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Chemical Agent in Water Treatment Process

Raw water contains a certain concentration of suspended and dissolved substances, mainly inorganic salts, colloids, and biological particles such as microorganisms and algae, which are deposited on the membrane and block the inlet channel. Dissolved substances are generally easily soluble salts (such as chloride) and insoluble salts (such as carbonate, sulfate, and silicate), etc. Insoluble salts, when exceeding their saturation limits, will precipitate from the water and form scales on the membrane surface, reducing RO membrane flux, increasing operating pressure, and leading to a decrease in product quality.

With the wide application of reverse osmosis technology in the field of water treatment, the fouling of RO membranes in operation is also receiving more and more attention. Therefore, it is necessary to add a water treatment dosing system in the pretreatment stage before raw water enters the RO membrane system to remove suspended particles, microorganisms, and excessive insoluble salts that may cause contamination of the RO membrane and reduce the tendency of membrane contamination. For example, flocculants to remove suspended impurities, pH regulators to adjust the pH of water, bactericides to remove microorganisms, reducing agents to reduce oxidizing impurities, scale inhibitors to remove scale, etc.

A row of dosing tank and dosing pump
Content
Flocculants & Coagulant Aids

Dosing order: Flocculant is added before pretreatment into quartz sand filter or activated carbon filter.

The main role of flocculants is to separate tiny particles of impurities. In the water treatment process, flocculants usually react with tiny particles in water to produce some chemical substances, which can cause tiny particles to gather into larger particles and form flocculants. Flocculants are denser than water, so they can be separated from water by precipitation or filtration.

There are many different types of flocculants, including inorganic and organic compounds. Inorganic flocculants include aluminum sulfate (alum), ferric chloride, and polymeric aluminum chloride, while organic flocculants include polyacrylamide and guar gum, among others. For different applications, different types of flocculants need to be selected to achieve the best separation effect. When selecting flocculants, a variety of factors need to be considered, such as water pH, temperature, type and concentration of suspended particles, etc.

Turbid water and water with impurities precipitated by the addition of flocculants

A coagulant aid, similar to a flocculant, is a chemical that helps gather dissolved substances in water into larger particles. Its main function is to coalesce dissolved substances in water into larger, heavier particles, thus making it easier to separate these particles from the water.

Flocculant Working Principle

Colloidal particles in water are tiny, surface hydrated, and surface charged to make them stable, and the flocculants are hydrolyzed into charged colloids with their surrounding ions to form a colloidal group with a double electric layer structure after being added to water. The rapid stirring method is used to promote collision opportunities and times between colloidal impurity particles in water and colloids hydrolyzed by the flocculant. The impurity particles in the water first lose stability under the action of flocculant, then coalesce with each other into larger particles, and then precipitate down or float up in the separation facility.

Flocculant working principle
pH Regulator

Dosing order: pH regulator is dosed before pretreatment into quartz sand filter or activated carbon filter or between primary RO system and secondary RO system.

The pH regulator is an important chemical in the RO system, mainly used to adjust the pH value of water, in order to better remove impurities and organic substances in water, so as to improve the desalination rate and extend the service life of the RO membrane. pH regulators can be divided into different types, such as alkaline regulators, acid regulators, buffering agents, and anti-scaling agents. Adding acid and alkali can prolong the service life of the RO membrane and improve the desalination rate of the RO membrane.

Alkaline regulators mainly include carbonate, sodium hydroxide, etc., which are used to neutralize acidic substances in water and raise the pH value of water, thereby reducing the corrosion effect of acidic substances in water on the RO membrane; acid regulators mainly include sulfuric acid, hydrochloric acid, phosphoric acid, etc., which are used to lower the pH value of water, thereby reducing the corrosion effect of alkaline substances in water on the RO membrane; buffers are used to stabilize the pH value of water and prevent the pH fluctuation of water. The main function of the anti-scaling agent is to combine calcium and magnesium plasma in water to prevent them from crystallizing on the surface of the RO membrane and forming a scale layer, thus protecting the RO membrane. The PH regulator of the RO system should be selected according to the specific situation, all to extend the service life of the RO membrane and improve the desalination rate.

Bactericides

Dosing order: Bactericides are added before pretreatment to the quartz sand filter or activated carbon filter.

Water treatment bactericides are chemicals used to destroy bacteria and other microorganisms in water. Common bactericides are divided into two categories: oxidizing bactericides and non-oxidizing bactericides. Oxidizing bactericides include chlorine, chlorine dioxide, hydrogen peroxide, bromine, and ozone. Non-oxidizing bactericides include quaternary ammonium compounds, bronopol, THPS, DBNPA, and glutaraldehyde. The following are examples of how oxidizing bactericides and non-oxidizing bactericides work.

As a common oxidizing bactericide, chlorine gas can be completely dissolved in water within a dozen seconds and quickly hydrolyzed into hypochlorous acid and hydrochloric acid, which will form hypochlorite and hydrogen ions in water. Hypochlorite is a strong oxidant that can destroy the cell membrane and proteins of bacteria. Hypochlorite can also oxidize enzymes and metabolites in bacterial cells, destroying the metabolic process of bacteria and accelerating their death. It should be noted that chlorine is a toxic gas and its concentration needs to be strictly controlled when used to avoid harm to the human body and the environment.

Reasons why chlorine can be used as an oxidizing bactericide

Quaternary ammonium compounds, as non-oxidizing bactericides, contain four long-chain alkyl groups and a positively charged nitrogen atom in the molecule. These positive charges can be associated with the adsorption of negatively charged bacterial bodies, and these ions can adsorb to the cell membrane surface, preventing respiration and glycolysis of bacteria, and leading to bacterial growth inhibition and death. Quaternary ammonium compounds can also denature proteins and cause the leaching of chlorine and phosphorus compounds from cells and lead to cell death.

HOCL sterilization process

When used, oxidizing bactericides are used at lower concentrations than non-oxidizing bactericides, but continued use of non-oxidizing bactericides may result in the formation of resistant colonies in the system, developing drug resistance, requiring the alternate use of two or more non-oxidizing bactericides. Many factors need to be considered when selecting a bactericide for use in a system, including contact time, concentration, temperature, pH, compatibility, and cost-effectiveness. Our technical department can advise on the selection of bactericides.

Reducing Agent

Dosing order: Reducing agent is added after the quartz sand filter or activated carbon filter in the pretreatment process and before the security filter.

The main purposes of the reducing agent are: to remove residual oxidant; to remove dissolved oxygen; and to reduce hexavalent chromium.

Reverse osmosis pretreatment often adds chlorine, an oxidizing bactericide to stop the growth of microorganisms, but residual chlorine will irreversibly oxidize and destroy the RO membrane, so the reducing agent must be added to remove residual chlorine. Generally, the most commonly used reducing agent to remove residual chlorine is sulfur dioxide or sodium bisulfite. Residual chlorine dissolved in water will produce hypochlorous acid, so the specific reduction reaction is as follows:

Chemical equation for the reduction of residual chlorine

Dissolved oxygen is the dissolved product of oxygen in water, and the removal of dissolved oxygen is generally done with sodium sulfite or ammonium bisulfite, with the following reaction:

Chemical equation for the reduction of oxygen

Industrial wastewater mainly contains trivalent chromium and hexavalent chromium. The pharmaceutical removal method is divided into two steps, the first reduction of hexavalent chromium to trivalent chromium, and then precipitation in the form of hydroxide, introducing the most common reduction of hexavalent chromium by sodium sulfite:

Chemical equation for the reduction of hexavalent chromium
Antiscalant

Dosing order: Antiscalant is added after the quartz sand filter or activated carbon filter in the pretreatment process and before the security filter.

Scale is formed by the combination of calcium ions and magnesium ions in water and carbonate. Scale is easy to form pipe fouling and causes unnecessary waste. Antiscallants are one of the common chemical methods used to stop scale generation.

Antiscalant is a surfactant negatively charged polymer that forms stable soluble chelates with calcium ions and magnesium ions, stabilizing more calcium ions and magnesium ions in water so that these ions are no longer easily deposited on the surface of pipes, equipment, etc. In addition, antiscalants can also destroy the structure of calcium carbonate crystals. Calcium carbonate crystals will be uniformly dispersed in water to avoid crystallization to form scale.

Antiscalant working principle