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.1 Normal primary and secondary treatment of these wastewaters eliminates the easily s
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Limits to Growth Michael A. Borowitzka
Introduction
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rban, industrial and agricultural wastewaters contain up to three magnitudes higher concentrations of total nitrogen and phosphorous, compared with natural water bodies.1Normal 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. Oswald1 estimates that the relative cost of tertiary treatment to remove POl-, NH4+ and N03- 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.M The possibility exists that the algae produced in these systems can be used as animal feed supplements,5.6 or be composted. The use of waste-grown algae may ultimately also have application in closed cycle life-support systems/'s or may be used in conjunction with power stations, not only to treat wastewaters, but also to act as a C01 sink for the amelioration of the impact of greenhouse gases.9-13 The biotreatment of wastewaters with algae to remove nutrients such as nitrogen and phosphorous, and to provide oxygen for aerobic bacteria was proposed over 40 years ago by Oswald and Gotaas.14 Since then there have been numerous laboratory and pilot studies of this process and several treatment plants using various algal treatment systems have been constructed.2.,15 Microalgal systems for the treatment of other wastewaters such as piggery effluent,'6-18 the effluent from food processing factories,19. 10 highly saline tannery wastewaters11 and agricultural wastes 22.,23 have also been studied. More recently, algae-based systems for the removal of toxic minerals such as Pb, Cd, Hg, Se, Sn, Ni, As and Br are also being developed. 14-28
Wastewater Treatment with Algae, edited by Yuk-Shan Wong and Nora F.Y. Tam.
© Springer - Verlag and Landes Bioscience 1998.
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Wastewater Treatment with Algae
Because of the large volumes to be treated, most algal wastewater treatment systems use large outdoor ponds. These may either be waste stabilization ponds (WSP) or high rate oxidation ponds (HROP). The WSPs usually consist of facultative and maturation ponds in series and rely to some degree on algae for their successful operation. A non-algal anaerobic pretreatment pond also may be included. The HROP system, on the other hand relies wholly on the interaction between algae and bacteria for complete treatment. Hybrid systems, consisting either of anaerobic pretreatment pon
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