256 Organic waste recycling: technology and management
advisable to use protecting covers, such as masks, hand gloves, etc., when
working in algal pond system.
The possibility of algal contamination with toxic substances and heavy
metals has also another adverse effect on public health. Heavy metals and
pesticides may be concentrated in algae by the process of bioaccumulation,
inducing impact on other consumers in food chain through biomagnification (as
discussed in section 2.5). The concentration of toxic substances in algae is
expected to be higher than in wastewater discharged into the pond. Therefore, to
avoid such risks, the treatment of wastewater up to the allowable concentration
of heavy metals and other toxic substances should be done prior to the feeding
of this wastewater to HRAP. The waste-grown algae to be used for human or
animal feed should be regularly monitored for the presence and concentrations
of these substances.
5.5.2 Public acceptance
Unprocessed freshwater algae tend to have strong smell and taste similar to
those detected in natural waters that are undergoing eutrophication. Algal
texture is also slimy and uninviting, making the direct dietary use unlikely for
human and animals (except for herbivorous fish). Therefore the conversion of
algal biomass into dietically acceptable forms or palatable material (such as the
pelletization) is important, and some encouraging results on the use of pelletized
algae as animal feed have been reported (section 5.4).
Considering the results on the acceptability and the nutritive value of waste-
grown algae, the prospect of their direct use as human protein supplement is
relatively remote. Although the strain Spirulina seems to have potential to be
used as human or animal feed, the main problem is that, whereas species control
is relatively simple with terrestrial crops so that the best suited species are
grown, it is almost impossible to control algal speciation in outdoor HRAP
cultures (Goldman 1979a). The uses of HRAP and the waste-grown algae
appear to be limited to solving specific environmental problems such as
wastewater treatment and other applications shown in Figure 5.2. These
limitations include the upper limit in algal yield of 30-40 g/(m^2 -day), the
economics of algal harvesting and the algal cell characteristics, which are the
key factors in determining the application of HRAP to a particular situation.