A coal chemical enterprise uses coal as raw material to produce methanol from syngas in coal gasification process, and methanol is then made into olefins. A large amount of chemical production wastewater is produced in its production process. After advanced treatment in the plant, a large amount of high salt wastewater will be produced. The ecological environment around coal chemical enterprises is often fragile, and enterprises are often built in places with shortage of water resources. If this part of high salt wastewater is directly discharged, it will cause great pollution and damage to the surrounding environment of the enterprise. Therefore, reverse osmosis technology is considered to treat this part of wastewater and reuse most of its water resources. Due to the high conductivity of the inlet water, the two-stage reverse osmosis system is adopted in this study. While making the product water meet the water quality requirements of reuse water, the concentrated water can enter the subsequent evaporation crystallization system for further treatment, which not only meets the water requirements and environmental protection requirements of the enterprise, but also realizes considerable economic benefits.
1、 Inlet water quality and treatment requirements
The high salt wastewater of the project comes from the reverse osmosis concentrated water generated by the upstream reuse unit, with water volume of 100 m3 / h, influent pH of 7 ~ 9 and conductivity of 50000 ~ 65000 μ S / cm, hardness 1000 mg / L, Cl - 12000 mg / L, it can be seen that: (1) the conductivity is high and the fluctuation range is large, and the high concentration of chloride ion has strong corrosivity to metal equipment
(2) High hardness, easy to produce inorganic salt scale on the membrane surface, affecting the operation of the system. The conductivity of the final product water of the project is required to be ≤ 500 μ S / cm, concentrated water conductivity > 100000 μ S/cm。
2、 Process selection and system design according to the inlet water quality characteristics and treatment requirements, the project adopts two-stage reverse osmosis system for reduction treatment,
The hardness of high salt wastewater is high, and the scaling of inorganic salt has a great impact on the reverse osmosis system. If it is not removed, it will cause serious pollution and blockage of reverse osmosis membrane. Therefore, it is necessary to soften the high salt wastewater first. Lime soda ash softening method is selected to remove the hardness of the water. Lime and soda ash react with calcium and magnesium in the water to form calcium carbonate and magnesium hydroxide precipitation, and the generated sludge is regularly discharged out of the system. SDI ≤ 5 is generally required for reverse osmosis influent. After softening and clarification of high salt wastewater, there are still some suspended particles in the supernatant. Therefore, it is necessary to further treat the effluent from the clarifier to ensure the normal operation of reverse osmosis. In this project, mechanical filter is used to further treat the produced water from the clarifier. In order to ensure that the hardness of the water entering the reverse osmosis reaches the standard, the project is equipped with an ion exchange system to further remove the residual hardness of high salt wastewater. The inlet water temperature is adjusted and stabilized at 25 ~ 30 ℃ through the heat exchanger to meet the requirements of inlet water temperature of reverse osmosis system. Before entering the primary reverse osmosis unit, it is necessary to add reducing agent (sodium bisulfite) and scale inhibitor (phosphorus containing small molecules). The role of reducing agent is to control the residual chlorine in reverse osmosis inlet water below 0.1mg/l and ensure that the reverse osmosis membrane is not oxidized. The role of scale inhibitor is to ensure the normal operation of reverse osmosis system without scaling and long-term stable operation of reverse osmosis system. In order to meet the design water quality requirements, the primary reverse osmosis concentrated water is used as the system concentrated water to the evaporation crystallization system for further treatment. The primary reverse osmosis produced water needs to enter the secondary reverse osmosis device for further treatment, and the secondary reverse osmosis device concentrated water returns to the RO water supply tank.
3、 System operation
After pre-treatment, the high salt wastewater of the project enters the two-stage reverse osmosis system. The water temperature is 30 ℃, pH is 9 ~ 10, hardness is about 10mg / L, turbidity is about 1ntu, Fe and Al are not detected.
3.1 system desalination rate and produced water quality
During the two-stage reverse osmosis operation, the produced water quality of the system is good, and all produced water quality indexes meet the design requirements. The produced water is reused as the make-up water of circulating water in the plant. Among them, when the recovery rate of primary reverse osmosis unit is about 50%, the desalination rate is basically maintained at more than 97.5%, and when the recovery rate of secondary reverse osmosis unit is about 92%, the desalination rate is basically maintained at more than 82%,
The desalination rate of primary ro decreases slightly with the increase of operation time. On the one hand, it is caused by the fluctuation of inlet water quality and operation control, on the other hand, it is caused by the local damage of membrane elements. During the daily monitoring process, it is successively found that the conductivity of produced water of several membrane shells increases abnormally, which should be caused by the leakage of security filter.
The conductivity of produced water of two-stage reverse osmosis is basically maintained at 200 μ Below s / cm,
The overall desalination rate of the secondary reverse osmosis device decreases less than that of the primary reverse osmosis device, and the conductivity of the produced water of the system can be basically maintained at 200 μ Below s / cm, it shows that the two-stage reverse osmosis system used in the project to treat high salt wastewater can ensure the overall stability of the system water production. During operation, the conductivity of secondary reverse osmosis concentrated water is 10000 ~ 12000 μ S / cm, which is much lower than that of primary RO, and has little impact on system operation after reflux.
3.2 system concentrated water quality and pressure
During the two-stage reverse osmosis operation, the pressure of the primary reverse osmosis system is 6.8 ~ 7.2mpa, and the conductivity of the concentrated water of the system is relatively stable, basically maintained at 100000 μ Above s / cm, the produced water of the system meets the requirements of system treatment and realizes the requirements of concentration and reuse of high salt wastewater treatment system. The monitoring results are shown in Figure 4.
The overall pressure of the system is constantly changing, which is due to the increase of pollutants accumulated in the system with the increase of operation time. After chemical cleaning, the system pressure will decrease. In the actual process, the stability of pretreatment shall also be ensured to avoid membrane element pollution.
3.3 system cleaning
During normal operation, reverse osmosis membrane elements will be polluted by microorganisms, colloidal particles and insoluble organic substances. During operation, these pollutants are deposited on the membrane surface, resulting in the rise of the pressure difference between inlet water and concentrated water. When the rise exceeds the design value, the membrane elements need to be cleaned. During cleaning, it is necessary to select appropriate cleaning agents for the above pollutants. In this project, acid cleaning agents and alkaline cleaning agents are used to clean the reverse osmosis membrane elements, and the reverse osmosis device is circulated and soaked off-line according to the actual pollution, and the cleaning solution is observed at any time until the cleaning requirements are met. Firstly, replace the raw water in the system with the cleaning solution with a lower flow rate. After completion, the constant cleaning solution temperature is 30 ℃, circulate in the system with a lower flow rate, circulate for 1h, and soak for 1H. Secondly, the cleaning solution with higher flow rate is circulated in the system for 1h to flush the cleaned pollutants. Finally, rinse the cleaning solution in the system with qualified product water for standby. Among them, the acidic cleaning agent is hydrochloric acid with mass fraction of 0.2%, and the alkaline cleaning agent is NaOH with mass fraction of 0.1% and sodium silicate with mass fraction of 0.02%. After chemical cleaning, rinse with RO produced water until the cleaning solution is completely washed out.
3.4 operation energy consumption
Calculate the power consumption and reagent consumption of the two-stage reverse osmosis system. The power consumption is the main part of the operation energy consumption. The reagent consumption is mainly reductant and scale inhibitor, which are calculated according to 3mg / L. when the reductant is calculated according to 1 yuan / kg, the scale inhibitor is calculated according to 26 yuan / kg and the electricity charge is calculated according to 0.5 yuan / (kW / h), the system operation cost is about 1.8 yuan / T.
4、 Conclusion
Through the successful application of the two-stage reverse osmosis system in the treatment of high salt wastewater in coal chemical industry, it can be seen that the two-stage reverse osmosis system can not only make the water production of the system meet the design requirements, but also ensure the stability of the water quality of the high salt water treatment system, which is more conducive to the recycling of the system water and save more water resources for enterprises. As a concentration and reuse unit in high salt wastewater treatment, the stable concentrated water index of the two-stage reverse osmosis system plays an important role in the water treatment system of the whole plant. It is an important link to realize the wastewater treatment requirements of the whole plant and even reach the zero discharge requirements, and has contributed to the environmental protection needs of the enterprise.