Updated: July 2002

1. Summary

The stickiness of sludge creates problems in the drying process, but is a benefit in the pelletizing process. Looking at the process in this way, this pelletizing/drying unit makes use of the stickiness of sludge, and enable the simultaneous drying and pelletizing of the sludge. Sludge originating from various waste water treatment plants is first put through a dehydrating process, followed by either treatment or disposal.
However, irrespective of recycling or disposal, handling of the sludge is posing a problem in most cases, because it is hard to deal with owing to its high moisture content. The development of this pelletizing/drying unit has been pursued with an emphasis on improved combustion as well as the recycling of sludge for other uses by reducing it to dense, dried pelletized materials.
Photo 1 illustrates dried pelletized materials.

Photo 1

2. Structure

Figure 1 illustrates the structure of the unit.

Fig. 1

3. Fundamentals

Utilizing the stickiness of the sludge, a thin film of sludge is applied to dried grains, and simultaneously dried with hot air thus producing these dried and pelletized material.
Figure 2 illustrates the drying and pelletizing mechanisms schematically.

Fig. 2

4. Features

- Produces dried and dense pelletized material
The dried material is consist of dense, hard grains containing hardly any fine powder, because the pelletization mechanism involves a coating and rolling motions that greatly enhances the density of the grains.
- Burns smell seldom emitted
Because the sludge is dried at roughly a wet-bulb temperature, denaturalization or thermal decomposition of organic substances never occurs, so there is seldom any burning odor.
The dried material exudes very little odor, allowing them to be suitably restored to green farmlands, and the negligible odor of exhaust gas that is easily treated.
- Low exhaust gas temperature and high thermal efficiency
Evaporation from the surface of the sludge tends to dry it inside the unit. To realize this, adequate hot air is applied.
Consequently, the temperature of the exhaust gas is low, and therefore the thermal efficiency is high, thereby making the unit compact in size.
- Copes with variations in the moisture content of the supply sludge and the temperature of the hot air
The quantity of the sludge to be supplied is always controlled by the detecting the torque applied to the agitator blade in order to keep the moisture content of the fluidized bed constant.
An automatic control responding to the variation in the moisture content of the supply sludge or the temperature of the hot air at the inlet port has become practicable.
Moreover, it allows the moisture content of the dried sludge to remain constant.

5. Application

Sewage mixed raw/digested sludge
Night soil excess activated sludge
Paper and pulp factory excess sludge
Other organic sludges