Treatment of Flue Gas Desulfurization Wastewater by an Integrated Membrane-Based Process for Approaching Zero Liquid Discharge

Separation of Chloride and Sulfate Ions from Desulfurization Wastewater Using Monovalent Anions Selective Electrodialysis

The high concentration of chloride ions in desulphurization wastewater is the primary limiting factor for its reusability. Monovalent anion selective electrodialysis (S-ED) enables the selective removal of chloride ions, thereby facilitating the reuse of desulfurization wastewater. In this study, different concentrations of NaCl and Na2SO4 were used to simulate different softened desulfurization wastewater. The effects of current density and NaCl and Na2SO4 concentration on ion flux, permselectivity (PSO42-Cl-) and specific energy consumption were studied. The results show that Selemion ASA membrane exhibits excellent permselectivity for Cl- and SO42-, with a significantly lower flux observed for SO42- compared to Cl-. Current density exerts a significant influence on ion flux; as the current density increases, the flux of SO42- also increases but at a lower rate than that of Cl-, resulting in an increase in permselectivity. When the current density reaches 25 mA/cm2, the permselectivity reaches a maximum of 50.4. The increase in NaCl concentration leads to a decrease in the SO42- flux; however, the permselectivity is reduced due to the elevated Cl-/SO42- ratio. The SO42- flux increases with the increase in Na2SO4 concentration, while the permselectivity increases with the decrease in Cl-/SO42- ratio.

Mapping the research on desulfurization wastewater: Insights from a bibliometric review (1991-2021)

Desulfurization wastewater in coal-fired power plants (CFPPs) is a great environmental challenge. This study aimed at the current status and future research trends of desulfurization wastewater by bibliometric analysis. The desulfurization wastewater featured with high sulfate (8000 mg/L), chlorite (8505 mg/L), magnesium (2882 mg/L) and calcium (969 mg/L) but low sodium (801.82 mg/L), and the concentrations of the main contaminants were critically summarized. There was an increasing trend in the annual publications of desulfurization wastewater in the period from 1991 to 2021, with an average growth rate of 15%. Water Science and Technology, Desalination and Water Treatment, Energy & Fuels, Chemosphere, and Journal of Hazardous Materials are the top 5 journals in this field. China was the most productive country (58.3% of global output) and the core country in the international cooperation network. Wordcloud analysis and keyword topic trend demonstrated that removal/treatment of pollutants dominated the global research in the field of desulfurization wastewater. The primary technologies for desulfurization wastewater treatment were systematically evaluated. The physicochemical treatment technologies occupied half of the total treatment methods, while membrane-based integrated processes showed potential applications for beneficial reuse. The challenges and outlook on desulfurization wastewater treatment for achieving zero liquid discharge are summarized.

Experimental study on the treatment of desulfurization wastewater from coal-fired power plant by spray evaporation

In this study, a pilot-scale evaporation tower system was built to treat the desulfurization wastewater by spray evaporation. The distribution characteristics of Cl^- in the wastewater evaporation process were investigated. Besides, the morphology and physicochemical property of solid evaporation products from desulfurization wastewater were analyzed. In addition, the emission characteristics of fine particulates were evaluated. The results indicated that the increase of salt content in desulfurization wastewater increased the mass concentrations of Cl^- in three phases, but the proportions of it remained almost unchanged, which were about 10%, 55%, and 35% in the gas phase, outlet solid phase, and bottom solid phase respectively. The increase of flue gas temperature can improve the content of Cl^- in the gas phase, while the increase of wastewater pH inhibited the formation of gaseous HCl. The solid evaporation products from desulfurization wastewater had a prismatic crystal structure, which mainly included the sulfate and chloride salts, and the main elements including O, Na, Mg, S, Cl, K, and Ca. Besides, the peak values of particle size distribution in the bottom solid phase and outlet solid phase were 7.67 and 0.32 μm, respectively. For the particulate matters in flue gas, the spray evaporation of desulfurization wastewater can reduce the particle concentration, promote particle agglomeration, reduce the number concentration of fine particles, and improve the removal effect of PM₁₀. When the inlet particle concentration was 7.62 g/m³, it can reduce the particle concentration at the tower outlet to 4.59 g/m³ and reduce the number and mass concentrations of PM₁₀ after ESP by about 43.8% and 36.8%.

Membrane Separation Used as Treatment of Alkaline Wastewater from a Maritime Scrubber Unit

Since 1 January 2020, the sulfur content allowed in exhaust gas plume generated by marine vessels decreased to 0.5% m/m. To be compliant, a hybrid scrubber was installed on-board, working in closed loop and generating a high volume of alkaline wastewater. The alkaline water suspension was treated by a silicon carbide multitubular membrane to remove pollutants, and to allow the water discharge into the natural environment. In this paper, membrane filtration behavior was analyzed for the maritime scrubber wastewater. A range of operating parameters were obtained for several feedwater quality-respecting industrial constraints. The objective was an improvement of (I) the water recovery rate, (II) the filtration duration, and (III) the permeate quality. Thus, in high-fouling water, a low permeate flow (60 L h^-1 m^-2) with frequent backflushing (every 20 min) was used to maintain membrane performance over time. In terms of water quality, the suspended solids and heavy metals were retained at more than 99% and 90%, respectively. Other seawater discharge criteria in terms of suspended solids concentration, pH, and polyaromatic hydrocarbons were validated. The recommended operating conditions from laboratory study at semi-industrial scale were then implemented on a vessel in real navigation conditions with results in agreement with expectations.

High Performance of Commercial PAC on the Simultaneous Desulfurization and Denitrification of Wastewater From a Coal-Fired Heating Plant

The flue gas desulfurization wastewater is highly saline and has too many refractory pollutants to be recycled during the desulfurization process of the coal-fired heating plant. Given that waste heat is abundant in coal-fired heating plants, a thermal treatment method was developed to simultaneously remove sulfates and nitrates from the wastewater, with the production of chemical-grade natroalunite and recycled water. The results showed that sulfates and nitrates were 50.3 and 10 g/L in the wastewater, respectively, and only 2.8% and 9.1% were removed after direct treatment at 270°C for 7 h; but these rates increased to 99.3% and 99.9%, respectively, with the addition of commercial poly aluminum chloride. Mass balance summarized that the treatment of 1 ton wastewater needed 0.1 ton PAC and produced 0.11 ton natroalunite and 0.92 ton recycle water. The removal of sulfates and nitrates was mainly done by the precipitation reaction of sulfates such as natroalunite and the redox reaction between nitrates and organics, respectively. Thermodynamic analysis demonstrated that the precipitate reaction occurred at 45°C and accelerated in the temperature range of 45–270°C, but became slow with the decrease of sulfate and Al concentrations in wastewater. Four other reagents were also used for wastewater treatment in comparison with PAC and showed the following order of performance: PAC > citrate calcium > limestone > subacetate aluminum > citrate ferric. This method provided a practical route to treat wastewater from flue gas desulfurization without generating secondary waste.

Seasonal Variations and Influencing Factors of Gross Nitrification Rate in Desert Steppe Soil

Nitrification plays an important role in nitrogen (N) turnover and N cycling. To date, there have been many studies on the net N turnover in grassland. However, few studies have specifically focused on the gross N turnover, which is mainly due to methodological limitations. Here, we set up an enclosed plot in a desert steppe and determined the gross nitrification (GN) rate of the soil by using the barometric process separation method. We found the seasonal dynamics of the GN in the desert steppe soil, such that the GN in the summer (117.65 ± 24.86 μg N kg−1 h−1) was significantly greater than in the spring and autumn (65.17 ± 7.33 μg N kg−1 h−1), and it peaked in July (213.75 ± 44.66 μg N kg−1 h−1). Additionally, the GN was lowest in the spring, with a means of 50.52 ± 3.95 μg N kg−1 h−1. The seasonal variation in the GN was different than the seasonal variation in the net nitrification rate, and the GN was generally much higher than the net nitrification rate. We further demonstrated that the soil moisture, temperature, bulk density and NH4+-N were the main factors that influenced the seasonal variations in the GN, and that the soil moisture had the greatest impact on the GN among all the factors measured.

Reverse Osmosis Membrane Combined with Ultrasonic Cleaning for Flue Gas Desulfurization Wastewater Treatment

Flue gas desulfurization (FGD) wastewater treatment is currently of interest, as stringent standards have been released in order to limit the pollution emissions from the energy industry, and concerns about water scarcity are also increasing. Reverse osmosis (RO) membrane is a promising alternative for highly efficient FGD wastewater treatment. However, membrane fouling strongly limits its application. This study developed a suitable treatment system by combining RO membrane with ultrasonic cleaning. The introduction of low-frequency and high-intensity ultrasonic cleaning improved the cleaning efficiency of membrane fouling, as the permeate flux recovered 49% of the reduced value within 10 min of cleaning. The lifespan of the membrane was also extended, as the time of permeate flux declined to the same level, increasing from 2 h to 4 h after ultrasonic cleaning. The effluent of the system could meet the standard of desulfurization wastewater treatment. The treatment system is feasible for FGD wastewater treatment at a laboratory scale. These findings proved that the combination of RO membrane and ultrasonic cleaning could be applied to FGD wastewater treatment. The study provided an efficient, cost-saving, and convenient way to develop the FGD wastewater treatment system.

Economic Analysis of Atomization Drying of Concentrated Solution Based on Zero Discharge of Desulphurization Wastewater

With the improvement of environmental protection requirements, more and more attention has been given to desulphurization wastewater with zero discharge in coal power plants. Atomization drying is part of the main zero discharge technologies at present. Economic analysis of the atomization drying of desulphurization wastewater is beneficial to the formulation of an appropriate operation scheme and to the reduction of operation costs. The economic analysis and sensitivity analysis of different operating conditions such as unit load, the handling capacity of concentrates, and the temperature of the extracted flue gas in the atomization drying process of concentrated desulfurized wastewater were carried out in this paper. The main cost of the drying process came from the influence of flue gas extraction on the overall heat transfer in the boiler, resulting in the decrease in power generation revenue, which can reach more than 80%. The operating cost of auxiliary machinery was relatively low. The cost of treatment for per ton of concentrates increased first and then decreased with the increase in temperature of the extracted flue gas, and it decreased with the increase in the handling capacity of the concentrates. The effect of a unit load on the treatment cost was also related to the temperature of the extracted flue gas, and the optimal flue gas temperature increase to higher temperatures as the unit load decreased. The minimum treatment costs per ton of concentrate ranged from CNY 143.54/t to CNY 158.77/t under different unit loads. Sensitivity analysis showed that the temperature of the extracted flue gas had the greatest impact on treatment cost, and its sensitivity coefficient was 0.0834. The ways in which to improve economic benefits were discussed.

Experimental and theoretical study of a new CDI device for the treatment of desulfurization wastewater

According to the characteristics of desulfurization wastewater, A new capacitive deionization (CDI) device was designed to study the desalination characteristics of desulfurization wastewater in this paper. The experiments investigated the desalination efficiency under different conditions which find that the best desalination efficiency is achieved at a voltage of 1.2V, pH=11 and 50°C. Besides, ion adsorption is more favorable under acidic and alkaline conditions. The anion and cation removal performance experiments showed that the order of cation removal is Mg²⁺>Na⁺>Ca²⁺>K⁺ and the order of anion removal is Cl^->CO₃^2->NO₃^->SO₄^2->HCO₃^-. The mechanism of CDI was studied and analyzed by the isothermal adsorption model and COMSOL simulation software. It was found that the Freundlich model and Redlich-Peterson model have a good fit with the experimental results. The experiments show that the CDI device has excellent stability. CDI device was used to treat actual desulfurization wastewater. Furthermore, the study provides theoretical support for the industrial application of CDI for desulfurization wastewater treatment in the future. Graphical abstract.

Application of disk tube reverse osmosis in wastewater treatment: A review

Disk tube reverse osmosis (DTRO), a modified module RO, has received growing attention in wastewater treatment. However, there is no comprehensive review of DTRO applications for wastewater treatment. In this study, China was found to be a major contributor to DTRO investigations. Specifically, 46 full- and 17 pilot-scale DTRO applications in China from the literature are summarized. The cumulative reported DTRO application scale in wastewater treatment amounted to ~16,500 m³/d by 2020 in China. Leachate and flue gas desulfurization (FGD) wastewater had the highest reported frequencies. Two-stage DTRO and "biological treatment + DTRO/two-stage DTRO" were the most reported processes in leachate treatment. Chemical oxygen demand (COD) and NH₄⁺-N were removed at 99.00-99.95% and 98.00-99.98%, respectively, in full scales in leachate treatment. DTRO was primarily utilized in the concentration unit in hypersaline wastewater treatment (e.g., FGD wastewater). Total dissolved solids (TDS) were removed at 94.69-96.87% and 85.95-96.5% in the full- and pilot-scales, respectively. The overall operating costs were 17.50-60.61 CNY/m³ and 0.69-8.75 CNY/kgCOD for leachate treatment and 26.94-52.28 CNY/m³ and 0.71-3.61 CNY/kgTDS for FGD wastewater treatment. The major components of operating costs were chemical costs (13.09 CNY/m³, 1.63 CNY/kgCOD) for two-stage DTRO and electricity costs (19.73 CNY/m³, 1.67 CNY/kgCOD) for the "biological treatment + DTRO/two-stage DTRO" process. DTRO has shown promising prospects for wastewater treatment.

Gaseous mercury re-emission from wet flue gas desulfurization wastewater aeration basins: A review

Wet flue gas desulfurization (WFGD) simultaneously removes Hg and SO₂ from coal-fired power plant flue gas streams. Hg⁰ re-emission occurs when the dissolved Hg(II) is converted to a volatile form (i.e., Hg⁰) that can be subsequently emitted into the ambient air from WFGD wastewater aeration basins. Others have shown that Hg⁰ re-emission depends on pH, temperature, ligands (Cl, Br, I, F, SO₃^2-, SO₄^2-, NO₃^-, SCN^-, and ClO^-), O₂, minerals (Se and As), and metals (Fe and Cu) in WFGD wastewater. Still others have shown Hg⁰ re-emission restriction via inhibitor addition (adsorbents and precipitators). This is the first review that summarizes the complex and inconsistently reported Hg⁰ re-emission mechanisms, updates misconceptions related to Hg(II) complexation and reduction, and reviews applications of inhibitors that convert aqueous Hg(II) into stable solid forms to prevent gaseous Hg⁰ formation and release.

Tio2 and carbon nanotubes composites modify capacitive deionization anodes to improve the dechlorination efficiency in desulfurization wastewater

A new capacitive deionization (CDI) technology was used to remove Cl^- from desulfurization wastewater. TiO₂ and carbon nanotubes (CNTs) were combined with N-methyl pyrrolidone (NMP) to form composites by a solvothermal method in which it is coated onto the CDI anode to improve dechlorination efficiency (DE). The morphology, surface area, wettability, crystal structure and chemical composition of the TiO₂/CNTs were characterized. They showed good hydrophilicity (contact angle: 85.9°), high specific surface area (96.68 m²/g) and high specific capacitance (87.6 F/g). The experimental results illustrated that the best DE was achieved by the composites (60%T/C) under 1.2 V with the maximum electrosorption capacity toward 6.5 mg/g, and the TiO₂/CNTs composites had excellent stability. Adsorption kinetics analysis was explored and analyzed. Furthermore, TiO₂/CNTs composites exhibited excellent DE in actual desulfurization wastewater. The catalysis and adsorption mechanisms of the TiO₂/CNTs anode were discussed in detail. This study provides a new direction for the application of TiO₂/CNTs composites as adsorption materials of CDI in the Cl^- of desulfurization wastewater.

Insight into the magnetic lime coagulation-membrane distillation process for desulfurization wastewater treatment: From pollutant removal feature to membrane fouling

The high suspended solid (SS) and salts were main issues for flue gas desulfurization wastewater (FGDW). A magnetic lime coagulation (MLC)-membrane distillation (MD) integrated process was firstly applied with a self-made poly (vinylidene fluoride-co-chlorotrifluoroethylene) (PVDF-CTFE) membrane and the pollutants remove feature and membrane fouling were discussed. The SS was nearly 100 % removed and magnetic seed significantly accelerate the settleability. The flux was 43.00 kg/m² h with a salt rejection >99 %. It was higher than 13 kg/m² h in the first 125 h during the 18d continuous test, and the rejection for all cations, anions, total organic carbon (TOC) and total inorganic carbon (TIC) were higher than 99.95 %, 99.00 %, 98.81 %, and 99.65 %, respectively. Humic substances and tryptophan with 100-5000 Da were main dissolved organic matter (DOM), which were significantly removed. However, membrane fouling and wetting happened after 150 h. Scaling was the main foulants, while the organic fouling and biofouling were also detected. A new "bricklaying model" was induced to depict the formation of foulant layer, the colloids, organic matters (OMs) and microbe communities act as the "concrete", while the inorganic crystals (magnesium and calcium oxysulphides) were the "bricks". This contribution offers a new method for FGDW treatment and the membrane fouling mechanism of MD process.

Membrane Technologies in Wastewater Treatment: A Review

In the face of water shortages, the world seeks to explore all available options in reducing the over exploitation of limited freshwater resources. One of the surest available water resources is wastewater. As the population grows, industrial, agricultural, and domestic activities increase accordingly in order to cater for the voluminous needs of man. These activities produce large volumes of wastewater from which water can be reclaimed to serve many purposes. Over the years, conventional wastewater treatment processes have succeeded to some extent in treating effluents for discharge purposes. However, improvements in wastewater treatment processes are necessary in order to make treated wastewater re-usable for industrial, agricultural, and domestic purposes. Membrane technology has emerged as a favorite choice for reclaiming water from different wastewater streams for re-use. This review looks at the trending membrane technologies in wastewater treatment, their advantages and disadvantages. It also discusses membrane fouling, membrane cleaning, and membrane modules. Finally, recommendations for future research pertaining to the application of membrane technology in wastewater treatment are made.

Feasibility of concentrating textile wastewater using a hybrid forward osmosis-membrane distillation (FO-MD) process: Performance and economic evaluation

The forward osmosis-membrane distillation (FO-MD) hybrid process has shown great promise in achieving zero liquid discharge in the textile industry, recovering valuable dye molecules while producing large amounts of clean water. However, the progress of this technology seems to have stagnated with the direct coupling of commercial asymmetric FO and MD membranes, because water management in the system is found to be rather complicated owing to the processing of the different membranes. Herein, we propose, for the first time, an FO-MD hybrid process using a custom-made self-standing and symmetric membrane and a hydrophobic polytetrafluoroethylene membrane in the FO and MD units, respectively. Three types of operation modes were investigated to systematically study the process performance in the concentration treatment of model textile wastewater; two commercial FO membranes were also tested for comparison. Owing to its low fouling propensity and lack of an internal concentration polarization effect, the water transfer rate of our symmetric FO membrane quickly reaches equilibrium with that in the MD unit, resulting in continuous and stable operation. Consequently, the hybrid process using the symmetric FO membrane was found to consume the least energy, as indicated by its lowest total cost in both lab- and large-scale systems. Overall, our study provides a new strategy for using a symmetric FO membrane in the FO-MD hybrid process and highlights its great potential for use in the treatment of textile wastewater.

Evaporation and concentration of desulfurization wastewater with waste heat from coal-fired power plants

The reduction of wet desulfurization wastewater is one of the important tasks of coal-fired power plants, and it is important for achieving "zero emissions." Evaporation and concentration (E&C) with waste heat is an effective way to reduce wastewater. Here, two typical types of industrial desulfurization wastewater are used to study the change rule of pH and total dissolved solids during wastewater concentration in a circulating evaporation tower. The results indicate that with the increase of concentration ratio, the pH of desulfurization wastewater is decreased rapidly and then is gradually stabilized at 2-3 when SO₂ or SO₃ is contained in flue gas, and the increase in conductivity is less for wastewater with higher SO₄^2- content. The characteristics of various ions are also analyzed, and the composition and microscopic morphology of the precipitates are characterized during concentration. The growth pattern of Ca²⁺ concentration is dependent on the ratio of Ca²⁺ and SO₄^2- in raw wastewater. When the concentration ratio is 7.21, the insoluble and slightly soluble substances undergo precipitation and the solid content is approximately 20%, which can help realize the concentration and reduction of desulfurization wastewater.