Wasteactivated

Waste-activated sludge, commonly abbreviated WAS, is the portion of biomass removed from the activated sludge in a wastewater treatment plant to maintain the desirable solids concentration and biological activity. The term is sometimes written as waste activated sludge or WAS. WAS comprises microorganisms and suspended solids that accumulate in the aeration basin as sludge grows, decays, and is intentionally withdrawn to control sludge age, mixed liquor suspended solids (MLSS), and system performance. In typical plants, a controllable fraction of the mixed liquor is wasted at intervals to keep the solids concentration and the effective residence time within target ranges; the exact rate depends on influent characteristics, plant design, and operating objectives.

After withdrawal, WAS is typically subjected to thickening to reduce volume, followed by stabilization which may involve anaerobic or aerobic digestion, to reduce odors and pathogens and improve biodegradability. The stabilized sludge can then be dewatered by centrifuges or belt presses and disposed of or reused as biosolids, subject to local regulations. In many designs, WAS is not treated in isolation from primary sludge, but is combined into the overall sludge handling stream.

Environmental and regulatory considerations include odor control, pathogen removal, and compliance with biosolids-use guidelines. Managing WAS is essential for maintaining the efficiency of the activated sludge process, preventing excessive biomass buildup, and ensuring downstream processing is feasible.