High salt removal rate circulating water sewage treatment

With the development of industrial production, water consumption is getting larger and larger, and the phenomenon of the insufficient water supply has appeared in many areas. Therefore, reasonable and saving water has become an important issue in the development of industrial production. Therefore, enterprises are generally required to treat and reuse part of their sewage by themselves, and the general reuse rate is required to be above 70%. 

Circulating water sewage is a kind of industrial wastewater. Because wastewater contains a variety of toxic substances, the pollution of the environment is very harmful to human health. It can only be discharged after taking corresponding purification measures for disposal. At present, the mainstream circulating water sewage (that is, the company’s clean sewage) has the following two treatment processes:

The first is to send the product water obtained after the circulating water sewage is treated by the double-alkali method to remove hardness, ultrafiltration, reverse osmosis (RO), and other processes to the circulating water system as supplementary water. The other is to send the product water obtained after the circulating water sewage is treated by the double-alkali method to remove hardness, ultrafiltration, nanofiltration (NF), and other processes to the circulating water system as supplementary water.

The circulating water sewage is subjected to dehardening filtration treatment, ozone oxidation treatment, denitrification treatment, ultrafiltration treatment and reverse osmosis treatment to obtain reverse osmosis product water, and the reverse osmosis product water is used for water replenishment of the circulating system.

The method realizes the recovery of water resources on the basis of effectively treating the circulating water sewage, and solves the problem that the circulating water sewage has a high content of hardness, suspended solids, COD, and salts and is difficult to treat. However, the water quality obtained by this scheme is too good, and it is a waste of resources to supplement water as circulating water.

After flocculation, sedimentation, and filtration of industrial circulating water sewage, part of it is directly reused as the replenishment water of the circulating water system, and the other part is reused after being treated by the nanofiltration system. The two are mixed in a certain proportion and added to the circulating water. , to achieve the purpose of reusing industrial circulating water and reducing the amount of fresh water in the system and the amount of sewage discharged.

In order for the water body to meet the influent requirements of the nanofiltration membrane, this part of the sewage needs to be pretreated first.

For example, before entering the nanofiltration membrane, the sewage described in the examples has also undergone the treatment of flocculation, filtration, and hollow fiber ultrafiltration. It can be seen that in the implementation of this method, in order to prevent the membrane from fouling and prolong the life of the membrane, the required pre-treatment Processing investment is high.

As for the nanofiltration membrane, there are also the defects that a large amount of acid and scale inhibitor need to be consumed during the operation process, and the operating pressure is too high, which leads to the increase of the operating cost, and the ion content of the effluent is far less than that of the circulating water treated by the nanofiltration membrane. The ion content that needs to be retained in the system not only cannot meet the conductivity requirements of circulating water but also causes a waste of resources to a certain extent.

At the same time, the use of nanofiltration membrane to treat the circulating water sewage can not reduce the chloride ion content in the circulating water system, and the accumulation of chloride ion content will lead to corrosion of circulating water pipes and equipment.

• Pretreatment of circulating water and sewage, that is, introducing circulating water and sewage (which may include clean rainwater in the plant, boiler sewage and other clean drainage) into the circulating water sewage pool for unified collection, and then pumping the sewage (or clean sewage) ) is sent to a multi-media filter for filtration and precipitation. Add coagulant PAC (addition amount is 100-1500ppm) and coagulant anion PAM (addition amount is 0.1-5ppm) at the front end of the integrated water purifier.The rear end of the integrated water purifier is filtered through the quartz sand filter layer to remove the suspended solids and part of the COD. The suspended solids in the outlet water of the integrated water purifier can reach less than 5mg/l, the turbidity is less than 3NTU, and the COD is less than 20mg /l, when the ammonia nitrogen is less than 3mg/l, the effluent enters the intermediate pool. 
• The purified water in the intermediate pool is pumped to the precision filter before the ultrafiltration device. After being treated by the precision filter, the suspended matter in the outlet water can reach less than 2mg/l, the turbidity is less than 1NTU, and the COD is less than 10mg/l.
• The effluent from the precision filter enters the ultrafiltration device. The ultrafiltration inlet water is pressurized to 0.2-0.3MPa by the ultrafiltration inlet water pump and then enters the ultrafiltration device. The ultrafiltration inlet water contains total hardness less than 500mg/l and calcium hardness less than 250mg /l (if it exceeds the above value, it is necessary to add a pre-hardness removal device such as a high-density sedimentation tank or a double-alkali method for hardness removal to ensure that the water production rate of the subsequent device is guaranteed), after ultrafiltration treatment, the effluent contains no suspended solids. Detected, SDI is less than 2.
• A small amount of ultrafiltration drainage is sent to the front end of the biochemical industry or directly into the circulating water as supplementary water.
• If the total hardness of ultrafiltration water is less than 200mg/l, it can go to the precision filter before the reverse osmosis device for further filtration, the filtered effluent enters the reverse osmosis device for treatment, and the reverse osmosis concentrated water is sent to the concentrated water reverse osmosis device for treatment. The water yield of the two-step reverse osmosis reaches 70-95%, the conductivity is 20-100us/cm, and the water is produced to the subsequent desalination unit to produce qualified desalinated water.
• If the total hardness of the filtered water is 200-500mg/l, it is necessary to add resin to remove the hardness, so that the total hardness of the permeate is less than 200mg/l before entering the RO and concentrated water RO device, otherwise the RO water production rate cannot be guaranteed.
• The concentrated water produced by the concentrated water reverse osmosis device is sent to the advanced sewage treatment device for further treatment or evaporation and crystallization to extract salt.
• Generally, petrochemical or coking enterprises have both circulating cooling water system and demineralized water system. The invention can deeply couple the circulating water sewage treatment system and the demineralized water system. The reverse osmosis device in the demineralized water station can remove most of the salt ions. Through the process treatment of the demineralized water station, the amount of ions removed can be reduced, and only a small amount of ions can be removed to provide high-quality supplementary water for the demineralized water station. This method not only saves money The operating cost can also save investment, such as the investment and daily operation of the multimedia and ultrafiltration devices at the front end of the general desalination station can be directly saved. 
• In addition, when the hardness of the ultrafiltration+RO+concentrated water RO device in the present invention is not high, the pre-hardness removing device may not be selected, which saves investment and operating costs.
• In order to ensure the production capacity of desalinated water and realize the true coupling of the desalted water device and the circulating water drainage device, the present invention can utilize the rainwater, boiler sewage and other clean sewage generated in the rainwater drainage system of the plant area.