What is the most important criterion for deciding when to backwash a mixed media filter?

The most important criterion for deciding when to backwash a mixed media filter is filter effluent quality. This is crucial because the primary function of a filter is to produce clean water by removing contaminants from the influent. If the quality of the effluent begins to degrade, it indicates that the filter media is becoming saturated with particles and may no longer be effective in providing adequate filtration.

Monitoring the effluent quality helps operators determine when the filter needs maintenance to ensure it continues to meet water quality standards. Maintaining high effluent quality is vital for protecting downstream processes and ensuring compliance with regulations.

While operating time, pressure drop, and the type of contaminant are all factors that might influence maintenance schedules or provide additional insights into filter performance, they do not directly assess the immediate efficacy of the filter in producing clean water. Thus, effluent quality remains the primary criterion in this context, as it directly reflects the filter's ability to do its job effectively.