ADVANCEES - Model CPP Series - Seawater Reverse Osmosis Pre-Treatment / Seawater Reverse Osmosis Post-treatment
Pre-treatment is important when working with RO and nanofiltration (NF) membranes due to the nature of their spiral wound design. The material is engineered in such a fashion to allow only one way flow through the system. As such the spiral wound design doesn't allow for backpulsing with water or air agitation to scour its surface and remove solids. Since accumulated material cannot be removed from the membrane surface systems they are highly susceptible to fouling (loss of production capacity).
Therefore, pretreatment is a necessity for any RO or NF system. Pretreatment in SWRO system has four major components:
- Screening of solids: Solids within the water must be removed and the water treated to prevent fouling of the membranes by fine particle or biological growth, and reduce the risk of damage to high-pressure pump components.
- Cartridge filtration - Generally string-wound polypropylene filters that remove between 1 - 5 micrometre sized particles.
- Dosing of oxidizing biocides such as chlorine to kill bacteria followed by bisulfite dosing to deactivate the chlorine which can destroy a thin-film composite membrane. There are also biofouling inhibitors which do not kill bacteria but simply prevent them from growing slime on the membrane surface.
- Prefiltration pH adjustment: If the pH, hardness and the alkalinity in the feedwater result in a scaling tendency when they are concentrated in the reject stream, acid is dosed to maintain carbonates in their soluble carbonic acid form.
CO3-2 + H3O+ = HCO3- + H2O
HCO3- + H3O+ = H2CO3 + H2O
- Carbonic acid cannot combine with calcium to form calcium carbonate scale. Calcium Carbonate Scaling tendency is estimated using the Langelier Saturation Index. Adding too much sulfuric acid to control carbonate scales may result in calcium sulfate, barium sulfate or strontium sulfate scale formation on the RO membrane.
- Prefiltration Antiscalants: Scale inhibitors (also known as antiscalants) prevent formation of all scales compared to acid which can only prevent formation of calcium carbonate and calcium phosphate scales. In addition to inhibiting carbonate and phosphate scales, antiscalants inhibit sulfate and fluoride scales, disperse colloids and metal oxides and specialty products exist to inhibit silica formation.
The desalinated water is very corrosive and is 'stabilized' to protect downstream pipelines and storages usually by adding lime or caustic to prevent corrosion of concrete or cement lined surfaces. Liming material is used in order to adjust pH at 6.8 to 8.1 to meet the potable water specifications, primarily for effective disinfection and for corrosion control.