Schlammalter

Schlammalter, in wastewater engineering, is the average time that the sludge solids remain in the biological treatment stage. It is commonly referred to as the solids residence time (SRT) and is a key parameter for activated sludge processes. It measures how long the active biomass stays in the system before being wasted or removed.

The SRT is conceptually defined as the mass of solids in the reactor, divided by the mass of solids wasted per unit time. In practice it is estimated from measurable quantities such as the mixed liquor suspended solids (MLSS), the volume of the aeration tank, and the rate and concentration of wasted sludge. A common approximation is SRT ≈ (MLSS × Volume) / (WasteFlow × MLSS_waste). This reflects that longer wasting intervals or higher solids concentrations increase the sludge age.

Typical values vary with treatment goals. For nitrification, common SRT ranges are about 5 to 15 days; for plants with longer nutrient removal targets, SRTs can be longer (often 10 to 25 days). Short SRTs reduce the residence time of slow-growing organisms and can limit complete nitrification or nutrient removal, while very long SRTs can lead to slower system response and potential buildup of certain biomass characteristics.

Schlammalter is related to, but distinct from, hydraulic retention time (HRT). HRT concerns water movement, while SRT concerns solids retention. The two are connected through the wasting rate and biomass production in the system.

Measurement and control rely on monitoring MLSS and MLVSS, and adjusting sludge wasting rate, reactor volume, or process conditions to achieve the desired SRT. Correctly managed Schlammalter influences treatment efficiency, sludge settleability, and nutrient removal performance.