Algal Systems

 Converting the dissolved nutrients in anaerobic lagoon or digester
 effluent to useful plant protein can be accomplished by algal culture. This
 can be done on a continuous basis in tropical or subtropical climates, and
 throughout much of the year in temperate climates. The high productivity of
 microalgae, coupled with the tolerance of many species to high concentrations
 of nitrogen and phosphorus, levels that are lethal to most aquatic plants,
 make them especially well suited for on-the-spot recycling of organic wastes.
 By engineering the algal growth units to maximize photosynthesis, the wastewater
 medium can be rapidly and effectively treated through the uptake of nutrients
 by the cells and photosynthetic oxygenation. After removal of the algal, cells,
 the renovated water can be recycled to the feeding operation. The dried algae
 typically contains 50% high-grade protein, and can be used to replace soybean
 meal as a feed supplement.

 Our research at the University of Florida Swine Unit indicates that dry
weight yields of algae at least an order of magnitude greater than soybean
can be obtained the year round. Using a culture of 0.1 ha receiving anaerobic
lagoon effluent at an average rate of 15,000 liters per day, we have obtained
daily crops of up to 30 kg (ash-free dry weight). Minimum detention time during
the spring and summer was four days. These crops represent conversion efficiency
of 3% of the total irradiance. During winter, yields dropped to about a quarter
as much, but because irradiance was reduced by more than half, conversion
efficiencies still remained in the neighborhood of 2% (Lincoln and Hill 1980).


Status of Current Research

 At present, the major obstacle to the use of algae in livestock production
is the high cost of harvesting the cells. Primary concentration of the suspended
algal cells, the most expensive unit operation, can be accomplished by chemical
coagulation followed either by settling or flotation. Our harvesting process,
which employs chemical flocculation and autoflotation, has proven technologically
feasible. However, the cost of the product is still too high to be competitive
with soybean meal.

 The fuel energy embodied in the standard chemical flocculant, alum
(Al (SO ) *18 H 0), is approximately 400 kcal/kg (Hagan and Roberts 1976).
Fieid studies have shown that the ratio of kg of alum used per kg of dry algae
recovered is very close to unity (Lincoln 1980). On a dry basis, algae contain
approximately 5,500 kcal/kg of combustible energy. Therefore, each kg of alum
invested in the harvesting process yields 12.5 times its own embodied fuel
energy in the form of algal energy. While this is a sizeable energy gain,
it is not reflected in the final cost of the dry algal product. Discounting
labor and amortization of capital, both of which are charged to waste management,
and considering only the price of chemicals and electric power, the algal
product costs $0.18 per kg. Soybean meal, by comparison, sells for slightly
more than $0.22 per kg.

 For an algae unit employing gravity flow to feed the culture, as ours
does, the cost of growing protein up to the time of harvest is essentially nil.
Water, nutrients, and hydraulic head are all supplied from outside by the normal
operation and design of the Swine Research Unit. It is therefore clear that