews the removal mechanisms and treatment performance in the use of FWs for the treatment of diverse industrial wastewaters. Our findings highlight that the performance of FWs relies on proper plant selection, design, aeration, season and temperature, plants harvesting and disposal, and maintenance. Well-designed FWs demonstrate remarkable effectiveness in removing organic matter (COD and BOD), suspended solids, nutrients, and heavy metals from industrial wastewater. This effectiveness is attributed to the intricate physical and metabolic interactions between plants and microbial communities within FWs. A significant portion of the reported applications of FWs revolve around the treatment of textile and oily wastewater. In particular, the application reports of FWs are mainly concentrated in temperate developing countries, where FWs can serve as a feasible and cost-effective industrial wastewater treatment technology, replacing high-cost traditional technologies. Furthermore, our analysis reveals that the treatment efficiency of FWs can be significantly enhanced through strategies like bacterial inoculation, aeration, and co-plantation of specific plant species. These techniques offer promising directions for further research. To advance the field, we recommend future research efforts focus on developing novel floating materials, optimizing the selection and combination of plants and microorganisms, exploring flexible disposal methods for harvested biomass, and designing multi-functional FW systems.

Advanced oxidation process (AOP) combined biological process for wastewater treatment: A review on advancements, feasibility and practicability of combined techniques

Complexity of wastewater is the most challenging phenomenon on successful degradation of pollutant via any wastewater treatment regime. Upon availability of numerous techniques, Advanced Oxidation Processes (AOP) is the most promising technique for treating industrial wastewater. Higher operating cost is the most promising factor that possess challenge for the industrial scale usage of the AOP process. Combination of biological process with AOP helps in achieving sustainable degradation of toxic pollutant in the wastewater. AOP result in complete or partial degradation of toxic emerging pollutants with the help of free radicals like hydroxyl, superoxide, hydroperoxyl and sulphate radicals. In addition to this the presence of bio-enzymes and microorganisms helps in sustainable degradation of pollutant in an economical and environmentally friendly strategy. In this review, a detailed discussion was conducted on various AOP, focusing on catalytic ozonation, electrochemical oxidation, Sono chemical and photocatalytic processes. With the need for sustainable solutions for wastewater treatment, the use of AOP in conjunction with biological process has innumerous opportunities for not only wastewater treatment but also the production of high value by-products. Further, the effect of AOP combined biological processes needs to be analyzed in real time for the different concentration of industrial wastewater and their benefits needs to be explored in future towards achieving SDGs.

Development of microbial fuel cell integrated constructed wetland (CMFC) for removal of paracetamol and diclofenac in hospital wastewater

Hospital wastewater management has become a significant concern across the globe due to the presence of pharmaceutically active compounds (PhACs) and other toxic substances, which can potentially disrupt ecosystems. The presence of recalcitrant PhACs in hospital wastewater increases the difficulty level for conventional wastewater treatment systems. Furthermore, incorporating advanced oxidation-based treatment systems increase capital and operation costs. To reduce treatment costs, low-cost innovative technology, i.e., composite constructed wetland and microbial fuel cell system (CMFC), has been developed for higher treatment efficiency of PhACs in hospital wastewater along with simultaneous bioelectricity generation as an additional outcome. In this study, influencing operating parameters, such as initial chemical oxygen demand (COD), electrode spacing, and substrate-to-water-depth ratio, were optimized for two plant species: water hyacinth (WH) and duckweed (DW). The optimized systems were run in batch and continuous mode for WH-CMFC and DW-CMFC to treat synthetic hospital wastewater with paracetamol and diclofenac, and the bioelectricity generation was monitored. DW-CMFC system depicted better treatment efficiency and voltage generation as compared to WH-CMFC. In continuous mode, the DW-CMFC system exhibited a removal of 95.3% COD, 97.1% paracetamol, and 87.5% diclofenac. WH-CMFC and DW-CMFC achieved power densities of around 21.26 mW/m2 and 42.93 mW/m2, respectively. The fate of PhACs during and after treatment and toxicity analysis of the transformation products formed were also carried out. Higher bio-electricity generation and efficient wastewater treatment of the DW-CMFC make it a sustainable option for hospital wastewater management.

Enhanced nitrogen removal from rural domestic sewage via partial nitrification-anammox in integrated vertical subsurface flow constructed wetland

This study aimed to improve the removal of nitrogen treating rural domestic sewage by developing a novel strategy for achieving partial nitrification-anammox (PNA) in an integrated vertical subsurface flow constructed wetland (VSFCW). The influent ammonia was oxidized to nitrite in the partial nitrification VSFCW (VSFCWPN), and 5 mg/L of hydroxylamine was added under the appropriate dissolved oxygen concentration level (1.2 ± 0.2 mg/L) to stabilize the average nitrite accumulation rate at 88.24% and maintain the effluent NO2--N/NH4+-N ratio at 1.26 ± 0.15. The effluent from VSFCWPN was introduced to the following chamber (VSFCWAN), where ammonia and nitrite were removed by the autotrophic anammox process. This implementation achieved high removal efficiencies for chemical oxygen demand, total nitrogen, and PO43--P, reaching 86.26%, 90.22%, and 78.94%, respectively, with influent concentrations of 120.75 mg/L, 60.02 mg/L, and 5.05 mg/L. Substrate samples were collected from 10 cm height (PN1, AN1) and 25 cm height (PN2, AN2). Microbial community analysis showed that Nitrosomonas dominated the community composition in VSFCWPN, with an increase from 1.61% in the inoculated sludgePN to 16.31% (PN1) and 12.09% (PN2). Meanwhile, Ca. Brocadia accounted for 44.81% (AN1) and 36.50% (AN2) in VSFCWAN. These results confirm the feasibility of the proposed strategy for establishing PNA and efficiently treating rural domestic sewage in an integrated VSFCW.

Quantitative prediction of the hydraulic performance of free water surface constructed wetlands by integrating numerical simulation and machine learning

Quantitative prediction of the design parameter-influenced hydraulic performance is significant for optimizing free water surface constructed wetlands (FWS CWs) to reduce point and non-point source pollution and improve land utilization. However, owing to limitations of the test conditions and data scale, a quantitative prediction model of the hydraulic performance under multiple design parameters has not yet been established. In this study, we integrated field test data, mechanism model, statistical regression, and machine learning (ML) to construct such quantitative prediction models. A FWS CW numerical model was established by integrating 13 groups of trace data from field tests. Subsequently, training, test and extension datasets comprising 125 (5^3), 25 (L₂₅(5⁶)) and 16 (L₁₆(4⁴)) data points, respectively, were generated via numerical simulation of multi-level value combination of three quantitative design parameters, namely, water depth, hydraulic loading rate (HLR), and aspect ratio. The short circuit index (φ₁₀), Morrill dispersion index (MDI), hydraulic efficiency (λ) and moment index (MI) were used as representative hydraulic performance indicators. Training set with large samples were analyzed to determine the variation rules of different hydraulic indicators. Based on the control variable method, φ₁₀, λ, and MI grew exponentially with increasing aspect ratio whereas MDI showed a decreasing trend; with increasing water depth, φ₁₀, λ, and MI showed polynomial decreases whereas MDI increased; with increasing HLR, φ₁₀, λ, and MI slowly increased linearly whereas MDI showed the opposite trend. Finally, we constructed models based on multivariate nonlinear regression (MNLR) and ML (random forest (RF), multilayer perceptron (MLP), and support vector regression. The coefficients of determination (R²) of the MNLR and ML models fitting the training and test sets were all greater than 0.9; however, the generalization abilities of different models in the extension set were different. The most robust MLP, MNLR without interaction term, and RF models were recommended as the preferred models to hydraulic performance prediction. The extreme importance of aspect ratio in hydraulic performance was revealed. Thus, gaps in the current understanding of multivariate quantitative prediction of the hydraulic performance of FWS CWs are addressed while providing an avenue for researching FWS CWs in different regions according to local conditions.

Ecological and human health risk assessment of polycyclic aromatic hydrocarbons (PAH) in Tigris river near the oil refineries in Iraq

Recent Iraqi battles against ISIS in 2014 and 2015 resulted in the destruction or severe damage to several refineries' infrastructure. This, along with other factors, has led to the release and accumulation of a wide range of hazardous chemicals into the environment, for instance, polycyclic aromatic hydrocarbons (PAH). Thus, for the first time, a comprehensive 16 PAHs measurements campaign over the course of six months near the oil refineries along the Tigris River and its estuaries was investigated. The 16 PAHs concentrations in surface water and the sediments for the following oil refineries: Baiji, Kirkuk, Al-Siniyah, Qayyarah, Al-Kasak, Daura, South Refineries Company, and Maysan were examined. The overall findings indicated that the 16 PAHs concentrations ranged from 567.8 to 3750.7 ng/L for water and 5619.2-12795.0 ng/g for sediment. Water samples in South Refineries Company recorded the highest PAH concentrations while Baiji oil refinery registered the highest PAH concentrations in the sediment samples. The percentages of high molecular weight PAH (5-6 rings) in water and sediment samples were the highest, ranging from 49.41% to 81.67% and from 39.06% to 89.39% of total PAH for water and sediment, respectively. The majority of 16 PAHs measured in water and sediment samples of the Tigris River were derived from pyrogenic sources. Based on sediment quality guidelines (SQGs), most sites showed a possible effect range with occasional biological effects of most of the PAH concentrations in all sediments' samples. The calculated incremental lifetime cancer risk (ILCR) value was high risk with adverse health effects, including cancer.

New insights into the bioremediation of petroleum contaminants: A systematic review

Petroleum product is an essential resource for energy, that has been exploited by wide range of industries and regular life. A carbonaceous contamination of marine and terrestrial environments caused by errant runoffs of consequential petroleum-derived contaminants. Additionally, petroleum hydrocarbons can have adverse effects on human health and global ecosystems and also have negative demographic consequences in petroleum industries. Key contaminants of petroleum products, primarily includes aliphatic hydrocarbons, benzene, toluene, ethylbenzene, and xylene (BTEX), polycyclic aromatic hydrocarbons (PAHs), resins, and asphaltenes. On environmental interaction, these pollutants result in ecotoxicity as well as human toxicity. Oxidative stress, mitochondrial damage, DNA mutations, and protein dysfunction are a few key causative mechanisms behind the toxic impacts. Henceforth, it becomes very evident to have certain remedial strategies which could help on eliminating these xenobiotics from the environment. This brings the efficacious application of bioremediation to remove or degrade pollutants from the ecosystems. In the recent scenario, extensive research and experimentation have been implemented towards bio-benign remediation of these petroleum-based pollutants, aiming to reduce the load of these toxic molecules in the environment. This review gives a detailed overview of petroleum pollutants, and their toxicity. Methods used for degrading them in the environment using microbes, periphytes, phyto-microbial interactions, genetically modified organisms, and nano-microbial remediation. All of these methods could have a significant impact on environmental management.

Review of hospital effluents: special emphasis on characterization, impact, and treatment of pollutants and antibiotic resistance

Health care institutions generate large volumes of liquid effluents from specific activities related to healthcare, analysis, and research. Their direct discharge into the environment has various negative effects on aquatic environments and human health, due to their high organic matter charges and the presence of various emerging contaminants such as disinfectants, drugs, bacteria, viruses, and parasites. Moreover, hospital effluents, by carrying antibiotics, contribute to the development of antibiotic-resistant microorganisms in the environment. This resistance has become a global issue that manifests itself variously in different countries, causing the transmission of different infections. In this respect, an effort is provided to protect water resources by current treatment methods that imply physical-chemical processes such as adsorption and advanced oxidation processes, biological processes such as activated sludge and membrane bioreactors and other hybrid techniques. The purpose of this review is to improve the knowledge on the composition and impact of hospital wastewater on man and the environment, highlighting the different treatment techniques appropriate to this type of disposal before discharge into the environment.

Appraising efficacy of existing and advanced technologies for the remediation of beta-blockers from wastewater: A review

The discharge of emerging pollutants, such as beta-blockers (BB), has been recognized as one of the major threats to the environment due to the ecotoxicity associated with these emerging pollutants. The BB are prescribed to treat high blood pressure and cardiovascular diseases; however, even at lower concentration, these pollutants can pose eco-toxic impacts towards aquatic organisms. Additionally, owing to their recalcitrant nature, BB are not effectively removed through conventional technologies, such as activated sludge process, trickling filter and moving bed bioreactor; thus, it is essential to understand the degradation mechanism of BB in established as well as embryonic technologies, like adsorption, electro-oxidation, Fenton process, ultraviolet-based advance oxidation process, ozonation, membrane systems, wetlands and algal treatment. In this regard, this review articulates the recalcitrant nature of BB and their associated removal technologies. Moreover, the major advantages and limitations of these BB removal technologies along with the recent advancements with regard to the application of innovative materials and strategies have also been elucidated. Therefore, the present review intends to aid the researchers in improving the BB removal efficiency of these technologies, thus alleviating the problem of the release of BB into the environment.

Application of a new baffled horizontal flow constructed wetland-filter unit (BHFCW-FU) for treatment and reuse of petrochemical industry wastewater

The shortage of water resources and generation of large quantum of wastewater has posed a significant concern to the environment and public health. Recent research on wastewater treatment has started to focus on reusing wastewater for different activities to reduce the stress on natural water resources. Constructed wetland (CWs) is a low-cost wastewater treatment option. However, some drawbacks include large areal requirements and the need for tertiary treatment units for reusable effluent. In this study, a novel composite baffled horizontal flow CW filter unit (BHFCW-FU) was developed to overcome the drawbacks of the conventional CW. The BHFCW-FU planted with Chrysopogon zizanioides provided a nine times longer flow path, and the adjoined variable depth dual media filter reduced the total area requirement and served as a polishing unit. On average, the BHFCW-FU with horizontal sub-surface flow regime could efficiently remove around 93.93%, 87.20%, and 66.25% of turbidity, phenol, and COD, respectively, from real petrochemical wastewater (initial turbidity: 29.6 NTU, phenol: 4.52 mg/L, and COD: 381 mg/L) and rendered the effluent quality reusable for irrigation, industrial, and other environmental purposes. In synthetic wastewater (initial turbidity: 754 NTU, phenol: 10.87 mg/L, and COD: 1691 mg/L), the removal efficiency of turbidity, phenol, and COD were 99.50%, 93.73%, and 87.05%, respectively. In-depth substrate characterization was done to study the removal mechanism. The developed BHFCW-FU required less space and maintenance, provided reusable effluent, and overcame the drawbacks of conventional CWs. Hence, it may show immense potential as an effective wastewater treatment.

Effect of partially hydrolyzed polyacrylamide (HPAM) on the bacterial communities of wetland rhizosphere soils and their efficiency in HPAM and alkane degradation

The effect of partially hydrolyzed polyacrylamide (HPAM) on structure and function of rhizosphere soil bacterial communities in constructed wetlands has been largely underinvestigated. In this study, we compare the effect of 250, 500, and 1000 mg/L of HPAM on bacterial community composition of Phragmites australis associated rhizosphere soils in an experimental wetland using MiSeq amplicon sequencing. Rhizosphere soils from the HPAM-free and the 500-mg/L-exposed treatments were used for laboratory experiments to further investigate the effect of HPAM on the soil's degradation and respiration activities. Soils treated with HPAM showed differences in bacterial communities with the dominance of Proteobacteria and the enrichment of potential hydrocarbon and HPAM-degrading bacteria. CO₂ generation was higher in the HPAM-free soils than in the HPAM pre-exposed soil, with a noticeable increase in both soils when oil was added. The addition of HPAM at different concentrations had a more pronounced effect on CO₂ evolution in the HPAM-pre-exposed soil. Soils were able to degrade between 37 ± 18.0 and 66 ± 6.7% of C₁₀ to C₃₀ alkanes after 28 days, except in the case of HPAM-pre-exposed soil treated with 500 mg/L where degradation reached 92 ± 4.3%. Both soils reduced HPAM concentration by 60 ± 15% of the initial amount in the 500 mg/L treatment, but by only ≤ 21 ± 7% in the 250-mg/L and 1000-mg/L treatments. In conclusion, the rhizosphere soils demonstrated the ability to adapt and retain their ability to degrade hydrocarbon in the presence of HPAM.

Free-living amoebae in an oil refinery wastewater treatment facility

Free Living Amoebae (FLA) are ubiquitous microorganisms reported from harsh environmental conditions. Oil refinery facilities consume vast volumes of water during their processes, generating a large amount of wastewater. The present study aimed to evaluate the wastewater treatment process in an oil refinery wastewater treatment facility (ORWWTF) for the presence of FLA. Water samples were collected from an oil refinery wastewater (ORWW) for nine months. After recording physical-chemical features, samples were cultivated onto non-nutrient agar (NNA). The discriminative fragments of the ribosomal RNA (rRNA) gene were amplified and sequenced to characterize the isolated FLA. Phylogenetic tree, and network analysis were employed to evaluate genetic relationships. The thermo- and osmotolerant tests were performed on the isolated FLA. Twenty-five (32.9%) samples were positive for FLA cultivation. Acanthamoeba spp., Vahlkampfiids, and Vermamoeba spp. were detected, of which Acanthamoeba species were predominant. There was no statistical correlation between pH, NH₃, PO₄, H₂S, and TDS with the presence of FLA. A statistical correlation between the presence of FLA and the type of wastewater treatment plants (WWTPs) was significant (P-value = 0.011). All Acanthamoeba spp. isolates belonged to the genotypes T4 (17/21; 80.95%) and T11 (4/21; 19.05%). Vahlkampfiids were Naegleria spp., (7/10; 70%), Tetramitus aberdonicus (1/10; 10%), Learamoeba spp., (1/10; 10%), and Vahlkampfia spp., (1/10; 10%). All three Vermamoeba spp. were V. vermiformis. The ORWW contains toxic materials, and a few microorganisms can stay active in these environments. This is the first study which isolates FLA from such super harsh conditions. For the first time, T. aberdonicus, and Learamoeba spp., were isolated from oily wastewater. Our findings signify the concern due to the distribution of potentially pathogenic FLA to downstream lands via treated wastewater that may be released after treatment processing.

A review on the treatment of septage and faecal sludge management: A special emphasis on constructed wetlands

The global concern of the pollution of freshwater resources is associated with faecal sludge (FS) disposal, which is an inevitable component of onsite wastewater management mostly in developing countries. The difficulties with its treatment facilities lies in its higher organic content and low dewaterability of various available treatment systems. Moreover, the higher variability in characteristics and quantity of FS generated at different locations creates hindrances in designing the treatment system. Among the several treatment options, the constructed wetlands (CW) are an organic/green approach towards sanitation of FS with low cost and higher efficiency. The present study is an in-depth literature review on the quality and quantity of FS and septage (stabilized FS) in different regions attributed to the wide variability of its characteristics. This paper highlights the treatment of FS in different systems with a special emphasis on CW systems. Different mechanisms and factors affecting the FS treatment efficacy in CW, such as DO/aeration, macrophytes, substrate, CW configuration, and other environmental parameters, have been studied meticulously. The cost analysis revealed CW to be an economic system, and it can enable hybridization with other technologies to develop a complete treatment system with pronounced efficiencies. Several process modifications, such as augmentation with aeration, recirculation, micro-organisms, and earthworms, can enhance the treatment efficacies of CWs. The present review exhibited that the widely used plant species is Phragmites, and the optimum solid loading rate (SLR) range is 50-250 kg TS/m²/yr. The various factors to construct an optimized CW system for FS treatment were attempted, which may bolster the necessary guidelines for field-scale applications.

A thorough observation of an ozonation catalyst under long-term practical operation: Deactivation mechanism and regeneration

Heterogeneous catalytic oxidation, as an efficient advanced treatment technology, has been gradually applied in industrial wastewater treatment. The fixed bed technique is one of the most popular catalytic ozonation methods. However, few studies have concentrated on the long-term operation effects on catalysts. In this study, we conducted long-term (~5 years) observations of the operation of the largest petrochemical wastewater treatment plant (treatment capacity 120,000 m³/d) with catalytic ozonation technology in China. A commercial catalyst, which uses Al₂O₃ pellets supporting copper oxide was applied in this plant. The results showed that the catalytic efficiency gradually decreased from 60.65% to 25.98% since 2018, and the ozone dosage to COD removal ratio (ozone/COD) also increased from 0.82 to 1.93 mg/mg as the running time continued. By means of the comparison and characterization of fresh catalyst and used catalyst, a "mucus layer" was formed by the adsorption of negatively charged extracellular polymeric substances on the positively charged catalyst surface and the interception of the catalyst layer. The mucus layer significantly reduced the catalytic efficiency by isolating ozone with catalytic active sites and releasing extra organic contaminants during the catalytic process resulting in 53.97% TOC increase in the batch test. Meanwhile, regeneration experiments revealed that the TOC removal efficiency was 4.76% and 43.48% in presence of washed catalysts and calcinated catalysts, respectively. Compared with the fresh catalyst, 73% of the catalytic activity was recovered for calcinated catalyst. Consequently, this study provides much practical information, showing positive effects on the promotion of catalytic ozonation application in actual wastewater treatment.

Current technologies and future perspectives for the treatment of complex petroleum refinery wastewater: A review

Petroleum refinery wastewater (PRW) is a complex mixture of hydrocarbons, sulphides, ammonia, oils, suspended and dissolved solids, and heavy metals. As these pollutants are toxic and recalcitrant, it is essential to address the above issue with efficient, economical, and eco-friendly technologies. In this review, initially, an overview of the characteristics of wastewater discharged from different petroleum refinery units is discussed. Further, various pre-treatment and post-treatment strategies for complex PRW are introduced. A segregated approach has been proposed to treat the crude desalting, sour, spent caustic, and oily wastewater of petroleum refineries. The combined systems (e.g., ozonation + moving bed biofilm reactor and photocatalysis + packed bed biofilm reactor) for the treatment of low biodegradability index wastewater (BOD₅/COD < 0.2) were discussed to construct a perspective map and implement the proposed system efficiently. The economic, toxicity, and biodegradability aspects are also introduced, along with research gaps and future scope.

Technical advances on current research trends and explore the future scope on nutrient recovery from waste-streams: a review and bibliometric analysis from 2000 to 2020

An exponentially growing global population has led to an increase in nutrient pollution in different aqueous bodies. Although different processes have successfully removed nutrients from wastewater on a large scale, a limited number of studies have been reported on efficiency, cost-effectiveness, and future potential of physical, chemical, and biological nutrient recovery methods to overcome the depletion of natural resources. Therefore, researchers need to understand current research trends by applying different approaches to investigate higher efficient nutrient recovery technologies. In this article, the research patterns and in-depth review of various nutrient recovery processes have been circumscribed with the application of bibliometric and attractive index (AAI) vs. activity index (AI) analysis. The performance, advantages, limitations, and future prospects of different nutrient recovery methods have also been addressed. More than 70% of study publications were published in the last decade in chemical and biological processes, which might be related to more rigorous effluent quality rules and increasing water pollution. The future prediction in the field of nutrient recovery has been predicted using S-curve analysis, and it was found that the number of publications in the saturated state in chemical methods was highest. However, the growth rate of the biological-based nutrient recovery methods is greater, which may be because of their huge research scope, cost-effectiveness, and easy operation methods. This study can assist researchers in understanding the current research scenario in nutrient recovery techniques and provide the research scope in nutrient recovery from wastewater in the future.

Recovery, regeneration and sustainable management of spent adsorbents from wastewater treatment streams: A review

Adsorption is the most widely adopted, effective, and reliable treatment process for the removal of inorganic and organic contaminants from wastewater. One of the major issues with the adsorption-treatment process for the removal of contaminants from wastewater streams is the recovery and sustainable management of spent adsorbents. This review focuses on the effectiveness of emerging adsorbents and how the spent adsorbents could be recovered, regenerated, and further managed through reuse or safe disposal. The critical analysis of both conventional and emerging adsorbents on organic and inorganic contaminants in wastewater systems are evaluated. The various recovery and regeneration techniques of spent adsorbents including magnetic separation, filtration, thermal desorption and decomposition, chemical desorption, supercritical fluid desorption, advanced oxidation process and microbial assisted adsorbent regeneration are discussed in detail. The current challenges for the recovery and regeneration of adsorbents and the methodologies used for solving those problems are covered. The spent adsorbents are managed through regeneration for reuse (such as soil amendment, capacitor, catalyst/catalyst support) or safe disposal involving incineration and landfilling. Sustainable management of spent adsorbents, including processes involved in the recovery and regeneration of adsorbents for reuse, is examined in the context of resource recovery and circular economy. Finally, the review ends with the current drawbacks in the recovery and management of the spent adsorbents and the future directions for the economic and environmental feasibility of the system for industrial-scale application.

Successful sulphide-driven partial denitrification: Efficiency, stability and resilience in SRT-controlled conditions

Partial denitrification is emerging as a valuable solution for NO₂^- supply in Anammox systems. When reduced sulphur compounds are used as electron donors, S-driven Partial Autotrophic Denitrification (PAD) can also be achieved, allowing for an integrated autotrophic nitrogen (N) and sulphur (S) removal from liquid and gaseous streams. The aim of the present work was to maximise NO₃^- reduction to NO₂^- coupled with complete HS^- oxidation, by the selective control of influent S/N ratio and sludge retention time (SRT). A 2.5-L chemostat was operated for 115 days and three operational phases were tested at decreasing SRT of 40, 23 and 13 h, testing S/N ratios in the range of 0.5-1 gS/gN. Successful sulphide-driven PAD was achieved and lead to average NO₂^- conversion efficiencies as high as77±17% at all the conditions tested, with the highest value of 99% at the lowest S/N of 0.58 gS/gN and SRT of 23 h. Respirometric tests showed that NO₃^- uptake rate was stable at 90±10 mgN/gVSS/h, when NO₃^- was present as sole electron acceptor or at NO₂^- levels as high as 120 mgN/l; on the contrary, NO₂^- uptake rates were very sensitive to the applied conditions. Metabarcoding analyses revealed that the microbial community was highly enriched in Sulphur Oxidizing Bacteria (SOB>80%) and stable S-limiting conditions appeared to favour Thiobacillus over Sulfurimonas genus. A preliminary assessment of N₂O potential emission was also performed. To the best of our knowledge, this is the first work evaluating the synergic effect of SRT and influent S/N ratio on nitrite accumulation in highly SOB-enriched systems and the NO₂^- conversion efficiencies achieved are among the highest reported in literature.

Improvement of heavy metal removal from urban runoff using modified pervious concrete

Heavy metals are one of the major chemical pollutant groups in urban runoff. The application of porous concrete is a potential alternative to conventional runoff management systems with the ability to remove heavy metals. Hence, a thorough understanding of the heavy metal removal mechanisms and constraints of conventional porous concrete opens a path for the development of effective modifications. This review critically discusses the major contributors in ordinary porous concrete which supports heavy metal removal. The effects of initial concentration, contact time and competing ions on heavy metal removal using porous concrete are also discussed. Additionally, the effect of decalcification, atmospheric carbonation, acid influent on heavy metal removal is reviewed. The major drawback of porous concrete is the high pH (>8.5) of the effluent water, decalcification of the porous concrete and leaching of adsorbed pollutants. Overall, the addition of adsorbent materials to the porous concrete increases removal efficiencies (7% - 65% increase) without neutralizing the effluent pH. Meanwhile, the addition of Reduced Graphene Oxide is successful in reducing the leachability of the removed heavy metals. The addition of pozzolanic materials can lower the effluent pH while maintaining similar removal efficiencies to unmodified porous concrete. Therefore, developing a novel method of neutralizing the effluent pH must be prioritized in future studies. Additionally, the toxicity that can occur due to the abrasion of modified porous concrete requires study in future research. Further, advanced characterization methods should be used in future studies to understand the mechanisms of removal via the modified porous concrete materials.

Pristine and engineered biochar for the removal of contaminants co-existing in several types of industrial wastewaters: A critical review

Biochar has been widely studied as an adsorbent for the removal of contaminants from wastewater due to its unique characteristics, such as having a large surface area, well-distributed pores and high abundance of surface functional groups. Critical review of the literature was performed to understand the state of research in utilizing biochars for industrial wastewater remediation with emphasis on pollutants that co-exist in wastewater from several industrial activities, such as textile, pharmaceutical and mining industries. Such pollutants include organic (such as synthetic dyes, phenolic compounds) and inorganic contaminants (such as cadmium, lead). Multiple correspondence analyses suggest that through batch equilibrium, columns or constructed wetlands, researchers have used mechanistic modelling of isotherms, kinetics, and thermodynamics to evaluate contaminant removal in either synthetic or real industrial wastewaters. The removal of organic and inorganic contaminants in wastewater by biochar follows several mechanisms: precipitation, surface complexation, ion exchange, cation-π interaction, and electrostatic attraction. Biochar production and modifications promote good adsorption capacity for those pollutants because biochar properties stemming from production were linked to specific adsorption mechanisms, such as hydrophobic and electrostatic interactions. For instance, adsorption capacity of malachite green ranged from 30.2 to 4066.9 mg g^-1 depending on feedstock type, pyrolysis temperature, and chemical modifications. Pyrolyzing biomass at above 500 °C might improve biochar quality to target co-existing pollutants. Treating biochars with acids can also improve pollutant removal, except that the contribution of precipitation is reduced for potentially toxic elements. Studies on artificial intelligence and machine learning are still in their infancy in wastewater remediation with biochars. Meanwhile, a framework for integrating artificial intelligence and machine learning into biochar wastewater remediation systems is proposed. The reutilization and disposal of spent biochar and the contaminant release from spent biochar are important areas that need to be further studied.

Technical, economic and environmental feasibility of resource recovery technologies from wastewater

An enormous amount of wastewater is generated across the world from different industrial or municipal sectors. Traditional wastewater treatment plants (WWTP) have primarily focused on the treatment of wastewater rather than the recovery of valuable resources. A shift from a linear to a circular economy may offer a unique platform for recovering valuable resources including energy, nutrients, and high-value goods from wastewater. However, transitioning from conventional frameworks to sustainable WWT systems remains a significant challenge. Thus, this review paper focuses on the avenues of resource recovery from WWTPs, by evaluating the potential for nutrients, water, and energy recovery from different types of wastewaters and sewage sludge. It discusses in detail a variety of available and advanced technologies for resource recovery. Further, the feasibility of these technologies from a sustainable standpoint is discussed, covering the technical, economic, and environmental facets.

Wastewater Treatment Using Constructed Wetland: Current Trends and Future Potential

Constructed wetlands (CW) is an environmentally friendly technique for removing pollutants from wastewater and has been applied to municipal wastewater, petroleum refinery wastewater, agriculture drainage, acid mine drainage, etc. The past decade has seen a remarkable number of innovations in the exponentially growing field of microbiology. This manuscript covers a critical review of key aspects of CW, such as various types of CW, the contaminants and their removal mechanisms, degradation pathways, challenges and opportunities, materials, applications, and theory with a focus on recent advances in the last three decades. In addition, an attempt has been taken to project future advances in the field of CW and facilitate these advances by framing key unsolved problems in CW. Guidelines are prepared for the fast-growing CW field through the standardization of key design aspects. This review covers the evaluation of the current state-of-the-art of CW technology and provides definitions and performance metric nomenclature in an effort to unify the fast-growing CW community. It also contains an outlook on the emerging trends in CW and proposes future research and development directions.

Submerged microfiltration membrane and activated carbon processes for recalcitrant compounds removal in oil refinery effluent as electrodialysis pre-treatment

The combination of suspended activated carbon (AC) and submerged microfiltration (SMF) processes was applied to polish a biotreated effluent generated in a refinery industry. Preliminary results indicated that Norit 1240 W AC was more suitable than Carbomafra AC brand for total organic carbon (TOC) removal due to the highest Freundlich adsorption constant value (1.97 ± 0.42 and 0.96 ± 0.23 (mg/g)(L/mg)^1/n, respectively), thus the first one was used in the combined system. Among all particle sizes of AC tested (0.041-1.01 mm), AC/SMF system was better performed, according to permeation flux, when applying granular AC instead of the powder one. On the other hand, the best response regarding TOC removal and absorbance at 254 nm (ABS_{254 nm}) reduction were observed when applying powder AC (89% and 97%, respectively). Statistical analysis with two-sample T-test (p-value <0.05) endorsed the need of both air purge (20 L/h) and backwash strategies (8 min of permeation and 10 seconds of backwash) to diminish fouling occurrence in the SMF system. Finally, it was found that 2 g/L of Norit 1240 W PAC (0.041 mm particle size) condition fitted the effluent to further electrodialysis reversal (EDR) process (3.4 mg/L TOC) with consistent normalized permeate flux after 5 h of permeation (0.76 ± 0.1 J/J₀).

Predicting the trend and utility of different photocatalysts for degradation of pharmaceutically active compounds: A special emphasis on photocatalytic materials, modifications, and performance comparison

The rapid rise in the healthcare sector has led to an increase in pharmaceutically active compounds (PhACs) in different aqueous bodies. The toxicity of the PhACs and their ability to persist after conventional treatment processes have escalated research in the field of photocatalytic treatment. Although different photocatalysts have been successful in degrading PhACs, their inherent drawbacks have severely limited their application on a large scale. A substantial amount of research has been aimed at overcoming the high cost of the photocatalytic material, low quantum yield, the formation of toxic end products, etc. Hence, to further research in this field, researchers must have a fair idea of the current trends in the application of different photocatalysts. In this article, the trends in the use of various photocatalysts for the removal of different PhACs have been circumscribed. The performance of different groups of photocatalysts to degrade PhACs from synthetic and real wastewater has been addressed. The drawbacks and advantages of these materials have been compared, and their future in the field of PhACs removal has been predicted using S-curve analysis. Zinc and titanium-based photocatalysts were efficient under UV irradiation, while bismuth and graphene-based materials exhibited exemplary performance in visible light. However, iron-based compounds were found to have the most promising future, which may be because of their magnetic properties, easy availability, low bandgap, etc. Different modification techniques, such as morphology modification, doping, heterojunction formation, etc., have also been discussed. This study may help researchers to clarify the current research status in the field of photocatalytic treatment of PhACs and provide valuable information for future research.

Study on clogging mechanisms of constructed wetlands from the perspective of wastewater electrical conductivity change under different substrate conditions

Constructed wetland (CW) has obvious advantages in wastewater treatment of medium and small towns. However, there is a lack of health monitoring research on CW system clogging. The electrical conductivity (EC) of wastewater purified by CW is related to the concentration of pollutants, which can reflect the CW clogging. The objectives of this study are to reveal the mechanisms of CWs substrate clogging from the perspective of wastewater EC changes, and provide an important reference for the health evaluation of CWs. The EC changes of nine CWs substrates (quartz sand, zeolite, gravel, coarse sand, straw biochar, sludge biochar, clay ceramsite, fly ash ceramsite and shale ceramsite) under different conditions (purified water, wastewater and wastewater + NaCl) were tested, and comparative analysis was used to reveal the influence of different substrate materials on the change of wastewater EC. The results show that the adsorption ability of substrate material isn't the main factor affecting the EC of wastewater, and the soluble component in the material is the important factor to cause the difference of EC increment. Under the condition of 0.4-1.0 g L^-1 NaCl concentration, the adsorption of substrate materials had little effect on the EC of wastewater, and the effect of NaCl used in CW tracer experiment was good. Quartz sand, coarse sand, gravel and sludge biochar have little influence on the change of wastewater EC. Other materials that have great influence on the change of wastewater EC can be treated by modifying or controlling the mixing ratio. The results are of great significance to reveal the clogging state of CW system and to carry out health assessment research.

Hydraulic characterization and removal of metals and nutrients in an aerated horizontal subsurface flow "racetrack" wetland treating primary-treated oil industry effluent

Constructed wetlands (CW) are an attractive technology due to their operational simplicity and low life-cycle cost. It has been applied for refinery effluent treatment but mostly single-stage designs (e.g., vertical or horizontal flow) have been tested. However, to achieve a good treatment efficiency for industrial effluents, different treatment conditions (both aerobic and anaerobic) are needed. This means that hybrid CW systems are typically required with a respectively increased area demand. In addition, a strong aerobic environment that facilitates the formation of iron, manganese, zinc and aluminum precipitates cannot be established with passive wetland systems, while the role of these oxyhydroxide compounds in the further co-precipitation and removal of heavy metals such as copper, nickel, lead, and chromium that can simplify the overall treatment of industrial wastewaters is poorly understood in CW. Therefore, this study tests for the first time an innovative CW design that combines an artificially aerated section with a non-aerated section in a single unit applied for oil refinery wastewater treatment. Four pilot units were tested with different design (i.e., planted/unplanted, aerated/non-aerated) and operational (two different hydraulic loading rates) characteristics to estimate the role of plants and artificial aeration and to identify the optimum design configuration. The pilot units received a primary refinery effluent, i.e., after passing through a dissolved air flotation unit. The first-order removal of heavy metals under aerobic conditions is evaluated, along with the removal of phenols and nutrients. High removal rates for Fe (96-98%), Mn (38-81%), Al (49-73%), and Zn (99-100%) generally as oxyhydroxide precipitates were found, while removal of Cu (61-80%), Ni (70-85%), Pb (96-99%) and Cr (60-92%) under aerobic conditions was also observed, likely through co-precipitation. Complete phenols and ammonia nitrogen removal was also found. The first-order rate coefficient (k) calculated from the collected data demonstrates that the tested CW represents an advanced wetland design reaching higher removal rates at a smaller area demand than the common CW systems.

Application of microbial electrochemical technologies for the treatment of petrochemical wastewater with concomitant valuable recovery: A review

Petrochemical industry is one of the major and rapidly growing industry that generates a variety of toxic and recalcitrant organic pollutants as by-products, which are not only harmful to the aquatic animals but also affects human health. The majority of the components of petrochemical wastewater (PW) are carcinogenic, genotoxic and phytotoxic in nature; hence, this complex wastewater generated from different petrochemical processes should be efficiently treated prior to its disposal in natural water bodies. The established technologies like advanced oxidation, membrane bioreactor, electrocoagulation and activated sludge process employed for the treatment of PW are highly energy intensive and incurs high capital and operation cost. Moreover, these technologies are not effective in completely eliminating petroleum hydrocarbons present in PW. Thus, to reduce the energy requirement and also to transform the chemical energy trapped in these organic matters present in this wastewater into bioelectricity and other value-added products, microbial electrochemical technologies (METs) can be efficaciously used, which would also compensate the treatment cost by transforming these pollutants into bioenergy and valuables. In this regard, this review elucidates the feasibility and application of different METs as an appropriate alternative for the treatment of PW. Furthermore, the numerous bottlenecks towards the real-life application and commercialization of pioneering METs have also been articulated.

A Review on the Treatment of Petroleum Refinery Wastewater Using Advanced Oxidation Processes

The petroleum industry is one of the most rapidly developing industries and is projected to grow faster in the coming years. The recent environmental activities and global requirements for cleaner methods are pushing the petroleum refining industries for the use of green techniques and industrial wastewater treatment. Petroleum industry wastewater contains a broad diversity of contaminants such as petroleum hydrocarbons, oil and grease, phenol, ammonia, sulfides, and other organic composites, etc. All of these compounds within discharged water from the petroleum industry exist in an extremely complicated form, which is unsafe for the environment. Conventional treatment systems treating refinery wastewater have shown major drawbacks including low efficiency, high capital and operating cost, and sensitivity to low biodegradability and toxicity. The advanced oxidation process (AOP) method is one of the methods applied for petroleum refinery wastewater treatment. The objective of this work is to review the current application of AOP technologies in the treatment of petroleum industry wastewater. The petroleum wastewater treatment using AOP methods includes Fenton and photo-Fenton, H2O2/UV, photocatalysis, ozonation, and biological processes. This review reports that the treatment efficiencies strongly depend on the chosen AOP type, the physical and chemical properties of target contaminants, and the operating conditions. It is reported that other mechanisms, as well as hydroxyl radical oxidation, might occur throughout the AOP treatment and donate to the decrease in target contaminants. Mainly, the recent advances in the AOP treatment of petroleum wastewater are discussed. Moreover, the review identifies scientific literature on knowledge gaps, and future research ways are provided to assess the effects of these technologies in the treatment of petroleum wastewater.

A review on hospital wastewater treatment: A special emphasis on occurrence and removal of pharmaceutically active compounds, resistant microorganisms, and SARS-CoV-2

The hospital wastewater imposes a potent threat to the security of human health concerning its high vulnerability towards the outbreak of several diseases. Furthermore, the outbreak of COVID-19 pandemic demanded a global attention towards monitoring viruses and other infectious pathogens in hospital wastewater and their removal. Apart from that, the presence of various recalcitrant organics, pharmaceutically active compounds (PhACs), etc. imparts a complex pollution load to water resources and ecosystem. In this review, an insight into the occurrence, persistence and removal of drug-resistant microorganisms and infectious viruses as well as other micro-pollutants have been documented. The performance of various pilot/full-scale studies have been evaluated in terms of removal of biochemical oxygen demand (BOD), chemical oxygen demand (COD), total suspended solids (TSS), PhACs, pathogens, etc. It was found that many biological processes, such as membrane bioreactor, activated sludge process, constructed wetlands, etc. provided more than 80% removal of BOD, COD, TSS, etc. However, the removal of several recalcitrant organic pollutants are less responsive to those processes and demands the application of tertiary treatments, such as adsorption, ozone treatment, UV treatment, etc. Antibiotic-resistant microorganisms, viruses were found to be persistent even after the treatment of hospital wastewater, and high dose of chlorination or UV treatment was required to inactivate them. This article circumscribes the various emerging technologies, which have been used to treat PhACs and pathogens. The present review also emphasized the global concern of the presence of SARS-CoV-2 RNA in hospital wastewater and its removal by the existing treatment facilities.

Evaluation of wastewater treatment performance to a field-scale constructed wetland system at clogged condition: A case study of ammonia manufacturing plant

Assessment of the treatment performance in the field-scale hybrid constructed wetland (CW) for ammonia manufacturing plant remains limited. After being in operations running on and off since 2014, the hybrid CW which treats effluent from the ammonia manufacturing plant in Peninsular, Malaysia has recently demonstrated the full clogging to the CW. It takes only 8 months to demonstrate a big deterioration of performance in 2019. Though the mechanism of clogging is not clear, which can be partially from inherent design problems or operational issues, nonetheless, it is important to evaluate how this clogging has impacted the effluent treatment performance and the continuous utilization of the CW. The purpose of this study is to evaluate the impact of the treatment performance on the ammoniacal nitrogen and COD removal when the CW is clogged. The result revealed that there is no impact on COD removal, but it has a substantial impact on the ammoniacal nitrogen removal. The ammoniacal nitrogen removal dropped to negative (outlet concentration is higher than inlet concentration) during the clogged period. Another observation is, the low removal rate also coincides with a high COD/N ratio, when the COD/N ratio increased to >2, the ammoniacal nitrogen removal rate dropped substantially, with the coefficient of determination, R² of 40.5%. The root cause for the clogging to develop in a short period of time is unidentified. However, it is still worth noting that COD and ammoniacal nitrogen efficiency did not behave the same at the clogged CW.

A review on performance of constructed wetlands in tropical and cold climate: Insights of mechanism, role of influencing factors, and system modification in low temperature

Constructed wetlands (CWs) are one of the most promising and sustainable alternatives for wastewater treatment that are being successfully implemented in several countries, especially in tropical and sub-tropical regions. The predominant mechanisms of removal of contaminants in CWs are microbial degradation, phytodegradation, phytoextraction, filtration, sedimentation, and adsorption, etc. Vertical flow subsurface CWs and hybrid CWs demonstrated promising results in terms of TN, BOD, and COD removal, while horizontal flow subsurface CWs were proficient in removal of TP. The performance of the CWs depends upon a various factors, such as hydraulic loading rate, pH, dissolved oxygen, temperature, etc. Among these, low temperature had the most antagonistic effect on the performance of the CWs because freezing ambient temperature lead to ice formation, hydraulic imperfections, malfunctioning of biotic and abiotic components, etc. Over the past three decades, thousands of studies have been conducted involving treatment of wastewater using CWs, among which only few have addressed the issues and concerns of cold climate representing a significant research gap in this field. Furthermore, the performance of CWs in terms of TN, TP, and COD removal was significantly lower in cold climates than that in tropical and sub-tropical climates. In order to find suitable remedies to overcome the challenges faced in cold climate various modifications, such as incorporating greenhouse structure, providing insulating materials, bio-augmentation, identification of suitable macrophytes, etc., in around 20 different scenarios have been studied. Greenhouse construction led to 20% increase in removal of TN and COD, while plant collocation accounted for up to 18% increase in the removal of COD. Artificial aeration, insulation and bio-augmentation also enhanced the performance of the CWs in low temperatures.

A comparative review on clean hydrogen production from wastewaters

This paper provides a unique review of hydrogen production methods with wastewater treatment to depict a clean and sustainable approach. Various methods for hydrogen production from wastewaters are identified and discussed with recent details by discussing the critical challenges, opportunities, and future directions. Five main performance sectors are considered in detail for each hydrogen production method of the recent case studies, including economic, environmental, social, technical, and reliability. Eight hydrogen production methods are reviewed, including anaerobic method, photo fermentation, dark fermentation, electrolysis, electrodialysis, photocatalysis, photoelectrochemical methods, and super water gasification. A comparative assessment of six reviewed methods for hydrogen production, including environmental, economic, energetic, and exergetic impacts, is evaluated. The comparative assessment results indicate that dark fermentation technology is the most economical method, and it is followed by microbial electrolysis and photofermentation. The most environmentally friendly method for the lowest global warming potential (GWP) is the microbial electrolysis method, and it is followed by photocatalysis and photoelectrochemical methods. Furthermore, the highest energy and exergy efficiencies have been recorded for the microbial electrolysis to be 68% and 64.7%, respectively.

Valorisation of soil contaminated by petroleum hydrocarbons and toxic metals in geopolymer mortar formation

Until the complete transition to a renewable energy sources based economy, the potential environmental hazards associated with petroleum refinery industries affecting water, air and soil seek sustainable solutions. In the present study contaminated soil from a refinery is used as an alternative source for producing useful building materials by geopolymerization. To this end, soil remediation by thermal desorption was initially applied. Thermal treatment was performed between 60 and 250 °C for short time intervals (10-30 min) in order to remove organic pollutants (Total Petroleum Hydrocarbons, TPHs and Polycyclic Aromatic Hydrocarbons, PAHs). Physical, chemical analyses, mineral phase composition, as well as thermogravimetric analysis were employed to characterize the sample. Moreover, removal efficiency of TPHs and PAHs was evaluated. Subsequently, the treated soil presenting the maximum elimination of TPH and PAHs was used in geopolymer mortar formation aiming to stabilize the toxic metals (TMs) and produce a possible profitable material. For geopolymer synthesis the substitution of metakaolin (MT) by treated soil at 0, 50, 70 and 100% was tested. The produced specimens were evaluated based on the 28 day compressive strength and metals leaching. Results showed that the geopolymer constructed by 50% MT-50% remediated soil at 250 °C for 30 min, had negligible content of organic pollutants, TMs were immobilized and exhibited increased strength thus giving significant recycling benefits. Valorisation of industrial residues to produce building materials is a promising solution for sustainable waste management.

Electrochemical treatment of real petrochemical effluent: current density effect and toxicological tests

This work aims to investigate the electrochemical treatment of petrochemical industry effluents (from the northwest region of Brazil) mediated by active chlorine species electrogenerated at ruthenium-titanium oxide supported in titanium (Ti/Ru_0.3Ti_0.7O₂) and boron doped diamond (BDD) anodes by applying 15 and 45 mA cm^-2. Chemical oxygen demand (COD) determinations and toxicity analyses were carried out in order to evaluate the process extension as well as the possible reuse of the wastewater after treatment. Toxicity was evaluated by assessing the inhibition of lettuce (Lactuca sativa) stem growth, seed germination, and the production of nitrite (NO^-₂) and nitrate (NO^-₃) species. Results clearly showed that the best COD reduction performances were reached at the BDD anode, achieving almost 100% of removal in a short time. Degradation of nitrogen-organic compounds generated NO^-₂ and NO^-₃ which act as nutrients for lettuce. Toxicity results also indicated that the electrogenerated active chlorine species are persistent in the effluent after the treatment, avoiding the stem growth, and consequently affecting the germination.