330 Organic waste recycling: technology and management
The slurry that is the by-product of biogas digestion can be used as organic
fertilizer for fish ponds and crops. Field experiments were conducted at the
Asian Institute of Technology, Bangkok, in which biogas slurry from four,
3.5m^3 biogas digester units fed with water hyacinth and nightsoil, was added to
fish (Tilapia) ponds. Extrapolated yields of Tilapia were found to be 2800-3700
kg/(ha-year) and this scheme was considered to have potential for rural waste
recycling in developing countries (Polprasert et al. 1982 and Edwards et al.
1987)
These initial studies have generated great interest in biogas production and
the recovery of fuel from aquatic weeds, especially for rural areas in developing
countries. As many developing countries have an inexhaustible supply of
aquatic weeds, this potential energy source deserves further investigation before
being commercially exploited.
7.6 Food potential
Since plants are a starting point of the food chains, they are naturally the source
of all food for animals and human. The major pathways involving the use of
aquatic weeds in food production are shown in Figure 7.8. The pathways
involving composting, mulching, green manuring, ash, and biogas digestion
were previously discussed in section 7.5. The discussion on the remaining
pathways is presented in this section.
7.6.1 Food for herbivorous fish
There are many species of herbivorous fish that feed on aquatic weeds (Edwards
1980; Mehta et al. 1976; NAS 1976), thus converting them to valuable food.
They basically fall into three categories:
- Grazers - if they eat stems and foliage.
- Mowers -if they devour the lower portions of aquatic plants and thus
cut them down. - Algal feeders - if they consume algae.
Algal feeders are not considered as they consume single-cell-protein algae,
which is outside the scope of this Chapter.