RO membrane plant description

The RO unit is the pivotal process since water production by membrane separation declines with time mainly due to fording and other factors discussed in Chapter 2. The RO system must be run under conditions that minimise decline in flux while maintaining high product water quality. The pre-treatment system must be stable and reliable to ensure the RO unit operates continuously without frequent shutdowns for cleaning to restore flux and rejection. A stable RO membrane performance, in turn, is required to ensure the pohshing system produces water that meets the product water specifications without frequent shutdowns for regeneration of ion-exchange resins. 

During the backwash phase, water is transferred from the water reuse tank to the GMF by the backwash pump at 170 m /h and 3 bar g. Water flows from the bottom upwards through the bed. A high superficial velocity of25—35 m/h is required to fluidise the media bed and remove dirt from the bed. The backwash phase lasts 15 min and is followed by a 2-min bed settle phase when no water flows to the filter. This allows the bed media to recover or rest before it is rinsed. During the rinse phase, the standby feed water pump transfers water to the media filter at 70 m /h and 3 bar g. Water flows downwards through the media bed and out from the bottom to drain. The rinse phase lasts 5 min. Once the GMF has been washed, it is placed in standby mode until the online unit needs to be washed. 

The filtered water is monitored for turbidity continuously by a turbidity analyzer. 

The turbidity must be less than 1.0 NTU to prevent foufing of RO membranes by particulate matter. A higher NTU value means the GMF must be washed. The filtered water is injected with chemicals as it flows to the RO system to prevent scaling of membranes by sparingly soluble salts. 

Chemical treatment consists of three weU-known treatment processes for ensuring rehable operation of RO membranes. First, a 20% sodium bisulphite (SBS) solution is injected by the chemical dosing pump (one pump is on standby). The iiyection rate is proportional to the RO feed water flow rate and is controlled by the PLC based on the chlorine concentration monitored by a chlorine analyser downstream of the in-line mixer. SBS like sodium sulphite and sodium metabisulphite is a reducing agent commonly used to dechlorinate RO feed or lower the chlorine concentration to less than 0.05 mg/1 in RO plants that use polyamide aromatic membranes. It takes 7.33 mg/1 of 20% NaHS03 solution to remove 1 ppm of residual chlorine in water on a stoichiometric basis. 

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