Desalination of sea water
Because of its high salt content, sea water can not be used directly. At present, two methods are used to desalt sea water, namely, distillation and reverse osmosis desalination equipment
Distillation is mainly used in large seawater desalination and energy rich areas. Reverse osmosis membrane is widely used and has a high desalination rate, so it is widely used. The reverse osmosis membrane is the first water extraction, preliminary treatment, reduce water turbidity, prevent bacteria, algae and other microorganisms, and then use the special high-pressure booster pump, the sea water into the reverse osmosis membrane, the water salinity is high, so the seawater reverse osmosis membrane must have high desalination rate, corrosion resistance, high pressure resistance, anti pollution and so on, through the reverse osmosis membrane treated seawater, the salt content is greatly reduced, the content of TDS from 36000 mg / 1 to 200 mg / L. The water quality after desalination is even better than that of tap water, so that it can be used by industry, commerce, residents and ships and ships.
1. Seawater Pretreatment
The pretreatment of water supply is the key to ensure the long term stable operation of RO system, whether it is desalination or brackish water desalination. In the preparation of Seawater Pretreatment plan, we should take full account of: there are a lot of microorganisms, bacteria and algae in the sea water. The propagation of bacteria, algae and the growth of microorganisms in the sea water will not only bring a lot of trouble to the water intake facilities, but also directly affect the normal operation of desalination equipment and process pipelines. Periodic high tide and low tide, large amount of sediment in the sea water, turbidity change greatly, and it is easy to cause the sea water pretreatment system to be unstable. The sea water is more corrosive, and the equipment, valves and pipe material used in the system must be screened, and the corrosion resistance is better.
2, seawater sterilization algae
In foreign countries, chemical reagents such as liquid chlorine, NaClO and CuSO4 are used to kill and kill algae. Considering the traffic and other factors, it is difficult to add chemical reagent to kill algae. In this project, the sodium hypochlorite generator is specially used in the process of equipment development. The sea water pump after the separation of a small pressurized water, into the sodium hypochlorite generator to generate NaClO in the DC electric field, face directly into the beach wells, to kill the bacteria in seawater, algae and microorganisms.
Because of the high hardness of sea water, the direct electrolysis of NaClO results in the problem of electrode fouling. In the process of development, the frequent reverse polarity (EDR) technique of electrodialysis was used for reference, that is, the polarity of electrode was changed every 5 ~ 10min, and the precipitation and precipitation of sodium hypochlorite generator were solved effectively.
3 coagulation filtration
Coagulation filtration is designed to remove colloids, suspended impurities, and turbidity in sea water. In reverse osmosis membrane separation engineering, the pollution index (FI) is used to measure the FI value of the feed water of reverse osmosis equipment <4. Because of the large proportion of sea water, high pH value and seasonal variation of water temperature, FeCl3 is used as coagulant in the system. It has the advantages of no temperature, big and strong alum, fast settling speed and so on.
The project adopts surface contact coagulation filtration technology, which consists of double layer filtration and activated carbon filtration. 2 double layer mechanical filters and 2 activated carbon filters are installed respectively, and the filtration rate is between 7 and 8m/s. The filter adopts steel coated with glue, marine paint, internal ABS water cap water distribution and 316L stainless steel tube arrangement gas. In order to reduce the Seawater Pretreatment System and the instantaneous load, and to improve the water recovery rate, a reverse flow infiltration system is adopted to filter the backwash water.
3. Reverse osmosis desalination
High salinity, high hardness, corrosion of equipment, water temperature and seasonal variation is called reverse osmosis seawater desalination system than the conventional desalination system is much more complex, project investment and energy consumption are much higher. Therefore, it is very important to reduce the cost of unit water production and ensure the stable operation of the system through careful process design and reasonable equipment allocation to reduce project investment and energy consumption.
4 、 chemical regulation treatment
In order to prevent the desalination process due to seawater concentration and formation of insoluble inorganic salts, such as CaCO3, CaSO4, in the reverse osmosis membrane surface system and pieces of pipe fouling in reverse osmosis seawater into the sediment, to add anti fouling agent before desalting system.
It is the most common and economical method for seawater desalination to add H2SO4 to regulate seawater pH and to decompose HCO-3 in sea water to prevent CaCO3 precipitation. Adding (NaPO3) 6 (SHMP) is an effective way to prevent the precipitation of CaSO4 (NaPO3), but in 6 scale while producing by-product of phosphate will encourage microbes, bacteria and algae growth, use has some limitations. The high price of special polymer scale inhibitor imported from western countries will directly affect the operation cost of desalination project. In this project, H2SO4 is chosen as the scale inhibitor, and the pH value of the feed water of the reverse osmosis system is controlled between 6.8 and 7. At the same time, the water recovery rate of the desalination system is controlled so as to prevent the precipitation of CaSO4.
Considering the reverse osmosis desalination system using composite membrane element with aromatic polyamide membrane material for its poor oxidation resistance of residual chlorine in water requirements was less than 0.1mg/L in reducing agent, so in seawater into the membrane system before the addition of NaHSO3, the control of sea water reverse osmosis device before the oxidation reduction potential (ORP), the in the seawater reverse osmosis device before the oxidation reduction potential (ORP) is 3 times the amount of water in the chlorine dosage in 280 ~ 320mV.NaHSO3.
5 、 chemical regulation treatment
In order to prevent the desalination of seawater during the desalination process, insoluble inorganic salts such as CaCO3 and CaSO are produced
