Energy consumption of agitators in activated sludge tanks – actual state and optimization potential

Depending on design capacity, agitators consume about 5 to 20% of the total energy consumption of a wastewater treatment plant. Based on inhabitant-specific energy consumption (kWh PE₁₂₀⁻¹ a⁻¹; PE₁₂₀ is population equivalent, assuming 120 g chemical oxygen demand per PE per day), power density (W m⁻³) and volume-specific energy consumption (Wh m⁻³ d⁻¹) as evaluation indicators, this paper provides a sound contribution to understanding energy consumption and energy optimization potentials of agitators. Basically, there are two ways to optimize agitator operation: the reduction of the power density and the reduction of the daily operating time. Energy saving options range from continuous mixing with low power densities of 1 W m⁻³ to mixing by means of short, intense energy pulses (impulse aeration, impulse stirring). However, the following correlation applies: the shorter the duration of energy input, the higher the power density on the respective volume-specific energy consumption isoline. Under favourable conditions with respect to tank volume, tank geometry, aeration and agitator position, mixing energy can be reduced to 24 Wh m⁻³ d⁻¹ and below. Additionally, it could be verified that power density of agitators stands in inverse relation to tank volume.