RECOVERY AND APPLICATION OF SOLIDS
FROM CRAB-PROCESSING WASTEWATER
Allen C. Chao, Associate Professor
Leon Tzou, Graduate Assistant
Clyde T. Young, Professor
Department of Civil Engineering
North Carolina State University
Raleigh, North Carolina 27650
INTRODUCTION
The seafood industry has been estimated to consume more than 2.5 billion gallons of water annually in processing the harvest into marketable products [1]. Most of the consumed water is discharged
as processing wastewaters which, in the past, received little treatment prior to their release into receiving bodies. However, with the advent of more stringent effluent limitations being imposed on industrial discharges, the seafood industry has planned three basic strategies to achieve the required treatment levels of their effluent without imposing too much financial burden on the industry. These
strategies are: (1) minimize the use of processing water; (2) recovery of the solid portions from the
waste discharge; and (3) recovery of the dissolved portions from the waste discharge.
To minimize the water usage, a research project has been undertaken by Carawan et al. [2] to
investigate methods that can be used by the food industry to reduce the water usage. The seafood
industry is one of the many food industries that can achieve savings of processing water by changing
or modifying the existing processing techniques and equipment. If the recommendations are followed,
the water usage and hence the resulting wastewater discharge can be reduced to its minimum quantity.
However, the need to provide adequate treatment of the wastewater discharge is not eliminated.
To recover the solid portions of substances from the wastewater discharge, the seafood industry is
currently employed screening and/or dissolved air flotation to remove coarse and some fine particles
[3]. But after the treatment, the effluents will still not meet proposed future standards [3,4]. Thus, it
leads to the conclusion that the soluble organic substances must be removed, or the seafood processing wastewaters will not meet future effluent limitations.
Biological systems such as extended aeration and lagoons are generally mentioned and recommended as additional treatment for removing soluble organic substances from seafood wastewaters
[5,6]. Since most seafood processing plants are small scale and scattered along coastal areas where
land is expensive the space limited, the biological method may not be the most suitable and cost-
effective system for treating seafood processing wastewaters. Further, most seafood industrial operations are dependent on the types of catch and are not operated continuously on a 12-month basis; it is,
therefore, difficult to maintain a trouble-free operation of a biological treatment system for treating
seafood wastewaters [6].
Unlike municipal and some industrial wastewaters, the pollutants in most seafood processing
wastewater are nutritious organic material. Since the wastewaters do not contain heavy metals and
other known toxicitants, the organic matter can be recovered from the wastewaters as usable byproducts. Use of a biological treatment system will simply convert these organic substances, at the
cost of energy consumption, into low-grade sludge which does not have much economic value. Further conditioning and disposal of the resulting biological sludge require additional costs and/or land
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