General water treatment methods and principles

Commonly used water treatment methods are: (1) sediment filtration method, (2) hard water softening method, (3) activated carbon adsorption method, (4) deionization method, (5) reverse osmosis method, (6) ultrafiltration method, (7) Distillation method, (8) Ultraviolet disinfection method, etc. Now the principles and functions of these treatment methods are explained here.

The purpose of sediment filtration method is to remove suspended particulate matter or colloidal matter from the water source. If these particulate matter is not removed, it will damage the other delicate filter membranes of the dialysis water or even block the waterway. This is the oldest and simplest water purification method, so this step is often used in the preliminary treatment of water purification, or when necessary, several more filters are 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, it will be blocked. The ions dissolved in the water cannot be blocked. If the filter is not replaced or cleaned for too long, more and more particulate matter will accumulate on the filter, and the water flow and water pressure will gradually decrease. People use the difference between the inlet water pressure and the outlet water pressure to judge the degree of clogging of the filter. Therefore, the filter should be backwashed regularly to remove the impurities accumulated on it, and the filter should be replaced within a fixed time.
There is also a problem with the sediment filtration method that is worth noting. Because particulate matter is constantly blocked and accumulated, bacteria may multiply here and release toxic substances through the filter, causing pyrogen reaction. Therefore, the filter should be replaced frequently. Principle When the pressure drop between the upper inlet and outlet water rises five times the original, the filter needs to be replaced.

The softening of hard water requires the use of ion exchange method. Its purpose is to use cation exchange resin to exchange calcium and magnesium ions in hard water with sodium ions,* to reduce the concentration of calcium and magnesium ions in the water source. The softening reaction formula is as follows:
Ca2++2Na-EX→Ca-EX2+2Na+1
Mg2++2Na-EX→Mg-EX2+2Na+1
“EX” in the formula represents ion exchange resins. These ion exchange resins combine Ca2+ and Mg2+ to release Na+ ions originally contained in them.
The ion exchange resins currently on the market are spherical synthetic organic polymer electrolytes. The resin matrix contains sodium chloride. In the process of hard water softening, sodium ions will be gradually used up, and the softening effect of the exchange resin will gradually decrease. At this time, regeneration is required. That is, add a certain concentration of brine at regular intervals, 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 cationic softening during the water treatment process, not only will calcium and magnesium bodies be deposited on the reverse osmosis membrane, which will reduce the efficiency or even destroy the reverse osmosis membrane, but the patient will also easily get hard water syndrome. Hard water softeners can also cause the problem of bacterial reproduction, so the equipment needs to have a back-flushing function. After a period of time, it will be back-flushed once to prevent too much impurities from adsorbing on it. Another noteworthy problem is hypernatremia, because the softening and re-reduction process of dialysis water is controlled by a timer. Normally, the reduction effect mostly occurs in the middle of the night. This is because the valve is in control. If it fails, a lot of salt water It will flood into the water source and cause hypernatremia in the patient.

Activated carbon is carbonized by dry distillation of wood, wood chips, fruit cores, coconut shells, coal or petroleum bottom residue at high temperature, and it needs to be activated with hot air or water vapor after it is made. Its main function is to remove chlorine and chloramines and other dissolved organic substances with a molecular weight of 60 to 300 Daltons. The surface of activated carbon is granular and the inside is porous. There are many capillaries about 10nm~lA in the pores. The internal surface area of ​​1g of activated carbon is as high as 700-1400m2, and 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 include the area of ​​activated carbon itself, the size of the pores, and the molecular weight and polarity of the removed organic matter (Polarity). It is mainly * physical adsorption capacity to exclude impurities, when the adsorption capacity reaches saturation , The impurities that are adsorbed too much will fall down and pollute the downstream water quality, so it is necessary to regularly use the method of backlash to remove the impurities adsorbed on it.
If the adsorption capacity of this activated carbon filter drops significantly, it must be replaced. Measuring the TOC concentration difference (or the difference in the number of bacteria) between the inlet and outlet water is one of the basis for considering the replacement of activated carbon. Some reverse osmosis membranes have poor tolerance to chlorine, so activated carbon must be treated before reverse osmosis, so that chlorine can be effectively adsorbed by the activated carbon, but the bacteria adsorbed by the holes on the activated carbon are easy to multiply and grow. The effect of activated carbon is limited in the removal of relatively large organic matter, so it must be reinforced with a reverse osmosis membrane.

RO-water Filtration with CIP system

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