Water treatment is generally divided into drinking water treatment and industrial water treatment. Drinking water treatment steps are generally physical processes (such as sedimentation and filtration), chemical processes (such as disinfection and coagulation), biological processes (such as slow sand filtration), industrial water treatment steps are generally the main process of industrial water treatment.
I. Definition of Water Treatment:
Water treatment is the process of improving water quality for specific end-use. End-uses may be drinking water, industrial water supply, irrigation, river maintenance, water recreation, and other uses, including safe return to the environment. Water treatment can remove contaminants and undesirable constituents or reduce their concentration to make the water suitable for its desired end-use. This treatment is essential to human health, benefiting people from both drinking and irrigation.
Basic Definition: The production of drinking water includes raw water sufficient to remove contaminants and has no long-term risk of short-term or adverse health effects. In general, the greatest microbial risk is associated with the ingestion of water contaminated with human or animal (including bird) feces. Feces can be a source of pathogens, viruses, protozoa, and worms. Destruction of microbial pathogens is essential, and reactive chemicals (e.g., suspended solids) are often used to remove bacteria, algae, viruses, fungi, and minerals, including iron and manganese. These substances continue to cause enormous damage in several underdeveloped countries that are unable to purify their water.
To ensure water quality, measures are taken to treat the water and transport and distribute the treated water. Therefore, it is common practice to leave residual disinfectants in the treated water to kill bacterial contamination in the distribution process.
Domestic water supplied for tap or other uses may be further treated before use, usually by an in-line treatment process. This treatment may include water softening or ion exchange. Many proprietary systems also claim to remove residual disinfectants and heavy metal ions.
Basic Processes: Processes related to contaminant removal include physical processes (e.g., sedimentation and filtration), chemical processes (e.g., disinfection and coagulation), and biological processes (e.g., slow sand filtration).
Combinations selected from the following processes are used worldwide for municipal drinking water treatment.
A tank with a sand filter removes precipitated iron (which does not work at the time).
Pre-chlorination can control algae and prevent biological growth.
When small amounts of manganese are present, aeration and pre-chlorination can remove dissolved iron.
Disinfection with chlorine, ozone, and ultraviolet light disinfects pathogens, viruses, and other pathogens.
Sedimentation for solids separation, i.e., removal of suspended solids trapped in flocs.
Filtration removes particles through the water, either through a washable, reusable sand bed or through a specially designed washable filter.
Removal of suspended solids by dissolved air flotation.
Coagulant adjuncts - also known as polyelectrolytes - improve coagulation to form a stronger coagulum.
Polyelectrolytes, or polymers as they are called in the field, are usually composed of positively or negatively charged materials, based purely on the water source's characteristics in the treatment plant.
These are often used combined with a primary coagulant such as ferric chloride, ferric sulfate, or alum.
The slow filtration of sand metabolizing organic matter with biofilms.
Well-developed technologies for drinking water allow for universal design and optional treatment technology pilot testing for specific water sources. Besides, private companies offer patented technology solutions to treat specific contaminants. Automation of water treatment is common in developed countries. The water source's quality across seasons, size, and environmental impact can determine capital and operating costs. The end use of the treated water determines the necessary quality monitoring technology, and local skills determine the level of automation adopted.
Brine can be treated to produce fresh water. Two main processes are used, reverse osmosis or distillation.  These two methods require more energy than treating local surface water and are usually only used in high salinity areas such as coastal areas or groundwater.
(7) Portable water purifiers.
Living away from drinking water sources usually requires some form of portable water treatment process. These complex variations range from the simple addition of disinfectant tablets to a hiker's water bottle to the complex multi-stage process of transporting it by boat or plane to a disaster area.
1、Basic Process: The two main processes for industrial water treatment are boiler water treatment and cooling water treatment. A large amount of proper water treatment can cause solids and bacteria to react in the piping and boiler rooms. If left untreated, steam boilers may develop scale or corrosion. Scale can lead to degraded and dangerous machine performance, while additional fuel is required to heat the same water level due to increased thermal resistance. Poor-quality effluent can become a breeding ground for bacteria such as Legionella, which can threaten public health.
Corrosion in low-pressure boilers can be caused by high levels of dissolved oxygen, acidity, and alkalinity. Therefore, water treatment should remove the dissolved oxygen to maintain the boiler water's proper pH and alkalinity. Without effective water treatment, the cooling water system may scale, corrode, and scaling, becoming a breeding ground for harmful bacteria. This reduces efficiency, shortens equipment life, and makes operation unreliable and unsafe.
2. Boiler water treatment.
Boiler water treatment is an industrial water treatment used to remove or chemically modify substances that may damage the boiler. Different treatment methods are used in different locations to avoid fouling, corrosion, or foaming. The external treatment of the raw water supply planned for use inside the boiler focuses on removing impurities before they reach the boiler. Internal treatment of the boiler concentrates on limiting the water's tendency to dissolve the boiler, keeping the impurities from causing trouble until they are removed from the boiler contamination.
3. Cooling water treatment.
Water cooling is the method of removing heat from parts and industrial equipment. In cases where air cooling is ineffective, water may be a more efficient heat transfer fluid. In most environments where people live, water has the advantage of a liquid with high thermal conductivity and high specific heat capacity. As well as the option of evaporative cooling. Low-cost coolant loops typically allow for single-use as waste but can be pressurized and recycled, eliminate evaporative losses, and provide greater portability and higher cleanliness. Non-pressurized recirculating coolant circuits using evaporative cooling need to drain the waste stream to remove evaporative concentrated impurities. Disadvantages of water-cooled systems include accelerated corrosion and maintenance requirements and reduced heat transfer by preventing biofouling and fouling formation. Chemical additives that reduce these drawbacks can be toxic to wastewater. Water cooling is commonly used to cool automotive internal combustion engines and large industrial facilities such as nuclear and steam power plants, hydroelectric generators, oil refineries, and chemical plants.
4. Main Technologies.
(1) Chemical treatment.
Chemical treatment is the technique of making industrial water suitable for use or discharge. These include chemical precipitation, chemical disinfection, chemical oxidation, advanced oxidation, ion exchange, and chemical neutralization.
(2) Physical treatment.
Filtration removes micro-particles from the water and can pass through sand, such as rapid gravity filters and mechanical filters. Dissolved air flotation removes floating solids from the water. This is achieved by dissolving air in the water under pressure and releasing water/air at atmospheric pressure in the flotation tank. The released air forms small air bubbles that attach to the float and float on the water's surface, where they can be removed from scum devices and overflow devices.
(3) Biological treatment.
Chronic sand filters use biological processes to purify raw water to produce drinking water. They work with complex biofilms that grow naturally on the sand surface. The gel-like biofilm, called hypocotyl or Schmutzdecke, is located a few millimeters above the sand layer. The Schmutzdecke consists of bacteria, fungi, protozoa, rotifers, and a range of aquatic insect larvae. As the biofilm ages, more algae may grow and larger aquatic organisms, including mosses, snails, and Annelida. As water passes through the hyphae, material particles are trapped in a viscous matrix, and soluble organic matter is adsorbed. Bacteria, fungi, and protozoa metabolize contaminants.
The depth of a slow sand filter is usually 1 to 2 meters, and the hydraulic load rate is 0.2 to 0.4 m3/m2 per hour. The filter loses performance as the biofilm thickens, reducing the flow rate. Refit the filter by removing the biofilm and thin sand. Water is poured back into the filter and recirculated to form a new biofilm. Also, the wet rake method involves agitating the sand and flushing the biofilm for treatment.
(4) Physicochemical treatment.
Chemical flocculants are used to produce flocs in the water and trap suspended solids. Chemical polyelectrolytes are used to increase the coagulation of suspended solids and improve removal. It consists of rapid mixing of the primary coagulant (e.g., ferric sulfate) and the coagulant cationic polymer before entering the coagulation cell. The water to be treated is rapidly mixed with the primary coagulant and polymer and placed into a flocculation tank where the water is slowly swirled or mixed with chemicals to form what is called Flocc, which settles to the bottom of the Floc basin. After the water mixes and forms flocs, it passes to the next stage of the settling basin. Here, the process has either a tube settler or a plate settler. Water flows upward through these tubes and plates, allowing freshwater to flow into the outflow logistics tank, which carries the settled water to the filter for further treatment. During the sinking phase, the tubes/plates have a large surface area for the Flocc to sink. These plates are usually at an angle of 30-45°, allowing Flocc particles to collect into the tubes and plates, ending up at the bottom of the settling tank. Typically, a sludge collection system collects all of the settling flocc or sludge, and then the waste is pumped or transferred to a tank or jar in the decant and disposed of. The settled water enters a filter and is then passed through a filter for storage in a water purification well where all the filtered water is collected for other chemical additions: pH regulators, chlorine, etc. After that, appropriate contact time and dissipation of the sludge is required. Afterward, with proper contact time and dissipation time, the water is discharged from the freshwater sink and header into storage tanks or distributed to the customer's faucet for use.