Alexis Vanderhasselt

Estimation of sludge sedimentation parameters from single batch settling curves

In 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 flux theory: (i) the traditional approach using zone settling velocity (VZS) 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 different 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 SBSCs. 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 fits to the SBSC. Here, the Cho model turned out to be the most effective one. When cross-validating the Cho model on the dilution experiment VZS-data, however, it was found to be less performing than the Vesilind model in describing the relationship between VZS 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 finding was that with all settling velocity models tested, practical identifiability problems appeared, indicating the need for better experimental designs. Finally, the flux curves associated with the SBSC-estimated Cho parameters were compared with the traditional Vesilind flux curves. Although both types of flux curves generally showed similar trends, the reliability of SBSC-based flux curve predictions is at present insufficient to warrant replacement of the traditional estimation of settling characteristics.

Short-term effects of additives on sludge sedimentation characteristics

The effect of different concentrations of additives on the settling properties of two types of activated sludge were evaluated by means of a Settlometer. In addition to the evaluation of the immediate positive effect of the additive, a method consisting of consecutive cycles of resuspension and evaluation of the settling properties, is proposed to study the stability of the beneficial effect of the additive. First, a sludge from industrial origin was supplemented with talc and with a polymer. Subsequently, a sludge from domestic origin was tested. Both additives were successful in improving the sedimentation characteristics of the sludges. The effect of talc was stable over a period of at least 14 h. However, the positive effect of the polymer was transient because after 4 h the beneficial effect disappeared. The experimental results from the talc addition were analysed by means of a physical model. The latter revealed that the observed increase of the initial settling velocity is smaller than but close to the increase related to the specific weight of the floc.

Sensors to monitor biological nitrogen removal and activated sludge settling

Abstract A brief overview of methods and sensors to characterize activated sludge is presented, summarizing techniques related to two important activated sludge processes: biological nitrogen removal (nitrification and denitrification) and sludge settling. Traditional off-line methods, typically applied in a laboratory environment to determine nitrifying/denitrifying sludge activities and sludge settling properties are briefly described. The main part of the paper covers a more detailed description of on-line sensors which were recently developed to continuously provide information about important activated sludge properties in a full-scale plant. The most important future work in this research field is the development of control strategies based on the data provided by these sensors.

Improvement of activated sludge performance by the addition of Nutriflok 50 S

Abstract By means of rigorously controlled lab-scale activated sludge systems operated in parallel, the effect of the addition of a nutritive supplement on the overall performance of these systems was verified. Nutriflok 50 S is a commercially available product, containing nutrients and flocculating agents. In a first series of tests by means of SCAS (semi-continuous activated sludge) reactors, fed with synthetic wastewater, the positive effect of the supplement (dosed at 5% of the chemical oxygen demand (COD) load) on the sludge volume index (SVI) (39% improvement) and the residual COD (40% improvement) was demonstrated. In a second series, an industrial wastewater was used in a benchscale set-up according to the OECD procedure. The overall efficiency and sludge sedimentation characteristics were compared for two parallel activated sludge systems continuously fed with pharmaceutical wastewater. In the control no additive was given; the other reactor received on a daily basis a dose of Nutriflok 50 S corresponding to 10% of the COD load. The performance of the Nutriflok 50 S supplemented reactor was found to be statistically better in terms of sedimentation characteristics, nitrification and COD removal than the control. Especially when operated under winter conditions (10°C) and at higher sludge loading rates (Bx = 0.26 kg COD/kg VSS.d) the COD removal efficiency was about 35% higher in the treated reactor. Moreover, nitrification could be maintained in the supplemented reactor at low temperatures, whilst it became minimal under those conditions in the control reactor. Also in the second series of tests, sludge sedimentation characteristics improved with Nutriflok 50 S. The SVI was 40% lower for the Nutriflok 50 S amended sludge and the recycled sludge was 27% more concentrated. When operated at higher temperatures (20–26°C) the amended sludge was less sensitive to denitrification linked flotation in the settler. Microscopic examination of the sludge revealed a more diverse microbial community and the presence of higher trophic organisms in the supplemented reactor. These results indicate a potential for considerable improvement, not only of the overall conversion efficiency, but also of the sludge management characteristics when Nutriflok 50 S is dosed to the activated sludge.