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Pollution Prevention and Control Technologies for Plating Operations
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.