Unplanted drying beds

Abstract

Unplanted sludge drying beds are shallow fi lters fi lled with sand and gravel with an under-drain at the bottom to collect leachate. Sludge is discharged onto the surface for dewatering (Figure 7.1). The drying process in a drying bed is based on drainage of liquid through the sand and gravel to the bottom of the bed, and evaporation of water from the surface of the sludge to the air. The design as well as the operation of the drying bed is fairly straightforward, provided the sludge loading rate is well selected and the inlet points for depositing the sludge onto the bed are properly designed. Depending on the faecal sludge (FS) characteristics, a variable fraction of approximately 50-80% of the sludge volume drains off as a liquid (or leachate), which needs to be collected and treated prior to discharge (Tilley et al., 2014). After reaching the desired dryness, the sludge is removed from the bed manually or mechanically. Further processing for stabilisation and pathogen reduction may be required depending on the intended enduse option. When considering the installation of a drying bed, the ease of operation and low cost needs to be considered against the relatively large footprint and odour potential.