AbstractÐIn this paper a comparison is made between two means of obtaining the parameters for the settling velocity models that are at the core of the solid¯ux theory: (i) the traditional approach using zone settling velocity (V ZS ) data obtained from a dilution experiment and (ii) a new direct parameter estimation method relying on a single batch settling curve (SBSC). For four distinct sludges, settling curves were recorded at dierent sludge concentrations (X ) and the Vesilind parameters were calculated in the traditional way. The value of the resulting model was evaluated by cross-validating it on its ability to describe complete SBSC's. Provided the dynamics of the sludge blanket descent were moderate a settler model incorporating these traditional Vesilind parameters could reasonably match the experimental batch settling curves. However, when the dynamics of the sludge blanket descent were fast, the Vesilind model failed. It was tried whether other settling velocity models could result in better ®ts to the SBSC. Here, the Cho model turned out to be the most eective one. When cross-validating the Cho model on the dilution experiment V ZS -data, however, it was found to be less performing than the Vesilind model in describing the relationship between V ZS and X. The fact that the Vesilind model is superior to the Cho model in describing such relationships, while the Cho model is better in describing complete settling curves, clearly points out that current settling models are still very empirical in nature. An important ®nding was that with all settling velocity models tested, practical identi®ability problems appeared, indicating the need for better experimental designs. Finally, the¯ux curves associated with the SBSC-estimated Cho parameters were compared with the traditional Vesilind ux curves. Although both types of¯ux curves generally showed similar trends, the reliability of SBSCbased¯ux curve predictions is at present insucient to warrant replacement of the traditional estimation of settling characteristics. # 1999 Elsevier Science Ltd. All rights reserved
The control of nitrogen removal processes in activated sludge systems requires not only on-line values of the parameters such as ammonium, nitrate and nitrite, but also an evaluation on the nitrifying and denitrifying biomass activity, influent toxicity and carbon source requirements for denitrification. This paper describes four different biosensors, two on denitrification and two on nitrification. They all measure the biological activity of the nitrifying or denitrifying bacteria. Three of them measure nitrogen and carbon species, the fourth biosensor measures influent toxicity. These instruments open new perspectives for efficient on-line monitoring of wastewater treatment plants.
Two novel sensors, Settlometer and Sehlumosed, were used to evaluate the beneficial effect of three additives on sludge-settling properties. Four different activated sludges were used in the study. The settling characteristics of the sludges were evaluated in automated batch settling tests by means of the start-up time (initial lag period), initial settling velocity, stirred sludge volume, and final transparency of the supernatant. Different sludges responded in distinct ways to the dosage of additives. Talc was typically more effective than coal in terms of improving settling velocity and reducing sludge volume. Chemical polymer surpassed the two particulate additives in terms of final transparency. Furthermore, different sludge sedimentation parameters of a particular sludge were not affected to the same extent by a specific additive. This substantiates the need to describe settling properties with more than one parameter. The absolute values of the sedimentation properties were found to depend on the type of sensor used. Water Environ. Res., 71,355 (1999).
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