Because they are economical reactors, properly designed ponds for waste management will grow in importance in the 21st Century. They will also be important for water and nutrient recycling and for animal feed production. Paddle wheel mixed algal growth ponds are not only a cost effective choice for wastewater oxidation but also are most efficient in fixing solar energy and in reclaiming water, nutrients, and energy from organic wastes. As the human population increases the amount of arable land will soon be less than one hectare per person and, at current world crop productivities of much less than 0.1 g/m2/d, one hectare of arable land per person will be insufficient to sustain a projected population of more than six billion by the year 2000. Because the amount of land is finite, the apparent solutions are to increase productivity and, ultimately, to attain a constant population. Microalgae in ponds can produce high protein biomass at a rate of 10 to 20 g/m2/d, productivities an order of magnitude greater than land crops. Algae can be combined with grains to produce valuable feed, which through poultry, animals, and fish can improve human nutrition. With improved human nutrition, financial security should increase and birth rates should decline; therefore, algaculture in ponds, together with conventional agriculture, can help reach a stable population and improve the quality of human life.

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