How to treat high-salt wastewater

With China's rapid economic development and the increasing discharge of high-salt wastewater, the difficulty of treating high-salt wastewater has increased. How to choose a high-salt wastewater treatment process? What are the advantages and disadvantages of high-salt wastewater treatment processes? How should these high-salt wastewaters be treated?

High-salt wastewater refers to wastewater containing organic matter and at least 3.5% (mass concentration) of total dissolved solids (TDS). This wastewater comes from a wide range of sources. One type is from various industrial production processes such as chemical, pharmaceutical, petroleum, papermaking, dairy processing, and food canning, which discharge a large amount of wastewater. The water contains not only high concentrations of organic pollutants but also a large amount of ions such as calcium, sodium, chlorine, and sulfate. Another type is that in order to make full use of water resources, some coastal cities directly use seawater as industrial production water or cooling water.

The treatment of high-salt wastewater is usually a "pretreatment-evaporation concentration crystallization desalination" process. According to the specific water volume, water quality, effluent requirements, investment, operating costs, and technical concepts, different pretreatment processes, technical equipment, and evaporation concentration crystallization desalination processes are selected in different situations. The following processes are summarized:

1. Chemical coagulation-flotation, sedimentation traditional pretreatment process

When the COD concentration of saline raw water is below 5000mg/L, and there is no requirement for the quality of crystalline salt, the traditional process is to pretreat the saline raw water through "adjustment-chemical coagulation-flotation, sedimentation", and then enter the "evaporation concentration crystallization desalination system". This method has low investment and operating costs, but the quality of the crystalline salt is poor and difficult to sell.

2. Fenton or electro-Fenton catalytic oxidation pretreatment process

Fenton's reagent contains H2O2 and Fe2+, which has a strong oxidation capacity for organic pollutants in wastewater, and the reaction speed is fast, the investment is low, and the effluent can be pretreated after sedimentation and purification.

However, the Fenton or electro-Fenton catalytic oxidation process requires specific reaction conditions: pH value 2-4, and produces a lot of iron-containing sludge, and the effluent will have color. It is more economical to use when the pH value of the saline raw water is low; otherwise, the process of "acid addition to reduce pH, alkali neutralization" will increase the operating cost. COD concentration around 10000 mg/L is better; if it is too high, multi-stage oxidation purification treatment is required, and the Fenton process has no advantage.

3. Double membrane pretreatment process

First, ultrafiltration is used with a pore size of 20-2000Ao (10-6.5-10-4.5cm) semi-permeable membrane, which can retain proteins, various enzymes, bacteria, colloids, and macromolecules in the concentrate, while water, solvents, small molecules, and ions forming salts can pass through the membrane and enter the permeate.

Because the amount of permeate water is reduced, but the amount of salt remains unchanged, the salt concentration in the permeate water increases. At this time, reverse osmosis is used with a pore size of 1-20Ao (10-7.5-10-6.5cm) semi-permeable membrane. Inorganic salts, sugars, amino acids, BOD, COD, etc. are retained in the concentrate, and only water and solvents enter the permeate, and the salt concentration in the concentrate further increases, and is sent to evaporation crystallization desalination.

The advantage of double membrane desalination is that it greatly reduces the amount of water for evaporation crystallization desalination, thus significantly reducing the operating cost and investment of evaporation crystallization desalination. However, the following issues should be noted:

Before ultrafiltration, pH should be adjusted to neutral, hardness removal, SS purification, etc.;

The salinity of the raw water is below 5000mg/L, otherwise the permeate water will be too low, and the desalination rate will also decrease;

The investment will be very high when the amount of saline raw water is large;

Because the membrane needs to be frequently washed, acid washed, and alkali washed for protection, the service life of the membrane is also limited, and the operating cost is also relatively high;

The biggest problem is what to do with the higher-polluted concentrate that is retained?! If valuable substances can be extracted or a large amount of biodegradable wastewater can be diluted and treated together, it is fine; otherwise, reuse will increase pollution accumulation; if incineration is used, the investment and operating costs are extremely high;

Wastewater with a salt content exceeding 5000mg/L can be directly evaporated and crystallized for desalination. There is no point in using membrane methods again, but it should be reminded that effective pretreatment should still be carried out before evaporation and crystallization desalination.

4. Ozone/catalytic/coagulation composite pretreatment process

Using ozone as a strong oxidant and combining catalysts and coagulants, sufficient cross-linking synergistic reactions can be carried out in a specific environment, which can break the ring chains and long chains in wastewater and improve the biodegradability of wastewater.

Creating suitable reaction conditions can also fully oxidize dissolved organic pollutants in wastewater, destroy colloids, chromophores, and odor groups in wastewater, remove COD, BOD, SS, odor, and some colors from wastewater, but cannot remove salt and a lot of ammonia nitrogen.

Because ozone is used as a strong oxidant and combined with catalysts and coagulants with oxidizing 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 the Fenton process. Multi-stage series operation can be used to ensure that the effluent meets the expected indicators.

How to choose the pretreatment process for saline wastewater:

Wastewater with a large amount of water and a salt content below 5000mg/L can first choose the double membrane method, and then desalinate after concentration;

Saline raw water with a pH value of 2-4 can first choose Fenton process pretreatment;

High-concentration COD with a pH value above 5 and a salt content greater than 5000mg/L can choose ozone/catalytic/coagulation composite pretreatment process;

If the raw water has high color or ammonia nitrogen, it must be decolorized and deammoniated separately;

Or a combination of several methods can be used for pretreatment.

5. Evaporation crystallization desalination process

For salt-containing solutions, due to their different solubilities, there are two schemes for crystallization from the solution. First, for systems whose solubility does not change much with temperature, the solvent is generally evaporated. Second, for systems whose solubility changes significantly with temperature, the solution is generally cooled.

Salt-containing wastewater is generally a mixture of various salts. Due to the common ion effect, its solubility curve and boiling point are different from those of a single system. Generally, its saturated solubility is lower than that of a single system, and the boiling point is higher than that of a single system at the same concentration. Therefore, to accurately grasp the solubility and boiling point of multi-component salts, experiments must be conducted, which is the key to the design of evaporative desalination.

The design of the end point of evaporation desalination concentration mainly depends on the matching of the subsequent separation equipment. If a horizontal spiral discharge centrifuge is used, the solid content of the solution from the evaporator should be around 10%. If a two-stage piston pusher centrifuge is used, the solid content of the solution from the evaporator is around 50%.
The design of the evaporation crystallizer is the key to the normal operation of the evaporation desalination device. The following factors should be considered during the design: crystal nucleus generation, supersaturation control, elimination of short-circuit temperature difference, immediate separation of large-particle salts, forced circulation method and flow rate, gas-liquid separation intensity, etc.