Recently, a chemical company sought a solution to meet the park's discharge standards for the 30 tons of polyester resin production wastewater it discharges every day.
The wastewater mainly includes high-concentration esterified wastewater (COD concentration of more than 32,000 mg/L), high-concentration vacuum wastewater (COD concentration of about 90,000 mg/L, the amount of water is small), and other low-concentration wastewater (such as domestic wastewater and cleaning wastewater).
Enterprises focus on esterified wastewater because of its high concentration, high concentration of biodegradable organic matter (such as alcohols, acids, organic solvents, etc.), lack of nutrients, and a pH value of 3-4. In order to ensure the efficient operation of the sewage treatment plant and avoid the impact on the biochemical treatment system, the COD concentration and B/C value of esterified wastewater need to be controlled.
Here are three possible ways to deal with it:
1. Evaporation method: The esterified wastewater is evaporated, and the effluent is mixed with other wastewater for biochemical treatment, but this method is costly.
2. Dilution method: dilute esterified wastewater with low-concentration wastewater, but this will lead to a significant increase in the treatment capacity of the sewage treatment station, which will also increase the cost.
3. Physicochemical treatment: First, the esterified wastewater is pretreated to reduce the COD concentration to the range of anaerobic biological treatment.
We adopted the third method, through the pretreatment process of "iron-carbon microelectrolysis-Fenton oxidation-coagulation precipitation", to achieve the purpose of pretreatment. The specific steps are as follows:
- Iron-carbon microelectrolysis: The galvanic cell composed of iron filings and carbon particles is used to undergo electrochemical reactions, and the charged colloids are destabilized and aggregated and settled through the action of microelectric fields, which improves the B/C value in wastewater and improves the biodegradability of wastewater.
- Fenton oxidation method: chemical oxidation pretreatment is carried out by adding H2O2 and using the strong oxidation of the mixed solution of hydrogen peroxide and Fe2+.
- Coagulation and precipitation: The pH of the raw water is adjusted by adding NaOH to meet the pH range required for flocculation and subsequent biochemical treatment.
In fact, depending on the water quality, we can reduce the pre-treatment process and solve more organic matter through biochemical treatment, thereby reducing the cost of wastewater treatment.
The pretreated esterified wastewater is mixed with other wastewater and then enters the conditioning tank and then enters the biochemical treatment system together: hydrolysis acidification - UASB - multi-stage contact oxidation tank.
- Hydrolysis and acidification: effective for refractory organic matter, has a certain removal rate of SS and COD, and can improve biodegradability, so as to optimize the entire wastewater treatment process.
- UASB: The UASB reactor undertakes most of the COD removal tasks, and the COD removal rate is maintained at more than 85% in actual operation, and there is no need for compressed air oxygenation and stirring, saving power consumption. In addition, it also has the advantages of high gas, solid and liquid separation efficiency, high organic load of equipment, strong impact resistance, simple operation and control, and stable operation of wastewater treatment.
- Multi-stage contact oxidation tank: The tank is filled with a filler, and aerobic microorganisms attach to the filler to form a biofilm, which is blown into the pool through a blower to fully degrade the pollutants in the wastewater under the action of microorganisms.
Through this series of rigorous treatment processes, the company has successfully achieved the discharge of 30 tons of polyester resin production wastewater per day.