Page 241 - ro membanes

224 10. COMPARISON OF GRANULAR MEDIA AND MEMBRANE PRETREATMENT
When algal cells break, they release easily biodegradable organic compounds, which could trigger accelerated growth and formation of biofilm of marine bacteria on the RO mem- branes. In turn, the accelerated biofilm formation could cause heavy fouling of the RO mem- branes and could result in a significant reduction of desalination plant production capacity within several weeks, and sometimes days, after the beginning of the algal bloom event. In such source water conditions, gravity downflow granular media filtration may be preferable over membrane pretreatment because it allows removing the algae from the source water with a minimum breakage of their cells. Therefore, gravity granular media filters have found much wider application than UF or MF filters for pretreatment of saline source waters orig- inating from shallow intakes, which are exposed to high-intensity algal bloom events.
Conversely, if the source water used for desalination originates from subsurface intake (e.g., beach wells), this water does not contain algae and its pretreatment using pressure- driven granular media filters or MF or UF membranes is usually more cost competitive than conventional gravity granular media filtration. In such conditions, pretreatment often could be completed adequately by using cartridge filters only.
10.3 SURFACE AREA REQUIREMENTS
Membrane pretreatment is typically more space efficient compared to granular media filtration. The smaller footprint benefits of membrane filtration are usually of greater impor- tance when the site available for building new or upgrading existing desalination plant is very limited or where the cost of new land acquisition is very high. Typically, the footprint of conventional single-stage dual media filtration system is 30e50 m2/1000 m3 day (1200e2100 ft2/MGD) of desalination plant production capacity.
The space benefits of membrane filtration are more significant for high-turbidity source waters where two-stage granular media filtration may be required to achieve comparable performance to a single-stage membrane pretreatment system. For example, difficult to treat source water, which necessitates the granular media filtration system to be designed for sur- face loading rates of less than 10 m3/m2 h (4 gpm/ft2) or where a two-stage granular media filtration is needed to produce comparable filter effluent, the membrane filtration systems may have up to 80% smaller footprint.
Under good to moderate source water quality conditions, when granular media filters are designed at a surface loading rate of 8.5e12.0 m3/m2 h (3.5e5.0 gpm/ft2), their surface is approximately 40%e50% larger than that of an ultra- or microfiltration systems producing similar filtered water quality. For better-than-average source water quality (silt density indexdSDI15 < 4) where granular media filters can perform adequately at surface loading rates of 15e20 m3/m2 h (6e8 gpm/ft2), the total footprint difference is usually only 20%e30% to the benefit of membrane pretreatment.
10.4 QUANTITY AND QUALITY OF GENERATED RESIDUALS
Conventional and membrane pretreatment systems differ significantly by the type, quality, and quantity the residuals generated during the filtration process (Table 10.1). Typically,