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scale waste water treatment plant

An on-line settlometer has been presented which automatically records sludge settling curves. From this curve the sensor deduces the initial hindered settling velocity (Vs). In order to evaluate the information gathered by the sensor, the instrument was used at an information rich full scale waste water treatment plant (Klagshamn, Sweden). This plant consists of a primary sedimentation tank followed by a single sludge post denitrification activated sludge system operated with an external carbon source. With the sensor a diurnal pattern could be detected in Vs. This pattern was mainly caused by the diurnal change in the sludge concentration. Using the Vesilind equation it was possible to standardise Vs for the sludge concentration. The variation in the standardised Vs’ was less than for the experimental Vs. Still it was possible to detect a decrease in Vs’ on nearly daily basis. At two of these instances rising sludge was detected in the settlometer during the 35 minutes lasting sedimentation period. The fact that this could be observed is remarkable as the water temperature was only 7,5 °C, the bulk liquid entering the settlometer was oxic and only a very low amount of carbon sources was found in the effluent. Hypotheses are put forward that could give an explanation for the occurrence of rising sludge. In the observed post-denitrification plant a positive correlation could be found between the airflow to the re-aeration tank and the sedimentation velocity.