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, collectively referred to as residual chlorine.), the uncharged hypochlorous acid monomer can enter through the cell membrane In the bacteria body, there is an oxidation reaction with the protein substances and nucleic acids in the bacteria body, which makes the bacteria metabolism disorder and kills the 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 of Wang Yonghui , it is proved that when the pH is close to neutral, hypochlorous acid and hypochlorite ions mainly exist, as shown in the figure: When the residual chlorine dose changes from 0.1mg/L gradually increased to 3.0mg/L, the pH value of the wastewater was controlled at about 7.5, and when the residual chlorine dose reached 1.0 mg/L, the removal rate of COD
began to decline; when the toxic dose reached 1.5mg/L, protozoa The quantity and form of the activated sludge did not change significantly, and 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 below 30%, the activated sludge flocs disintegrated, and the primary Animals died in large numbers. 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. 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! In the Sewtopia technical exchange group, there is a small partner whose influent residual chlorine exceeds 2mg/L, and denitrification is abnormal, which can be used as evidence! 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 residual chlorine. The principle is the law of gas partial pressure (Henry's law), and there is a dynamic balance of sodium hypochlorite in water (the formula is Don’t write, let’s imagine) can release part of chlorine gas, which is why the tap water smells like disinfectant, and pre-aeration can blow out chlorine gas through gas partial pressure, resulting in the continued decomposition of hypochlorous acid! The pre-aeration measures are suitable for the sewage plants with aeration and agitation 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 the activated carbon, and its full capacity cannot be exerted! 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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