In June 2015, the client contracted the services of Prime Chemicals Corporation for replacement of brackish RO membranes.
The RO unit was fed by bore water pretreated through a multimedia sand and carbon filter followed by a cartridge filter. Prior to start-up of the RO unit upon membrane re- placement, SDI (Silt Density Index) tests were carried out in the pretreatment steps.
The SDI measures the fouling potential of water on the RO membrane. Membrane fouling is caused by particles and colloidal material which are present in the feed water and are concentrated at the membrane surface. As a rule of thumb, an SDI of less than 5.0 is considered acceptable while a value less than 3.0 is recommended for bore waters.
The SDI test revealed only a nominal improvement in SDI across the sand filter. Across the cartridge filter did the SDI improve from around 5.7 to 5.4.
To complex matters, presence of dissolved iron at around
1.0 mg/l was detected in the feed water. RO membranes feed water guidelines allow a maximum iron content of 0.05 mg/l.
Application: Process Water
SDI Reduction in RO feed water
Iron removal in RO feed water
To address both these issues simultaneously, a Manganese Greensand filter was proposed. Although alternate media were available for iron removal, due to its smaller effective size (0.30—0.35 mm), Greensand media was preferred for better SDI reduction.
Media filter was provided in a proprietary FRP housing with provision for backwash and rinse. Batch regeneration option with Potassium Permanganate was preferred.
Tests were carried out upon installation of the greensand filter. Much to the customer’s and our satisfaction, the SDI downstream of the greensand filter had improved to 3.4, while iron was also found to be within the recommended limits.
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