Section 18. MISCELLANEOUS WASTES
USING SOLAR ENERGY
FOR DRYING INDUSTRIAL SLUDGE IN JAPAN
Allen C. Chao, Associate Professor
Department of Civil Engineering
North Carolina State University
Raleigh, North Carolina 27650
Fusakazu Ai, Professor
Kengo Watanabe, Associate Professor
Department of Agricultural Engineering
Tokyo University of Agriculture and Technology
Tokyo, Japan
The concept of using solar energy for drying waste sludge is not new in wastewater treatment. For decades, sludge drying beds have been adopted by modern wastewater treatment
plants to reduce the moisture content of biologically treated sludge material. The sludge to
be dewatered is spread onto the drying bed and left there for an extended period of time to
be dried directly under the sun. The practice requires very little energy but is extremely inefficient, and results in very large requirements of land space, especially for regions with cold
and wet weather.
To save land space requirements and speed up the sludge dewatering process, mechanical
sludge dewatering methods such as vacuum filtration and centrifugation have been adopted
to replace sludge drying beds. Among the many disadvantages of the mechanical methods
is their high energy requirements, which make their operation relatively expensive. It was
estimated that 55-60% of a wastewater treatment facility's operation costs are devoted
to sludge handling [ 1 ].
The organic matter contained in the sludge can be incinerated to obtain heat energy, but
direct incineration of mechanically dewatered sludge is often not feasible because its
moisture is too high [2,3]. Too much energy is needed to dry the sludge before it can be
effectively incinerated.
A simple and effective solar energy sludge drying system has recently been developed at
North Carolina State University (NCSU) to reduce the energy requirement and operating
costs of dewatering and drying sludge. Using the concept of Huang and Ozisik for using
solar energy to dry agricultural crops [4], a bench-scale model with a capacity of 3.5 kg has
been built and tested for drying municipal sludge. The results indicated that after the initial
dewatering stage in which free water was removed by gravity drain, the sludge could be dried
by use of the hot air that was generated in a solar collector. The experimental work was
conducted in March 1981 at Raleigh, North Carolina (latitude 35 40'40"N). The drying air
had an average temperature of 20-30 C higher than the ambient temperature during the
daytime when the sun was shining. The dried sludge is in the form of small pellets that have
dry surface layers and a small core region of relatively higher moisture content. The overall moisture content is approximately 75% after a drying time of 14 hours [51. These
pellets can be burned easily like charcoal.
The aforementioned experimental results have drawn the attention of some Japanese
researchers. Japan is extremely limited in land space and energy sources. The solar sludge
drying system could assist in alleviating the sludge disposal problem. Because of the extremely high land costs, a direct landfill of the dewatered sludge and other types of solid
wastes would be considered very costly. Therefore, it is necessary to reduce the volume of
the material to be buried. The incineration process can be used to achieve such an objective.
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