8 Organic Waste Recycling: Technology and Management
ponds. Although the algal cells produced during wastewater treatment contain
about 50 percent protein, their small size, generally less than 10 ȝm has caused
some difficulties for the available harvesting techniques which as yet are not
economically viable. Aquatic macrophytes such as duckweeds, water lettuce or
water hyacinth grow well in polluted waters and, after harvesting, can be used as
animals feed supplements or in producing compost fertilizer.
There are basically three techniques for reusing organic wastes in fish culture:
by fertilization of fish ponds with human or animal manure; by rearing fish in
effluent-fertilized fish ponds; or by rearing fish directly in waste stabilization
ponds. Fish farming is considered to have a great potential for developing countries
because fish can be easily harvested and have a high market value. However, to
safeguard public health in those countries where fish are raised on wastes, it is
essential to have good hygiene in all stages of fish handling and processing, and to
ensure that fish are consumed only after being well cooked.
Chitin and chitosan are non-toxic, biodegradable polymers of high molecular
weight and of same chemical structure. They are a nitrogenous polysaccharide
which can be isolated from shells of crustaceans, such as shrimps and crabs. Chitin
is a linear chain of acetyl glucosamine groups which is insoluble in water. Chitosin
is obtained by removing acetyl group (CH 3 -CO) from chitin molecules (a process
called deacetylation). Chitosan has cationic characteristics, is soluble in most
diluted acids and is able to form gel, granule, fiber and surface coating.
Chitin and chitosan have many useful applications in the environmental, food,
cosmetic and pharmaceutical fields. They have been used in wastewater treatment
and as food additives, disinfectants and soil conditioners. Chitin and chitosan are
being used as components in the manufacturing of several cosmetic and medicinal
products.
Since crustacean shells, abundant in nature, are usually disposed of as waste
materials from households and food industries, converting them into valuable
products, such as chitin and chitosan, should help to protect the environment and
bringing financial incentives to the people. More details of chitin and chitosan
production and their application are given in Chapter 6.
1.2.4 Indirect reuse of wastewater
The discharge of wastewater into rivers or streams can result in the self-purification
process in which the microbial activities (mainly those of bacteria) decompose and
stabilize the organic compounds present in the wastewater. Therefore, at a station
downstream and far enough from the point of wastewater discharge, the river water
can be reused in irrigation or as a source of water supply for communities located
downstream. Figure 1.3 depicts typical patterns of dissolved oxygen (DO) sag
along distance of flow of a stream receiving organic waste discharge. Type 1