Closing Nutrient Cycles


 The following discussion focuses on a particular area of appropriate
technology, that of recycling animal wastes, and concentrates on a narrow
application of such technology, namely, the use of algae as a source of feed
protein. By focusing on the practical aspects of real systems operating on
a field scale, my aim is to lend substance to the larger concept of rejoining
industrial society with natural systems. It was on this larger concept that
the present investigation was originally based, and upon which it continues
to be modified.


Livestock Production

 An appropriate interface of industrial society with renewable nature is,
oddly enough, at the point of disposal of manure from the confined livestock
operations which have become the mainstay of the modern livestock industry.
In terms of magnitude, the problem of such waste disposal is of global signifi-
cance. The volume of manure produced each day in the U.S. is roughly ten
times that of human wastes. The U.S. daily output of manure nitrogen amounts
to 30 billion grams (Miner 1969, Hill 1974, Calvert 1974). If this were
converted to edible plant protein, it would be sufficient to meet the minimum
daily protein requirement of every human on earth.

 In contrast to municipal sewage which is subjected to energy intensive
treatment, animal wastes are managed without the equivalent of sewage treatment
plants. Some 45% of this waste stream is collectible, and it is typically
in the form of wastewater up to ten times as concentrated as sewage. It there-
fore constitutes an environmental hazard of ever increasing magnitude. On the
other hand, with the application of appropriate technology and proper management
practices, it could be transformed into a resource of far-reaching economic
importance. Although manure has traditionally been used as a fertilizer in
commercial agriculture, its use for this purpose has declined markedly in
recent years, partly because chemical fertilizers have become more cost
effective, and partly because current flushing methods produce wastewater
volumes too large to transport to distant crop lands.

 In intensive animal production units, retention of liquid wastes in
anaerobic lagoons followed by facultative lagoons has become standard practice.
Bacterial action reduces oxygen demand of the wastes and mineralizes organic
components which are then dispersed as gaseous C02, methane, and ammonia.
Certain of the remaining nutrients are subsequently converted to algae in the
facultative lagoon. However, the final effluent still contains high levels
of suspended solids and dissolved nutrients, hence cannot be discharged into
surface waters.

 Recently there has been a move toward replacing anaerobic lagoons with
methane generators. These allow the recovery of methane which can be used as
a fuel. Although some 50% of the free energy contained in the wastes can be
recovered, theater treatment problem is not solved, since the digester
effluent generally contains higher concentrations of nitrogen and phosphorus
than the influent.


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