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RO Pretreatment/SDI Reduction

Global investment in Reverse Osmosis (RO) has been growing at a double-digit rate for years. This growth is primarily driven by municipal water desalination applications, process water treatment and water reuse. The high growth rates will continue to exceed expectations due to global awareness of environmental protection requirements. The process of wastewater for reuse is expanding in industries because the ever more restrictive wastewater and sewerage regulations have made it an economic choice.  In some cases, large industrial operations, including power plants, steel manufacturing and a variety of heavy industries, are adopting Zero Liquid Discharge (ZLD) because they can no longer afford or are not permitted to discharge.

RO has widely been selected in recent years because costs have fallen during the past two decades. However, an RO system is only as good as the effectiveness of its pretreatment. unhoused modulesThe influents to the RO require the removal of colloidal particles, bacteria and other contaminants to protect the RO membranes from fouling and eventual failure. The cleaner the feed stream entering the RO process, the longer the membranes will last; so, the practice of using the cheapest pretreatment media is giving way to the use of the more effective MF or UF. Despite a rapidly growing uptake in new projects, two issues continue to limit the wider adoption of MF/UF membrane pre-treatment, (1) hollow-fiber/spiral-wound cartridges often require additional processes to produce a feed suitable for RO, and (2) inefficient operating periods at reduced flux due to caking require higher maintenance with additional operational steps to control physical fouling like a chemical enhanced backwash or clean-in-place on a regular basis.

With its open tubular channels, in recent years’ tubular membrane filters (TMF) have excelled for applications with high solids concentration as well as large particle sizes. This is especially true for many industrial applications that require pH adjustment, water softening, or the addition of flocculent/coagulant/polymers. TMF reduces colloidal fouling, silt density index (SDI) and turbidity of the feed to the RO unit.

RO systems employ a membrane replacement strategy to maintain consistent performance. Since membrane pretreatment provides an improved feed water pretreatment quality, lower RO replacement rates are realized and reduced cleaning frequencies can be employed, providing significant operational benefits and reduced costs. The potential benefits offered by membrane pretreatment compared to conventional pretreatments, such as multi-media filter, bag filter, or cartridge filter, can be summarized below:
  • Lower suspended solids and less biological content, resulting in improved RO operation
  • RO membrane cleaning cost savings in cleaning chemicals
  • Lower RO pressure drops from fouling, resulting in lower energy costs
  • Longer RO membrane life
  • Increased flux rates in the RO system
  • Shorter plant footprint size resulting in reduced capital investment
  • Lower chemical and sludge handling costs
From a life cycle cost analysis, the effects of these benefits for sustainable and reliable performance of the overall RO systems economically outweigh any concern over relatively higher capital and replacement costs as compared to many conventional pretreatments. The WTS system provided by FSI employs TMF is an ideal system for RO Pretreatment/SDI Reduction.