Disinfection efforts have been intensified in all aspects of life, such as water supply and sewage discharge. Many of these disinfectants flow into the sewage treatment plant through the municipal pipe network, resulting in a large increase in the amount of disinfectant entering the sewage treatment plant. Chlorine-based disinfectants are mainly used, and the indicators show that the residual chlorine entering the sewage treatment plant has increased! 1. The effect of residual chlorine on activated sludge 1) The bactericidal effect of residual chlorine After the salt form is thrown into the water body, it will be hydrolyzed to generate free available chlorine, including chlorine gas, hypochlorous acid and hypochlorite ions, which are collectively called residual chlorine.) The uncharged hypochlorous acid monomer can enter the bacteria through the cell membrane In the body, it oxidizes with protein substances and nucleic acids in bacteria, causing bacterial metabolic disorders and killing bacteria. The higher the concentration of hypochlorite, the stronger the bactericidal ability. 2) The amount of residual chlorine on activated sludge According to the research, it is proved that when the pH is close to neutral, hypochlorous acid and hypochlorite ions mainly exist. When the residual chlorine dose gradually increases from 0.1mg/L to 3.0mg /L, the pH value of wastewater is controlled at about 7.5, and when the residual chlorine dose reaches 1.0 mg/L, the COD
and BOD removal rates begin to decline; when the toxic dose reaches 1.5 mg/L, the number and form of protozoa do not change significantly The COD and BOD removal rates dropped to 39% and 69% respectively; when the toxic dose exceeded 2.5mg/L, the COD removal rate dropped to below 30%, the activated sludge flocs disintegrated, and a large number of protozoa died. However, the residual chlorine value in this study is based on the traditional activated sludge method. For sewage treatment plants that require denitrification, it cannot be simply considered that the residual chlorine in the influent is 1.0mg/L. It has an impact on the system. If In addition to the effect of internal reflux (generally control r=200 or more), the author believes that the residual chlorine in the influent of sewage treatment plants with denitrification process is not more than 2mg/L, which is acceptable! 2. How to deal with the residual chlorine in the influent However, according to the description of the small partners in the sewage top state technology group, when the residual chlorine rises above 0.5mg/L, it will start to affect the denitrification, and it needs to supplement the carbon source to maintain it, which will lead to an increase in cost! Therefore, there is an urgent need for pretreatment measures to deal with residual chlorine: 1) Pre-aeration Pre-aeration in the influent can remove part of the residual chlorine. The principle is the law of partial pressure of the gas. There is a dynamic balance in the sodium hypochlorite in the water that can release part of the chlorine gas. Why? The reason why the tap water will smell the smell of disinfectant, and the pre-aeration can blow off the chlorine gas through the partial pressure of the gas, resulting in the continued decomposition of hypochlorous acid! The pre-aeration measures are suitable for the sewage plants with aeration and stirring in the adjustment tank or the process of aeration sedimentation tank! 2) Dosing activated carbon Activated carbon removes residual chlorine. The principle is the same as that of adding reducing substances, because the surface of activated carbon has unsaturated groups and is reductive, and removes residual chlorine through the oxidation-reduction effect of unsaturated groups and residual chlorine. Chlorine works! But because there are more SS in the water, it is easy to block the capillary pores of activated carbon, and it cannot exert its full capacity! However, the epidemic period is so long, do you have so much reserves? 3) Dosing reducing substances Dosing reducing substances to remove residual chlorine, the principle is the same as that of activated carbon, through redox reaction to remove residual chlorine, personally recommend ferrous salts for reducing substances, because ferrous salts are used as flocculants , There may be more reserves in some sewage treatment plants! Some friends suggested adding sodium sulfite, the effect is no problem, but the question is can you buy it? Is the price affordable? 4) Increase sludge concentration/increase internal and external reflux. The principle of increasing sludge concentration is that since there are not so many defenses, we can only use physical resistance. High sludge concentration has a certain resistance to the impact of influent water; increase internal and external reflux (Sludge reflux and mixed liquid reflux) are used to dilute part of the residual chlorine, allowing high-concentration sludge to directly contact residual chlorine in the influent, reducing the pressure of subsequent sludge! It is best to use it together to increase sludge concentration/internal and external return flow, which can achieve twice the result with half the effort. This method is the most recommended by the author, because every sewage treatment plant can be operated, simple and convenient!
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