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Water SA

On-line version ISSN 1816-7950
Print version ISSN 0378-4738

Water SA vol.34 n.2 Pretoria Feb. 2008


Batch and automated SVI measurements based on short-term temperature variations



WH RössleI; WA PretoriusII

IERWAT Chair in Wastewater Management, Department of Chemical Engineering (Water Utilisation Division), University of Pretoria, Pretoria 0001, South Africa
IIPretWatSpes, Ysterhoutlaan 59, Val de Grace 0184, South Africa





Effects of short-term temperature variations on the sludge volume index (SVI) are evaluated with batch and automated mixed liquor suspended solids (MLSS) settling tests. The test-cylinder environment and meteorological conditions have a direct influence on the MLSS sample temperature (Ts). A T change of 4.3°C over the 30 min settling test duration results in an inverse SVI change of 63.0 m/g, at an average SVI decrease of 14.8 m/g per 1°C T increase. T compensation or control during routine SVI tests is not common practice, partially due to a lack of temperature-controlled equipment and an absence of Ts-based MLSS settling models. A practical solution is found to reduce Ts variations experienced before and during batch MLSS settling tests. An automated MLSS settling meter is used to demonstrate a semi-continuous on-line method to determine SVI at the operational reactor temperature (T) of a full-scale plant. Basic and best-fit SVI models are obtained from the SVI data generated over diurnal periods, based on MLSS concentration and T fluctuations. These SVI models confirm the inverse dependence of SVI on temperature for the site-specific conditions. A diurnal T fluctuation of 1.8°C results in an SVI change of 26.6 m/g, at an average -14.8 m/g SVI change per 1°C Tr variation.

Keywords: activated sludge, mixed liquor suspended solids (MLSS), model, sludge volume index (SVI), settleability, temperature, wastewater


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Received 9 November 2007
Accepted in revised form 12 March 2008