60    DEWATERING AND DRYING RESIDUES USING
SOLAR HEAT ENERGY
Allen C. Chao, Associate Professor
Department of Civil Engineering
North Carolina State University
Raleigh, North Carolina 27695
Kengo Watanabe, Associate Professor
Department of Environmental Resources
Tokyo University of Agriculture & Technology
Fuchu, Tokyo, JAPAN
Seishu Tojo, Assistant Professor
Department of Environmental Resources
Tokyo University of Agriculture & Technology
Fuchu, Tokyo, JAPAN
INTRODUCTION
A drying bed can be considered as a very primitive solar sludge drying device. It allows initial
mechanical dewatering to remove some excess free water that is contained in the sludge. The sludge
drying is achieved by utilizing the solar thermal energy during the subsequent drying period. Drying
beds have been used for many decades without significant changes in their designs and operations.
The major advantage of using the drying bed is that very little energy and caring are required during
the drying period. The sludge is dried to a high solid content that cannot be achieved using mechanical
dewatering methods. Its major drawback, however, is that the efficiency of using solar heat energy is
extremely low thus a large land space is inevitably required. In some parts of the world especially
Asian countries such as Japan and China, drying beds are seldom used because of high land cost.
For some agricultural or industrial sludge, mechanical dewatering may not be adequate to reduce
the water content level to an acceptable level. Thermal drying of the sludge is needed to achieve an
even lower moisture content level. A joint effort between North Carolina State University and Tokyo
University of Agriculture and Technology has been undertaken for many years in order to devise a
system that will efficiently utilize the solar heat energy for drying a variety of industrial, municipal or
agricultural solid residues so that the final products are easier to store, transport and/or incinerate.
The passive system that was proposed for curing tobacco leaves and grains has been modified for
handling and drying sludge-like industrial and agricultural solid residues. The proposed system is
simple in its construction but its major advantage is that the heat generated is directly used thus
avoiding the problem of heat loss that is often associated with other types of solar energy system.
Additionally, the drying system utilizes a patented "drum surface drying" method to enhance the
drying process. The equipment, test results and simulation of the proposed system are delineated in
this paper.
EQUIPMENT
The proposed drying system is similar to those reported in previous literature'"3 with slight modifications. As shown schematically in Figure 1, the system consists of a solar housing, heat absorbers
and a rotary sludge drying drum. The solar housing is made of semicircular structure covered with
transparent vinyl sheet to allow penetration of solar radiation. The floor bed inside the transparent
housing is covered with vinyl sheet at the bottom to prevent evaporation of water from the soil, a
middle layer of clay sand and a top layer of pebbles. The pebbles are painted black to function as the
heat absorbers. A cylindrical steel drum is placed inside the transparent housing. One end of the drum
is sealed but provided with a loading port allowing manual loading and discharge of the solids to be
dried, and the other end is connected to an exhaust fan with a 100 mm dia. pipe. Four rollers are used
47th Purdue Industrial Waste Conference Proceedings, 1992 Lewis Publishers, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.
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