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Sludge is dried to reduce its moisture content, and this demands a source of thermal energy (or heat).
The heat source is often either an upstream anaerobic digester or a downstream thermochemical process.
There are three primary reasons why drying sewage sludge may be advantageous:
Sludges of different origins and characteristics exhibit different behaviours when drying. These are most readily observed by correlating the mass water flux against the moisture content of the sludge. This correlation normally reveals the three different drying phases:
Sludge drying is intended to remove water, as opposed to destroying the solids as with a thermochemical process.
The solids temperature is generally kept between 60 and 93 °C. The air temperature is kept substantially higher than this range to encourage water evaporation. The drying process is energy-intensive, demanding up to 1400 kWh energy per tonne of water evaporated, although energy consumption can be reduced by up to 20% using energy recovery.
Sludge drying processes generally take three configurations:
For conduction (or conductive, or indirect) drying, the sludge is heated indirectly by bringing it into contact with a surface which is heated either by steam or a hot oil system.
Paddle, disc and thin film dryers are examples of conductive dryers which apply heat indirectly to the sludge solids through coming into contact with a rotating heated surface.
Convective (or direct) sludge dryers are the most commonly used type of dryer for sewage sludge.
The sludge particles are in direct contact with hot gas – air or steam – which provides the heat required to evaporate the water. Since the process demands a significant volume of air, the air is often recirculated prior to venting, to conserve energy.
Solar dryers make use of renewable solar energy to dry sludge. They take the form of large greenhouses into which the sludge is fed either continuously or in batches.
Solar drying provides the lowest energy demand of all dryers, as low as 50 kWh/t. Although heating is provided at zero or minimal cost, energy is demanded by both the mechanical agitation of the sludge solids and deodorisation of the exhaust air.
