Page 51 - ro membanes

34 2. MEMBRANE FOULANTS AND SALINE WATER PRETREATMENT
TOC has increased significantly. The TN and TP levels, while slightly elevated, increased at a lower magnitude than TOC (i.e., TOC 1⁄4 4.6 mg/L; TN 1⁄4 0.8 mg/L and TP 1⁄4 0.7 mg/L, with TOC:TN:TP ratio of 6.6:1.1:1.0). Under this nutrient imbalanced environment, marine bacteria contained in the source water have switched from passive (non-EPS forming) state to an active state, where they have created large amount of EPS, which in turn has resulted in rapid biofouling of the membrane elements of the RO test facility in Carlsbad fed with seawater during the algal bloom period.
In the example case of nutrient balance presented above, the DP of the RO system has increased with approximately 1 bar (14.5 psi) over two-week period only. Under normal non-algal bloom conditions, such DP increase is observed over a period of 4e5 months. The accelerated fouling and associated drastic increase in DP during this test period required membrane cleaning after only 1 month of operation because the RO system productivity have decreased with over 15% from its steady-state average production level.
It is interesting to point out that not every condition of nutrient imbalance would trigger significant fouling effect on the RO membranes. While bacteria may switch from passive to active mode when the TOC:TN:TP ratio in source water is imbalanced, the amount of biomass created by their growth is limited by the TOC content in the source water. For example, during a short-term (three-week) period of mild algal bloom at the same test location in Carlsbad, the TOC of the source water has increased to 1.2 mg/L only. While the TOC:TN:TP was imbal- anced, the elevated biofouling resulted in DP increase of only 0.28 bars (4.0 lb/in2) over the 3-week algal bloom period and then the DP has stabilized back to normal (i.e., average DP of 0.3 bars or 4.3 lb/in.2 month). This mild algal bloom event did not trigger the need for more frequent RO-membrane system cleaning.
2.6.3.4 Source Water Temperature
Elevated source water temperature increases biological activity and, therefore, it acceler- ates biofouling. The effect of temperature on biofouling increases significantly when temper- ature exceeds a particular threshold, which for most marine bacteria is between 25C (77F) and 28C (82.4F). It is interesting to note that the growth rate of many anaerobic bacteria in- creases dramatically when the source water has temperature between 33 and 37C (91 and 99F). Since many side products of the metabolism of anaerobic bacteria are fine solids that are very difficult to remove by conventional filtration, and which tend to create acceler- ated fouling of UF and MF membrane pretreatment systems, it is advisable to control the source water temperature, if possible, in order to avoid this temperature to enter into the range for optimum growth of anaerobic bacteria.
Natural ambient saline water sources usually have temperatures outside of the range for optimal growth of anaerobic bacteria. Such temperature, however, may be reached when ambient source water is blended with cooling water from thermal desalination plants, which is practiced at a number of desalination facilities in the Middle East that have both RO and thermal desalination plants sharing common intake and outfall.
2.6.4 Biofouling Measurement Parameters and Methods
At present, there are no simple and inexpensive online methods to measure the biofouling potential of saline source waters. Therefore, biofouling potential and rate in practice are