A Summary of Common Water Treatment Methods for Different Water Qualities!

In the environmental water treatment industry, commonly used water treatment methods generally include:
(1) Sediment filtration, (2) Water softening, (3) Activated carbon adsorption, (4) Deionization, (5) Reverse osmosis, (6) Ultrafiltration, (7) Distillation, (8) UV disinfection, and (9) Biochemical methods. The principles and functions of these water treatment methods are introduced below.

I. Sediment Filtration
The purpose of sediment filtration is to thoroughly remove suspended particles or colloidal substances from the water source. If these particles are not removed, they can damage the precision filtration membranes used in dialysis water systems or even cause blockages in the water lines. This is the oldest and simplest water purification method, so this step is commonly used as a preliminary treatment in water purification. When necessary, additional filters may be installed in the piping system to remove larger impurities.            
 There are many types of filters used to remove suspended particulate matter, such as mesh filters, sand filters (e.g., quartz sand), or membrane filters. Any particles larger than the pore size of these filters will be trapped.　  
 There is another issue worth noting regarding sediment filtration: as particulate matter is continuously trapped and accumulates, bacteria may multiply on the surface of these materials and release toxic substances through the filter, causing pyrogenic reactions. Therefore, filters must be replaced frequently. As a general rule, filters should be replaced when the pressure difference between the inlet and outlet water reaches five times the original value.

II. Hard Water Softening Method (Water Softening Equipment)
Hard water softening requires the use of ion exchange. The process utilizes cation exchange resin to replace calcium and magnesium ions in hard water with sodium ions, thereby reducing the concentration of calcium and magnesium ions in the water source. The softening reaction is as follows:            
Ca²⁺ + 2Na⁻-EX → Ca⁻-EX²⁺ + 2Na⁺ 
Mg²⁺ + 2Na⁻-EX → Mg⁻-EX²⁺ + 2Na⁺ 
The ion exchange resins currently available on the market are spherical synthetic organic polymer electrolytes.　   
If cation softening is not performed during water treatment, not only will calcium and magnesium deposits form on the reverse osmosis membrane—reducing its efficiency or even damaging it—but consumers are also at risk of developing hard water syndrome. Hard water softeners can also lead to bacterial growth, so the equipment must include a backwash function; it should be backwashed periodically to prevent excessive impurity buildup.

III. Activated Carbon Filter
Activated carbon is produced by pyrolyzing and carbonizing materials such as wood, wood chips, fruit pits, coconut shells, coal, or petroleum residues at high temperatures; after production, it must be activated using hot air or steam. Its primary function is to remove chlorine, chloramine, and other dissolved organic substances with molecular weights ranging from 60 to 300 daltons. Activated carbon has a granular surface and a porous interior, with numerous capillaries approximately 10 nm to 100 nm in size within the pores. One gram of activated carbon has an internal surface area as high as 700–1,400 m², and it is on these capillary and granular surfaces that adsorption occurs.

IV. Deionization Method
The purpose of the deionization method is to remove inorganic ions dissolved in water. Like water softeners, it utilizes the principle of ion exchange resins. Two types of resins are used here: cation exchange resins and anion exchange resins. Cation exchange resins use hydrogen ions (H+) to exchange for cations, while anion exchange resins use hydroxide ions (OH-) to exchange for anions.   　Once the adsorption capacity of these resins is exhausted, they must be regenerated; cation exchange resins require strong acids for regeneration, whereas anion exchange resins require strong bases. If the anion exchange resin is exhausted and not regenerated, fluoride—which has the weakest adsorption capacity—will gradually appear in the dialysis water, leading to rickets, osteoporosis, and other bone disorders. If the cation exchange resin is exhausted, hydrogen ions will also appear in the dialysis water, increasing the water’s acidity. Therefore, the effectiveness of the deionization function must be monitored regularly. This is generally assessed by measuring the water’s resistivity or conductivity. It is important to note that the ion exchange resins used in deionization can also promote bacterial growth, leading to bacteremia.

V. Reverse Osmosis
Reverse osmosis effectively removes inorganic and organic substances, bacteria, pyrogens, and other particles dissolved in water, making it the most critical step in the treatment of dialysis water.      Reverse osmosis can achieve purification down to the ionic level. Common semi-permeable membrane materials used in reverse osmosis water treatment include cellulose-based membranes, aromatic polyamides, polyimides, and polyfuranes. As for their structural configurations, they include spiral-wound, hollow-fiber, and tubular types.      
If proper pretreatment is not performed before reverse osmosis, contaminants such as calcium, magnesium, and iron ions can easily accumulate on the membrane, causing a decline in reverse osmosis performance. Some membranes (such as polyamide) are susceptible to damage from chlorine and chloramine; therefore, pretreatment steps such as activated carbon filtration and water softening must be implemented before the reverse osmosis membrane. Consequently, it is highly recommended to include this step when preparing water for hemodialysis.

VI. Ultrafiltration
Ultrafiltration is similar to reverse osmosis in that it also uses a semi-permeable membrane; however, it cannot control the removal of ions because the membrane pore size is larger, ranging from approximately 10 to 200 Å. It can only remove bacteria, viruses, pyrogens, and particulate matter, but cannot filter out water-soluble ions.      
The primary function of ultrafiltration is to serve as a pretreatment for reverse osmosis to prevent bacterial contamination of the reverse osmosis membranes. It can also be used as the final step in water treatment to prevent upstream water from becoming contaminated by bacteria in the piping system. Generally, the effectiveness of the ultrafiltration membrane is determined by the pressure difference between the inlet and outlet. Similar to activated carbon, backwashing is typically used to remove impurities adhering to the membrane.

VII. Distillation
Distillation is an ancient yet effective water treatment method. It can remove any non-volatile impurities but cannot eliminate volatile contaminants. It requires a large storage tank, and this tank, along with the distribution pipes, is a major source of contamination. Currently, this method is not used for treating water for hemodialysis.

VIII. Ultraviolet Disinfection
Ultraviolet disinfection is one of the most commonly used methods today. Its mechanism of action involves destroying the genetic material (nucleic acids) of bacteria, preventing them from reproducing. The most significant reaction is the formation of dimers from the pyrimidine bases within the nucleic acid molecules. Typically, low-pressure mercury discharge lamps (germicidal lamps) are used to generate artificial ultraviolet energy at a wavelength of 253.7 nm. The principle of ultraviolet germicidal lamps is the same as that of fluorescent lamps, except that the interior of the tube is not coated with a phosphor, and the tube is made of quartz glass with high ultraviolet transmittance. Generally, ultraviolet devices are classified by application into irradiation-type, immersion-type, and flow-through-type systems.

IX. Biochemical Methods
Biochemical water treatment methods utilize various naturally occurring bacteria and microorganisms to decompose and convert organic matter in wastewater into harmless substances, thereby purifying the wastewater. Biochemical water treatment methods can be categorized into the activated sludge process, biofilm process, biological oxidation towers, land treatment systems, and anaerobic biological water treatment methods.
Biochemical water treatment process flow:
Raw water → Bar screen → Equalization tank → Contact oxidation tank → Sedimentation tank → Filtration → Disinfection → Effluent.