Nine kinds of water treatment basic processes. Yes, it is. And do you know what are they? Now, let’s continue to reading.
Precipitation is usually a multi-step process. It is used to reduce turbidity and suspended matter in water. This multi-step process involves the addition of a chemical coagulant or pH adjuster to react to form flocs.
The flocs precipitate in the sedimentation tank due to gravity. Or filter out when water passes through the height difference filter. The precipitation process effectively removes particles larger than 25 μm.
2. Lime – Soda Softening
A method of reducing the calcium and magnesium content by adding lime (CaO) and soda ash (Na2CO3) to water is called a lime softening method.
The goal is to precipitate calcium hydroxide and magnesium hydroxide (hardness) in the water. This process costs less. But the effect is not ideal.
The water hardness usually produced is 50-120 ppm (3-7 gpg). The inadequacy of this process is that the pH of the treated water is high, generally ranging from 8.5 to 10.0.
3. Mechanical Filter (Multi-media Filter)
A mechanical filter is a pressure vessel filled with a specified thickness of filter material. When filling a single filter, it is called a single layer mechanical filter. When filling different types of filter media, it is called a double or multi-media filter.
In the water quality pretreatment system, quartz sand of different particle sizes in the multi-media filter pressure vessel is loaded at a certain level. The flocculated raw water passes through the filter layer from top to bottom under a certain pressure. Thereby the suspended matter in the water is removed.
The multi-media filter can effectively remove suspended solids, fine particles, full-valent iron and colloids, algae and organic matter in raw water. Its effluent SDI (pollution index) is less than or equal to 5. The result is fully capable of meeting the water ingress requirements of the reverse osmosis unit.
4. Activated Carbon Filter
The activated carbon filter is a pressure vessel filled with a coarse quartz sand cushion and high-quality activated carbon.
In the water quality pretreatment system, the activated carbon filter is capable of adsorbing chlorine that cannot be removed in the pre-stage filtration. This prevents the reverse osmosis membrane from being oxidatively degraded.
At the same time, it also adsorbs pollutants such as small molecules and organic substances leaking from the front stage. It has obvious adsorption and removal effects on odor, colloid and pigment, heavy metal ions and COD in water. The SDI value of the RO influent can be further reduced to ensure SDI < 5 and TOC < 2.O ppm.
Ion exchange softening devices are the most commonly used equipment in water treatment chemicals processes. Its role is to remove scale-forming calcium and magnesium ions from hard water.
In many cases, soluble ions (iron ions) can be removed using a demineralized water device. Standard soft water equipment has four main components: resin column, resin, salting device, and a valve controller.
The resin column of the soft water equipment contains a treated ion exchange resin – small particles of polystyrene. Initially, this resin particle adsorbed sodium ions during the regeneration process. This resin has a much greater affinity for multivalent ions (calcium ions, magnesium ions).
When hard water flows through the resin, calcium ions and magnesium ions are adsorbed on the resin. At the same time, it desorbs ions until it reaches equilibrium. At this time, the soft water equipment completes the exchange of sodium ions and calcium and magnesium ions in the water.
At the time of regeneration, the NaCl solution flows through the resin. Hard ions are replaced by sodium ions. The use of high concentrations of brine weakens this affinity between the resin and the hard ions. This regeneration process can be repeated indefinitely without damaging the resin.
The softener is a simple ion exchange process. It solves the very common form of water pollution: hardness. Recycling with NaCl is a simple but inexpensive process. At the same time, automatic regeneration can be achieved without the need for strong chemical reagents.
6. Anion and Cation Exchangers
The anion and cation exchangers are exchangers for respectively filling an anion resin and a cation resin. A cation exchanger is used to remove positively charged ions (cations) and an anion resin is used to remove negatively charged ions (anions).
The cation exchange resin replaces the H+ ion with a cation. Such as calcium, magnesium and sodium ions. The anion exchange resin is substituted with an OH-ion to an anion. Such as chloride, sulfate, and bicarbonate. The displaced H+ and OH– combine to form water, removing ions from the water.
The exchange capacity of the resin is limited. Regeneration must be carried out after its exchange capacity is exhausted. The depletion of exchange capacity occurs when the adsorbed ions reach equilibrium.
The regeneration of the cationic resin is carried out using an acid. It is usually regenerated with hydrochloric acid. That is, filling with H+ ions. The regeneration of the anionic resin generally uses sodium hydroxide. That is, it is filled with OH– ions.
7. Mixed Bed
Regeneration can be carried out outside the column using exchange columns and deionization equipment, or in a column by installing regenerative deionization equipment and regeneration equipment and chemical reagents.
The mixed bed is the abbreviation of mixed ion exchange column. It is a device designed for ion exchange technology.
The mixed bed is a mixture of a certain proportion of cation and anion exchange resin packed in the same exchange device. The exchange and removal of ions in the fluid is completed.
The specific gravity of the cation resin is larger than that of the anion resin. Therefore, in the mixed bed, the anion resin is in the upper cation resin. Generally, the ratio of male and female resin loading is 1:2. There is also a loading ratio of 1:1.5. The choice of different resins can be considered as appropriate.
Mixed beds are also divided into in-vivo synchronous regenerative mixed beds and in vitro regenerative mixed beds. The synchronous regenerative mixed bed is carried out in a mixed bed during operation and throughout the regeneration process. The resin does not move out of the equipment during regeneration. And the cation and anion resin are simultaneously regenerated. Therefore, fewer accessory devices are required and the operation is simple. Synchronous regeneration has the following advantages.
(1) The effluent water quality is excellent. The effluent pH is close to neutral.
(2) The effluent water quality is stable. Changes in short-term operating conditions (such as influent water quality or composition, operating flow rate, etc.) have little effect on the quality of mixed bed effluent.
(3) The effect of the intermittent operation on the quality of effluent water is small. The time required to return to the water quality before the shutdown is relatively short.
(4) The exchange endpoint is obvious.
EDI works by removing unwanted ions by exchanging hydroxyl or hydroxide ions. These ions are then delivered to the wastewater stream.
The ion exchange reaction is carried out in a purification chamber of the module. The anion exchange resin releases hydroxide ions (OH–) and anions from dissolved salts (such as chloride, Cl–). Likewise, the cation exchange resin releases hydrogen ions (H+) and cations are obtained from dissolved salts (such as sodium, Na+).
A direct current (DC) electric field is applied through an anode (+) and a cathode (-) placed at one end of the assembly. The voltage drives these absorbed ions to move along the surface of the resin sphere. Then through the film into the concentrated water chamber.
Negatively charged anions such as OH–, Cl– are attracted to the anode (+). These ions pass through the anion selective membrane and then enter the adjacent concentrate chamber.
Positively charged cations (such as H+, Na+) are attracted to the cathode (-). These ions pass through the cation selective membrane into the adjacent concentrated water chamber.
The ions transported from both directions neutralize each other. The current flowing from the power source is proportional to the number of moving ions. Both streams (H+ and OH–) trend ions are delivered and added to the required current.
The water flows through two different types of chambers, and the ions in the purification chamber are depleted. At the same time, it is collected into the adjacent concentrated water stream, which removes the removed ions from the assembly.
The use of ion exchange resins in purification chambers and/or concentrated water chambers is a key to EDI technology and patents.
An important phenomenon also occurs in the purification chamber. In certain areas where the potential gradient is high, electrochemical “decomposition” can cause water to produce large amounts of H+ and OH- ions. The H+ and OH– ions produced in these regions allow the resin to be continuously regenerated in the mixed ion exchange resin, and the formation of the film does not require the addition of a chemical agent.
9. Reverse Osmosis System
Reverse osmosis is a side stream filtration. The raw water traverses the membrane under pressure. A portion of the raw water permeates through the membrane, while the remaining raw water flows out of the system along the tangential direction of the membrane without filtration.
The filtered water stream is referred to as “permeate water” due to permeation through the membrane. Another stream of water is called “concentrated water” by taking away the concentrated contaminants blocked by the membrane. Because the raw water stream and the concentrated water stream are parallel to the membrane rather than perpendicular to the membrane. So this process is called “side flow” or “tangential flow”.
Reverse osmosis is one of the most widely used membrane separation processes that use pressure to permeable water through the membrane, while soluble salts, colloids, organic matter, and microorganisms are trapped on the membrane surface and discharged with concentrated water. It effectively removes all organic matter and 90%-99% of ions.
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