On the classical concept of water treatment

I. common basic concepts

1. Conductivity: the number of ions that can be moved between two electrodes with an area of one square centimeter and a distance of one centimeter apart, called conductivity, unit: μ s / cm.

2. Resistance: reciprocal of conductivity, unit: m Ω· cm.

3. Hardness: refers to the content of calcium and magnesium ions in the water source, which is usually expressed in GPG.

4. PH value: the relative content of acid and alkali in the solution. PH is a measure of the negative logarithm (log) of the concentration of hydrogen ions in water. PH value is divided into 0-14 blocks. If pH value is 7.0, water is neutral; if pH value is less than 7.0, water is acidic; if pH value is greater than 7.0, water is neutral. Then the water is alkaline.

5. Total solids (TS): refers to the sum of total dissolved solids (TDS) and total suspended solids (TSS).

6. Alkalinity: a general term used to describe the content of carbonate, bicarbonate and hydroxide.

7. Total organic carbon (TOC): total organic carbon (TOC) is the unit of measurement of organic pollution in water in mg / L. TOC is a direct unit of measurement of oxidizable organic matter.

8. Activated carbon: granular activated carbon, used to remove peculiar smell, odor, chlorine, chloramine and some organic matters in water.

9. NTU: scattering turbidity unit - the turbidity of low turbid water measured by scattering turbidimeter after passing a beam of light through sample water.

10. Permeation: water flows naturally from the side of low concentration solution to the side of high concentration solution through semi permeable membrane until the energy reaches balance.

11. SDI: sediment density index - used to measure the number of suspended solids in raw water used in reverse osmosis system.

12. Resin: specially manufactured polymer ball, used in ion exchange system to remove dissolved salt in aqueous solution.

13. LSI: Langelier saturation Index-A calculation formula used to predict calcium carbonate precipitation under specified conditions, temperature, pH value, TDS, hardness and alkalinity.

14. Endotoxin: an antipyretic pyrogenic substance, especially the lipopolysaccharide found in the cell walls of living or inanimate bacteria.

15. Ozone: an unstable and highly active form of oxygen. It is produced by natural lightning or high-voltage charge passing through the air. It is an excellent oxidant and disinfectant.

II. Basic treatment process

1. precipitation

Precipitation is usually a multi-step process to reduce turbidity and suspended matter in water. This multi-step process involves adding a chemical coagulant or pH regulator to react to form floccules, which settle down in the settling tank due to gravity, or filter out when water passes through the height difference filter. The precipitation process can effectively remove particles larger than 25 μ M.

2. Lime soda softening

Adding lime (CAO) and soda (Na2CO3) to water to reduce the content of calcium and magnesium is called lime softening method. Its goal is to precipitate calcium hydroxide and magnesium hydroxide (hardness) in water. The cost of this technology is less, but the effect is barely. Generally, the hardness of water produced is 50-120ppm (3-7gpg). The disadvantage of this process is that the pH value of the treated water is high, generally in the range of 8.5 ~ 10.0.

3. Mechanical filter (multi-media filter)

The mechanical filter is a pressure vessel filled with filter material of specified thickness. It is a single-layer mechanical filter when filling a single filter material and a double-layer or multi-media filter when filling different kinds of filter materials.

Function: in the water quality pretreatment system, the quartz sand with different particle sizes in the pressure vessel of the multi-media filter is filled according to a certain level. The flocculated raw water passes through the filter material layer from top to bottom under a certain pressure, so as to intercept and remove the suspended solids in the water. The multi-media filter can effectively remove the suspended solids, fine particles, full valence iron, colloid, bacteria and algae and organic matters in the original water. 。 Its effluent SDI15 (pollution index) is less than or equal to 5, which can fully meet the requirements of reverse osmosis device.

4. Activated carbon filter

The pressure vessel of activated carbon filter is a kind of pressure vessel filled with coarse quartz sand cushion and high quality activated carbon.

Function: in the water quality pretreatment system, the activated carbon filter can absorb the residual chlorine that cannot be removed in the previous stage filtration to prevent the later stage reverse osmosis membrane from being oxidized and degraded, and also absorb the small molecule organic matter and other pollutants leaked from the previous stage, which has obvious adsorption and removal effect on the peculiar smell, colloid and pigment, heavy metal ion, COD, etc. in the water. The SDI value of RO inlet water can be further reduced to ensure SDI < 5 and TOC < 2.oppm.

5. softener

Ion exchange softening device is the most commonly used equipment in the process of water treatment. Its function is to remove calcium and magnesium ions which form scale in hard water. In many cases, soluble ions (iron ions) can be removed by using softened water equipment. Standard soft water equipment has four main parts: resin column, resin, salt adding device, valve controller.

The resin column of soft water equipment is equipped with treated ion exchange resin polystyrene small particles. The resin particles initially adsorbed sodium ions in the regeneration process. The resin has much greater affinity for multivalent ions such as calcium and magnesium ions. Therefore, when the hard water flows through the resin, calcium and magnesium ions will be adsorbed on the resin, and at the same time desorption ions, until the equilibrium state is reached. At this time, the soft water equipment completes the exchange of sodium ions and calcium and magnesium ions in water.

During regeneration, the NaCl solution flows through the resin, and the hard ions are replaced by sodium ions. The affinity between resin and hard ions was weakened by using high concentration brine. This regeneration process can be repeated indefinitely without damaging the resin.

Softener is a simple ion exchange process, which solves the very common form of water pollution: hardness. Using NaCl for regeneration is a simple but not expensive process. It can also realize automatic regeneration without strong chemical reagent.

6. Anion and cation exchanger

The anion and cation exchanger are respectively filled with anion resin and cation resin. The cation exchanger is used to remove positively charged ions (cations), and the anion resin is used to remove negatively charged ions (anions).

Cation exchange resin is to replace H + ions with cations, such as calcium, magnesium and sodium ions; anion exchange resin is to replace Oh - ions with anions, such as chloride ions, sulfate ions and bicarbonate ions. The replaced H + and oh - are synthesized into water to remove the ions in water.

The exchange capacity of resin is limited, and it must be regenerated after its exchange capacity is exhausted. When the equilibrium state between the adsorbed ions appears when the exchange capacity is exhausted, the regeneration of cation resin is treated by acid, generally by hydrochloric acid, i.e. filling with H + ions. Generally, sodium hydroxide is used for regeneration of anion resin, that is, it is filled with Oh ion. Regeneration can be carried out outside the column by using the regenerated exchange column deionization equipment, or inside the column by installing the renewable deionization equipment, regeneration equipment and chemicals.

7. mixed bed

Mixed bed is the abbreviation of mixed ion exchange column, which is designed for ion exchange technology. The so-called mixed bed is that a certain proportion of cation and anion exchange resins are mixed and loaded in the same exchange device to exchange and remove ions in the fluid. Because the specific gravity of cation resin is larger than that of anion resin, the anion resin is under the upper cation resin in the mixed bed. Generally, the filling ratio of cation resin and anion resin is 1:2, and there are also filling ratios of 1:1.5, which can be selected according to different resins.

The mixed bed is also divided into internal synchronous regeneration mixed bed and external regeneration mixed bed. The synchronous regeneration type mixed bed is operated in the mixed bed and the whole regeneration process is carried out in the mixed bed. During regeneration, the resin is not removed from the equipment, and the cation and anion resins are regenerated at the same time. Therefore, the auxiliary equipment required is less and the operation is simple, with the following advantages:

(1) the quality of effluent is excellent, and the pH value of effluent is close to neutral.

(2) the outlet water quality is stable, and the change of short-term operation conditions (such as inlet water quality or components, operation flow rate, etc.) has little effect on the outlet water quality of the mixed bed.

(3) the impact of intermittent operation on the effluent quality is small, and the time required to recover to the water quality before shutdown is relatively short.

(4) the end point of exchange was obvious.

8. EDI

EDI works by exchanging hydroxyl ions or hydroxyl ions to remove unwanted ions, which are then transported to the waste water flow. The ion exchange reaction is carried out in the purification chamber of the module, where the anion exchange resin releases the hydroxide ion (OH -) to obtain the anion from the dissolved salt (such as chloride, Cl -). Similarly, the cation exchange resin releases hydrogen ions (H +) to obtain cations from dissolved salts (such as sodium, Na +).

A direct current (DC) electric field is applied by an anode (+) and a cathode (-) placed at one end of the assembly. The voltage drives these absorbed ions along the surface of the resin ball, and then through the film into the concentrated water chamber.

Negative anions (such as OH -, Cl -) are attracted to the anode (+). These ions pass through the anion selective film and enter the adjacent concentrated water chamber, but not through the adjacent cation selective film and stay in the concentrated water chamber, and are properly treated. Positive cations (such as H +, Na +) are attracted to the cathode (-) in the fresh water chamber. These ions pass through the cation selective membrane and enter the adjacent concentrated water chamber, where they are blocked by the adjacent anion selective membrane and properly treated.

In the concentrated water chamber, it is still electrically neutral. The ions transported from both directions neutralize each other. The current flowing from the source is proportional to the number of moving ions. Both streams (H + and oh -) trend ions are transported and added to the required current.

When water flows through two different types of chambers, the ions in the purification chamber will be exhausted and collected into the nearby concentrated water flow, which will take the removed ions away from the module.

The use of ion exchange resin in purification room and / or concentrated water room is a key to EDI technology and patent. An important phenomenon will occur in the purification room. In the specific area with high potential gradient, electrochemical "decomposition" can make water produce a large number of H + and oh - ions. The H + and oh - ions produced in these regions can regenerate the resin continuously in the mixed ion exchange resin, and form a film without additional chemical reagents.

9. reverse osmosis system

Reverse osmosis is a kind of side flow filtration, in which raw water passes through the membrane under the action of pressure, part of which passes through the membrane, while the rest of the raw water flows out of the system along the tangent direction of the membrane without filtration. The filtered water flow is called "permeated water" because it permeates the membrane; the other water flow is called "concentrated water" because it takes away the concentrated pollutants blocked by the membrane. Because the raw water flow and concentrated water flow are parallel to the membrane, rather than perpendicular to the membrane, this process is called "side flow" or "tangential flow".

Reverse osmosis is one of the most widely used membrane separation processes. It uses pressure to make water penetrate the membrane, while soluble salts, colloids, organic matters and microorganisms are trapped on the surface of the membrane and discharged with concentrated water. It can effectively remove all organic matters and 90% ~ 99% ions.