Dewatered sludge is generally easier to handle than thickened or liquid sludge. For some options for disposal or further treatment dewatering is necessary:

• mechanical sludge drying

• sludge composting

• landfilling

• trucking over longer distances

Several techniques are used in dewatering devices for removing moisture. Some of these techniques rely on natural evaporation and percolation to dewater the solids. In mechanical dewatering devices, mechanically assisted physical means (filtration, squeezing, capillary action, centrifugal separation, compaction) are used to dewater the sludge more quickly. The most important mechanical devices are:

• solid-bowl centrifuge (25 – 30%TS)

• belt-filter press (25 – 30%TS)

• recessed-plate filter press (30 – 40%TS)

These techniques are not discussed here since they are economically and ecologically not feasible in small and rural waste water treatment plants.

A technique close to nature and very effective is dewatering in drying beds (figure 5). The principal advantages of drying beds are low costs, infrequent attention required, and high solids content in the dried product, especially in arid climates. Disadvantages are the large space required, effects of climatic changes on drying characteristics, labour-intensive sludge removal, insects and potential odours. The most common type are sand drying beds, where the sludge is placed on a bed 200 to 300 mm layer and is allowed to dry. Sludge dewaters by drainage through a drainage system consisting of gravel (200mm), coarse (75mm), fine sand (150 mm) and drainage pipes and by evaporation from the surface exposed to the air. To prevent water from percolating into the soil there has to be a sealing foil.

The drying area is partitioned into individual beds of a convenient size so that one or two beds will be filled in a normal loading cycle (depends on gravity thickener volume). The interior partitions commonly consist of two or three planks, one on top of the other, to a height of approx. 400 to 500 mm. They can be made of concrete, as well. The outer boundaries may be of similar construction or may be earthen embankments or concrete.

Table 4: Typical area requirements for open sludge drying beds (may differ due to extreme climates)

Figure 5: Sludge drying beds

An alternative to this simple sludge drying are sludge humification beds, where the drying process is supported by plants (figure 6). Furthermore biological activity is enhanced, stabilisation is continued and a soil-like character of the sludge is achieved.

Sludge humification beds are constructed like sludge drying beds, only the depth is increased to approx. 1m. Two different kinds of plants are use: reed or grass. Reed is planted into the humification beds and small amounts of sludge (2 – 2.5 kgTS/m²) are applied every two weeks and after 8 to 10 years the dewatered and humificated sludge can be taken out and used further.

Sludge humification with grass works a little different. A higher amount of sludge is applied to drying beds (20 – 25 kgTS/m²) and after some days of drainage grass is seeded into the sludge manually. The grass grows and the roots penetrate the sludge. They provide ventilation and dewatering in deeper layers (not only at the surface like at sludge drying). After a few months (3 – 7) another layer of sludge is applied onto the grass and grass is seeded again. This can be repeated for 2 to 5 years. The sludge is dewatered up to 40 – 60 %TS and pathogens are reduced significantly. An example of sludge after humification can be seen in figure 7.

Figure 6: Sludge humification plant in operation

Figure 7: Sludge after humification