Velocity and solids distribution in circular secondary clarifiers: Full scale measurements and numerical modelling

Results of full-scale measurements of flow velocity and solids distribution in a circular secondary clarifier are presented and evaluated using numerical modelling. The flow velocity distribution was measured by means of an ultrasonic velocity profile monitor. A two phase three dimensional model was applied to gain a better understanding of the observed flow patterns. The calculation was carried out for a two dimensional field assuming axisymmetric conditions. The momentum, continuity and k– turbulence equations were solved. Solid transport equations for a coupled system were employed. Circulating currents were found by modelling and in practice. The full scale velocity components in radial direction as well as the solids distribution showed a satisfactory agreement to the predicted values. At the outer tank rim no settled sludge was predicted by modelling whereas a very thin sludge blanket was found in reality, however. It is assumed that conveyance of the settled sludge by the scraper has not been properly described for the very outer tank zone. The solids content at the tank bottom was around 12 g l−1. The maximum radial velocity components varied in the range of 10 to 17 mm s−1. High velocity components of up to 48 mm s−1 towards the tank bottom were found in the inlet region. By numerical modelling those high numbers could not be matched. The calculated values were being in the range of 21 mm s−1.