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2.7 COMBINED EFFECT OF VARIOUS FOULANTS ON MEMBRANE PERFORMANCE 39
FIGURE 2.5 Algal bloom event dominated by Pseudo-nitzchia.
NOM and do not contain measurable amount of algae. Therefore, the TOC level has to be considered as supplemental rather than as a primary criterion for occurrence of algal bloom. Not every algal bloom has significant impact on desalination plant operations. Most pretreatment processes, including granular media and membrane filters, can typically handle low intensity algal blooms with minimum to no impact on plant operation and
RO-membrane fouling rate.
Single-stage gravity granular media filters and conservatively designed single-stage vac-
uum- and pressure-driven UF and MF filters can also provide adequate pretreatment during moderate algal blooms. If the intake area, however, experiences frequent high-intensity or se- vere algal blooms, two-stage pretreatment system would need to be employed to maintain plant productivity and reliable performance.
The occurrence of algal bloom can be verified by observation of the algal community in the source water by examining water sample under microscope. Under normal non-algal bloom conditions, the algae community is very diverse and contains variety of species in comparable numbers or algal cells per species. Under algal bloom condition, over 75% of the algal cells in the water sample are comprised by one dominating species of algae. For example, Fig. 2.5 illustrates algal bloom condition dominated by the algal species Pseudo-nitzschia. Excessive growth of this species is a frequent cause of red tides along the Pacific Ocean coast of California and Australia, as well as at a number of other locations worldwide. While exam- ination of Fig. 2.5 shows the presence of a number of algal species in the source water sample, the majority of the algal cells visible on this figure have elongated needle-shape, which is characteristic for the Pseudo-nitzshia species.
2.7 COMBINED EFFECT OF VARIOUS FOULANTS ON MEMBRANE PERFORMANCE
Membrane fouling is a complex process, which often results from the additive impacts of several types of foulants. Particulate fouling usually causes a relatively quick and definitive