Treatment of wastewater from an alkaline cleaning solution by combined coagulation and photo-Fenton processes

Treatment of Olefin plant spent caustic by combination of Fenton-like and foam fractionation methods in a bench scale

Spent Merox caustic (SMC) is a hazardous waste that is produced during the Merox desulfurization process in the petroleum refinery industry and should be treated before discharging to environment. In the present study, treatment of SMC was investigated by three methods including Fenton-like process, foam fractionation, and a combination of both processes. Immobilized TiO₂/Fe⁰ on modified silica nanoparticles was used as a heterogeneous Fenton-like catalyst. The chemical and physical characteristics of the catalyst were determined using Fourier-transform infrared spectroscopy, X-ray diffraction, diffuse reflectance spectroscopy, thermogravimetric analysis, differential scanning calorimetry, and transmission electron microscopy techniques. The treatment performance of the combined method was measured as a cost-effective method with chemical oxygen demand (COD) removal percentage. The effect of parameters including pH, gas flow rate, surfactant type and concentration of hydrogen peroxide, catalyst, and chelate were investigated. It is found that the prepared heterogeneous catalyst has high activity for the treatment of SMC. In addition, the results showed that the combined method achieved 97.6 ± 0.5% COD removal, while the measured values for Fenton or foam fractionation methods alone did not exceed 85.5 ± 1% and 47.2 ± 0.4%, respectively. The advantage of combination process over foam fractionation was the use of an advanced oxidation process in the separating column to eliminate or reduce the secondary phase contamination load. Besides, the role of the column in the effective contact of contaminants with the rising bubbles improved the degradation performance of the proposed process and reduced the consumption of hydrogen peroxide by 46% compared to the Fenton-like method.

Comparison on treatment strategy for chemical cleaning wastewater: Pollutants removal, process design and techno-economic analysis

Chemical cleaning wastewater (CCW) usually consists of pickling wastewater (PW) and alkaline cleaning wastewater (ACW), and the strategy of separate treatment or combined treatment affects pollutant removal efficiency and cost. In this study, separate and combined treatment of real PW and ACW generated from an on-site cleaning campaign were investigated. A neutralization - fluoride removal - coagulation - oxidation process was constructed for PW and mixed wastewater (MW) treatment, and operational conditions of each process were optimized. The optimal mixing ratio of PW and ACW in the primary neutralization tank was 3:7, which obtained a near neutral pH, efficient chromaticity and turbidity removal and good settling performance. The neutralized MW and PW were both adjusted pH to 9.5 to precipitate metal ions as hydroxides. After fluoride precipitated as CaF₂, the fluoride removal rates of MW and PW were both 99.9%, respectively, and polyaluminum chloride was dosed to improve the settleability of CaF₂. Then sodium hypochlorite oxidization was employed to remove NH₃-N and soluble COD. Techno-economic analysis based on pilot-scale tests showed that separate treatment of PW and ACW obtained better effluent quality than combined treatment. The total cost of combined treatment (37.44 $/m³) was greatly higher than that of separate treatment of PW and ACW (18.20 $/m³). This study proposed a cost-effective strategy for CCW treatment, and suggested that neutralization with acidic or alkaline wastewater should be systematically considered for technical and economic feasibility.

Treatment of chemical cleaning wastewater and cost optimization by response surface methodology coupled nonlinear programming

The real alkaline cleaning wastewater (ACW) was treated by a process consisting of neutralization, NaClO oxidation and aluminum sulfate (AS) coagulation, and a novel response surface methodology coupled nonlinear programming (RSM-NLP) approach was developed and used to optimize the oxidation-coagulation process under constraints of relevant discharge standards. Sulfuric acid neutralization effectively removed chemical oxygen demand (COD), surfactant alkylphenol ethoxylates (OP-10) and silicate at the optimum pH of 7.0, with efficiencies of 62.3%, >82.7% and 94.2%, respectively. Coagulation and adsorption by colloidal hydrated silica formed during neutralization were the major removal mechanisms. NaClO oxidation achieved almost complete removal of COD, but was ineffective for the removal of surfactant OP-10. AS coagulation followed by oxidation can efficiently remove OP-10 with the formation of Si-O-Al compounds. The optimum conditions for COD ≤100 mg/L were obtained at hypochlorite to COD molar ratio of 2.25, pH of 10.0 and AS dosage of 0.65 g Al/L, with minimum cost of 9.58 $/m³ ACW. This study shows that the integrative RSM-NLP approach could effectively optimize the oxidation-coagulation process, and is attractive for techno-economic optimization of systems with multiple factors and threshold requirements for response variables.

Coagulation-flocculation sequential with Fenton or Photo-Fenton processes as an alternative for the industrial textile wastewater treatment

In this study, the industrial textile wastewater was treated using a chemical-based technique (coagulation-flocculation, C-F) sequential with an advanced oxidation process (AOP: Fenton or Photo-Fenton). During the C-F, Al₂(SO₄)₃ was used as coagulant and its optimal dose was determined using the jar test. The following operational conditions of C-F, maximizing the organic matter removal, were determined: 700 mg/L of Al₂(SO₄)₃ at pH = 9.96. Thus, the C-F allowed to remove 98% of turbidity, 48% of Chemical Oxygen Demand (COD), and let to increase in the BOD₅/COD ratio from 0.137 to 0.212. Subsequently, the C-F effluent was treated using each of AOPs. Their performances were optimized by the Response Surface Methodology (RSM) coupled with a Box-Behnken experimental design (BBD). The following optimal conditions of both Fenton (Fe²⁺/H₂O₂) and Photo-Fenton (Fe²⁺/H₂O₂/UV) processes were found: Fe²⁺ concentration = 1 mM, H₂O₂ dose = 2 mL/L (19.6 mM), and pH = 3. The combination of C-F pre-treatment with the Fenton reagent, at optimized conditions, let to remove 74% of COD during 90 min of the process. The C-F sequential with Photo-Fenton process let to reach 87% of COD removal, in the same time. Moreover, the BOD₅/COD ratio increased from 0.212 to 0.68 and from 0.212 to 0.74 using Fenton and Photo-Fenton processes, respectively. Thus, the enhancement of biodegradability with the physico-chemical treatment was proved. The depletion of H₂O₂ was monitored during kinetic study. Strategies for improving the reaction efficiency, based on the H₂O₂ evolution, were also tested.