During backwash, what bed expansion percentage must be achieved for filters using granular activated carbon?

Achieving a 50% bed expansion during the backwash process for filters using granular activated carbon is essential for effective cleaning. This level of expansion allows for the thorough flushing of accumulated solids, organic materials, and other contaminants trapped within the filter media. During backwashing, water flows upward through the filter bed, lifting the carbon granules and creating space between them. At 50% expansion, the particles have enough room to move freely, which enhances the removal of particulate matter and helps to redistribute the filter media evenly before normal operation resumes. This ensures optimal performance for the activated carbon, which is crucial for effective water treatment, particularly in removing specific contaminants and odors.

In contrast, lower expansion percentages may not provide adequate cleaning, leaving behind residual contaminants. Higher percentages, while seemingly beneficial, might lead to unnecessary wear and tear on the media or could cause operational issues. Hence, targeting a 50% bed expansion is the industry's standard practice for optimizing the efficiency and longevity of granular activated carbon filters during backwash processes.