Application of NaClO in Water Treatment

I. Properties of NaClO
Sodium Hypochlorite is an inorganic compound with the chemical formula NaClO. It has strong oxidizing properties and is easily soluble in water and alcohol. Solid NaClO is a white powder,and general industrial products are colorless or light yellow liquids. The effective chlorine content is 10%-12%,and it has a pungent odor. NaClO solution [containing more than 5% effective chlorine] is a hazardous chemical and must be stored and used in accordance with relevant requirements for safe operation.

II. The role of NaClO
(1) Effect of disinfection
The disinfection mechanism of NaClO is mainly the following two:
One is that NaClO is hydrolyzed into hypochlorous acid in water,and then hypochlorous acid decomposes to generate new ecological oxygen. The strong oxidizing property of ecological oxygen denatures the proteins of bacteria and viruses,thereby killing pathogenic microorganisms:
NaClO+H2O=NaOH+HClO
HClO=HCl+{O}
Secondly,hypochlorous acid can not only react with the cell wall,but also because of its small molecule and no charge,it can invade the cell and oxidize the protein,destroying its phosphate dehydrogenase,causing its sugar metabolism to be disordered and cause death:
R-NH-R+HClO=RNC+H2O
The dosage of NaClO disinfectant should be determined according to the disinfection object,water quality,pollution degree and the desired disinfection effect. Under normal circumstances,the dosage for water supply engineering disinfection is 1-3 mg/L; the dosage for reclaimed water engineering disinfection is 5-10 mg/L; the dosage for hospital sewage disinfection is 30-50 mg/L for primary treatment effluent,15-25 mg/L for secondary treatment effluent; and 15-20 mg/L for swimming pool disinfection. The specific dosage should be adjusted according to the actual situation.
(2) Oxidation and coagulant,algae removal,iron and manganese effects
Chlorine disinfectants generally have a coagulant effect due to their strong oxidizing properties. Specifically,the oxidation coagulant function of NaClO mainly oxidizes organic matter in water and converts it into a form that is easy to flocculate,thereby improving the work efficiency of subsequent treatment units. However,this coagulant effect is more commonly used in water treatment,but is relatively less used in sewage treatment.
For raw water containing algae,NaClO has an algae removal effect,which can destroy the algae cell wall and inhibit its growth. But in fact,its algae removal effect is relatively limited. In addition,NaClO also has the ability to oxidize iron and manganese substances,which can play a role in subsequent flocculation and precipitation removal. However,similar to the algae removal effect,its oxidation ability for iron and manganese is weak,and the pH value requirement is high.
(3) Decolorization and bleaching effects
The decolorization and bleaching effects of NaClO mainly rely on its oxidation properties. It is generally used in combination with coagulation and sedimentation. It can effectively decompose pigments and organic matter in water. For example,it can oxidize the iron in water from divalent to trivalent,and then precipitate the trivalent iron through coagulation to complete the decolorization of the water,or oxidize some colored organic matter in the water and then coagulate and precipitate it to play a decolorization role. However,this effect is also affected by the pH value. Too low or too high a pH value will reduce its decolorization efficiency.
(4) The role of membrane cleaning
Membrane fouling is a technical problem that has not been well solved so far,and microorganisms and their metabolites are important factors that cause membrane fouling during membrane treatment. In the membrane system,NaClO can effectively remove organic and inorganic pollutants on the membrane surface to maintain the permeability and treatment efficiency of the membrane.
NaClO cleaning mainly cleans and maintains the membrane system through oxidation,sterilization,decomposition and removal of pollutants to ensure its normal operation and treatment efficiency. In actual operation,it is necessary to reasonably select the concentration,cleaning time and method of NaClO according to specific conditions and requirements to achieve the best cleaning effect.
III. Preparation of NaClO
The chemical reaction for large-scale industrial production of NaClO is:
Cl2+2NaOH=NaCl+NaClO +H2O
The effective chlorine content of its finished NaClO is 10%-13%.
In addition,NaClO can be produced by electrolyzing dilute salt water. The working principle is:
NaCl+H2O→NaClO+H2↑
Electrode reaction:
Anode：2Cl--2e→Cl2
Cathode：2H++2e→H2
Solution reaction：2NaOH+Cl2→NaCl+NaClO+H2O
The concentration of NaClO produced by electrolysis is generally 0.6%-0.9%. Generally,for every kg of effective chlorine produced,about 3-4.5 kg of salt is consumed.
IV. Dosage of NaClO
The dosage should be determined by comprehensively considering factors such as raw water quality,microbial contamination level,treatment objectives and economic costs. The most suitable dosage can be obtained through gradual optimization through the scientific process of laboratory test,pilot test and on-site commissioning.
The dosing methods of NaClO include continuous dosing,intermittent dosing and pulse dosing. The specific choice of dosage method requires comprehensive and detailed consideration based on actual application scenarios and needs.
The disinfection effect of NaClO is significantly affected by pH value. In an acidic environment,NaClO can exhibit the best disinfection effect. Therefore,during the process of adding NaClO,it is necessary to closely monitor and reasonably regulate the pH value of the water body to ensure the maximum disinfection effect.
After NaClO is added to the water body,it takes a certain amount of time to fully mix and react,so as to achieve the desired disinfection goal. In actual operation,measures such as optimizing the mixer design,adjusting the water flow rate,and accurately setting the dosing point position can be taken to ensure that NaClO and the water body are fully mixed and effectively reacted.
In order to ensure the stability and effectiveness of the NaClO dosing process,a complete monitoring system can be established to monitor the dosage and water quality indicators (such as residual chlorine content,total bacteria count,etc.) in real time,and provide timely feedback and adjustments based on the monitoring results.
V. Storage of NaClO
As a chemical substance,NaClO is not suitable for long-term storage in solid or solution form,and its storage conditions need to be strictly controlled in a light-proof and low-temperature environment. In the process of electrolytic production of NaClO solution,it is recommended to adopt an immediate production and immediate use strategy to reduce storage time. When the temperature is below 30°C,the daily loss of effective chlorine in NaClO solution is between 0.1-0.15 mg/L; when the ambient temperature exceeds 30°C,the daily loss of effective chlorine can increase to 0.3-0.7 mg/L. In view of this,if a certain amount of NaClO reserves need to be maintained for emergency use,it is recommended that the storage time in summer should not exceed 24 hours,and in winter should not exceed one week.
VI. Conclusion
As an important chemical disinfectant,NaClO plays an irreplaceable role in water treatment and other fields. Mastering its main characteristics and scientifically and rationally controlling the dosing process are of great significance for ensuring water quality safety and improving disinfection efficiency. In the future,with the continuous advancement of science and technology and the increasing requirements for environmental protection,the dosing process of NaClO will continue to be optimized and improved to better adapt to various complex and changing application scenarios and needs.