Sludge treatment need; sand drying bed

Necessity of sludge treatment

Sludge is the semi-solid residue (about 95-99% water) removed from screens, sedimentation tanks, filters and activated sludge units. Raw sludge is a highly putrescible mixture of organic matter, pathogens and heavy metals, so it must be treated before disposal.

1. To prevent nuisance: raw sludge decomposes quickly and gives foul odours and attracts flies and insects.
2. To protect public health: it carries pathogens (bacteria, viruses, helminth eggs) that could spread disease.
3. To reduce volume: untreated sludge has a very large volume; thickening and digestion cut the volume to 1/3-1/10 and make transport and disposal cheaper.
4. To stabilise the organic matter: digestion changes the unstable organic matter into inert, stable matter.
5. To prevent pollution of land, groundwater and streams by leachate.
6. To recover resources: biogas for energy and dried sludge as manure.

Sludge drying bed (dewatering)

Digested sludge (about 90-95% water) is dewatered by spreading it in thin layers on sand drying beds, where water is lost by drainage and evaporation.

 Section                          Plan
  sludge  ->  _____ 20-30 cm layer     inlet pipes
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~    +----+----+----+
  sand 20-30 cm (0.3-0.75 mm)     |bed |bed |bed |  6-8 m x 30 m
  gravel 20-30 cm (graded)        +----+----+----+
  open-jointed underdrain pipes -> filtrate back to plant inlet

Construction:

• Beds are rectangular (6-8 m wide, 20-30 m long), with brick or concrete walls 0.5-0.9 m high.
• Layers from the top: 20-30 cm of coarse sand (effective size 0.3-0.75 mm), 20-30 cm of graded gravel, and an underdrain of open-jointed or perforated pipes laid at a slope of 1 in 100. The filtrate goes back to the treatment plant.
• The area allowed is about 0.1-0.25 m² per person (covered beds are used in rainy climates).

Working:

1. Digested sludge is applied 20-30 cm deep.
2. Water drains through the sand in 1-3 days (about 50-75% of the water), and the rest evaporates by sun and wind.
3. After 10-15 days (in dry weather) the cake has 30-40% solids and cracks; it is removed by hand with forks and the bed is reused. The dried cake is used as manure or for landfill.
4. The sand is renewed after about 10-12 cycles.