Which statement best describes young sludge in the activated sludge process?

• A. Excessive white billowy sludge, low MLSS, low sludge age, high F/M.
• B. Dense dark sludge with high MLSS.
• C. Sludge with low solids but very old age.
• D. No sludge formation occurs during steady state.

Answer: (A)

Young sludge is biomass that has recently entered the aeration basin and is still actively growing. It appears fluffy and white, with a low solids concentration (low MLSS) because the biomass hasn’t aged or settled into dense aggregates yet. Because there is abundant readily biodegradable substrate relative to the small, newly formed biomass, the food-to-microorganism ratio is high (high F/M) and the sludge age is low. This combination describes light, billowy, low-density sludge typical of young, developing biomass. Older sludge, by contrast, tends to be dense and dark with high MLSS, and very old age with low solids isn’t characteristic of active, young sludge. And sludge formation does occur at steady state in an active system.