1. Seawater Desalination and Purification: Membrane Protection and Energy Optimization

Seawater desalination reverse osmosis membranes are extremely sensitive to feed water quality. Once suspended solids, algae, colloids, and microorganisms enter the membrane system, they can cause severe fouling within just a few hours. This condition leads to a significant drop in permeate output. Therefore, modern large-scale seawater desalination plants commonly adopt a step-by-step pretreatment process consisting of "dissolved air flotation + multi-media filtration + cartridge filtration + ultrafiltration." The fine pores of the ultrafiltration membrane can effectively reject bacteria, viruses, and macromolecular organic matter, and keep the SDI of the treated water consistently below 3. To cope with high turbidity spikes caused by seasonal red tides or storm surges, the pretreatment system must also be equipped with an emergency powdered activated carbon dosing device to adsorb dissolved organic matter. These measures ensure that the RO system receives stable, high-quality feed water under all operating conditions, which is a critical step in extending membrane service life.

The core desalination stage is carried out by high-pressure RO trains. Seawater is pressurized to 5.5–6.5 MPa; water molecules pass through the membranes while the vast majority of salts are rejected. Approximately 60% of the seawater is converted into fresh water, while the remaining 40% becomes high-pressure concentrated brine, which contains enormous pressure energy. The system, by integrating pressure exchangers or turbocharger energy recovery devices, can efficiently transfer this energy to the feed side, thereby dramatically reducing the specific energy consumption from over 8 kWh per ton of water to just 2.5–3 kWh. The technical challenge in this stage lies in the fact that the hydraulic pressure differences among the high-pressure pump, the energy recovery device, and the membrane elements must be precisely matched; any fluctuation will lead to a sharp drop in energy efficiency or cause water hammer damage to the membrane elements. Optimizing the system to approach the energy consumption limit places extremely high demands on automatic control precision.

The fresh water produced by RO contains almost no salts, and its total hardness is far too low, leaving it in a highly corrosive state. If conveyed directly, such water would severely erode the water supply piping network. Therefore, post-treatment of the fresh water is also an indispensable part of the water treatment equipment. The permeate first passes through a decarbonation tower and then enters a mineralizing filter filled with limestone or dolomite; alternatively, the system precisely doses carbon dioxide and lime milk to rebuild the water's alkalinity and hardness, bringing the saturation index into balance. Finally, the system injects an appropriate amount of sodium hypochlorite to ensure the necessary residual chlorine is maintained in the distribution network. This post-treatment stage may seem simple, but it is critical to the lifespan of the entire water supply system and to public health. It is the final purification step that transforms desalinated water from an "industrial product" into "municipal water supply."