Fertilizer and energy economy

Fertilizer and energy economy

General Considerations

The incentives for energy recovery from wastes are strongest in rural areas of developing countries where the level of energy consumption is low. In the Federal Republic of Germany, for instance, conversion of all available organic waste to methane would produce not more than 1 per cent of the energy annually required (11). In contrast, small amounts of methane sufficient to cook one meal a day and to allow reading for a few hours per night would have a strong impact in areas doomed to remain deprived of electricity.

With respect to recycling minerals, the pressures are somewhat more diversified. In developed countries, the health hazards (methemoglobinemia, carcinogenic nitrosamines) associated with increasing nitrate concentrations in water resources, coupled with the eutrophication hazard, are still leading to costly tertiary treatment, creating an irrational situation in that one may find nitrogen-fixing industries and oxidative waste treatment plants side by side, keeping one another in business at great expense. Obviously, urban areas having large food imports cannot easily return the waste minerals to the land where the food was grown unless these minerals are sufficiently concentrated. As this does not appear to be too difficult a task for chemical technology, some engineering innovation is bound, sooner or later, to cause a reversal of tertiary treatment from mineral dissipation to mineral recycling.

In developing countries, drastic technological innovation is not necessary for the rural areas because the transport problem is much smaller and high fertilizer prices that often have to be paid in hard currency are strong incentives to move from the time-honoured methods of waste-burning and direct manuring to composting and biogas production. Burning is inefficient energy utilization, causes air pollution, and wastes the minerals, while direct manuring has health hazards, leads to nitrogen losses during storage, and wastes the energy of the manure. Thus, it would appear that, through minimization of mineral losses, rural communities can reach the point of mineral self-sufficiency by merely importing quantities of minerals that compensate for minerals lost in exported foods. For nitrogen, microbiological N-fixation is an attractive alternative.

Conclusions

1. Collection of wastes in undiluted form offers the best opportunity for energy and mineral recovery, and on this account merits high priority
2. Except for cases where effluents from oxidative treatment of diluted wastes can be used for irrigation, mineral economy can only be maximized by composting, anaerobic digestion, or photosynthetic treatment, provided the resulting algae in the latter are harvested.
3. While both the theory and practice of composting need much more study, present knowledge (12) is sufficiently advanced to promote composting practices in which animal and domestic wastes are mixed with soil, solid refuse, and/ or agricultural wastes to achieve the proper C:N:P ratio. Water content, access of air, minimization of nitrogen losses, and capacity to reach sufficiently high temperatures of 40 to 70 C for killing pathogens are critical elements of the process. It is labour-intensive, and in the microbiological "selfheating" process, only part of the energy contained in the waste is put to use for disinfection; the remainder is lost in the form of residual organic matter. However, depending on soil quality, this may be beneficial to agricultural land.
4. Anaerobic digestion maximizes both energy and mineral economy in a felicitous manner for the following reasons (13, 14):

• The process exploits energy better than burning or composting does; the resulting CH4/CO2 mixture can be easily stored, transported, and employed in stoves, lamps, and motors.
• The residual sludge containing the waste minerals is less hazardous, bulky, or obnoxious, and more easily transportable than the original waste.
• Almost all organic wastes can be subjected to anaerobic digestion provided a proper C:N:P ratio is achieved.
• It reduces the need for firewood collection, and hence helps counteract erosion by deforestation.
• It can be carried out at a scale ranging from a small farm (Figure 5) to a large city.

Figure. 5. Cross Section through Biogas Plant with Cylindrical Digestion Tank. Output of residual sludge (fertilizer) proceeds by gravity flow, following input of waste.

• The process can be integrated with other waste treatment or utilization processes; its resulting sludge can serve as a basis for photosynthetic production of algae, which, in turn, can be used for fertilizer, fodder, food, or as a source of biogas through digestion (Figure 6).

Figure. 6. Simplified Scheme Indicating Various Combinations of Digestion and Photosynthesis, for the Production of Fodder, Fertilizer, and Biogas

• Operation is simple, provided regular feeding is maintained, and the design of the container allows for the effect of seasonal temperature changes on efficiency of digestion.

While sufficient numbers of prototypes are available for promoting the extension of the method from Asia to Africa and Latin America, it should be noted that this long-neglected process stands to be improved greatly by the research efforts now focused upon it. Improved stirring, operation at high temperatures of up to 60 C, and multiple-stage digestion are all showing promise. In addition, anaerobic treatment of dissolved organic wastes from the potato starch and sugar industries has recently been found feasible and considerably cheaper than oxidative treatment (15). In these more sophisticated processes, advantage was taken of the fact that anaerobic processes have low cell yields and low nutrient requirements, and can be expected to operate at much higher cell densities than oxidative ones, as there is no need for aeration. High cell densities could be obtained by retention of flocci and microbial films that settle on solid surfaces.

The single disadvantage of the method emerges when dung for fuel becomes scarce, a situation which caused dissatisfaction among the poorest rural inhabitants of India in areas where biogas installations had been built.

5. Present photosynthetic waste treatment methods, though effective from the point of view of health and water economy, have not reached the stage where they can be used in rural areas for energy recovery, mineral recycling, and fodder and food production. In addition to the many well-functioning types of oxidation ponds in existence, we should also recognize the convergent development of industrial algal cell production in Asia, based on synthetic mineral culture media and aimed at the markets of health foods and pet bird and fish foods. Also, there are the traditional practices for harvesting Spirulina from natural habitats where it has persisted for centuries as an important part of the food chain (Figure 7). High priority should be given to research and development of new methods resulting from the "mariage à trois" of these three developments. Special attention needs to be focused on: (i) maintenance of the predominance of the desired alga; (ii) cheap harvesting methods, and (iii) the utilization of wastes as substrate. In this regard, the studies of Oswald and Shelev and coworkers, reported recently at the first international symposium on this subject (16, 17), show great promise.

Figure. 7. Simplified Food Chain in Lake Nakuru, Kenya