How do we know that fuel has been remediated through polishing?

Before and after the polishing process, fuel samples are taken as one of the visual aids to remediation. During the filtration phase the filters are removed and inspected for contamination. Upon inspection of the filters, the determination is made whether or not to continue. The filtration process will continue until all traces of contamination are no longer present in the filters. After tank cleaning and fuel polishing, fuel samples can be taken and sent to a third-party lab for fuel quality results.