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2.6 MICROBIAL FOULANTS 37
2.6.4.5 Algal Count and Profile
Algal count is a measure of the number of algal particles per unit volume of source water. The total algal count is the number of all algal cells contained in 1 mL or 1 L of source water. Typically, algal concentration (total algal count) in saline source waters below 10,000 cells/L does not present challenge for desalination plant performance (e.g., pretreatment system and RO-membrane fouling). Total algal count can be measured by online instrumentation or in laboratory setting. Higher content of algae in the source water may have three key perfor- mance impacts:
1. Reduction of pretreatment system filtration capacity and solids removal efficiency due to the filter overload;
2. Accelerated fouling of cartridge filters as a result of the increased content of fine solids (i.e., pico- and microalgae) in the source water and elevated biogrowth on the filter surface;
3. Loss of productivity and increase of DP of the RO membrane elements due to biofouling.
In addition to the total algal content, algal profile indicates the number, size, and type of
algal species contained in the source water and is completed in laboratory setting by experi- enced biologist familiar with the aquatic species of the type of the analyzed source water. The algal count of individual species in the water is a very useful tool to verify the presence of algal bloom and identify the size and type of algae triggering the bloom.
Knowing algal size distribution is of critical importance for pretreatment system design and operation because it allows determining what pretreatment method or combination of methods will need to be applied and how the pretreatment facilities would have to be sized and configured. For example, if dissolved air flotation is used for pretreatment, than the DAF clarifiers have to be designed such that the size of the bubbles they create closely match the size of the majority of algae contained in the source water. If DAF design is not reflective of the actual size of algae in the water, this type of pretreatment facilities would have very limited algal removal efficiency and will be of limited or no benefit to plant operations.
2.6.5 Threshold Levels of Microbial Foulants
Table 2.7 presents a list of source water quality parameters used for characterizing partic- ulate content that are recommended to be measured when deciding upon the type of pretreat- ment needed for a given source water.
Typically, algal bloom is defined as an event in which over 75% of the algae are from the same species. Knowing the type and size of the dominant algal species is critical to opti- mizing the source water chemical conditioning and pretreatment approach. Table 2.8 provides threshold levels of algal content, which characterize algal bloom severity and asso- ciated biofouling potential of the saline source water.
It is important to note that the levels of these parameters are thresholds and they do not neces- sarily correlate with each other for a given source water. For example, usually, there is no statis- tically significant correlation of the algal content measured as chlorophyll a and as number of algal cells per liter of source water. However, if the source water contains chlorophyll a at concentration higher than 0.5 mg/L, and/or if the total algal count of the source water is higher than 10,000 cells/L, then it is very likely the intake area of the desalination plant is experiencing algal