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Sulphidic spent caustic (SSC) is an alkaline waste stream which is generated during caustic scrubbing of liquefied petroleum gas and ethylene products. Due to presence of high concentrations of sulphides and phenols, the waste stream requires proper treatment before mixing with the low strength wastewater streams produced from other refinery operations. Electrochemical process is an emerging treatment method that can work efficiently at ambient conditions. The present study reports performance of electro-Fenton (EF) process for the treatment of synthetic SSC wastewater (sulphides = 10 g L -1 , phenol = 2 g L -1 and pH = 12.9). The EF runs were carried out for 2 h duration in a reactor equipped with iron electrodes. The effects of H 2 O 2 dose (0.26-1.3 M), current density (1-20 mA cm -2 ), pH (4.5-12.9) and stirring speed (100-1000 rpm) were investigated on removal of pollutants. The H 2 O 2 was rapidly consumed in initial 30 min during which the significant fraction of the pollutants was degraded or removed. The optimum conditions for EF process were found to be as follows: pH = 4.5, H 2 O 2 dose = 1.05 M, current density = 5 mA cm -2 and stirring speed = 500 rpm. At these conditions, the maximum sulphide and phenol removals from the wastewater were 98% and 91%, respectively. The results will be helpful to the wastewater treatment plant operators worldwide dealing with high concentrations of such pollutants., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

The present study investigated the performance of a chromium-based advanced oxidation process using chromium (as Cr 3+ or Cr 6+ ) and H 2 O 2 for the treatment of synthetic and simulated textile wastewaters. With the Cr 3+ /H 2 O 2 system, the maximum total organic carbon (TOC) and color removals from the synthetic dye wastewater (Remazol Brilliant Violet 5R dye concentration = 100 mg/L) were 75% and 99%, respectively, within 30 min duration ([Cr 3+ ]:[H 2 O 2 ] = 1:30, stoichiometric H 2 O 2 dose = 2.01 ml/L and pH = 7). Whereas the same catalyst and oxidant combination resulted in chemical oxygen demand (COD) and color removals of ~ 46%, and 84%, respectively, after 3 h of reaction at the optimized reaction conditions (i.e., [Cr 3+ ]:[H 2 O 2 ] = 1:50, stoichiometric H 2 O 2 dose = 11.6 ml/L and pH = 7) from the simulated textile wastewater (initial pH = 10.2, and COD = 1820 mg/L). Further, the addition of stoichiometric H 2 O 2 dose to the pretreated wastewater and pH adjustment increased the overall COD removal to 77%. Both oxidation and precipitation reactions were found responsible for organics removal from the wastewater. The other alternative involving activated carbon adsorption as second step, was not found as effective as the above scheme. The data on COD removal from simulated textile wastewater could be fit adequately in the retarded first-order kinetic model. Based on the COD and color removal results and preliminary cost analysis, this can be suggested that the Cr 3+ /H 2 O 2 oxidation process followed by pH adjustment and further H 2 O 2  treatment was the best option for the removal of COD and color from the simulated combined textile wastewater., (© 2022. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

In the present study, sewage sludge (SS) was used to synthesize activated carbon (AC) which was further utilized as adsorbent for the removal of ciprofloxacin (CPX) from synthetic wastewater. The adsorbent was prepared by chemical activation method using ZnCl 2 as activating agent. Design of experiments (DOE) approach was explored to determine the optimum operating conditions for the synthesis of AC and CPX removal from the wastewater. The optimum conditions for AC synthesis (i.e., carbonization temperature = ~500°C, activation time = 30 min, and impregnation ratio = 2.26) were decided based on results for three response parameters, that is, adsorbent yield, methylene blue removal, and iodine number. The synthesized adsorbent showed ~93% CPX removal (initial CPX concentration = 100 mg/L) at the following optimum conditions: adsorbent dose = 1.31 g/L, pH = 7 and reaction time = 12 h. Langmuir isotherm model was best fit to the equilibrium adsorption data (maximum adsorption capacity of SS derived AC = 102 mg/g) whereas pseudo-second order model showed the best fit to adsorption kinetic data (adsorption capacity = 77.5 mg/g). An effort was also made to reduce fresh water requirement for adsorbent synthesis by recycling the wastewater produced during chemical activation of SS. PRACTITIONER POINTS: Experiment design approach was used for optimization of adsorbent preparation conditions and CPX removal conditions by waste derived adsorbent. Sewage sludge derived adsorbent had BET surface area of 564 m 2 /g which is comparable to commercial activated carbon. 93% CPX adsorption with the sewage sludge derived adsorbent at optimum conditions. Langmuir model better suited the CPX adsorption data. Wastewater recycling and ZnO recovery from wastewater produced during adsorbent synthesis were performed., (© 2021 Water Environment Federation.)

Catalytic advanced oxidation processes (AOPs) utilising UV irradiation have been reported to be highly efficient for the degradation of recalcitrant compounds. The focus of present study was to evaluate the potential of steel industry waste as an alternative to homogenous iron salts often used in AOPs. Iron rich sludge from effluent treatment plant (ETP) of a steel industry was subjected to acid washing and acid washing followed by calcination before using as catalyst for the oxidative removal of chlorinated organics present in synthetic and simulated bleaching effluents. The maximum total organic carbon removals of 64% and 25% from 4-chlorophenol solution and simulated pulp bleaching effluent, respectively were observed during photo-catalytic oxidation with acid washed steel sludge at stoichiometric H 2 O 2 dose after 2 h reaction (catalyst dose = 1 g/L, and pH = 4.4). Low iron leaching (<2%) was observed from the catalyst even in acidic conditions and it could be reused twice without significant loss of catalytic activity., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

In this study, the performance of activated sludge derived granular activated carbon (SGAC) was investigated for ciprofloxacin (CPX) removal from synthetic and simulated wastewaters in a fixed-bed adsorption column operated in continuous mode. The adsorbent was synthesized using chemical activation using ZnCl 2 as activating agent. Its surface area and pore volume were found comparable to that of the commercial granular activated carbon (CGAC). The maximum saturation adsorption capacities for CPX were ~16 mg/g and ~14 mg/g, respectively, with SGAC column under identical operating conditions (CPX concentration = 50 mg/L, bed height = 4 cm and wastewater flow rate = 1.5 mL/min) for synthetic and simulated wastewaters. The presence of other organics reduced CPX adsorption capacity of SGAC. The breakthrough curve data for both wastewaters could be adequately fit in Thomas and Yoon-Nelson kinetic models. The addition of H 2 O 2 in wastewater showed no considerable improvement in CPX removal. However, H 2 O 2 oxidation of spent adsorbent exhibited better results compared to thermal treatment for adsorbent regeneration. The results showed that sewage sludge can be recycled as an efficient adsorbent for the removal of recalcitrant organic pollutants from wastewater., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

The present experimental study reports the performance of tea waste (TW) derived adsorbent for the adsorption of sodium diclofenac (SD) from aqueous solution (SD concentration = 10-50 mg/L). The waste-derived activated carbon was prepared by chemical activation process of raw waste using H 2 SO 4 , KOH, ZnCl 2 , and K 2 CO 3 as activating agents (TW: activating agent = 1:1 by weight). Subsequently, the oven-dried material was carbonized at 600-°C temperature for 2 h. The synthesized adsorbents were porous and their Brunauer-Emmett-Teller (BET) surface area was ranged 115-865 m 2 /g. Among all synthesized adsorbents, the adsorbent activated by ZnCl 2 exhibited the highest adsorption capacity (= 62 mg/g), though it was much lower compared to 91 mg/g obtained with commercial activated carbon (CAC) (SD concentration = 30 mg/L, adsorbent dose = 300 mg/L and initial wastewater pH = 6.47). SD equilibrium data could be described by Langmuir isotherm adequately, while pseudo-second-order rate model showed better fit to the time based adsorption data. Low activation energy of the adsorption process suggests the reaction to be temperature independent. Thermodynamic parameters showed the spontaneous and endothermic nature of adsorption process conducted in the presence of waste derived adsorbent.

The bleaching effluent discharged from a pulp and paper mill contains chlorinated organic compounds which are toxic to living matter. Physicochemical treatments such as coagulation and different advanced oxidation processes (AOPs) were employed for combined bleaching effluent generated from the first two stages (i.e. chlorination and alkali extraction) (pH = 3.5, chemical oxygen demand (COD) = 1,920 mg/L, and total organic carbon (TOC) = 663 mg/L). At optimum conditions (pH = 7.5, polyaluminium chloride (PAC) dose = 3.84 g/L and slow mixing time = 25 min), ∼68% removal in UV254 and ∼23% TOC removal was obtained during coagulation. Among various AOPs, UV/Fe 2+ /TiO 2 /H 2 O 2 system showed the highest TOC and COD removals (∼78%) after 2 h duration (Fe 2+ :H 2 O 2 molar ratio = 1:100). After the AOP process, chloride ion concentration and biodegradability of the treated wastewater was increased to 2,762 mg/L and 0.46 from an initial value of 2,131 mg/L and 0.29, respectively. The wastewater and sludge analysis showed oxidation and adsorption as the major mechanisms for organics removal. Upon reuse of the regenerated catalysts, TOC removal was reduced significantly. It was found that three times more sludge per unit TOC removal was generated after coagulation in comparison to that produced after UV/Fe 2+ /TiO 2 /H 2 O 2 treatment.

The present study was performed to investigate the performance of activated carbon-supported copper and manganese base catalyst for catalytic wet oxidation (CWO) of pulping effluent. CWO reaction was performed in a high pressure reactor (capacity = 0.7 l) at temperatures ranging from 120 to 190 °C and oxygen partial pressures of 0.5 to 0.9 MPa with the catalyst concentration of 3 g/l for 3 h duration. With Cu/Mn/AC catalyst at 190 °C temperature and 0.9 MPa oxygen partial pressures, the maximum chemical oxygen demand (COD), total organic carbon (TOC), lignin, and color removals of 73, 71, 86, and 85 %, respectively, were achieved compared to only 52, 51, 53, and 54 % removals during the non-catalytic process. Biodegradability (in terms of 5-day biochemical oxygen demand (BOD 5 ) to COD ratio) of the pulping effluent was improved to 0.38 from an initial value of 0.16 after the catalytic reaction. The adsorbed carbonaceous fraction on the used catalyst was also determined which contributed meager TOC reduction of 3-4 %. The leaching test showed dissolution of the metals (i.e., Cu and Mn) from the catalysts in the wastewater during CWO reaction at 190 °C temperature and 0.9 MPa oxygen partial pressures. In the future, the investigations should focus on the catalyst reusability.

Biological treatment of high-strength nitrogenous wastewater is challenging due to low growth rate of autotrophic nitrifiers. This study reports bioaugmentation of Thiosphaera pantotropha capable of simultaneously performing heterotrophic nitrification and aerobic denitrification (SND) in sequencing batch reactors (SBRs). SBRs fed with 1:1 organic-nitrogen (N) and NH 4 + -N were started up with activated sludge and T. pantotropha by gradual increase in N concentration. Sludge bulking problems initially observed could be overcome through improved aeration and mixing and change in carbon source. N removal decreased with increase in initial nitrogen concentration, and only 50-60 % removal could be achieved at the highest N concentration of 1000 mg L -1 at 12-h cycle time. SND accounted for 28 % nitrogen loss. Reducing the settling time to 5-10 min and addition of divalent metal ions gradually improved the settling characteristics of sludge. Sludge aggregates of 0.05-0.2 mm diameter, much smaller than typical aerobic granules, were formed and progressive increase in settling velocity, specific gravity, Ca 2+ , Mg 2+ , protein, and polysaccharides was observed over time. Granulation facilitated total nitrogen (TN) removal at a constant rate over the entire 12-h cycle and thus increased TN removal up to 70 %. Concentrations of NO 2 - -N and NO 3 - -N were consistently low indicating effective denitrification. Nitrogen removal was possibly limited by urea hydrolysis/nitrification. Presence of T. pantotropha in the SBRs was confirmed through biochemical tests and 16S rDNA analysis.

The present study deals with the non-catalytic and catalytic wet oxidation (CWO) for the removal of persistent organic compounds from the pulping effluent. Two activated carbon-supported heterogeneous catalysts (Cu/Ce/AC and Cu/Mn/AC) were used for CWO after characterization by the following techniques: temperature-programmed reduction, Fourier transform infrared spectroscopy and thermo-gravimetric analysis. The oxidation reaction was performed in a batch high-pressure reactor (capacity = 0.7  L) at moderate oxidation conditions (temperature = 190°C and oxygen pressure = 0.9 MPa). With Cu/Ce/AC catalyst, the maximum chemical oxygen demand (COD), total organic carbon (TOC) and lignin removals of 79%, 77% and 88% were achieved compared to only 50% removal during the non-catalytic process. The 5-day biochemical oxygen demand (BOD5) to COD ratio (a measure for biodegradability) of the pulping effluent was improved to 0.52 from an initial value of 0.16. The mass balance calculations for solid recovered after CWO reaction showed 8% and 10% deduction in catalyst mass primarily attributed to the loss of carbon and metal leaching. After the CWO process, carbon deposition was also observed on the recovered catalyst which was responsible for around 3-4% TOC reduction.

In the present study, the performance of sewage sludge based adsorbents was examined for the removal of two recalcitrant pollutants (i.e. lignin and amoxicillin) from synthetic wastewater solutions (adsorbate concentration=50-250 mg/l). The effect of various reaction parameters such as wastewater pH, adsorbent dosage and temperature was studied. Possible mechanisms for the adsorption process have been proposed which depends upon the behaviour of adsorbent surface and adsorbate molecules under specific reaction conditions. Three-parameter Redlich-Peterson isotherm model was found the best fit to the equilibrium data. Pseudo first and second order models validated the kinetic data for lignin and amoxicillin adsorption systems, respectively and the corresponding activation energy was 3.5-4.5 and 12-22 kJ/mol. The nature of adsorption was elucidated from the thermodynamic parameters., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

In the present study, catalytic wet oxidation (CWO) was investigated for the destruction of organic pollutants in the thermally pretreated effluent from a pulp and paper mill under moderate temperature and pressure conditions. The thermal pretreatment studies were conducted at atmospheric pressure and 368K using copper sulfate as a catalyst. The thermal pretreatment reduced COD by about 61%. The filtrate of the thermal pretreatment step was used at pH 8.0 for CWO at 383-443K temperature and a total pressure of 0.85MPa for 4h. Catalysts used for the reaction include copper sulfate, 5% CuO/95% activated carbon, 60% CuO/40% MnO(2), and 60% CuO/40% CeO(2). Maximum COD reduction was found to be 89% during CWO step using 5% CuO/95% activated carbon with a catalyst loading of 8gl(-1) at 443K and 0.85MPa total pressure. Overall COD reduction for the pretreatment and the CWO was found to be 96%. Besides this, 60% CuO/40% CeO(2) catalyst also exhibited the similar activity as that of obtained with 5% CuO/95% activated carbon catalyst at 423K temperature and 0.85MPa total pressure. The pH of the solution during the experimental runs decreases initially due to the formation of carboxylic acid and then increases due to the decomposition of acids.