|Bioconversion of Organic Residues for Rural Communities (UNU, 1979)|
|Micro-organisms as tools for rural processing of organic residues|
Other sources of biomass deserving mention are marine plants, such as giant seaweeds (kelp), from the waters of the tropical and temperate oceans, and the so-called weeds, such as the water hyacinth.
Although the technology has not been proven, ocean-based kelp farming has some attraction for two reasons: (i) kelp is fairly efficient in converting sunlight into stored energy (2 per cent), and (ii) land and terrestrial waters are not constraints. Experimental data on marine plants have been collected by Wilcox of the US Navy's Ocean Food and Energy Farm Project (34). All aspects of this programme are interesting, but the only data involving microbiology are those concerned with the production of methane as a source of energy from kelp by anaerobic microbial digestion. In addition to methane, certain by-products remain in the sludge and liquid of the digester, which can be used for nitrogen fertilizer or animal feed supplements
There are no major nutritional deficiencies in kelp for mesophilic, anaerobic, methaneproducing microorganisms, so they grow readily on a slurry of the material. One precaution is that the salt concentration of the raw slurry must be reduced. An interesting research project would be to develop strains of methane-producing micro-organisms that are more salt-tolerant.
Economic studies indicate that the cost of methane production from kelp fermentation may range from US$2 to US$7 per GJ (per million BTU), depending on credit values received from feeds and other by-products. Thus, entry to the fuels market for kelp-derived methane will require research to provide a cheaper product, and capitalization. If preparatory methods for handling the kelp, and the fermentation, could be carried out in the open ocean where wind, wave, and solar energy could be used, the cost of the methane could be reduced below current land based fermentation processes.
One of the most prolific plant colonizers of rivers and lakes is the water hyacinth, which has spread in recent years from its natural habitat in South America to at least 50 tropical and sub-tropical countries around the globe. A few plants can multiply and spread over an area of 120 yd(2) of water in several months, depending on the nutrients in, and temperature of, the water, and the plant mass may represent many hundred tons of hyacinth.
Such water weeds are an environmental disaster in some countries because they interfere with water transportation and fishing, and they can be a health hazard as well by providing a suitable breeding site for the malarial mosquito. Ironically, the water hyacinth may be a promising candidate for solving needs of animal feed, energy, and control of water pollution, and in this regard, micro-organisms can play an important part.
Water hyacinths contain most of the essential nutrients for animal growth, but making a palatable feed from them is not easy because of the high moisture content of the plants. Research indicates, however, that the plants can be converted into silage by placing chopped plants in a closed container and allowing them to undergo microbial fermentation for about a month. Such silage has been shown to be highly palatable to sheep and other animals.
Potentially, the water hyacinth may be used as a source of energy, and for the purification of sewage. Considerable research has been done on these subjects, especially by Wolverton and McDonald at the NASA Space Technology Laboratories in Bay St. Louis, Mississippi (35). Biogas or methane production from the microbial anaerobic decomposition of water hyacinths has been investigated only on a laboratory scale. Many factors, such as carbon to nitrogen (C/N) ratios and temperature, affect the amount of gas and residue produced from the microbial digestion of the plant material. Based on research, it has been calculated that one hectare of water hyacinths can produce enough biomass each day to generate between 90 and 180 m³ of methane gas, and at the same time 0.5 ton of residue useful as a fertilizer. Further research is needed on the use of water weeds as a substrate for microorganisms.