[Hongjie Water Service] Explanation of dosing principles in each stage of the water treatment system

Boiler water treatment dosing system

1. Ammonia: Adjusting the pH value

2. Hydrazine: deoxygenation

These two chemicals are pumped to the deaerator through a dosing pump to regulate the pH value of the feedwater and remove oxygen from the water.

3. Phosphate (trisodium phosphate) is directly added to the boiler drum. It also regulates pH to prevent scaling.

The overall function of all reagents is to minimize scaling and corrosion of the internal piping of the boiler.

Raw water tank addition (sodium hypochlorite, NaOC): The chemical name of the bleach is hypochlorite (sodium hypochlorite, NaOCI), which is a strong oxidant and a cheap and easily available disinfectant. Its bactericidal effect is the decomposition of sodium hypochlorite into hypochlorite, which is unstable and decomposes into new ecological oxygen and chlorine in aqueous solution, causing strong oxidation of bacteria and leading to death. It is effective in killing both bacteria and bacteriophages.

Adding flocculants before pre-treatment: Flocculants mainly include inorganic flocculants, organic polymer flocculants, and microbial flocculants, which are mainly used for treating various types of wastewater. Specifically, organic polymer flocculants have a wide range of applications in treating refinery wastewater, other industrial wastewater, high suspended solids wastewater, and solid-liquid separation with cationic flocculants. Especially the acrylamide series organic polymer flocculants have shown superiority in water treatment due to their strong molecular weight and bridging ability. Microbial flocculants have a wide range of flocculation, high flocculation activity, and extensive action conditions, mostly unaffected by ion strength, pH value, and temperature. Therefore, they can be widely used in sewage and industrial wastewater treatment. Adding flocculants is to quickly and thoroughly separate water and impurities.

Ultrafiltration backwashing dosing: sodium hydroxide, hydrochloric acid, sodium hypochlorite

Sodium hydroxide and sodium hypochlorite do not react, while hydrochloric acid and sodium hypochlorite undergo HCL+NACLO=NACL+HCLO. Under acidic conditions, HCLO is unstable and can easily generate CL2, which is toxic and poses a safety hazard. Hydrochloric acid and sodium hydroxide neutralize and react, so they cannot be added together. Sodium hypochlorite is mainly used for oxidation and sterilization, and microorganisms are organic matter. Alkali washing mainly eliminates organic matter. Alkali has the effect of softening and loosening sediment, so the combined effect of the two is better, Alkali washing should also be beneficial for subsequent acid washing to remove dirt

Remove inorganic salt deposition. Whether to acid wash or alkali wash first mainly depends on the types of membrane pollutants

Reverse osmosis dosing: add scale inhibitor at the inlet of the pressure pump, and add NAHS03 reducing agent before RO inlet: (commonly used reducing agent sodium bisulfite)

To ensure the long-term stable operation of the reverse osmosis device and prevent the surface of the membrane from being contaminated and damaged by microbial oxidants and suspended impurities, a reduction and dosing treatment of oxidants is carried out before RO inlet to eliminate the impact of oxidants on the membrane; To prevent fouling on the membrane surface and reduce the desalination performance of the membrane, a scale inhibitor dosing system is installed at the inlet of each high-pressure pump to improve the water production flux of the membrane.

Alkali addition is less commonly used in primary reverse osmosis. Inject alkaline solution into the reverse osmosis influent to increase pH- The only commonly used alkaline agent is sodium hydroxide (NaOH), which is convenient to purchase and easily soluble in water- Industrial grade sodium hydroxide without other additives can meet the needs. The commercial sodium hydroxide contains 100% flake alkali, as well as 20% and 50% liquid alkali. When adding alkali to adjust the pH, it is important to note that an increase in pH will increase LSI and reduce the solubility of calcium carbonate, iron, and manganese. The most common alkali addition application is the secondary RO system.

In the secondary reverse osmosis system, the water produced by the primary RO is supplied to the secondary RO as raw water. The secondary reverse osmosis process "polishes" the water produced by the primary reverse osmosis, and the water quality of the secondary RO production can reach 4 megaohms.

There are four reasons for adding alkali to the secondary RO inlet water:

a. Above pH 8.2, all carbon dioxide is converted into carbonate ions, which can be removed by reverse osmosis. And carbon dioxide itself is a gas that will permeate through

Free entry of RO produced water through the liquid causes improper load on downstream ion exchange bed polishing treatment.

b. Some TOC components are more easily removed at high pH.

c. The solubility and removal rate of silica are higher at high pH (especially above 9).

d. The removal rate of boron is also high at high pH (especially above 9). There is a special case of alkaline application, commonly known as the HERO (High Efficiency Reverse Osmosis System) process, which adjusts the inlet pH to 9 or 10. Primary reverse osmosis is used to treat brackish water, which can have pollution issues at high pH (such as hardness, alkalinity, iron, manganese, etc.). Pretreatment typically uses weakly acidic cationic resin systems and degassing devices to remove these pollutants.

The free chlorine in RO and NF influents needs to be reduced to below 0.05ppm in order to meet the requirements of polyamide composite membranes. There are two pre-treatment methods for chlorine removal, granular activated carbon adsorption and the use of reducing agents such as sodium sulfite. In small systems (50-100 gpm), activated carbon filters are generally used, which makes the investment cost relatively reasonable. It is recommended to use high-quality activated carbon that has been pickled to remove hardness and metal ions. The fine powder content should be very low, otherwise it may cause pollution to the membrane. The newly installed carbon filter material must be thoroughly rinsed until the carbon powder is completely removed, which usually takes several hours or even days. We cannot rely on 5 μ M's security filter is used to protect the reverse osmosis membrane from carbon powder contamination. The advantage of a carbon filter is that it can remove organic matter that can cause membrane fouling, making it more reliable for the treatment of all incoming water than adding chemicals. But its disadvantage is that carbon can become the feed for microorganisms, and bacteria can grow in the carbon filter, resulting in biological pollution of the reverse osmosis membrane.

Sodium bisulfite

Sodium bisulfite (SBS) is a typical reducing agent used in larger RO units. Dissolve solid sodium metabisulfite in water to prepare a solution. The purity of commercial sodium metabisulfite is 97.5-9%, and the dry storage period is 6 months. SBS solution is unstable in the air and reacts with oxygen, so it is recommended to use 2% of the solution for 3-7 days, and solutions below 10% for 7-14 days. In theory, 1.47ppm SBS (or 0.70ppm sodium bisulfite) can reduce 1.0ppm of chlorine. When designing, the safety factor of the industrial brackish water system was taken into account, and the addition amount of SBS was set at 1.8-3.0ppm per 1.0ppm of chlorine. The injection port of SBS should be upstream of the membrane element, and the distance should be set to ensure a reaction time of 29 seconds before entering the membrane element. It is recommended to use an appropriate online mixing device (static mixer).

If cationic coagulants or filter aids are used in pre-treatment, special attention should be paid when using anionic scale inhibitors. It will produce a viscous pollutant, which will increase the operating pressure and make cleaning of this pollutant very difficult.

Sodium hexametaphosphate

Sodium hexametaphosphate (SHMP) was a common scale inhibitor used in reverse osmosis in the early stages, but with the emergence of specialized scale inhibitors, its use has greatly decreased.

There are some limitations to the use of SHMP. Prepare a solution every 2-3 days as exposure to air can cause hydrolysis, which not only reduces the scale inhibition effect but also increases the possibility of calcium phosphate scaling. The use of SHMP can reduce calcium carbonate scaling, and the LSI can reach+1.0.

Scale inhibitor

The scale inhibitor hinders the growth of salt crystals in RO influent and concentrated water, thus allowing insoluble salts to exceed their saturation solubility in concentrated water. The use of scale inhibitors can replace or be combined with acid addition. There are many factors that can affect the formation of mineral scaling. A decrease in temperature will reduce the solubility of scaling minerals (except for calcium carbonate, which, contrary to most substances, decreases with increasing temperature), while an increase in TDS will increase the solubility of insoluble salts (this is because high ion strength interferes with the formation of crystal seeds:

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