The treatment of high salt wastewater is usually a “pre-treatment evaporation concentration crystallization desalination” process.
According to the specific water quantity, water quality, effluent requirements, investment, operation cost and technical concept, different pretreatment process, technical equipment and evaporation concentration crystallization desalination process are selected under different conditions.
1. Traditional pretreatment process of coagulating air flotation and sedimentation.
When the COD concentration of salt bearing raw water is below 5000mg / L, and there is no requirement for the quality of crystal salt, the traditional process is that the salt bearing raw water is pretreated by “regulation, dosing, coagulation, air flotation and sedimentation”, and then enters the “evaporation, concentration, crystallization and desalination system”.
The method has the advantages of low investment, low operation cost and poor crystal salt quality.
2. Pretreatment process of Fenton or electro Fenton Catalytic Oxidation.
enton reagent contains H2O2 and Fe2 +, which has a strong oxidation capacity for organic pollutants in wastewater, and the reaction speed is fast and the investment is low. After sedimentation and purification, the effluent can achieve the purpose of pretreatment.
However, Fenton or electro Fenton Catalytic oxidation process requires specific reaction conditions: pH value is 2-4, and more iron sludge is produced, and the effluent will have color.
When the pH value of salt containing raw water is low, it is more economical to use, otherwise the process of “adding acid to reduce pH, adding alkali to neutralize” will increase the operation cost.
If the COD concentration is about 10000 mg / L, it is still good. If it is too high, it will be treated by multi-stage oxidation purification, and Fenton process will have no advantage.
3. double membrane pretreatment process.
First, a semipermeable membrane with a pore size of 20-2000ao (10-6.5-10-4.5cm) is used for ultrafiltration, which can intercept proteins, various enzymes, bacteria and other colloidal substances and macromolecular substances in the concentrate, while water, solvents, small molecules and salt forming ions can enter the permeated water through the membrane.
Because of the decrease of the water content and the unchanged of the salt content, the salt concentration in the permeated water increases.
At this time, a semi permeable membrane with a pore diameter of 1-20ao (10-7.5-10-6.5cm) is used for reverse osmosis.
Inorganic salts, sugars, amino acids, BOD, COD, etc. are retained in the concentrated solution. Only water and solvent enter the permeated water, and the concentration of salt in the concentrated solution is further increased, which is sent to the evaporation crystallization desalination.
The advantage of double membrane desalination is that it greatly reduces the amount of water for desalination by evaporative crystallization, thus significantly reducing the operating cost and investment of evaporative crystallization desalination.
But pay attention to the following problems:
- Before ultrafiltration, adjust pH to neutral, remove hardness, remove SS purification, etc.
- The salt content of raw water is less than 5000mg / L, otherwise the water penetration is too low and the desalination rate is also reduced.
- When the amount of salt bearing raw water is large, the investment will be high.
- Due to the frequent water washing, acid pickling and alkali washing protection of the membrane, the service life of the membrane is also limited, and the operation cost is relatively high.
Therefore, a johnson wedge wire screen filtration system should be installed for pretreatment to filter out large particles of impurities.
For the wastewater with salt content more than 5000mg / L, it can be directly evaporated and crystallized for desalting. It is not meaningful to use membrane method again, but it should be reminded that: Effective pretreatment should be carried out before desalting by evaporation crystallization.
4. Ozone / Catalysis / coagulation composite pretreatment process
Taking ozone as strong oxidant, combined with catalyst and coagulant, and carrying out sufficient cross-linking and synergistic reaction in a specific environment, can break the ring chain and long chain in the wastewater and improve the biodegradability of the wastewater.
Creating appropriate reaction conditions can also fully oxidize the dissolved organic pollutants in the waste water, destroy the colloid, chromophore and odorant in the waste water, remove the COD, BOD, SS, peculiar smell and some colors in the waste water, but can not remove the salt and more ammonia nitrogen.
Due to the use of ozone as a strong oxidant and catalyst and coagulant with complex oxidation properties, the amount of sludge produced in the whole process of removing organic pollutants is very small, and the reaction environment, form and process are much simpler than Fenton process, which can be operated in series at multiple levels to ensure that the effluent reaches the expected index.
How to select the pretreatment process of salty wastewater:
- For wastewater with large water volume and salt content less than 5000mg / L, double membrane method is preferred, and desalination is conducted after concentration.
- Fenton process is the first choice for pretreatment of raw salt water with pH value of 2 ~ 4.
- For high concentration cod with pH above 5 and salt content more than 5000mg / L, the combined pretreatment process of ozone / Catalysis / coagulation can be selected.
- If the color of salty raw water is high or ammonia nitrogen is high, decolorization and deamination must be carried out separately.
5. Desalting process by evaporation crystallization
For salt solution, there are two ways to crystallize and precipitate from the solution due to its different solubility. The first is to evaporate the solvent for the system with small solubility with temperature, and the second is to cool the solution for the system with large solubility with temperature.
Salt containing wastewater is generally a mixture of various salts. Due to the existence of the same ion effect, its solubility curve and boiling point of solution are different from that of a single system. Generally, its saturated solubility is lower than that of a single system, and its boiling point is higher than that of a single system at the same concentration.
Therefore, the solubility and boiling point of multicomponent salt must be obtained through experiments, which is the key to the design of evaporation desalination.
For the design of the end point of evaporation, desalting and concentration, it mainly depends on the matching of the subsequent separation equipment. The horizontal screw discharge centrifuge is selected, and the solid content of the solution out of the evaporator is about 10%. The two-stage piston pusher centrifuge is selected, and the solid content of the solution out of the evaporator is about 50%.
[Sieve drum for centrifuge: Centrifuge Screen Basket]
The design of the evaporation crystallizer is the key to the normal operation of the evaporation desalination device. The following factors shall be considered in the design: The formation of crystal nucleus, the control of supersaturation, the elimination of short-circuit temperature difference, the instant separation of large particle salt, the way and flow rate of forced circulation, the intensity of gas-liquid separation, etc.