Treatment of phenol-containing wastewater from the paint industry using mixed clarification tanks

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Paint wastewater primarily originates from the water used in wet painting booths to clean the air in those areas; the paint particles and organic solvents present in the air are transferred into the water, thus forming paint wastewater. Wastewater contains a large amount of paint particles, and its quality depends on the paints used (primarily nitrocellulose paints, amino paints, alkyd paints, and epoxy paints), solvents (such as ethanol, acetone, esters, and benzene), and additives. I. Sources of paint wastewater and main pollutants 1. Sources of paint wastewater and harmful substances Paint wastewater primarily originates from the pre-degreasing, degreasing, surface treatment, phosphating, and passivation processes involved in vehicle preprocessing ； Cathodic electrophoresis process, as well as intermediate coating and topcoat spraying processes. 2. The main toxic and harmful substances present in paint wastewater are as follows: Pre-treatment for painting: sulfates, phosphates, emulsified oils, surfactants, Ni2+, Zn2+. Primer: low-solvent cathodic electrophoretic paint films, lead-free cathodic electrophoretic paint films, pigments, powders, epoxy resins, butanol, ethylene glycol monobutyl ether, isopropanol, dimethylethanolamine, polybutadiene resins, dimethylethanol, paints, etc. Middle coat, top coat: organic solvents such as xylene and nail polish remover, paint film, pigments, powders. II. Main causes of paint wastewater generation: Degreasing waste liquid. Acidification is used as a pre-treatment method to demulsify the degreasing waste liquid; inorganic acids are added to this liquid to lower its pH to 2–3, which causes the long-chain fatty acid soaps present in the emulsifiers to release fatty acids. These long-chain fatty acids are insoluble in water but soluble in oil, thereby enabling the demulsification and separation of oil from the degreasing waste liquid. Furthermore, the addition of acid causes the anionic surfactants in the degreased waste liquid to decompose easily in the acidic solution, losing their stability and the original balance between hydrophilicity and hydrophilicity, thereby achieving demulsification. After pretreatment, CODCr decreased from 2500–4000 mg/L to 1500–2400 mg/L, with a removal rate of around 40% ； Meanwhile, the oil content was reduced from 300–950 mg/L to 50–70 mg/L, with a removal rate as high as 90%–95%. Electrophoretic wastewater contains a large amount of high-molecular-weight organic substances; the CODCr level can reach up to 20,000 mg/L. It also includes plenty of electrophoretic sludge, which exist in the water as fine suspended particles or as negatively charged colloids. Appropriate cationic polyacrylamide (PAM) and polyaluminum chloride (PAC) are added during treatment as coagulants, taking advantage of the adsorption and bridging effects of these flocculants to rapidly remove pollutants from wastewater. During pretreatment, the pH of the electrophoresis waste liquid should be between 11 and 12 to achieve good precipitation effects. The CODCr of the effluent after the reaction is around 2000 mg/L. For paint wastewater, pretreatment is first carried out using Fenton’s reagent (H2O2 + FeSO4) to oxidize and decompose the organic substances present in it; the removal efficiency of CODCr in this step is around 30%. Afterwards, PAC and PAM are added to facilitate coagulation and sedimentation. Through these two treatment steps, the overall removal rate of CODCr can reach 60%–80%, reducing the concentration from 3000–20000 mg/L to 1200–4000 mg/L. The effluent is discharged into the mixed wastewater regulating tank. Mixed clarifiers are used to treat phenol-containing wastewater from the paint industry. After extraction and separation in mixed clarifiers, since the volume of wastewater is small, thorough mass transfer and separation can take place within these clarifiers, allowing a significant portion of the harmful substances to be effectively recovered. Due to the reactive chemical properties of formaldehyde and phenol, a Fenton extractant is used to oxidize them into organic compounds with low biological toxicity that are easily absorbed and degraded by organisms; as a result, the COD can be reduced to below 1000 mg/L after oxidation. At the same time, it can also break down difficult-to-degrade large molecular organic compounds into easily degradable small molecular organic compounds, thereby improving the biodegradability of wastewater and facilitating subsequent aerobic biological treatment. Coagulation treatment can remove suspended solids from the influent water; the coagulated flocs help to adsorb some dissolved organic substances, and it also enables efficient removal of color.