In the reverse osmosis (RO) pretreatment system, processes including sand filtration, ultrafiltration, activated carbon filtration, softening, fine filtration and sterilization require optimized design of their respective operation modes and parameters. In addition, the optimal sequence arrangement among these processes is also critical. A rational process sequence reflects the system design level, and is also an important measure to fully exert the function of each process and improve the overall performance of the system.

In pretreatment systems centered on coagulation–sand filtration, this process features low filter media cost, small filter media loss, excellent performance in suspended solids interception and turbidity reduction, and no performance attenuation. It naturally serves as theleading processfor treating general raw water in pretreatment systems. Given the limited COD removal efficiency of coagulation–sand filtration, biochemical processes such as biological aerated filters (BAF) can be installed upstream to reduce raw water COD when necessary.

In pretreatment systems centered on ultrafiltration (UF), the main role of UF is to intercept suspended solids, colloids and large‑particle organic matter, with functions similar to coagulation–sand filtration, and a comparable position in the flow scheme. However, UF has higher filtration precision and higher cost, and suffers severe performance degradation after fouling. For raw water with high turbidity and high COD, high‑efficiency upstream processes such as disc filtration, fiber filtration or biological aerated filters are required. Generally, microfiltration (MF) needs upstream pretreatment with filtration precision coarser than 500 μm, and ultrafiltration needs upstream pretreatment coarser than 100 μm.

Process Positions of Activated Carbon Filtration and Softening

Sand filtration or ultrafiltration protects activated carbon by intercepting suspended solids, colloids and large‑particle organic matter. Activated carbon filtration has dual functions:adsorbing organic matterandreducing oxidants. For organic adsorption, activated carbon captures small‑particle organics via its large inner pore surface area, and adsorbs colloids and large‑particle organics on its limited outer surface. Since adhesion of colloids and large‑particle organics blocks the access of small‑particle organics to inner pores, activated carbon is more suitable for adsorbing small‑particle organics. Therefore, activated carbon filtration is normally placedafter coagulation–sand filtration or ultrafiltration, and before resin softening.

Removal of small‑particle organics by activated carbon not only protects RO membranes from organic fouling but also effectively prevents softening resins from organic contamination.