Pretreatment and post-treatment for seawater treatment system
Due to the nature of its spiral wound design, pretreatment is important when using RO and membranes. This material is designed in such a way that only one way is allowed to flow through the system. Therefore, the spiral wound design does not allow agitating backflushing with water or air to wash its surface and remove solids. Since the accumulated substances cannot be removed from the membrane surface system, they are extremely prone to fouling (loss of production capacity). Therefore, pre-processing is necessary for any RO or NF system. The pretreatment of seawater reverse osmosis system has four main components:
Solid screening: The solids in the water must be removed and the water must be treated to prevent the membrane from being contaminated by fine particles or biological growth, and to reduce the risk of damage to the high-pressure pump components.
Filter element filtration-usually a wire-wound polypropylene filter, which can remove particles 1 – 5 microns in size.
Add oxidizing bactericide (such as chlorine) to kill the bacteria, and then add bisulfite to inactivate the chlorine that can damage the film composite membrane. There are also biofouling inhibitors that do not kill bacteria, but only prevent them from growing mucus on the surface of the membrane.
Pre-filter pH adjustment: If the pH, hardness, and alkalinity of the feed water cause a tendency to scale when they are concentrated in the waste liquid, add acid to keep the carbonate in its soluble carbonic acid form.
CO3-2 + H3O+ = HCO3- + H2O
HCO3- + H3O+ = H2CO3 + H2O
Carbonic acid cannot combine with calcium to form calcium carbonate scale. The Langelier saturation index is used to estimate the tendency of calcium carbonate to foul. Adding too much sulfuric acid to control carbonate scale may cause calcium sulfate, barium sulfate or strontium sulfate scale to form on the RO membrane.
Pre-filtering scale inhibitor: Compared with the acid that can only prevent calcium carbonate and calcium phosphate scale formation, scale inhibitors (also called scale inhibitors) can prevent the formation of all scales. In addition to inhibiting carbonate and phosphate fouling, scale inhibitors can also inhibit sulfate and fluoride fouling, dispersing colloids and metal oxides, and some special products can inhibit the formation of silica.
Desalinated water is highly corrosive and is usually "stabilized" by adding lime or caustic alkali to protect downstream pipelines and storage to prevent corrosion of concrete or cement lining surfaces. Lime material is used to adjust the pH value from 6.8 to 8.1 to meet drinking water specifications, and is mainly used for effective disinfection and corrosion control.
Post-treatment includes stabilizing the water and preparing it for distribution. The desalination process is a very effective biological barrier to pathogens, but disinfection is used to ensure a "safe" water supply. Disinfection (sometimes called sterilization or sterilization) is used to kill any bacterial protozoa and viruses that bypass the desalination process and enter the product water. Sterilization can be through ultraviolet radiation, using ultraviolet lamps directly on the product, or through chlorination or chloramination (chlorine and ammonia). In many countries/regions, chlorination or chloramination is used to provide "residual" disinfectants in the water system to prevent contamination of the water system from contaminants entering the system.