Improving the heat transfer properties of waste activated sludge by advanced oxidation processes

The disposal of Waste Activated Sludge is a major problem. The paper presents several disposal routes. The ultimate steps in these routes are indirect drying and subsequent incineration. Although the design of the indirect dryers is straightforward from a required heat input, operating temperature difference between sludge bed and heating medium and the heat transfer coefficient, this coefficient remains largely unknown. The present research determines this heat transfer coefficient through measuring the apparent thermal conductivity of the sludge flocs at various degrees of dryness. Values of the calculated convective heat transfer coefficient are dependent on the characteristics of the sludge cake and on the mixing mechanisms of the dryer, represented by an average contact time between drying/dried sludge bed and the heat transfer surface. The experimental results are used to calculate the apparent thermal conductivity of the sludge (between 0.35 and 0.5 W/mK), the convective heat transfer coefficient and the overall heat transfer coefficient (including particle convective and bed conductive resistances). This overall coefficient is needed in the design of sludge dryers. The paper moreover shows that sludge peroxidation using the Fenton reaction improves both the dewatering characteristics by some 15 to 20 % , and the heat transfer coefficient by 13 to 20 %. Peroxidation using peroxymonosulphate achieves about half these gains. A final assessment compares both a the design of drying untreated and peroxidetreated sludge, and the comparison of the design approach with industrial drying data. Results are in fair agreement and stress the importance of limiting the presence of a static layer of dried sludge on the heat transferring surface, as is achieved in better way in a fluidized bed dryer than in a multiple hearth dryer.