Extending and calibrating a mechanistic hindered and compression settling model for activated sludge using in-depth batch experiments.

Currently, no mechanistic model is available in wastewater industry that can accurately describe the batch settling behaviour of activated sludge. Such a model, which is based on the fundamental mass and force balances for water and solids, is extended and applied in this work and excellently describes batch settling experiments for sludges originating from two different wastewater treatment plants. The mechanistic model contains a Kynch batch density function f(bk) (hindered settling) and an effective solids stress function sigma e (compression). Initial settling velocities were obtained from detailed spatio-temporal dynamic solids concentration profiles measured with the aid of a radiotracer [De Clercq, J., Jacobs, J., Kinnear, D.J., Nopens, I., Dierckx, R.A., Defrancq, J., Vanrolleghem, P.A., 2005. Detailed spatio-temporal solids concentration profiling during batch settling of activated sludge using a radiotracer. Water Res. 39(10), 2125-2135]. Moreover, inverse modelling calculations were performed using the same data set. Both calculations showed that (1) the power function of Cole gave acceptable results and (2) a single effective solids stress function could be found when a time-dependent compression solids concentration C C was considered. This compression solids concentration is found just below the sludge blanket and is readily calculated from the solids concentration profiles. Given these time-evolutions, the effective solids stress values exhibit a uniform logarithmic relationship with the difference between the solids concentration and the compression solids concentration. The descriptive power of the model indicates a good potential for wider applicability of the model.

Cobalt sorption onto anaerobic granular sludge: isotherm and spatial localization analysis.

This study investigated the effect of different feeding regimes on the cobalt sorption capacity of anaerobic granular sludge from a full-scale bioreactor treating paper mill wastewater. Adsorption experiments were done with non-fed granules in monometal (only Co) and competitive conditions (Co and Ni in equimolar concentrations). In order to modify the extracellular polymeric substances and sulfides content of the granules, the sludge was fed for 30 days with glucose (pH 7, 30 degrees C, organic loading rate=1.2 g glucose l(-1) day-1) in the presence (COD/SO4(2-)=1) or absence of sulfate. The partitioning of the sorbed cobalt between the exchangeable, carbonates, organic matter/sulfides and residual fractions was determined using a sequential extraction procedure (modified Tessier). Experimental equilibrium sorption data for cobalt were analysed by the Langmuir, Freundlich and Redlich-Peterson isotherm equations. The total Langmuir maximal sorption capacity of the sludge fed with glucose and sulfate loaded with cobalt alone displayed a significantly higher maximal cobalt sorption (Qmax =18.76 mg g-1 TSS) than the sludge fed with glucose alone (Qmax =13.21 mg g-1 TSS), essentially due to an increased sorption capacity of the exchangeable (30-107%) and organic/sulfides fractions (70-30%). Environmental scanning electron microscopy coupled with an energy dispersive X-ray analysis of granular cross-sections showed that mainly iron minerals (i.e. iron sulfides) were involved in the cobalt accumulation. Moreover, the sorbed cobalt was mainly located at the edge of the granules. The sorption characteristics of the exchangeable and carbonates fractions fitted well to the Redlich-Peterson model (intermediate multi-layer sorption behaviour), whereas the sorption characteristics of the organic matter/sulfides and residual fractions fitted well to the Langmuir model (monolayer sorption behaviour). The organic matter/sulfides fraction displayed the highest affinity for cobalt for the three sludge types investigated.

Detailed spatio-temporal solids concentration profiling during batch settling of activated sludge using a radiotracer.

In building and tuning good settling models for secondary clarifiers of wastewater treatment plants, there is a need for measured continuous solids concentration profiles during batch settling. Conventional measuring techniques have difficulties in recording this kind of data, either because they are invasive, or because of the low solids concentration and/or solids density of activated sludge. This paper investigates a novel non-invasive measurement technique borrowed from nuclear medicine, using a solids radiotracer and gamma cameras, to obtain solids concentration profiles during the batch settling of activated sludge, in a pilot-scale column with a height of 1m. The technique does not disturb the settling process, does not alter the settling characteristics, gives profiles every minute and every few millimeters, and is capable of measuring in a range of 0-25 g/l with high accuracy. Dynamic solids concentration profile measurements were performed for sludges of different wastewater treatment plants, and at different initial concentrations. The results show a quantitative representation of the settling process, and reveal hindered and compression settling.