Page 53 - ro membanes

36 2. MEMBRANE FOULANTS AND SALINE WATER PRETREATMENT
biofilm-forming microorganisms such that the flow pattern within the membrane elements changes and the feed flow is completely blocked in some portions of the feed channels and increased in others. Flow channeling caused by random blockages of the feed channels and spacers results in sharp increase in salt concentration in the affected areas, which in turn triggers precipitation of sparingly soluble salts such as calcium carbonate and sulfate in the feed channels and further exacerbates and accelerates the membrane fouling problem. All of these impacts of membrane-fouling result in an increased head loss of source water traveling through the feed/brine spacers, which exhibits itself as an increase in difference between the feed pressure entering the membrane vessels and the pressure of the blend of feed water and concentrate exiting the pressure vessels (e.g., the DP).
Typically, DP of RO systems desalinating surface water collected by open intake increases with 0.1e0.4 bars (1.0e5.8 lbs/in.2)/month. However, if the RO system is exposed to heavy biofouling (e.g., during intensive algal bloom events), DP increase of the same magnitude could be observed within 1e2 weeks. The DP of RO systems using source waters collected by subsurface intake (e.g., saline water beach wells or brackish water pro- duction wells) could be significantly lowerd0.04e0.20 bars (0.58e2.90 lbs/in2). However, source waters collected by most of these systems have very low TOC concentration (typically <0.5 mg/L).
It should be pointed out that DP is not a parameter that measures biofouling exclusively. This parameter is reflective for all five types of fouling (particulate, colloidal, mineral, NOM, and microbial), which usually occur simultaneously, but at different rates. However, except for particulates and hydrocarbons, all other types of foulants have a lower rate of impact on the DP, and usually, if the pretreatment system performance and source water quality have not changed significantly in terms of turbidity and hydrocarbon content, the most likely type of fouling that can cause measureable DP increase within a short period of time (i.e., 1 week) is biofouling. Therefore, DP increase rate, especially during algal bloom events, is most often representative of the biofouling rate of the membranes.
2.6.4.4 Chlorophyll a
Chlorophyll a concentration of source water is an indicator of the content of algae with green pigmentation in the water. This parameter is measured using fluorometer or spectro- photometer. The content of chlorophyll a is proportional to the light transmission through the water sample at a given wavelength, which is detected by the instrument and converted into concentration units, typically either micrograms per liter (mg/L) or milligrams per liter (mg/L).
As a rule of thumb, source waters with chlorophyll a content below 0.5 mg/L have low biofouling potential, and algae levels are indicative of non-algal-bloom conditions. During severe algal blooms, the chlorophyll a level could exceed 10 mg/L.
It should be pointed out that content of algae and chlorophyll a naturally varies diurnally and seasonally, and also changes with depth. In general, algal content is proportional to the intensity of solar irradiation and typically increases significantly in the summer, as compared to the average annual algal content.
Tracking chlorophyll a on a daily basis and trending the collected data allow an operator to determine the occurrence of algal bloom events, because during algal blooms, the chlorophyll a content in the source water usually increases several times in a matter of only several days.