Page 65 - ro membanes

48 3. DIAGNOSTICS OF MEMBRANE FOULING AND SCALING
RO-performance data review includes analysis of the plant normalized performance data (e.g., flow, salt passage) for the last month and for periods in the past of similar problems. The purpose of this analysis is to determine whether:
1. The problem is instantaneous (i.e., during startup/shutdown), it is instrumentation related, or it is mechanical/equipment related.
2. The problem is gradualdmembrane fouling or degradation has occurred slowly over time.
3. The salt passage has increasedde.g., which RO trains/vessels have excessive salt
passage.
4. The plant production has decreased and/or the feed pressure has increased since the
last membrane cleaning.
5. The membrane maintenance history points out to significant changes in:
a. Cleaning frequency and chemicals,
b. Membrane replacement frequency.
6. Past problems with equipment and source water quality have occurred.
7. Instrumentation and calibration track record reveals frequent malfunction of monitoring
equipment.
Data review and analysis above allows to determine whether the performance challenges
of the RO system are:
• Membrane/vessel integrity loss-related problemsddeteriorating the product quality.
• Membrane productivity loss-related problemsddeteriorating normalized plant produc-
tion flow.
Membrane integrity diagnostics includes two steps:
1. Completion of conductivity profile for all pressure vessels within a malfunctioning RO train to identify which vessels have highest conductivity/largest deviation from the average train conductivity.
2. Probing of the problematic vessels to identify which specific membrane elements or other internal components (e.g., O-rings, interconnectors, brine seals) inside the vessel are malfunctioning.
3.3.2 Vessel Profiling and Probing
Vessel profiling is completed by collecting permeate from all vessels though their sampling ports (Fig. 3.6) and analyzing the conductivity of the collected samples to identify vessels with conductivity which significantly differ from the rest.
For the vessels that are identified to have significantly higher than average RO train salinity, vessel probing test is completed. This test consists of insertion of plastic tubing into the vessel permeate tube to determine the conductivity level along the length of the vessel as shown in Fig. 3.7. Permeate plug is removed from the end cap and tubing is inserted through the length of the vessel. Probe tubing is pushed along the length of the vessel moving from the last to the first (front) element within the vessel. Samples are collected near the brine seal and permeate tube point of connection between the membranes. A more detailed procedure for vessel probing is presented elsewhere (AWWA, 2007).