How to calculate the dosage of chemicals in water treatment chemical dosing equipment

I. Scale Inhibitor Dosage

The recovery rate of the primary reverse osmosis system in the desalination water treatment system is calculated at 75%. Under conditions of 20-50℃, this water quality has a strong tendency to scale. This indicates that an appropriate amount of membrane dispersant must be added to ensure the long-term safe and stable operation of the reverse osmosis system and extend the service life of the membrane. The secondary reverse osmosis feed water is the primary reverse osmosis product water, with low hardness and alkalinity, so no scale inhibitor needs to be added. According to the calculation of the reverse osmosis dedicated software: the recommended dosage under a recovery rate of 75% is: 3ppm (based on influent), daily dosage = reagent concentration × influent volume × 24h ≈ 8.64 kg (influent volume calculated at 120m3/h).

II. pH Adjustment

The pH adjustment system uses an X015 diaphragm pump and a 120L water tank. A 0.1%~0.5% NaOH solution is prepared in the water tank, and the drug is added through the diaphragm pump. Adjust the amount of alkali added according to the pH value and conductivity of the product water to achieve a moderate value, and determine the concentration added according to the actual conductivity of the secondary product water. The diaphragm pump runs synchronously with the secondary reverse osmosis. General principles for adjusting the amount of alkali added:

1. When the conductivity rises sharply, it indicates that the amount of alkali added is too large.

2. When the conductivity is relatively stable but high, it indicates that the amount of alkali added is too small.

3. When the amount of alkali added is too small and then increased, the conductivity drops sharply, but rises again after dropping to a certain extent, indicating that the amount of alkali added is too large.

4. If the alkali diaphragm pump scale cannot reach the alkali addition amount even when adjusted to the maximum, it indicates that the alkali concentration in the tank is too low.

5. If the alkali pump scale is below 20 and the adjustment sensitivity is too high, it indicates that the alkali concentration in the tank is too high.

III. Biocide Addition

1. Since the raw water is municipal tap water, there are fewer bacteria in the system. However, with the increase in temperature, especially in summer, it will affect the normal operation of the reverse osmosis membrane. Therefore, a certain amount of biocide should be added to control the growth of bacteria and protect the reverse osmosis membrane from microbial damage. This reagent is continuously added to the system pretreatment to maintain the residual chlorine content in the influent to inhibit the growth of bacteria. Therefore, the dosage should be determined according to the actual degree of biological pollution of the system. The recommended dosage is 2ppm (based on influent).

2. Daily dosage of the desalination water treatment system = reagent concentration × influent volume × 24h ≈ 5.76 kg (influent volume calculated at 120m3/h).

IV. Reductant Addition

1. In order to prevent the oxidizing biocide from entering the reverse osmosis membrane and oxidizing the membrane elements, a reductant addition system is set up before the reverse osmosis system. The model of the reagent used is DCL95. The specific dosage depends on the residual chlorine content in the reverse osmosis system influent after adding the oxidizing biocide. It is generally about 3-5 times the remaining chlorine content, with a dosage of 2ppm.

2. Daily dosage of the desalination water treatment system = reagent concentration × influent volume × 24h ≈ 5.76 kg (influent volume calculated at 120m3/h). Note that excessive addition of reductant will also cause system blockage. Therefore, daily residual chlorine monitoring must be strengthened to adjust the dosage ratio between the oxidizing biocide and the reductant, so that the reductant can fully react with the oxidizing substances to prevent damage to the membrane elements, while avoiding excessive residual amounts that bring additional pollution to the system.