Water treatment methods and principles

1.Water treatment methods and principles of deionization

The purpose of deionization is to remove the inorganic ions dissolved in water. Like hard water softeners, it also uses the principle of ion exchange resin. Two resins are used here-cation exchange resin and anion exchange resin. Cation exchange resins use hydrogen ions (H +) to exchange cations; while anion exchange resins use hydroxide ions (OH-) to exchange anions. Hydrogen ions and hydroxide ions combine with each other to form neutral water. The reaction equation is as follows:

M + x + xH-Re → M-M-Rex + xH + 1

A-z + zOH-Re → A-Rez + zOH-1

In the above formula, M + x represents cation, x represents valence number, M + x cation exchanges with H-Re hydrogen ion on cation resin, Az represents anion, z represents valence number, after Az is combined with anion exchange resin, OH- ions are released. The combination of H + ion and OH- ion becomes neutral water.

These resins also need to be reduced after the adsorption capacity is exhausted. The cation exchange resin needs a strong acid to reduce; on the contrary, the anion needs a strong base to reduce. The adsorption capacity of cation exchange resin for various cations is different, and their strength and relative relationship are as follows:

Ba2 +> Pb2 +> Sr2 +> Ca2 +> Ni2 +> Cd2 +> CU2 +> Co2 +> Zn2 +> Mg2 +> Ag1 +> Cs1 +> K1 +> NH41 +> Na1 +> H1 +

The affinity strength of the anion exchange resin and each anion is as follows:

S02-4 +> I-> NO3-> NO2-> Cl-> HCO3-> OH-> F-

If the anion exchange resin is exhausted without reduction, the weakest adsorbed fluorine will gradually appear in the dialysis water, causing rickets, osteoporosis and other bone lesions; if the cation exchange resin is exhausted, hydrogen ions It will also appear in the dialysis water, resulting in an increase in the acidity of the water quality, so it is necessary to constantly monitor whether the deionization function is effective. It is generally judged by the resistivity or conductivity of water quality. It is worth noting that the ion exchange resin used in the deionization method can also cause the reproduction of bacteria and bacteremia.

2. Water treatment method and principle of hard water softening method

The softening of hard water requires the use of an ion exchange method. Its purpose is to use cation exchange resin to exchange calcium and magnesium ions in hard water with sodium ions, thereby reducing the concentration of calcium and magnesium ions in the water source. Its softening reaction formula is as follows:

Ca2 ++ 2Na-EX → Ca-EX2 + 2Na + 1

Mg2 ++ 2Na-EX → Mg-EX2 + 2Na + 1

The EX in the formula represents an ion exchange resin. After these ion exchange resins combine Ca2 + and Mg2 +, the Na + ions originally contained therein are released.

The ion exchange resins currently on the market are spherical synthetic organic polymer electrolytes. The resin matrix (resin matrix) contains sodium chloride. During the softening of hard water, sodium ions will be gradually used up and the softening effect of the exchange resin will gradually decrease. At this time, regeneration work is required. That is, a specific concentration of brine is added every fixed time, generally 10%, and the reaction method is as follows:

Ca-EX2 + 2Na + (concentrated brine) → 2Na-EX + Ca2 +

Mg-EX2 + 2Na + (concentrated brine) → 2Na-EX + Mg2 +

If there is no softening of cations during water treatment, not only calcium and magnesium bodies will be deposited on the reverse osmosis membrane to reduce the efficiency or even destroy the reverse osmosis membrane, but patients are also easy to get hard water syndrome. The hard water softener can also cause the problem of bacterial reproduction, so the equipment needs to have a backwash function, which must be backwashed after a period of time to prevent too many impurities from adsorbing on it. Another notable problem is hypernatremia, because the softening and re-reduction process of dialysis water is controlled by a timer. Normally, the reduction effect occurs mostly in the middle of the night, which is controlled by the valve. If a malfunction occurs, a large amount of saline It will pour into the water source, and cause the patient’s hypernatremia.

3. Water treatment method and principle of activated carbon

Activated carbon filtration (activated carbon filtration) is made by carbonization of wood, residual wood chips, fruit core, coconut shell, coal or petroleum bottom residue at high temperature, and it needs to be activated by hot air or steam after it is made. Its main role is to remove chlorine and chloramines and other dissolved organic matter with a molecular weight of 60 to 300 Daltons. The surface of the activated carbon is granular, and the inside is porous. There are many capillaries of about 1Onm ~ lA in the pores. The internal surface area of ​​1g of activated carbon is as high as 700-1400m2. The inner surface of these capillaries and the surface of the particles are where the adsorption is. The factors that affect the ability of activated carbon to remove organic matter are the area of ​​activated carbon itself, the size of pores, the molecular weight of the removed organic matter and its polarity (Polarity). It mainly relies on physical adsorption capacity to remove impurities, when the adsorption capacity reaches saturation If too much impurities are adsorbed, it will fall down and pollute the downstream water quality. Therefore, the impurities adsorbed on it must be removed regularly by backwashing.

If the adsorption capacity of this activated carbon filter drops significantly, it must be renewed. Measuring the difference in TOC concentration (or the difference in the number of bacteria) between the influent and effluent is one of the reasons for considering the replacement of activated carbon. Some reverse osmosis membranes have poor tolerance to chlorine, so activated carbon treatment is required before reverse osmosis, so that chlorine can be effectively adsorbed by activated carbon, but the bacteria adsorbed on the pores of activated carbon are prone to multiply and grow. The removal of larger organic matter has limited effectiveness of activated carbon, so it must be reinforced behind by a reverse osmosis membrane.

4. Precipitation filtration method of water treatment method and principle

The purpose of the sediment filtration method is to remove suspended particulate matter or colloidal substances in the water source. If these particulate matter are not removed, it will cause damage to other dialysis water precision filter membranes or even block the waterway. This is the oldest and simplest water purification method, so this step is usually used in the preliminary treatment of water purification, or if necessary, several more filters will be added to the pipeline to remove larger impurities. There are many types of filters used to filter suspended particulate matter, such as mesh filters, sand filters (such as quartz sand, etc.), or membrane filters. As long as the particle size is larger than the size of these holes, they will be blocked. The ions dissolved in water cannot be stopped. If the filter has not been replaced or cleaned for a long time, the accumulation of particulate matter on the filter will increase, and the water flow and pressure will gradually decrease. People just use the difference between the inlet water pressure and the outlet water pressure to judge the degree of filter clogging. Therefore, the filter should be backwashed regularly to eliminate impurities accumulated on it, and the filter should be replaced within a fixed time.

There is another problem in the sediment filtration method that is worth noting, because the particulate matter is constantly blocked and accumulated, there may be bacteria in these substances to multiply, and release toxic substances through the filter, causing pyrogen reaction, so the filter must be replaced frequently, principle When the pressure difference between the inlet and outlet water rises to five times, the filter needs to be replaced.

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