Bart Peeters

Mechanical Dewatering and Thermal Drying of Sludge in a Single Apparatus

This article describes the Centridry® technology (marketed in the UK by Euroby Ltd.) used at the Monsanto wastewater treatment plant in Antwerp, Belgium, for handling excess wastewater-activated sludge. In one single enclosed machine, sludge with an initial solids concentration of 1.5 to 4% w/w dry substance is mechanically dewatered with a high-solids decanter centrifuge and thermally dried in a flash dryer, yielding a final product with a concentration of solids ranging from 80 to 95% w/w dry substance. Operational experience with varying sludge characteristics affecting the fouling sensitivity of the system is presented for the Centridry technology at the Monsanto plant in Antwerp.

Addition of polyaluminiumchloride (PACl) to waste activated sludge to mitigate the negative effects of its sticky phase in dewatering-drying operations

This paper presents a new application of polyaluminiumchloride (PACl) as a conditioner for waste activated sludge prior its dewatering and drying. It is demonstrated at lab scale with a shear test-based protocol that a dose ranging from 50 to 150 g PACl/kg MLSS (mixed liquor suspended solids) mitigates the stickiness of partially dried sludge with a dry solids content between 25 and 60 %DS (dry solids). E.g., at a solids dryness of 46% DS the shear stress required to have the pre-consolidated sludge slip over a steel surface is reduced with 35%. The salient feature of PACl is further supported by torque data from a full scale decanter centrifuge used to dewater waste sludge. The maximal torque developed by the screw conveyor inside the decanter centrifuge is substantially reduced with 20% in the case the sludge feed is conditioned with PACl. The beneficial effect of waste sludge conditioning with PACl is proposed to be the result of the bound water associated with the aluminium polymers in PACl solutions which act as a type of lubrication for the intrinsically sticky sludge solids during the course of drying. It can be anticipated that PACl addition to waste sludge will become a technically feasible and very effective method to avoid worldwide fouling problems in direct sludge dryers, and to reduce torque issues in indirect sludge dryers as well as in sludge decanter centrifuges.

Modeling of the fouling probability of an activated sludge dryer

Abstract Better understanding (and so better control) of the stickiness phenomenon of drying sludge is critical for proper operation of a variety of industrial dryers, as is the case for the combined centrifuge-dryer system used on the Monsanto Europe site in Antwerp, Belgium. In this paper a binary logistic regression analysis is performed on a 4-years data base, which resulted in an empirical model for the evaluation of the dryer fouling probability as function of changes in the Sludge Volume Index (SVI) and the dosing of clay additive and tertiary (flotation) sludge to the main secondary (biological) sludge feed to the system. According to the research results, a decreasing SVI increases the probability of obtaining fouling in the dryer. Furthermore, in periods of low SVI, the fouling probability can be reduced significantly by reducing the clay dosing. This extended analysis confirms the hypothesis formulated earlier (Peeters et al. , 2009a) that a wetter cake (induced by high SVI and low clay dosing) at the beginning of the drying stage of this system is better to avoid drying sludge fouling the dryer wall. Copyright

Centrifuge focus: do particle size changes affect acetate recovery?

Bart Peeters, Gert Callaerts, Benno Van den Bogaert and Jan Crommen from Monsanto Antwerp (Belgium) advocate a detailed investigation of the effect of slurry feed conditions when optimising the performance of a solid bowl decanter centrifuge.