Membrane Desalination Case Process Analysis

1. Process

2. Pretreatment process

Impurities in seawater: According to their size, they can be divided into dissolved matter, colloids and suspended matter.

(1) Coagulation and sedimentation: folded plate flocculation + inclined tube sedimentation

Through experimental research on the treatment effects of different coagulant dosages and no coagulant dosage, and combining engineering experience with current water quality conditions, ferric chloride was selected as an economical and applicable coagulant with a dosage of 3ppm and a water turbidity of less than 4NTU.

Adopt tubular static mixer: directly installed on the water inlet pipe, the mixing efficiency can reach more than 94%.

A folded plate flocculation tank with enhanced coagulation effect is used: the flocculation time is short and the tank volume is small.

Use a heterogeneous flow inclined tube sedimentation tank: the advantages are high sedimentation efficiency, small tank body and small footprint.

The sedimentation tank and reaction tank are built together, with a total designed water volume of 2900m3/h.

• Total flocculation time: t=22min.

• Sedimentation tank surface load: 5.5m3/m2·h.

• Water turbidity ≤5NTU

(2) Valveless filter

By utilizing high-level water inlet and outlet and siphon backwashing process, there is no need to add power equipment. It has the advantages of low cost, energy saving, simple operation and management, and the fine filtration can meet the requirements.

The valveless filter is used in conjunction with the clarifier, and is mainly composed of a top flushing water tank, a middle filter chamber, a bottom water collection chamber, a water inlet device and a flushing siphon device.

8 valveless filters are configured and divided into two groups. The overall dimensions of each valveless filter are 10.5 meters × 5.5 meters. The interior has a double-chamber structure and is filled with anthracite and quartz sand filter media. The single-chamber filtration area is 4.7 meters × 4.7 meters. The designed filtration rate is 7.3m/h. The water output of each valveless filter is about 370m3/h, and the turbidity of the produced water is ≤1NTU. The effluent flows directly into the clear water tank using the high head difference.

The effluent quality of the valveless filter is slightly lower than that of the V-type filter, but it can fully meet the requirements of the mechanical filter. The operation of the first phase for more than 8 years shows that the valveless filter is simple to operate and manage, has low failure rate, and saves maintenance costs, making it very suitable as a pre-treatment for mechanical filters.

(3) Mechanical filter

44 vertical mechanical filters are configured, with a water output of 2640m3/h. Each is a Φ3200 vertical mechanical filter, with a designed flow rate of 7.5m/h and an output of 60m3/h.

Each filter is equipped with a flow meter, inlet and outlet water pressure instruments and manual/pneumatic regulating valves. The filter is filled with quartz sand and anthracite of different grades. Seawater is filtered through the filter material, and the outlet water SDI value is less than 5, meeting the inlet water quality requirements of the reverse osmosis membrane element.

3. Desalination process

The reverse osmosis system is divided into 5 sets, each with a capacity of 5000m3/d. Through model analysis and experimental verification, the RO membrane stack combination structure is designed as an 8×8 vertical and horizontal arrangement.

3.1 The membrane stack combination adopts the large opening interface connection of the membrane shell, and the raw water or concentrated water interfaces of multiple large opening membrane shells are connected in series using copy forests. In order to ensure the convenience of disassembly and installation of the membrane stack, each branch pipe is connected to the main pipe with a double copy forest.

3.2 This structure makes the membrane stack compact, the water distribution more uniform, the disassembly and installation more convenient, and reduces the construction cost.

3.3 Through continuous monitoring of the produced water quality since operation, the water quality of the equipment is stable, safe and reliable.

In the entire membrane stack system, the 21 rows of membrane shells arranged vertically have relatively uniform water conductivity, and the difference between the membrane shells in each row is kept within 20%. The 6 rows of membrane shells arranged horizontally also have relatively uniform water conductivity, and the difference between the water conductivity of the membrane shells in each row is also kept within 20%, which further verifies that the water distribution method of the middle water inlet is relatively reasonable.

4. Energy recovery

The water production rate of this seawater desalination system is 40%, and the remaining 60% of the seawater becomes high-pressure concentrated seawater. The energy recovery device uses high-pressure concentrated water to pressurize the feed seawater, which can reduce the capacity of the high-pressure pump by 55-60%, reduce the energy consumption of seawater desalination by about 50%, and reduce the energy consumption per ton of water to below 2.2 degrees.

5. Post-mineralization treatment

This project adopts the dissolved mineral method and, through a large number of static and dynamic test studies, adds CO2 and calcium carbonate to the RO produced water to make the water quality meet the requirements.

The design process is to inject CO2 by increasing the pressure in the RO water production pipeline. CO2 reacts positively with the permeate, causing the pH value of the permeate to be acidic.

The permeate then flows by gravity into the post-mineralization tank filled with calcium carbonate particles, causing the CO2 dissolved in the water and free in the pipeline to react with the calcium carbonate particles, thereby increasing the alkalinity and hardness of the reverse osmosis water.