Interest in producing heat and power using municipal wastewater sewage sludge as a fuel is increasing worldwide. Since its water content is initially high, sludge must be dewatered and further dried if it is to serve as an effective fuel for combustion. However, to maximize net energy production, the drying processes must use as little energy as possible.
The water content in sewage sludge comprises both unbound and bound water. Unbound water content is typically extracted using a number of mechanical dewatering techniques. In terms of total solids content (TS), dewatering processes can take sludge from an initial 3–5% to a more solid 25–45% TS with minimal energy expenditure. However, this level of dryness is not sufficient for effective combustion.
To produce an effective fuel, TS levels must be increased. Achieving high level of dryness involves removing any remaining unbound water and substantial bound water content as well. Heat is normally applied to accomplish this by changing the phase of the water from liquid to vapor. Although dewatering is energy-efficient, thermal drying is not. The energy used to thermally dry sludge can be two orders of magnitude greater than the energy used for dewatering. Therefore, to expend as little energy as possible to achieve the needed dryness, conventional dewatering processes clearly must be improved.
This paper describes work carried out to identify promising ways to efficiently enhance the dewatering and drying of sewage sludge. Available dewatering approaches were reviewed and experiments were carried out to examine the relative effects of temperature, atmospheric pressure, and high-power ultrasound. The high-power ultrasound approach seemed to be particularly effective. The mechanisms involved include atomization, microstructural effects, cavitation, and the sponge effect, which work to reduce both internal and external resistances. Applied in the right way, ultrasound could become a very effective way to enhance mechanical dewatering.