Your web browser is out of date.
Update your browser for better security, speed and to get the best experience on this website.
Sludge characteristics depend on the original feedwater quality and the type of unit operation from which sludge is discharged.
Sludge treatment and processing comprises sludge drying, dewatering, thickening, stabilisation, conditioning, thermochemical processes and anaerobic digestion.
Sludge characteristics depend on both the original feedwater quality and the type of the unit operation from which the sludge is discharged.
For conventional municipal wastewater treatment, the two main sludge streams are from the primary sedimentation stage, producing primary sludge, and the secondary biological treatment stage, producing waste activated sludge (WAS) if the biological treatment is based on the activated sludge process.
Sludge stabilisation processes are those which reduce its odour and putrescence and its level of pathogenic organisms. This is generally achieved either chemically or biologically.
Sludge thickening and dewatering processes normally require pre-treatment to assist the separation of the water from the solids. Pre-treatment is referred to as 'conditioning', which can take two forms − chemical or thermal.
Anaerobic digestion is the most widely used sludge stabilisation process. AD stabilises the sludge biologically in the absence of air. As with most sludge and wastewater unit processes, the design and performance of AD depends on the feed characteristics.
In sludge stabilisation, the amount of volatile solids is reduced by converting them to a biogas (methane CH4, carbon dioxide CO2 and water H2O). The biogas then needs further processing for recovery and reuse of its methane content.
Thickening processes remove water from the sludge to reduce its volume by mechanical means at relatively low energy consumption rates. Processes used include sedimentation, dissolved air flotation, centrifugation and draining through a belt or a rotating drum.
Thickening processes concentrate the sludge by removing part of the free water so that the finished product retains the liquid, free-flowing characteristics of the feed sludge and so can still be conveyed by pumping.
Sludge dewatering employs a variety of different processes, such as centrifugation or pressing, to reduce the sludge moisture content by mechanical means. The energy consumption is higher than that associated with thickening processes and the moisture content of the processed product correspondingly lower.
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.
Thermochemical methods employ high temperatures combined with, for some processes, high pressures for the end treatment of sludge.
These processes are all able to recover resources from the sludge, primarily energy but also potentially phosphorus, whilst also substantially reducing the sludge solids.
