The photosynthetic potential of algae and certain other aquatic plants in nutrient-rich waste waters has been tantalizing the biologists for centuries. This concept involves the conversion of diluted nutrients from animal wastes, through photosynthesis, into higher and lower plants such as water hyacinth, Lemnacae spp., algae and several other water plants.
According to Dymond (1949), water hyacinth can produce, 2,500 t/ha of green matter, or 150 t/ha of dry matter. The plant has a high moisture content, ranging from 92 to 96.5% (Miner et al. 1974). It is widely grown in South Asia and elsewhere by pig farmers, who use the plant as a source of forage.
Water hyacinth has a great capacity for removing nutrients from water, and its evapotranspiration is 3.2 to 3.7 times greater than that from open water surface.
Several successful studies by Boyd (1969) showed that apart from water hyacinth, water lettuce and Hydrilla also grow and yield well on nutrient-rich waters.
The system being labour-intensive, its economic viability depends directly on labour costs.
A biological treatment of diluted pig waste by Lemna minor L. and Euglena spp. combined with the production of fish and shellfish has been introduced by Stanley and Madewell (1975). This system may, under specific conditions, be an economically feasible waste disposal system, because dry Lemnacae spp. can be considered as a protein concentrate (33% crude protein) and are also a very rich source of carotenes and xanthophylls (Lautner and Müller, 1954; Müller and Lautner, 1954).
Growing algae on liquid animal waste effluent has been practised for several decades, but very few projects have reached the scale of commercial application. In Table 102 are shown economic estimates, prepared by Martin and Madewell (1971), indicating the potential of blue-green algae production.
| Assumptions as to yield 50 tons/ha dry wt., 90% of available minerals utilized | ||
| Capital costs/ha* | ||
| Land levelling (or shore installations) | $ | 1,000 |
| Pump-seration units (50/ha) | $ | 10,000 |
| Piping and valves (300m/ha) | $ | 2,000 |
| Frames, channels, repair jigs, etc. | $ | 2,000 |
| Central facilities (pro-rated) ($ 2,570/ha) | $ | 1,000 |
| $ | 16,000 | |
| Operating costs/yr./ha | ||
| Labour (50 workers per 1,000 ha, $ 1,000 annual wage) | $ | 50 |
| Water and minerals | $ | 200 |
| Power (at 1–2¢ per kWh) | $ | 300 |
| Plastics replacement and materials for repairs | $ | 150 |
| $ | 700 | |
| Total costs | ||
| (10-yr. amortization, 8% interest on capital) | $ 3,500/yr./ha or about $ 70/ton | |
Source: Martin and Madewell, 1971.
The theoretical potential of algae protein production, as compared with other protein sources, is striking (Oswald, 1962):
| System | Protein yield (t/ha) |
| Algae | 30 |
| Soybean | 2.45 |
| Livestock | 0.173 |
Dugan, Golueke and Oswald (1968 and 1972) reported an algae production of 68–90 t/ha, (on DM) for a cost of 2c per 12 eggs, algae being considered as the main protein food for layers. Earlier, however, Hart and Golueke (1964, 1965) had found dehydration of algae for animal feeding uneconomical.
A different approach involving production of methane and algae from poultry manure was designed by Dugan et al. (1969). The schematic flow of the proposed ecologically closed cycle is shown in Figure 14.
Oregon researchers (Gasper et al., 1975) introduced a pilot plant converting pig manure into SCP, comprising algae + bacteria, and methane. The excessive waste heat derived from the thermal power plant is used to enhance the recovery of nutrients from animal wastes.
The high-temperature strain of Chlorella vulgaris 211/8k was used as a production organism. It was observed that the high inorganic nitrogen and organic carbon content of pig waste favoured bacterial, rather than algae, growth. Chlorella remained the predominant species in the presence of other opportunist invaders such as Chlamydomonas sp., Euglena sp., Ankistrodesmus sp., Scenedesmus sp. and Vorticella sp. Only Chlamydomonas was able to take over to the detriment of Chlorella.
It was observed that this problem can be related to the presence of antibiotics. Clumping of Chlorella in the outdoor basins was always associated with pig manure derived from finishing pigs not fed an antibiotic supplement. It was concluded that residual antibiotics in pig waste had a significant effect on clumping of Chlorella and eventual takeover by Chlamydomonas.
The authors concluded that 30–35% of the waste nitrogen can be converted into SCP, but that production of bacterial biomass appears to be easier and less complicated than production of algae, which requires close control over nutrient supply in the solution, retention time and culture depths.
Figure 14 — Algae production from poultry waste (From Dugan et al., 1969).
Development status
Photosynthetic recovery is technically feasible, but its commercial viability depends upon the marketing potential of the final product. When marketed for human consumption commercial viability is high, while for feed it is marginal or nil.
Reliability and applicability
Higher water plants (water hyacinth, Lemnacae, etc.) are easy to grow, but algae require a sophisticated technological approach. The application potential is related to climatological and geographical factors: warm climates with a long photosynthetic day have greater potential.
Health hazards
Accumulation of heavy metals, pesticides and pathogens from faecal waste may become a problem, because processing eliminates only the microbial contaminants.
