Page 285 - ro membanes

268 12. REVERSE OSMOSIS SYSTEM DESIGN AND PRETREATMENT
FIGURE 12.9 Point Lisas seawater desalination plant schematic.
The size of the individual RO trains depends on the overall production capacity of the SWRO plant and typically varies between 2000 m3/day (0.5 MGD) and 21,000 m3/day (5.5 MGD). The main advantage of this RO train-based configuration is that it is modular and allows for relatively easy flow distribution and service of the individual trains. Since typically the size of the individual RO train does not exceed 10%e20% of the total plant production capac- ity, train shut down for maintenance (membrane cleaning and replacement, and equipment service) is handled either by using a standby RO train or by temporary increase of the pro- duction capacity of the RO trains remaining in service.
The RO-train based configuration is very suitable when the SWRO plant is designed and intended to operate at a constant production output. At present, most of the existing large municipal SWRO plants worldwide are designed to supplement existing conventional water supply sources rather than to be the primary or the only source of water supply for a given area. Therefore, the operation of these SWRO plants does not need to have the flexibility to follow the actual diurnal and monthly fluctuations in product water demand and most of the existing plants are designed to operate at constant production capacity.
In the future, the SWRO is likely to become a prime rather than a supplemental source of water supply for many coastal communities pressured by population growth, especially for large urbanized or industrial areas with limited traditional local sources of freshwater supply (i.e., groundwater or river, or lake water). The SWRO systems servicing such areas have to be designed to have the operational flexibility of matching desalination plant production with the product water-demand fluctuations.
Shift of the SWRO plant operational paradigm from constant to variable production flow requires a change of the typical SWRO configuration from one that is most suitable for constant