Author: Site Editor Publish Time: 2023-01-30 Origin: Site
The usual pre-treatment system consists of the following, coarse filtration (~80 microns) to remove large particles, addition of oxidants such as sodium hypochlorite, followed by precision filtration through a multi-media filter or clarifier, then addition of oxidants such as sodium bisulphite to reduce residual chlorine, and finally the installation of a security filter before the inlet of the high pressure pump.
The role of the security filter is, as the name suggests, to act as the ultimate insurance measure against the damaging effects of occasional large particles on the impeller and membrane elements of the high pressure pump. Water sources containing high levels of suspended particles usually require a higher degree of pre-treatment to meet the required influent requirements; water sources with high hardness content are recommended to be softened or acid and scale inhibitors added, etc. For water sources with high microbial and organic content, activated carbon or anti-pollution membrane elements are also required.
Reverse osmosis (RO) is very dense and has a very high removal rate for viruses, phages and bacteria, at least 3log or more (>99.9% removal rate). However, it must also be noted that in many cases, microbial re-growth on the produced water side of the membrane can still occur, depending on how it is assembled, monitored and maintained, meaning that the ability of a system to remove microorganisms depends crucially on the design, operation and management of the system rather than the nature of the membrane elements themselves.
The higher the temperature, the higher the water yield and vice versa. When operating at higher temperatures, the operating pressure should be reduced to keep the water yield constant and vice versa. Please refer to the relevant section for the temperature correction factor TCF for changes in water production.
Particle and colloid fouling in a reverse osmosis or nanofiltration system can seriously affect the water yield of the membrane and sometimes reduce the desalination rate.
Early symptoms of colloidal fouling are an increase in system differential pressure. The source of particles or colloids in the membrane feed water varies from site to site and often includes bacteria, sludge, colloidal silica, iron corrosion products etc. Drugs used in the pretreatment section such as aluminium polymerisation and ferric chloride or cationic polyelectric media can also cause fouling if they are not effectively removed in the clarifier or media filter.
In addition, cationic polyelectric media can also react with anionic scale inhibitors and their precipitates can foul the membrane elements, the tendency of such fouling in water or whether the pretreatment is qualified is evaluated using SDI15, please refer to the relevant section for details.