What is the purpose of backwash in a filtration system?

• A. To clean filter media and restore filtration efficiency by reversing flow and removing trapped solids.
• B. To maintain correct pH during filtration.
• C. To sanitize the filter media.
• D. To increase filtration resistance.

Answer: (A)

Backwash cleans the filter media and restores filtration efficiency by reversing the flow through the filter and flushing out the trapped solids. As filtration proceeds, solids build up in the media pores, reducing void space and increasing resistance to flow. Reversing the flow lifts and suspends those solids, carrying them away to waste so the media can reclaim its ability to trap new contaminants and keep the effluent quality consistent.

This isn't about pH control, which is managed by chemical dosing; it isn't about sanitizing the media, which is done through disinfection or sanitization steps; and it doesn't aim to increase filtration resistance—backwashing reduces resistance by removing accumulated solids.