Key points and techniques for pretreatment of printing wastewater
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Nowadays, green printing has become the development trend of the printing industry. Whether the printing wastewater meets the standard discharge is an important project to measure the degree of green printing promotion of printing enterprises. The printing wastewater includes the PS plate developing waste liquid, the offset printing machine rubber roller cleaning wastewater, etc., and has the characteristics of high COD value in addition to acid and alkali, high chroma, and large fluctuation of waste water. The COD value of ordinary domestic wastewater is less than 800mg/L, and the COD value of printing wastewater exceeds 20000mg/L, which is difficult to handle. In order to further promote the green printing process of our company and improve the level of green printing, our company has analyzed and summarized the effective treatment method for printing wastewater through long-term exploration and practice, that is, strengthening the pretreatment of wastewater in the process, and willing to cooperate with the industry. Share with peers.
Principle and basic method of printing wastewater pretreatment
The pretreatment of printing wastewater refers to the pretreatment of wastewater before entering the traditional precipitation method, biological method and other processing steps, which is the precondition for ensuring the stability of the printing wastewater treatment process.
The pretreatment process of printing wastewater mainly includes pH adjustment and coagulation. pH adjustment is a key step in the pretreatment of printing wastewater, which will directly affect the coagulation effect and the next step of wastewater treatment. Generally, the pH of printing wastewater is 5.0-9.0, which can be adjusted by adding H2SO4 or NaOH. The specific control range of pH value varies depending on the water quality and the processing link. Our company's experience is to adjust the pH value of printing wastewater to 8.5-9.0. Time is the best.
Subsequently, a coagulant polyaluminum chloride (PAC) and a coagulant polyacrylamide (PAM) were added to the printing wastewater after adjusting the pH. These two additives are water-soluble high-molecular polymers. When mixed with printing wastewater, they can destabilize, aggregate, coagulate and adsorb fine suspended particles and colloidal ions in wastewater under the bridging action and polymerization of ions. Flocculation, final precipitation, to achieve the purification effect, this process can strongly remove the fine toxic substances and heavy metal ions in the printing wastewater.
Experimental Exploration on the Best Method of Printing Wastewater Pretreatment
The printing wastewater component of our company is complex and the pH value is large. There is a delay and error in measuring the pH value of printing wastewater by pH potential controller. Therefore, the best way to test the pH of the printing wastewater is the beaker test. The specific operation is as follows: take a sample of raw water every 5 to 10 minutes, adjust the pH value by adding appropriate amount of H2SO4 or NaOH reagent, and then use pH test paper for detection. According to the test result, the pH value of the sample is finely adjusted to make the pH value Within the scope of the request. The beaker test method can avoid the misjudgment of pH value due to the delay and error of the pH potential controller, and further inject the incompletely processed printing wastewater into the next process, affecting the coagulation effect and the final treatment effect of the printing wastewater.
Table 1 shows the effect of coagulation treatment on printing wastewater of different pH values (the raw water COD value is about 18,000 to 20,000 mg/L). The pH value of our company's printing wastewater raw water is between 5.0 and 7.0, and it is adjusted to the pH range corresponding to the four groups of tests. Then, according to the long-term experience, an appropriate amount of coagulant is added to observe the water quality after physicalization and measure the pH value. . It can be seen from Table 1 that the pH value of the printing wastewater directly affects the coagulation treatment effect, especially from the comparison between the first group and the second group of two samples, the daily addition amount of the coagulant is different. The ratio is about double, but for the printing wastewater with the high pH value (the second group), the final treatment effect is not good.
It can be seen that controlling the pH value of printing wastewater is crucial, not only related to the stability of the whole process of printing wastewater pretreatment, but also saving the use of coagulant and reducing the printing wastewater treatment cost while ensuring the discharge of water quality.