Deep concentration technology of high salt wastewater

Deep concentration technology of high salt wastewater
As a large industrial water user, thermal power plants are faced with great challenges in water pollution control. Desulfurization wastewater, acid-base recycling wastewater and concentrated drainage of circulating water are high salt wastewater and cannot be reused in other systems. Therefore, high salt wastewater has become the focus and difficulty of water pollution prevention and control. In view of the large amount of water in high-salt wastewater, water consumption can not be achieved in one step, priority is given to the deep concentration of high-salt wastewater before the implementation of zero discharge. Deep concentration reduction refers to the use of technical means to separate part of the water in the high-salt wastewater, leaving the end wastewater with a higher salt content, and the end wastewater can not be further concentrated and can be directly considered for curing treatment. The purpose of the deep concentration reduction is to reduce the amount of wastewater entering the subsequent curing treatment to reduce the total investment and operating costs of the wastewater treatment system. At present, some domestic engineering cases use different depth concentration processes.

In this paper, the technical and economic comparison of DTRO, ED, multi-effect flash and low-temperature flue gas concentration process is made, which provides a reference for the same type of wastewater renovation projects.
1. Introduction of deep concentration technology

1.1 Disc Tube Reverse Osmosis Membrane (DTRO)

Dish tube reverse osmosis (DTRO) is a membrane component specially used to treat high-concentration and high-salt wastewater. Practical engineering cases have been used for decades in the treatment of high-salt wastewater landfill leachate. Its core technology is DTRO membrane column, which is stacked with a hydraulic guide disk, fixed with a central pull rod and an end plate, and then placed in a pressure casing. It forms a membrane column. DTRO's unique open flow channel is completely different from the traditional coiled membrane assembly structure. During operation, raw water reaches the upper flange of the membrane column through the channel between the lower flange and the sleeve at the bottom of the membrane column, and enters the deflector from the upper flange. The raw water flows from one side of the diaphragm installed between the deflector plates to the other side at a very high speed, and then flows out from the grass in the center of the deflector plate below. Entering the next diaphragm, a double "S" shaped route is formed from the profile, and the final effluent at the end of the membrane column is the concentrated liquid. DTRO is widely used in landfill leachate treatment, and there are many application cases in zero discharge of thermal power plant wastewater, and its operation is more reliable.

1.2 Electrodialysis (ED)

Electrodialysis (ED) is a combination of electrolytic dialysis diffusion processes, which is also one of the membrane separation technologies. Using the selective permeability of ion exchange membrane, that is, the positive membrane theoretically only allows cations to pass through, and the negative membrane theoretically only allows anions to pass through. Under the action of external direct current electric field, the negative and cation move towards the anode and cathode respectively. If the fixed charge of the membrane is opposite to the charge of the ion, the ion can pass through. Thus the purpose of concentration and separation of the solution can be realized. The purpose of desalting is achieved in the fresh water chamber, the concentrated water is obtained in the concentrated water chamber, and the reverse ion migration is the main process of electrodialysis desalting. Electrodialysis technology can concentrate the solution to the mass fraction of TDS 18%-22%, and can run stably for a long time.

1.3 Multi-effect flash evaporation

The main working principle of multi-effect evaporation is to realize multiple utilization of steam heat energy by using the secondary steam generated by the former evaporator as the heat source of the later evaporator, so as to improve the utilization rate of heat energy. In thermal power plants, power steam can be derived from auxiliary steam. The power steam heats and condenses the single-effect evaporator for reuse. The waste water is evaporated and concentrated by the one-effect evaporator and forms the secondary steam as the heat source of the two-effect evaporator. The concentrated wastewater is further concentrated in the influent two-effect evaporator, and the secondary steam is used as the heat source of the three-effect evaporator. And so on, the concentrated wastewater goes into the next effect evaporator to continue to concentrate. Finally, the steam from the final effect evaporator finally enters the condenser and condenses into water, that is, the distilled water after desalting, and the water quality is very good, which can be used as cooling tower or even boiler recharge water; The final concentrated brine can be continued into the subsequent concentration system or directly through the thickener and centrifuge for solid-liquid separation. According to the flow direction of secondary steam and feed liquid, it is divided into parallel flow, advection flow, countercurrent and cross-flow operation, in practical application, according to the production requirements and different physical and chemical properties of various materials to choose different processes.

1.4 Low temperature flue gas evaporation

Low-temperature flue gas evaporation and concentration treatment is to draw part of the low-temperature flue gas after the induced draft fan into the evaporation concentrator as a heat source. The high salt waste water is diverted to the evaporating concentrator and is vaporized and concentrated by direct contact with the flue gas for heat exchange. The saturated wet flue gas is treated by the defogger in the evaporating concentrator and then enters the main flue. After low temperature flue gas evaporation treatment, high-salt wastewater is formed into condensate water and concentrated liquid. The condensate water can be reused as desulfurization process water, and the concentrated liquid enters the terminal wastewater treatment process unit. In low temperature flue gas evaporation and concentration using large flow circulation evaporation, the working principle is the same as desulfurization absorption tower, so that the wastewater concentration of 5~10 times.

2, a variety of deep concentration technology comparison

The effect of deep concentration and reduction directly affects the curing treatment of end wastewater and is the key unit to achieve zero discharge, so it is necessary to select a suitable deep reduction and reduction program. For a domestic power plant depth optimization of water use and water pollution prevention and control transformation project, the installed capacity of the whole plant is 4x1000MW, and the remaining high salt wastewater that cannot be reused after deep optimization of the whole plant is 66m3/h, mainly desulfurization wastewater, a small amount of acid and alkali recycling wastewater, and the TDS(salt content) after mixing is 42000mg/L. The comparison of various deep concentration technologies with chloride ion concentration of 8000mg/L is shown in Table 2. See FIG. 1 to FIG. 4 for each process flow chart.

2.1 Raw water quality conditions

DTRO process and ED process are both membrane concentration processes, which use membrane technology for salt removal and salt separation, so their application is not limited by the amount of wastewater. However, in order to ensure the stable operation of the membrane treatment system, necessary pretreatment (including turbidity removal, hard removal, etc.) should be carried out before the membrane concentration reduction. The multi-effect flash process has low requirements for influent water turbidity and hardness, and can not be pre-treated, but from the perspective of long-term stable operation, the solid content is controlled to be less than 1%. The operation principle of the low-temperature flue gas evaporation process is similar to that of the desulfurization absorption tower. After atomizing through the nozzle, it fully contacts with the low-temperature flue gas to ensure that the nozzle is not blocked and the suspended matter is controlled below 500mg/L.

2.2 Floor Space

Compared with the DTRO process, ED process and multi-effect flash process, the low-temperature flue gas evaporation process does not need to support the new concentration workshop, a single set of low-temperature flue gas evaporation process covers a small area, directly arranged around the desulfurization absorption tower, and the control system can be directly incorporated into the desulfurization system, which covers the lowest space. DTRO process and ED process concentration workshop in addition to the new softening system, supporting the new membrane system washing, backwashing, dosing and other facilities, occupies the largest area, need to consider the overall layout of the site. The multi-effect flash process requires no softening system and occupies a relatively moderate space.

2.3 Comparison of applicability

DTRO process and ED process system have the advantages of high technical maturity, wide application range, and relatively independent system, but the system is more complex and the investment cost of high-salt wastewater treatment is relatively high, and it is suitable for zero-discharge concentration and reduction treatment of large-volume end wastewater. However, the inlet water quality needs to be softened, and the overall operating cost is the highest. The multi-effect flash process system is relatively independent, has a high degree of automation, and does not need pretreatment process. It has certain application performance in thermal power plants, but its performance and anti-fouling and blockage need further verification. The low-temperature flue gas evaporation process uses the residual heat from the tail of the power plant, has less energy efficiency, and the evaporation concentration reduction is greatly affected by the load of the unit, so it is suitable for the end waste water concentration reduction project with less water.

3. Analysis of economic indicators of various deep concentration processes

Economic index analysis of various deep concentration processes of high salt wastewater from 4x1000MW unit of a power plant in China. As can be seen from Table 3, for the deep concentration stage of high-salt wastewater, the investment of new pretreatment softening units supporting DTRO process and ED process accounted for nearly 40%. Among the operating costs, the main cost came from the operation cost of the pretreatment softening unit, and the operating cost was the highest. Therefore, raw water pretreatment softening restricted the development of membrane concentration process.

The main investment cost of multi-effect flash process is concentrated in the flash equipment, and the operating cost is mainly the steam consumption cost. Low-temperature flue gas evaporation and concentration process has the lowest operating cost, the use of desulphurization inlet flue gas waste heat, and the addition of low-temperature flue gas evaporation tower into the desulphurization system can avoid the addition of workshop operators and reduce operating costs.

4. Conclusion

According to the technical and economic comparison of four kinds of deep concentration technology of high salt wastewater, the four technology schemes can meet the requirements of deep concentration and realize the minimum amount of end wastewater. But different process routes have advantages and disadvantages.

(1) The application performance of DTRO process is the most, the process is mature, and the effluent water quality is good. However, due to the problems such as small effluent water, high operating pressure and salt content of the water quality, the single membrane needs to be equipped with NF+SWRO for primary pre-concentration. Raw water needs to be pre-treated and softened, and the operation and maintenance cost is high.

(2) The ED process is simple, the inlet water is not limited by the salt content, the concentration rate is high, but the effluent water quality is poor, and a new level of reverse osmosis can be added to further improve the water quality. Similarly, raw water needs to be pre-treated and softened, and the operation and maintenance cost is high.

(3) The multi-effect flash process system is independent, the water quality is good, no pretreatment is required, but the steam consumption is high, the relevant application performance is less, the time is short, and the long-term operation reliability still needs to be further confirmed.

(4) Low-temperature flue gas evaporation process, using desulphurization to import low-temperature flue gas, has the lowest energy efficiency and the lowest renovation investment cost, which shows obvious advantages in the technical and economic comparison of this project. However, the application is few and the time is short, and the long-term operation reliability still needs to be further confirmed.

(5) The selection of the deep concentration process of high-salt wastewater is crucial to the realization of zero discharge of wastewater, and new technologies are emerging in an endless stream. In actual projects, the appropriate transformation process is selected according to different external conditions, wastewater quality conditions and other factors for comprehensive comparison, so as to achieve the best concentration effect.