What is the BEST indication that the pre-treatment filter (or filters) of a reverse- osmosis freshwater generator must be serviced by cleaning or replacement?

Correct Answer: Option C — An excessive pressure drop across the pre-treatment filter.

An excessive pressure drop across the pre-treatment filter is the primary indicator of fouling or clogging. Monitoring the differential pressure is a standard operational requirement to ensure adequate flow to the high-pressure pump and to protect the downstream membrane modules from particulate damage and premature fouling.