On-line version ISSN 0378-4738
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 mℓ/g, at an average SVI decrease of 14.8 mℓ/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 mℓ/g, at an average -14.8 mℓ/g SVI change per 1°C Tr variation.
Keywords : activated sludge; mixed liquor suspended solids (MLSS); model; sludge volume index (SVI); settleability; temperature; wastewater.