Page 172 - ro membanes

8.2 THE FILTER OPERATION CYCLE TABLE 8.1 Target Pretreated Water Quality
155
Parameters
Turbidity (daily avg./max) SDI15
Total organic carbon pH (min)/(max) ORP
Chlorine residual Total hydrocarbons
Concentrations/Levels
<0.1/0.5
<3 (at least 95% of the
time)
<5 (at all times) <1.0
4.0/9.0
Less than 200
0.02 0.04
Units
NTU No unit
mg/L pH units mV mg/L mg/L
aim to protect the downstream RO membranes from premature damage, loss of permeability and productivity and salt rejection, and excessive frequency of cleaning and replacement.
8.2 THE FILTER OPERATION CYCLE
Granular media filtration is a cyclical process that incorporates two sequential modes of operation: (1) source water processing (filtration) mode; and (2) filter media backwash mode.
8.2.1 Source Water Filtration
During the filtration cycle the water moves in the direction of size gradation of the media and suspended solids and organics in the water are retained on and around the media grains. Depending on the direction of the flow through the media, filters are classified as upflow (from bottom to top) or downflow (from the top to the bottom). As the filtration cycle is continuous, the solids removed from the source water are accumulated in the filtration media and slowly fill up the media cavities around the filter grains.
As the feed water is filtered through the media, the content of solids and silt in this water decreases. Usually, well-operating filters remove 90%e99% of the suspended solids, particulate organics, and silt contained in the saline source water. Some of the aquatic microorganisms (e.g., algae, bacteria, and some viruses) are also retained on the filter media. These microorganisms consume a portion of the dissolved organics in the source water as it passes through the filtration bed. The efficiency of the filters to remove dissolved organics is a function of three main factors: media depth, surface loading rate, and temperature. Removal of dissolved organics by the filters increases with depth and temperature, and with the decrease of the filter loading rate. Elevated temperature increases the dissolved organic content removal because it accelerates the growth rate of the microorganisms retained in the media. The higher depth and lower filter loading rate increase the contact time between the microorganisms in the media bed and the dissolved organics in the water, creating opportunity for their enhanced assimilation.