In dual media filtration, if the ratio of anthracite size to sand size is close to 2, which outcomes may occur?

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

Answer: (B)

In dual-media filtration, the top coarse layer and the bottom finer layer are designed to stay in separate beds so the water passes through progressively finer pores. The size ratio between these layers helps keep that separation and supports effective depth filtration. If the anthracite size is about twice the sand size, the interface between the layers tends to become unstable and the media can mix. This intermixing undermines the distinct pore structure of each layer, allowing fines and organic matter to move into the lower layer and form mudballs—chunks of accumulated material that clog pores. As these blocks clog more pores, the flow resistance increases, leading to higher head losses. So a ratio near two tends to produce excessive head losses and mudball formation, which is why this is the most likely outcome.