Development of microwave-assisted dehydration system for sludge waste treatment

Abstract

A laboratory scale of batch system and continuous system was developed for sludge treatment. Sludge sample from local municipal wastewater treatment plant was chosen as the test material. Okara was chosen as the surrogate sludge sample for the continuous system. This sample was taken from a local beverage manufacturer. Municipal wastewater treatment plants operated by Indah Water Konsortium (IWK) generate a large quantity of sludge. Malaysia generates 3.2 millions cubic meters of domestic de-watered sludge annually, and this figure will increase to 4.3 millions by the year 2005. In applying current drying method of sludge wastes using sand bed, which characterised as surface heating, all the heat must pass through its surface to the sludge (external transport phenomena). The difficulty has always been to transfer the heat from the surface into the core of the sludge (internal transport phenomena), which is limited entirely by its inherent properties of poor thermal diffusivity. Microwave drying is characterised as volumetric heating. The microwaves penetrate directly into total mass of sludge and heat volumetrically from inside out. The internal vapour formation creates a higher pressure gradient between the core and the surface, thus increasing the internal transport phenomena. From this study, the microwave energy can be reached deep to the core of sample at smaller radius. Due to partly loss of energy in microwave drying system, the highest efficiency of microwave drying for batch system is 92% from the ideal value. For the continuous system, moisture extraction rate of 1.02 kg H2O/kWh was achieved when drying was carried out at microwave power of 10.8 kW and belt speed at 3.5 m/h. Microwave energy intensity, feeding rate, conveyor belt speed and product moisture content were inter-related for continuous microwave system.