Limits to Growth

Urban, industrial and agricultural wastewaters contain up to three magnitudes higher concentrations of total nitrogen and phosphorous, compared with natural water bodies. Normal primary and secondary treatment of these wastewaters eliminates the easily settled materials and oxidizes the organic material present, but does not remove the nutrients which will cause eutrophication of the rivers or lakes into which these wastewaters may be discharged. Tertiary treatment of the effluent is therefore required, and both chemical and physical methods which are used are very expensive. Oswald estimates that the relative cost of tertiary treatment to remove PO 4 3− , NH 4 + and NO 3 − is about 4 times the cost of primary treatment. Higher orders of treatment, such as quaternary treatment required to remove refractory organics and organic and inorganic toxicants and quinary treatment to remove inorganic salts and heavy metals, are 8 to 16 times as expensive as primary treatment. Algae can be used as a biological alternative tertiary treatment and also for the removal of heavy metals and possibly other toxic substances. The possibility exists that the algae produced in these systems can be used as animal feed supplements, or be composted. The use of waste-grown algae may ultimately also have application in closed cycle life-support systems, or may be used in conjunction with power stations, not only to treat wastewaters, but also to act as a CO2 sink for the amelioration of the impact of greenhouse gases