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2.6 MICROBIAL FOULANTS 35 monitored by surrogate parameters such as total organic carbon, TOC:TN:TP ratio, RO train
DP rate increase, chlorophyll a, and algal count and profile. 2.6.4.1 Total Organic Carbon
Most commonly, TOC concentration is used as an indicator of the biofouling potential of saline water, while the rate of DP increase is applied as an indicator of biofouling rate. Oper- ational practice indicates that as the TOC concentration of the source saline water exceeds a certain threshold, typically 2 mg/L (Huang et al., 2013), the biofouling potential of this water increases dramatically. The same group of researchers have shown a strong statistically sig- nificant correlation between TOC and membrane fouling. In addition, their research study points out to chlorophyll a as a statistically significant indictor of source water biofouling potential during algal blooms.
2.6.4.2 TOC:TN:TP Ratio
Typically, TOC:TN:TP ratio is measured weekly to check whether the saline water poten- tial is indicating appreciable increase of the biofoluing potential of the saline source water. If ratios are higher than 20% of 1:1:1, then typically the biofouling potential of the source water is elevated to the point that activates the generation of EPS by bacteria, which in turn increases significantly their ability to foul the RO membrane elements.
2.6.4.3 RO Train Differential Pressure Rate Increase
The most commonly used surrogate measure for the rate of biofouling of RO membrane elements is the RO system DP (i.e., the difference between RO system’s feed and concentrate pressures, DP). Usually, in full-scale desalination plants the DP of all RO system trains is tracked continuously.
Biofouling of the feed channels and spacers of spiral-wound membranes (Fig. 2.4) typically results in significant increase in the membrane DP.
As biofilm forming bacteria colonize the RO membrane surface, often sections of the feed channels and spacers between the membrane leafs are blocked over time by patches of
FIGURE 2.4 Biofilm accumulation in RO-membrane spacers.
