Page 73 - ro membanes

56
3. DIAGNOSTICS OF MEMBRANE FOULING AND SCALING
TABLE 3.2 Example Results From Bubble and Vacuum Tests of a Set of Seven SWRO Elements
Serial Number
A609590 position 1 A609572 position 2 A609568 position 3 A609656 position 4 A611240 position 5 A619550 position 6 A608298 position 7
Bubble Feed End
(lead) NL NL NL NL NL NL (tail) NL
Test Brine End
NL NL NL NL NL NL CT
Vacuum Test Start Finish
20.9 20.2 20.9 20.1 20.9 20.4 20.9 20.4 20.9 20.2 20.9 20.4 20.9 18.5
3.4.4 Standard Membrane Performance Test
All RO-membrane elements are tested at a preset standard salinity, pressure, temperature, permeate recovery, and pH range after they are manufactured to confirm that they meet per- formance specifications in terms of permeate flow, salt rejection, and differential pressure (such test is referred to as “factory test”). The standard test conditions are different for SWRO, BWRO, and NF elements and they sometimes also vary among membrane manufac- turers. When a given membrane element is purchased, its documentation contains the results of its original standard factory test.
As a part of the comprehensive performance analysis, the membrane elements provided for evaluation are retested at the same conditions as they were tested originally by the manu- facturer and the values of membrane rejection, flow, and salt passage are compared. Signif- icant reduction of membrane production and increase in differential pressure (over 8% per year of operation) are typically indicative of measurable membrane fouling.
Over 20% increase in salt passage could be attributed to damage of the membrane ele- ments by oxidants, loss of mechanical integrity and/or membrane fouling. Comparison with the results of the other performance tests described herein is used to identify the most probable cause of the membrane performance problems.
If all membrane elements within the vessel have comparably high loss of production, this observation is indicative of propagation of the fouling throughout the length of the mem- brane vessel. If salt passage of all membranes in a given vessel has increased by approxi- mately the same level, then it is very likely that these membranes have been exposed to oxidant. If only individual elements are exhibiting elevated salt passage, such phenomenon is likely caused by mechanical integrity loss of the elements.
3.4.5 Membrane Cleaning Test
Sample membrane elements are cleaned applying various combinations of cleaning chem- icals and durations of soak and flush cycles to identify what is the optimal combination of