Combustion of sewage sludge with heat recovery

Sludge from sewage treatment plants can accumulate significant amounts of heavy metals, organic toxins and pathogenic micro organisms. Traditional use of sludge in agriculture as a fertiliser is increasingly frustrated by the presence of these impurities. Alternatives for agricultural use have been studied and compared using technical feasibility, total costs and ecological consequences as criteria. From the results it can be concluded that an increasing proportion of the sludge will have to be combusted.

Three alternative process scemes are discussed, consisting of the following components: —filtration —heat recovery —drying —fluidized bed combustion — flue gas purification. The three schemes differ in the way the heat recovery is carried out: —by direct heat transfer between flue gas and the combustion air, —by drying the filtered sludge in a fluidized bed dryer heated by the flue gasses and —by multiple effect drying of the filtered sludge in a set of fluidized bed dryers operated at different pressures.

In an automated computer procedure, the physical dimensions of the equipment components are calculated. By a similar method the capital investments, capital costs, running costs and consequently the integral costs of treatment are calculated.

The results indicate that heat recovery is an important means to decrease the total costs of combustion. As direct heat transfer between the flue gas and the combustion air allows only a limited fraction of the heat of combustion to be recycled, this option is always more expensive. On the other hand, the application of multiple effect drying requires large investments and its costs are somewhat higher than those for an installation which uses only one stage drying.