Pollution Prevention and Control Technologies for Plating
Section 3 - Chemical Recovery
3.7 REVERSE OSMOSIS
3.7.7 Operational and Maintenance Experience
The message from the Users Survey respondents with respect to
operational and maintenance problems is very clear: RO membranes
are highly susceptible to fouling and have relatively short life-spans.
As stated by one respondent: "Membrane problems, not enough
filtration with (original) unit. Had to add much, much extra filtration
ahead of (the RO) unit" (PS 008). The shop using RO as an
end-of pipe technology also had membrane fouling problems and
frequent membrane replacement (every 3 mth.) (PS 233). They also
indicated that their efforts to clean the membrane have been unsuccessful.
Short membrane life was identified as a problem by another respondent
(PS 172). Membrane fouling by algae was identified as a problem
by one respondent (PS 172). Another identified high caustic and
carbonate levels as the cause of fouling (PS 089). This shop suggested
that carbonate levels must be below 45 g/l or "some kind
of caustic membrane washing must be applied."
RO membranes are susceptible to fouling by suspended solids in
the feed stream or materials that precipitate during processing.
Solids in the feed stream can be controlled by installing prefilters
that are properly selected for the solids encountered and sized
for the loading. Precipitation is inhibited by changing operational
parameters such as pH. Because pH adjustment of plating chemicals
is not usually possible, applications where precipitation may
occur should be investigated through bench and/or pilot testing
before purchasing equipment.
Two respondents reported no operational or maintenance problems
and a percentage downtime of less than 1% (PS 131, PS 230).
A respondent that has eliminated use of their RO unit, indicated
that the process "increased solution purification requirements
with attendant solution losses, yielding questionable benefits
from the technology" (PS 172). It should be noted that any
recovery technology that returns both bath chemicals and contaminants
to the process tank will cause this problem. However, when contaminants
can be removed from the process bath (e.g., carbonates), the benefits
of recovery will often outweigh any losses.
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