© 2007 IWA Publishing. Organic Waste Recycling: Technology and Management. Authored by
Chongrak Polprasert. ISBN: 9781843391210. Published by IWA Publishing, London, UK.
6
Fish, chitin, and chitosan production
Among several waste recycling methods, the reuse of human and animal wastes
for the production of algae and fish has been extensively investigated. Although
the algal cells photosynthetically produced during sewage treatment contain
about 50% protein, their small sizes, generally less than 10 ȝm, have caused
some difficulties for the available harvesting techniques, which as yet are not
economically viable (see Chapter 5). Apart from aesthetic reasons, one of the
drawbacks concerning the direct utilization of the waste-grown algae as an
animal feed, except Spirulina, has been the low digestibility of the algal cell
walls. Thus the culture of phytoplankton-feeding (herbivorous) fish in the same
pond to graze on the algae, or feeding the algal-laden water to herbivorous-fish
ponds is attractive, in order to produce the fish protein biomass which is easily
harvestable for animal (or human) feed.
There are basically three techniques for reusing organic wastes in
aquaculture: by fertilization of fish ponds with excreta, sludge or manure; by
rearing fish in effluent-fertilized fish ponds; and by rearing fish directly in waste
stabilization ponds (such as in maturation ponds). A World Bank report
(Edwards 1985) cited several Asian countries, especially China, where excreta