Wastewater facilities need reliable solids stabilization and storage to control odors, support biosolids quality, and deliver consistent feed to downstream dewatering—often while managing tight energy budgets and limited footprints. Aerobic digesters and sludge holding tanks address these challenges by stabilizing waste activated sludge (WAS) and keeping biosolids well-mixed and manageable for further processing or disposal.
Aerobic digestion
Aerobic digestion is a wastewater solids stabilization process that uses oxygen and microbial activity to reduce volatile solids, odors, and pathogens while producing a more stable biosolids product. Volatile solids are reduced through endogenous respiration, where some microorganisms begin to decay and are consumed by other microorganisms. Aerobic digestion is commonly used at small- to medium-sized wastewater treatment plants to treat waste activated sludge (WAS). It can be operated with continuous aeration or with aerobic/anoxic cycling.
Conventional approach to aerobic digestion
Aeration-only operation is simple and supports steady biological oxidation using diffused aeration, blowers, and—when needed—controls such as dissolved oxygen (DO) sensors. However, in aeration-only designs the aeration system is used both to provide the oxygen needed for endogenous respiration and to fully mix the basin, which can result in higher energy costs than necessary for oxygen demand alone.
Optimized aerobic digestion
Cycling between aerobic and anoxic conditions can reduce energy demand, improve nitrogen removal through alternating nitrification and denitrification, and help stabilize pH without chemical addition. Cyclic operation typically includes dedicated mixing equipment along with process controls to manage oxygen levels and cycle timing. This allows the aeration system to be right-sized for oxygen transfer while the mixing system provides the energy needed to mix the tank. Conditions in aerobic digesters are often mixing-limited, meaning oxygen demand can be significantly lower than the mixing energy required.
The best approach depends on plant goals, sludge characteristics, basin configuration, blower capacity, and the desired balance of operating flexibility, stability, and lifecycle cost.
Sludge holding
Sludge holding tanks provide storage of biosolids and can also serve as a location for thickening prior to further processing or disposal. Effective mixing maintains a uniform sludge concentration, helps prevent stratification and solids deposition, and supports a homogeneous feed to downstream dewatering equipment.
How we can help
Xylem works closely with consulting engineers and municipalities and understands aerobic digestion and sludge holding challenges. We offer a comprehensive portfolio of products and experienced professionals to help identify the best options for new treatment plants, retrofits, or upgrades. Depending on energy, nutrient removal, and sludge dewaterability priorities, Xylem can help you design the best solution for your application.
