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Pretreatment of sea water desalination

HomeApplicationPretreatment of sea water desalination

1. Technique purpose

The aim of Seawater Pretreatment is to remove the impurities such as mud sand, colloid and microorganism in the surface sea water, and the Seawater Pretreatment ensures the long-term stable operation of the reverse osmosis system.


When the Seawater Pretreatment process is designed, the quality of seawater after pretreatment should meet the requirements of influent quality of reverse osmosis membrane components.


2. Treatment process

(1) coagulant filtration or shallow air floatation

Coagulation filtration is designed to remove colloid and suspended impurities in seawater and reduce turbidity. Because of the great sea water ratio and high pH value, FeCl3 is chosen as coagulant and the dosage is 1 ~ 2mg/L. After mixing, the iron salt and the colloid impurities in the sea water form larger alum, and then filtered by mechanical filter, the pollution index (SDI15) of the water quality is less than 5, and the turbidity is less than 1.
The first phase of the project was equipped with 9 mechanical filters, a single mechanical filter design diameter of 3200mm, lined with 5mm natural rubber, coated with anticorrosive ship paint, and a water cap on the bottom, filled with multi-media filter material according to the grading, and a single filter designed for 67m3/h.
In order to save water consumption, reverse osmosis concentrated water is used as machine filter backwash water. The system has concentrated sump, backwash pump and roots blower. The air scrubbing speed is 16L/m2.s and the water backwash flow is 15L/m2.s.
Shallow air floatation is also widely used in seawater desalination, such as Caofeidian will desalination pretreatment and shallow air floatation, but both shallow and ordinary air floatation, dissolved gas system is the core. At present, there are only a few or two in China.
(2) add medicine to eliminate residual chlorine and prevent fouling and sedimentation on reverse osmosis membrane surface.
The presence of oxidizing agents such as chlorine in seawater can reduce the performance of reverse osmosis membrane elements, so seawater must control free chlorine <0.1mg/L before entering the reverse osmosis membrane. By adding 1.5-2mg/L Sodium Bisulfite to the metering pump, the residual chlorine in the sea water reacts with the Sodium Bisulfite to form acid and neutral salt, thus eliminating the effect of the residual chlorine on the reverse osmosis membrane.
The sea water contains high concentration of Ca2+, Mg2+, HCO3-, SO42- plasma. In the process of reverse osmosis seawater desalination, the seawater is concentrated, and it is easy to form the precipitation of insoluble inorganic salts on the surface of the reverse osmosis membrane. According to the water recovery rate of the original water quality and the reverse osmosis device, the Stiff & Davis index is calculated, and the scaling precipitation trend is judged, and the scale inhibitor is added before the seawater enters the reverse osmosis device. After a comprehensive evaluation of the scale inhibition effect and price of several scale inhibitors, the dosage of sulphuric acid as a scale inhibitor is 15 ~ 20mg/L, and the pH value of seawater is controlled in the range of 7 ~ 7.5. It can effectively prevent the insoluble inorganic salts from forming scale precipitation in the surface of the reverse osmosis membrane.
(3) security filtering
The safety filter before the reverse osmosis device is designed to protect the high-pressure pump, the energy recovery device and the membrane components for safe and long-term operation. According to the filter water and filtration precision design and calculation of the specifications of the security filter, four diameters of 800 security filter, the filter material is 316L, the inner lining plastic, the single security filter is equipped with 70 57-1000-5 mu m insert filter core, and the designed effluent is 150m3/h.



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