In dual-media filtration, if the ratio of anthracite size to sand size is close to 2, what is a likely outcome?

• A. 2, too much intermixing, poorer quality filtrate
• B. 2, excessive head losses, mudball formation
• C. 4, excessive head losses, mudball formation
• D. 4, too much intermixing, poorer quality filtrate

Answer: (B)

In dual-media filtration, the size relationship between the top (anthracite) and bottom (sand) media determines how well the two layers separate and how fines are trapped during filtration. When the anthracite size is about twice the size of the sand, the boundary between layers is not as effective at preventing fines from moving between them, and the system can start to clog more quickly as solids accumulate.

This increased accumulation raises the resistance to flow, so the head losses across the filter become larger. At the same time, fines can clump together under flow and shear forces to form mud-like aggregates, or mudballs, that plug pore spaces and disrupt smooth filtration. Over time, these effects degrade filtration performance and require more frequent backwashing or media rebalancing.

So, a ratio near two tends to lead to higher head loss and potential mudball formation due to reduced effective porosity and increased tendency for fines to consolidate within the bed.