Summary of ultra practical water treatment knowledge

1. Several basic concepts of water treatment: SDI, LSI, KSP
SDI: The pollution index is an indicator that measures the pre-treatment effect of the system, with an SDI<6.7. For deep well water, the reverse osmosis device requires an inlet SDI<5.
LSI: Langelier Saturation Index, which measures the scaling tendency of reverse osmosis devices. If LSI=0, the system has no scaling or corrosion tendency; LSI>0, system tends to scale; LSI<0, the system has a tendency for corrosion.
For reverse osmosis systems, the LSI value is required to be no greater than 0. The LSI value of the system can be reduced by adding acid or by reducing the water recovery rate of the system.
Ksp: solubility equilibrium constant. The reverse osmosis device selectively penetrates the solvent and solute in the raw water, resulting in concentration on the concentrated water side due to the reduction of solvent. When the concentration product of dissolved solids on the concentrated water side is greater than the solubility equilibrium constant, crystallization and precipitation will occur, posing a threat to the reverse osmosis device. Increasing the solubility equilibrium constant of the system can be achieved by adding scale inhibitors, which can increase the solubility of dissolved solids.

2. How can the LSI index be effectively controlled?
The effective control of the system's LSI index can be achieved through the following aspects:
1) The system's LSI index can be reduced by reducing the system's water recovery rate.
2) The system LSI index can be reduced by adding acid.
3) The solubility of dissolved salts in the system can be increased by adding corresponding agents, such as TRISPE1000 scale inhibitor.
4) It is possible to reduce or pre remove easily structured ions in the water, such as softening the system inlet water through a softening column.

3. What are the pre-treatment equipment?
The pre-treatment equipment includes mechanical filters, high-efficiency fiber filters, activated carbon filters, precision filters, ultrafiltration, microfiltration, sodium ion softeners, iron and manganese removal filters, dosing devices, raw water tanks, and aeration tanks.

4. What are the pre desalination equipment?
The pre desalination equipment includes an electrodialysis device and a reverse osmosis device.

5. What are the deep desalination equipment available?
There are deep desalination equipment such as anion exchangers, cation exchangers, mixed ion exchangers, distillation units, and EDI units.

6. How are mechanical filters selected? What is its working principle?
The selection of mechanical filters is based on the total water inflow of the system to choose the size and combination method of the filters (if one mechanical filter is not enough, multiple filters can be used in parallel or in reserve). The filler inside the mechanical filter is composed of many different particle sizes of refined quartz sand arranged in strict order from large to small, thus forming a good quartz sand grading.
When a filter is first put into use, its filtering effect is often not very good because it does not form a "bridging" at the beginning. The so-called "bridging" refers to a interception network composed of suspended solids in water, which intercepts suspended solids of the same particle size, and then intercepts smaller suspended solids, forming a reverse particle filtration process that first intercepts large particulate matter and then intercepts small particulate matter.
Once the filter forms a "bridge", the filtering effect is very good. As the operation time increases, the filtering accuracy becomes higher, the interception net becomes thicker, and the pressure difference between the inlet and outlet becomes larger. When the pressure difference reaches 1kg/cm2, the filter should be backwashed.
It is best to equip compressed air to scrub quartz sand during the backwashing process. General engineering experience is that mechanical filters with a diameter less than 2500mm do not require compressed air; Mechanical filters with a diameter greater than 2500mm must be scrubbed with compressed air to achieve satisfactory cleaning results; The backwash flow rate is generally 3-4 times the design capacity of the filter.

7. How are precision filters selected? How many methods are there for filter cartridges?
The selection of precision filters is matched with the total water inflow, and the diameter of the precision filter is selected based on the total water inflow. For a filter element with a filtration accuracy of 40 "5um, the water yield per unit is approximately 2m3/h.
The types of filter elements roughly include polypropylene filter elements, honeycomb filter elements, spray melt filter elements, folding filter elements, etc.

8. How to remove iron from water?
The iron in groundwater is generally divalent ferrous, so it must be oxidized to trivalent iron. The oxidation process is completed through aeration, and the aeration device fully contacts the water with oxygen to produce natural oxidation; The aerated water undergoes the iron removal process through a iron and manganese removal filter. If most of the iron in the water is trivalent iron, it does not need to be aerated and directly enters the iron and manganese removal filter for removal.

9. Why do some water types need to be equipped with a carbon dioxide remover after passing through a cation exchanger?
The exchange of metal ions in water with H+ions on the cation resin results in the entry of H+ions into the water. Therefore, the effluent of the cation exchanger is slightly acidic, causing most of the HCO3- in the water to be converted into H2CO3 and further into CO2 gas.
Due to the low solubility of CO2 gas, firstly, it provides good conditions for degassing. Secondly, if degassing is not carried out, H2CO3 will exchange with anion exchange resin, increasing the burden on the anion exchanger and shortening the water production cycle of the anion exchanger.
Usually, the carbon dioxide remover is placed after the cation exchanger, before the anion exchanger, and also before some pre desalination systems such as reverse osmosis. However, in some places, a carbon dioxide remover is not needed, all of which depend on the water quality and type of the user.

10. What are the methods of anti-corrosion?
There are various anti-corrosion methods such as rubber lining, epoxy, plastic lining, and enamel.