Settling characterisation using on-line sensors at a full-scale wastewatertreatment plant

An on-line settlometer is presented which automatically records sludge sett ling curves. From these curves the sensor deduces the initial hindered zone settling velocity (Vs). In order to evaluate the information gathered by t he sensor, the instrument was used at an information-rich full-scale wastew ater treatment plant (Klagshamn, Sweden). This plant is operated as a singl e sludge post-denitrification activated sludge system with an external carb on source. With the sensor a diurnal pattern of Vs could be detected. This pattern was mainly caused by the diurnal change of the sludge concentration . Using the Vesilind equation it was possible to standardise Vs to the slud ge concentration (Vs'). The variation in the standardised Vs' was less comp ared to the experimental Vs. Still, it was possible to detect a sudden drop in Vs' on nearly daily basis. Ar two of these instances rising sludge was even detected in the settlometer during the 35 min lasting sedimentation pe riod. The fact that this could be observed is remarkable as the water tempe rature was only 7.5 degrees C, the bulk liquid entering the settlometer was aerobic and only a very low amount of carbon sources was found in the effl uent. Hypotheses are put forward that could give an explanation for the occ urrence of rising sludge under these circumstances. In the observed post-de nitrification plant a positive correlation could he found between the airfl ow to the reaeration tank and the sedimentation velocity.