Water evaporation requires 800 – 900 kWh/t heat. Little heat is recovered. Sludge dryers also consume power.
- High-temperature dryers, such as drum, fluidized-bed or contact dryers, require high temperature heat, usually supplied as fossil fuel.
- Medium-temperature dryers, such as our HUBER Belt Dryer BT+, are operated with waste heat. If it is supplied at a temperature of 80 – 90 °C, disinfection can be achieved.
- Low-temperature dryers, such as our HUBER Solar Active Dryer SRT, have a large footprint, but need no or little heat.
Fossil fuel is far too expensive to be burned for sludge drying. If there is a lack of heat, it should be provided from a power-heat-cogeneration system, generating 0.6 kWh of power for every 1.0 kWh of heat. Since power is at least 3 times as expensive as heat, the benefit/cost-ratio is almost doubled.
Footprint and costs of solar dryers can by reduced by additional supply of low-temperature heat, e.g. recovered from effluent with a heat exchanger / heat pump. Heat pumps supply about 4 kW of heat per 1 kW of power.