Algal production 253pointed out by Goldman (1979a), it is rather impossible to control algal
speciation in outdoor cultures for sustained periods, except in unique chemical
environments that rarely exist in nature. Due to the rapid generation of algae,
certain species tend to dominate through natural selection regardless of which
algae is used as an inoculum. Invariably, the thick-walled and multicellular
freshwater species such as Chlorella, Scenedesmus and Micractinium tend to
become dominant over time (Goldman 1979b).
The direct use of algae for human food supplement may be unnecessary, if the
algae can be fed to the animals (pigs, poultry, and fish, etc.) and these animals are
used as food for human. Hintz et al. (1966) reported that the waste-grown algae
(Chlorella and Scenedesmus) were 73 percent digestible when fed to ruminant
animals such as cattle and sheep, and were only 54 percent digestible when fed to
pigs. The digestible energy content for the ruminants was 2.6 kcal per g algae.
These algae were found to supply adequate protein to supplement barley for pigs.
Alfafa-algae pellets, when fed to lambs, resulted in higher weight gains than alfafa
pellets alone.
Algae are basically not palatable to most of the livestocks, but this may be
overcome by pelletizing the processed algae with usual feed of particular animal,
such as steam barley in the case of cattle. In general, the waste-grown algae appear
to have potential as a livestock feed because of the high contents of protein and
other valuable substances (Tables 5.10 and 5.11).
Edwardset al. (1981a) reported the use of waste-grown algae as feed for
herbivorous fish (Tilapia). Extrapolated fish yields approaching 20 tons/(ha-year)
were obtained in the 4m^3 concrete pond system based on 3 month growing periods
under ambient, tropical conditions. A linear relationship was established between
fish yields and means algal concentration in the fish ponds, in which an algal
concentration of 70 mg/L in the pond water was considered to be high enough to
produce good fish growth. Higher algal concentrations were not recommended
since it might lead to zero dissolved oxygen concentrations in the early morning
hours.
5.4.2 Algae for fertilizer
Algae may be used directly or indirectly as fertilizer in agriculture. In direct use,
algae is cultured in HRAP and then irrigated to crops. This method is simpler,
but requires more time because the algal cells need to be decomposed in the soil
first. In term of indirect use, algae is harvested and composted and then applied
to soil as fertilizer. The application of algal-laden water to crops should be
undertaken with due respect to public health considerations and guidelines as
proposed in Table 2.27.