Technical and functional descriptions of common water treatment methods

basic concepts

In short, “water treatment” is the process of removing some substances in the water that are not needed for production and living through physical and chemical methods.
It is a process of sedimentation, filtration, coagulation, flocculation, corrosion inhibition, scale inhibition and other special purpose water quality adjustment processes.
Due to the close relationship between social production, life and water, the range of applications involved in the field of water treatment is very wide, forming a huge industrial application.
It is often said that water treatment includes sewage treatment and drinking water treatment. Commonly used water treatment agents include polyaluminum chloride, polyaluminum ferric chloride, basic aluminum chloride, polyacrylamide, activated carbon, and various filter materials.

Wastewater treatment generally includes the following three treatment stages: The first stage is the removal of stones, sand and fat, iron ions, manganese ions, grease and so on. It is contained in the sewage by mechanical treatment (such as grid, sedimentation or air flotation). The secondary treatment is a biological treatment. Pollutants in sewage are degraded by microorganisms and converted into sludge. Tertiary treatment is the advanced treatment of sewage, including the removal of nutrients and disinfection of sewage by chlorination, ultraviolet radiation or ozone technology. Depending on the treatment objectives and water quality, some wastewater treatment processes may not include all of the above.

The mechanical (primary) processing section includes structures such as grids, grit chambers and primary sedimentation tanks, designed to remove coarse particles and suspended solids. The treatment principle is to achieve solid-liquid separation by physical methods to separate pollutants from sewage. This is a commonly used sewage treatment method. Mechanical (primary) treatment is a necessary item for all wastewater treatment processes (although some processes sometimes omit the primary sedimentation tank). The typical removal rates of BOD5 and SS in primary sewage treatment are 25% and 50%, respectively. Aerated sedimentation tanks are generally not recommended for biological phosphorus and nitrogen removal wastewater treatment plants to avoid rapid degradation of organic matter removal. In the case that the quality characteristics of the raw sewage are not conducive to the removal of phosphorus and nitrogen, according to the subsequent process of the special injection of water quality, it is necessary to carefully analyze and consider the settings and methods of the initial sedimentation to ensure and improve the water intake in the subsequent processes such as phosphorus and nitrogen removal. quality.

Sewage biochemical treatment is a secondary treatment. Its main purpose is to remove insoluble suspended matter and soluble biodegradable organic matter. Its process composition is various and can be divided into treatment methods such as activated sludge method, anaerobic digestion method, anaerobic digestion method, A2 / O method, SBR method, oxidation ditch method, stable pond method, land treatment method and the like. Recently, most municipal sewage treatment plants use activated sludge. The principle of biological treatment is to complete the decomposition of organic matter and the synthesis of organisms through biological action, especially the action of microorganisms, and convert organic pollutants into harmless gaseous products (CO2), liquid products (water) and organic matter. Solid product (microbial population or biological sludge); the remaining biological sludge is separated from solids and liquids in sedimentation tanks and removed from the purified sewage.

In the process of sewage biochemical treatment, the factors affecting microbial activity can be divided into two categories: matrix and environment:

  1. the matrix includes nutrients, such as carbon-based organic compounds, such as carbon source materials, nitrogen sources, phosphorus sources and other nutrients, and trace elements such as iron, zinc, and manganese; in addition, it includes some toxic and harmful chemicals Such as phenols, benzenes and other compounds, but also includes some heavy metal ions such as copper, cadmium, lead ions and so on.
  2. The main environmental factors are:
    1. Temperature. The effect of temperature on microorganisms is very wide. Although some types of bacteria are also active in high temperature environment (50 ℃ ~ 70 ℃) and low temperature environment (-5 ~ 0 ℃), most of the microorganisms in sewage treatment are the most A suitable growth temperature range is 20-30 ° C. In a suitable temperature range, the physiological activity of microorganisms is strong, and its activity increases with increasing temperature, and the better the treatment effect. Beyond this range, the activity of microorganisms becomes worse, and the biological reaction process will be affected. Generally, the maximum and minimum limits for controlling the progress of the reaction are 35 ° C and 10 ° C, respectively.
    2. PH value. The most suitable pH range for microorganisms in activated sludge system is 6.5-8.5. Excessive acidic or alkaline environments are not conducive to the survival and growth of microorganisms. In severe cases, sludge flocs will be destroyed and the bacterial micelles will disintegrate. The treatment effect deteriorated sharply.
    3. Dissolved oxygen. For aerobic biological reactions, maintaining a certain concentration of dissolved oxygen in the mixture is essential. When the dissolved oxygen in the environment is higher than 0.3mg / l, both facultative and aerobic bacteria perform aerobic respiration; when the dissolved oxygen is lower than 0.2-0.3mg / l and close to zero, the facultative bacteria turn into anorexia. Oxygen respiration, most of the aerobic bacteria basically stop breathing, and some aerobic bacteria (mostly filamentous bacteria) may also grow well, and often lead to sludge swelling after occupying an advantage in the system. Generally, the dissolved oxygen at the outlet of the aeration tank is preferably maintained at about 2 mg / l. If it is too high, the energy consumption is increased, which is uneconomical.

Among all influencing factors, matrix factors and pH values ​​are determined by the quality of the incoming water. The control of these factors mainly depends on daily monitoring and the strict implementation of relevant regulations and regulations. For the general urban sewage, most of these factors will not have much impact, and the parameters can basically be maintained in an appropriate range. The change in temperature is related to the climate. For a 10,000-ton city sewage treatment plant, especially when activated sludge technology is used, it is difficult to implement temperature control, which is not very economically and engineeringly feasible. Therefore, it is generally through the appropriate selection of design parameters to meet the processing requirements of different temperature changes to achieve the processing goals. Therefore, the main goal of process control lies on the activated sludge itself and the environmental factors that can be changed by means of regulation. The main task of control is to take appropriate measures to overcome the influence of external factors on the activated sludge system, so that it can Continuously and steadily.
The process control of the biological reaction system lies in the selection of control objects or control parameters, which is closely related to the processing technology or the processing target.

As mentioned earlier, dissolved oxygen is a very important indicator parameter in the type and process of biological reactions. It can intuitively and quickly reflect the operating status of the entire system, convenient operation management, and simpler installation and maintenance of instruments and meters. This is why the newly built sewage treatment plants in China have basically realized on-site and online monitoring of dissolved oxygen.

Tertiary processing

The tertiary treatment is the advanced treatment of water. Currently, not many practical applications have been put into practice in China’s sewage treatment plants. It removes the nitrogen and phosphorus from the secondary treated water, removes the remaining pollutants in the water by activated carbon adsorption method or reverse osmosis method, and disinfects bacteria and viruses with ozone or chlorine disinfection, and then sends the treated water into the Water channels serve as water sources for flushing toilets, spraying streets, watering green belts, industrial water, and fire protection.
It can be seen that the role of the sewage treatment process is only through biodegradation and solid-liquid separation. While the sewage is purified, the pollutants are concentrated in the sludge, including the primary sedimentation sludge produced in the primary treatment section Residual activated sludge from the secondary treatment section and chemical sludge from the tertiary treatment. Because these sludges contain a large amount of organic matter and pathogens, and are extremely prone to decay and odor, it is easy to cause secondary pollution, and the task of eliminating pollution has not been completed. The sludge must be properly disposed of by a certain volume reduction, reduction and stabilization and harmless treatment well. The success of sludge treatment and disposal has an important impact on the sewage plant and must be paid attention to. If the sludge is not treated, the sludge will have to be discharged with the treated effluent, and the purification effect of the sewage plant will be offset. Therefore, in the actual application process, the sludge treatment in the sewage treatment process is also quite critical.

General water treatment methods and principles

Common 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 will be explained here one by one.

  1. the sediment filtration method
    The purpose of the sediment filtration method is to remove suspended particulate matter or colloidal matter from the water source. If these particulate matter are not removed, it can cause damage to other precision filtration membranes for dialysis water or even block the waterway. This is the oldest and simplest method of water purification, so this step is commonly used in the preliminary treatment of water purification, or if necessary, several 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 size of the particles is larger than these holes, they will be blocked. The ions dissolved in water cannot be stopped. If the filter is not replaced or cleaned for too long, the particulate matter accumulated on the filter will increase, and the water flow and water 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 blockage. Therefore, the filter should be periodically backwashed to eliminate impurities accumulated on it, and the filter should be replaced within a fixed time.
    The sediment filtration method has another problem worth noting, because particulate matter is continuously blocked and accumulated. These materials may have bacteria breeding there, and release toxic substances through the filter, causing a pyrogen reaction, so the filter must be replaced frequently. Principle When the pressure drop between the inlet water and the outlet water increases five times, the filter needs to be replaced.
  2. Hard water softening method
    The softening of hard water requires an ion exchange method. Its purpose is to use a cation exchange resin to exchange calcium and magnesium ions in hard water with sodium ions, so as to reduce the concentration of calcium and magnesium ions in the water. Its softening reaction is as follows:
    Ca2 ++ 2Na-EX → Ca-EX2 + 2Na + 1
    Mg2 ++ 2Na-EX → Mg-EX2 + 2Na + 1
    EX in the formula represents an ion exchange resin. After these ion exchange resins combine Ca2 + and Mg2 +, Na + ions originally contained therein are released.
    The ion exchange resins currently on the market are spherical synthetic organic polymer electrolytes. The 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 needed. That is, a specific concentration of saline is added at regular intervals, usually 10%, and the reaction method is as follows:
    Ca-EX2 + 2Na + (concentrated saline) → 2Na-EX + Ca2 +
    Mg-EX2 + 2Na + (concentrated saline) → 2Na-EX + Mg2 +
    If there is no softening of cations in the water treatment process, not only the calcium and magnesium bodies will be deposited on the reverse osmosis membrane, which will reduce the efficiency or even destroy the reverse osmosis membrane. At the same time, patients will easily get hard water syndrome. Hard water softeners can also cause bacterial reproduction problems, so the device needs to have a backflush function. After a period of time, backflush 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 occurs in the middle of the night. This is the control of the valve. If a failure occurs, a large amount of saline It will flood into the water source, which will cause the patient’s hypernatremia.
  3. activated carbon
    Activated carbon is made by carbonizing carbonized materials such as wood, residual wood chips, fruit kernels, coconut shells, coal or petroleum bottom slag under high temperature, and it needs to be activated with hot air or water vapor after making. Its main function is to remove chlorine and chlorammonia and other dissolved organic materials with molecular weights ranging from 60 to 300 Daltons. The surface of the activated carbon is granular, and the interior is porous. There are many capillaries with a size of 1 nm to 1 A in the pores. The internal surface area of ​​1 g of activated carbon is as high as 700-1400 m2. 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 organics include the area of ​​the activated carbon itself, the size of the pores, the molecular weight of the removed organics, and their polarity (Polarity). It mainly excludes physical adsorption capacity to exclude impurities. When the adsorption capacity reaches saturation Too much impurities will fall down and pollute the downstream water quality, so the backwash must be used regularly to remove the impurities adsorbed on it.
    If the adsorption capacity of this activated carbon filter is significantly reduced, it must be renewed. Determining the difference in TOC concentration (or the difference in the number of bacteria) of the incoming and outgoing water is one of the basis for considering the replacement of activated carbon. Some reverse osmosis membranes are not tolerant to chlorine, so you must have activated carbon treatment before reverse osmosis, so that chlorine can be effectively adsorbed by activated carbon, but the bacteria adsorbed by the holes on the activated carbon are prone to multiply and grow. The removal of larger organic matter, the effectiveness of activated carbon is limited, so the reverse osmosis membrane must be reinforced at the back.
  4. the deionization method
    The purpose of the deionization method is to exclude inorganic ions dissolved in water. Like the hard water softener, it is also based on the principle of ion exchange resin. Two resins are used here-a cation exchange resin and an 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 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 a cation, x represents a valence number, M + x cations exchange with H-Re hydrogen ions on the cationic resin, Az represents an anion, z represents a valence number, and Az is combined with an anion exchange resin. OH- ions are released. H + ions combine with OH- ions to form neutral water.
    These resins also need to be reduced after their adsorption capacity is exhausted. Cation exchange resins need strong acids to reduce them; conversely, anions need strong rhenium to reduce them. Cation exchange resins have different adsorption capacities for various cations, 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 anion exchange resin with each anion is as follows:
    S02-4 +> I-> NO3-> NO2-> Cl-> HCO3-> OH-> F-
    If the anion exchange resin is used up without reduction, the weakest fluorine will gradually appear in the dialysis water, causing chondropathy, osteoporosis and other bone diseases; if the cation exchange resin is exhausted, hydrogen ions It will also appear in dialysis water, causing the acidity of the water to increase, so the effectiveness of the deionization function needs to be constantly monitored. It is generally judged by the resistivity or conductivity of the water quality. It is worth noting that the ion exchange resin used in the deionization method can also cause the reproduction of bacteria and cause bacteremia.
  5. Reverse Osmosis
    Reverse osmosis method can effectively remove inorganic substances, organic substances, bacteria, pyrogens and other particles dissolved in water. It is the most important part in the treatment of dialysis water. To understand the “reverse osmosis” principle, we must first explain the concept of “osmosis”. The so-called osmosis refers to the use of a semi-permeable membrane to separate two different concentrations of the solution, where the solute cannot penetrate the semi-permeable membrane, the lower concentration One water molecule will pass through the semi-permeable membrane to reach the other one with higher concentration, until the concentration on both sides is equal. Before the equilibrium is reached, pressure can be gradually applied to the one with higher concentration, and the aforementioned state of water molecule movement will be Temporarily stop, the pressure required at this time is called “osmotic pressure”. If the applied force is greater than the osmotic pressure, the movement of water will go in the opposite direction, that is, from a high concentration example to a low concentration. On the one hand, this phenomenon is called “reverse osmosis.” The purification effect of reverse osmosis can reach the level of ions, and the rejection rate for monovalent ions can reach 90% -98%, and Divalent ions (divalent ions) can reach 95% -99% (can prevent the passage of substances with molecular weight greater than 200 Dalton).
    Semi-permeable membrane materials commonly used for reverse osmosis water treatment include cellulosic, aromatic polyamides, polyimide or polyfuranes, etc. As for its structural shape, it has spiral wound and hollow fiber type (hollow fiber) and tubular (tubular). As for the cellulose film of these materials, it has the advantage of high chlorine resistance, but the life will be shortened under the conditions of pH (pH ≥8.0) or the presence of bacteria. The disadvantage of polyamide is its poor resistance to chlorine and chlorammonia. As for which material is better, there is no conclusion yet.
    If the pre-treatment is not done before reverse osmosis, there is a tendency for dirt to accumulate on the osmotic membrane, such as calcium, magnesium, iron and other ions, causing the decline in reverse osmosis function; some membranes (such as polyamide) are easily damaged by chlorine and chloramine. Therefore, there must be pretreatments such as activated carbon and softener before the reverse osmosis membrane. Although reverse osmosis is more expensive, because the pore size of the reverse osmosis membrane is generally below 10A, it can exclude bacteria, viruses, pyrogens, and even various dissolved ions. Therefore, it is best to prepare this step in the preparation of hemodialysis water.
  6. ultrafiltration method
    Ultrafiltration is similar to reverse osmosis. It also uses a semi-permeable membrane, but it cannot control the removal of ions because the membrane has a larger pore size, about 10-200A. It can only exclude bacteria, viruses, pyrogens, and particulate matter, etc. It cannot filter water-soluble ions. The main role of the ultrafiltration method is to act as a pretreatment for the reverse osmosis method to prevent the reverse osmosis membrane from being contaminated by bacteria. It can also be used as a final step in water treatment to prevent upstream water from being contaminated with bacteria in the pipeline. Generally, the difference between the inlet water pressure and the outlet water pressure is used to determine whether the ultrafiltration membrane is effective. Similar to activated carbon, usually the backwash method is used to remove impurities attached to it.
  7. distillation
    Distillation is an ancient but effective water treatment method. It can remove any non-volatile impurities, but it cannot exclude volatile pollutants. It requires a large water tank to store it. This water tank and conveying pipe are An important cause of contamination. At present, hemodialysis water is not treated in this way.
  8. ultraviolet disinfection method
    Ultraviolet disinfection is one of the commonly used methods. Its sterilization mechanism is to destroy the biological genetic material of bacterial nucleic acid and prevent it from reproducing. The most important reaction is that the pyrimidine base in the nucleic acid molecule becomes a dimer. . Generally, it uses artificial 253.7nm wavelength ultraviolet energy of low-pressure mercury discharge lamp (sterilization lamp). The principle of the ultraviolet germicidal lamp is the same as that of the fluorescent lamp, except that the inside of the lamp tube is not coated with fluorescent substances, and the material of the lamp tube is quartz glass with high ultraviolet transmission rate. General ultraviolet devices are divided into irradiation type, immersion type and running type according to the application.
    Water treatment ultraviolet sterilization lamp used in the hemodialysis dilution water is used in the pipeline between the water storage tank and the dialysis machine, that is, all dialysis water must be exposed to ultraviolet radiation before use to achieve complete sterilization effect. Pseudomonas aeruginosa and coliform bacteria are the most sensitive to ultraviolet light; conversely, the more tolerant are the bacillus subtilis. Because the UV disinfection method is safe, economical, less selective to bacteria, and the water quality will not change, this method has been widely used in recent years. For example, drinking water on ships often uses this disinfection method. All the bacteria in the water, such as Escherichia, Basella, Salmonella, etc., can kill 360 degrees of sterilization by diving into the water center, which is three times as effective as the surface disinfection lamp. It can eliminate algae in the water, with significant effects and convenient use. The UV germicidal lamp is suitable for: filtration of various sizes of fishing grounds, water treatment, large and small pools, swimming pools, hot springs. The sterilization efficiency can reach 99% -99.99%.

Three ways of wild water treatment

Regarding whether the water in the wild should be treated after drinking, everyone has different ideas. Some people think that there are few people in the wild, that there will be any pollution, and it is all right to drink. In fact, as long as it can be reached by amateur players like us, what else can there really be no pollution? In fact, even if there is no pollution, it does not mean that there is no virus, bacteria, or various harmful substances in the water. Under the circumstances, it is best to deal with it. The current water treatment methods are mainly boiling, water purification drugs and filters boiling. This is the most practical and effective method. The disadvantage is that it wastes fuel and is time consuming. Water purification tablets are considered to be chemically purified. Most of them use chlorine and iodine. The mainstream in the market is iodine. The storage of iodine tablets needs to be protected from light and moisture. The advantages of using water purification tablets are cheap, convenient and lightweight. It is time consuming. Generally, you can drink it after about 20 minutes after adding water purification tablets. The disadvantage is that there is a strange smell and color. This point can be solved by post-processing, such as the yellow bottle above, which is used to deodorize. Or add Guozhen or something. The second is that there is a major virus that cannot be removed, cryptosporidium, which is probably the most common water parasite. Third, it is easy to cause allergies and chemical reactions with certain foods or utensils. In general, water purification tablets still have a large market because of their lightness, cheapness, and small size. It is important to wait at least 20 minutes in use. Second, do not use it continuously for more than a week. Purification filters There are actually two categories, filters and filtration purifiers. The main difference is that the filters remove bacteria and parasites by filtering. In addition to removing bacteria and parasites, the filter purifier can also remove viruses. (Classification based on REI) Filters General filter purifiers are based on filtering and then chemically remove viruses, such as iodine, so they also have the disadvantages of water purification tablets that were previously purified chemically . The post above is the only water purifier that uses no chemical method to remove the virus. The advantage of using a purification filter device is that you can drink water immediately, almost no waiting, and it is also the safest method of water purification (if using a filter purifier). The disadvantage is that the price is expensive, rarely less than 60 US dollars, the later cost is high, and the filter element also needs to spend money. Large size and weight, almost all above 1 lbs. It also needs frequent maintenance during use and is easy to be blocked. Each of the three methods has its own strengths, and how to choose is your opinion.

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Further reading:

common water treatment methods

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