Many standards or specifications in water purification systems guarantee the water we are using is safe or suitable for industrial production. There are large amount of ions and impurities in the water from underground or earth surface, such as sand, organics, microorganisms and even some harmful substances. And all of these impurities affects the quality of water: for the drinking water, excessive amount of impurities in water is harmful to the body; for the water for industrial usage, the substandard water affects quality of production, and causes great loss.
The exist of impurities in raw water affects the treatment result of product water, no matter where the raw water comes from. In order to ensure the treatment result and the steady operation of following precision treatment devices, pretreatment process is important to the overall system of water purification. The common measures of pretrteatment are coagulation, clarification, filtration, softening and pH adjusting. Let’s be detailed as follows:
1.SS causes the membrane fouling and reduces the water yield in purification system.
Contact coagulation or filtration can be adopted when the concentration of SS in raw water is less than 50 ppm. The flocculant enters filter via pump and pipelines. The common filter in pretreatment is multi-media filter.
2. The hard raw water causes membrane scaling.
When the concentration of carbonate is high in raw water, lime can be added to soften the water at the same time with turbidity removal. The most common measures to prevent scaling is cation exchange resin and antiscalant dosing.
3. Organics in raw water reduces the lifespan of anion exchange resin and RO membrane flux.
Chlorine adding, coagulation and clarification can be adopted to remove the organic substance in raw water. If the outlet water via these measures still can not meet the requirements of follow-up process, activated carbon filtration can be added to a further organics removal.
4. Free chlorine changes the natural composition RO membrane.
When the free chlorine in raw water exceeds the inlet requirements of following equipment, activated carbon filtration or Na2SO3 can be added.
5. Microorganisms accumulates and reproduce in the surface of membrane, which corrodes the membrane and causes irreversible damage.
The common pretreatment measures to remove the microorganisms by chlorine dosing or adding ultra-filter device.
6. Fe and Mn ions with high concentration is easy to form hydroxide colloid on the surface of nano-filtration or RO membrane, and this sedimentation affects the membrane flux. Aeration device and filtration device can be added to remove the Fe and Mn ions.
In water purification systems, there should be one or two security filter before the raw water enter the RO system or EDI system, which guarantees the water quality meets the requirements.