Proceedings

Influence of premixing on the rheological properties and drying characteristics of municipal sewage sludge

Charlou, C., Sauceau, M. and Arlabosse, P., University of Toulouse, France

(free)

Abstract:

The sticky phase of the sludge, which occurs in disc and paddle dryers between 45 and 60% of dry solid contents, is still misunderstood. To investigate the influence of the initial structure of the product on the sticky phase, a pre-treatment in a planetary mixer has been applied to an extended aeration sewage sludge with 20% of dry matter. The impact of this preprocessing on the sludge structure was characterized through particle size analysis and rheology. Preprocessing decreases the apparent viscosity and the size of the particles in the pasty sludge. The rheological behavior follows the Ostwald and Herschel-Bulkley models. The flow behavior index remains constant while the consistency index and the yield stress decreases with the preprocessing treatment. Drying experiments, performed in a batch agitated dryer, emphasizes that the preprocessing has no influence on the location and the magnitude of the sticky phase.

Introduction: 

Sludge management is an ever-increasing problem worldwide due to environmental pollution and energy consumption. 1,125,000 and 10,135,745 tons of dry matter were produced in year 2007 in France and in Europe (Salado 2008), respectively. Sewage sludge is commonly land spread as complementary fertilizer in agriculture, subjected to a thermal treatment, or disposed of in landfills. In this context, drying cannot be overlook: conventional combustion technologies and landfilling require a minimal dry solid content, which can only be reached by partial drying of the mechanically dewatered sludge. For those that promote sludge use in agriculture, drying produces dried pellets easy to store, handle and transport. Moreover the product is stabilized and sometimes free of pathogens. Many technologies are available on the market but indirect dryers have by far the lowest specific energy consumption (Arlabosse, Ferrasse et al. 2011). In these dryers, the heat necessary to vaporize the moisture is transferred to the sludge through a heated surface, over which the product is transported. These dryers are composed of an internal rotor inserted in a cylindrical double wall external envelope. The design of the rotor, which is specific to each technology, has a crucial importance on the overcoming of the sticky phase and thus on the operation of the drying process. In spite of several hundred of references worldwide and many years of dryer operation, the sticky phase of the sludge, which occurs between 45 and 60% of dry solid contents (Ferrasse, Arlabosse et al. 2002), is still misunderstood and the design of these rotors not optimal. Recently, multiple negative experience feedbacks came to compromise the development of these technologies. Research is needed to better understand this sticky phase and the mechanisms involved in the transition from a continuous material towards a granular packing. To investigate if the initial structure of the product has an influence on the sticky phase, a pre-treatment in a planetary mixer for 20 minutes has been applied to conventional municipal sewage sludge. The impact of this preprocessing on the sludge structure was characterized through particle size analysis and rheology. Drying experiments were also performed in a batch agitated dryer, specifically designed to measure the mechanical torque required to maintain a constant stirrer velocity in the drying system. Indeed, a combination of dynamic and flow measurements in a rotating rheometer could have been a way to understand the evolution of the sludge structure during drying. But their range of operation is limited and, as the sludge concentration increases, viscosity measurement becomes problematic.

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