Journal of Applied Phycology 12: 277–284, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 277 Treatment of high strength organic wastewater by a mixed culture of photosynthetic microorganisms James C. Ogbonna 1,2∗ , Hitoshi Yoshizawa 1 & Hideo Tanaka 1,2 1 Institute of Applied Biochemistry, University of Tsukuba, Tsukuba City, Japan 2 CREST, Japan Science and Technology Corporation (JST), Tokyo, Japan ( ∗ Author for correspondence; phone +81-298-53-6646; fax +81-298-53-4605; e-mail jogbonna@sakura.cc.tsukuba.ac.jp) Received 3 October 1999; revised 16 February 2000; accepted 18 February 2000 Key words: organic wastewater treatment, photosynthetic cells, monoculture, mixed culture, photoheterotrophic, heterotrophic Abstract The growth characteristics and nutrient removal from synthetic wastewater by Rhodobacter sphaeroides, Chlorella sorokiniana and Spirulina platensis were investigated under aerobic dark (heterotrophic) and aerobic light (pho- toheterotrophic) conditions. Both in terms of economy and efficiency, aerobic dark conditions were the best for wastewater treatment using R. sphaeroides and C. sorokiniana, but light was necessary with S. platensis. Neither growth nor nutrient removal characteristics of the cells were affected in synthetic wastewater with as high as 10 000 ppm acetate, 1000 ppm propionate, 700 ppm nitrate and 100 ppm phosphate. Although R. sphaeroides and C. sorokiniana showed good growth in synthetic wastewater containing 400 ppm of ammonia, S. platensis was completely inhibited. When grown as a monoculture, none of the strains could simultaneously remove acetate, propionate, ammonia, nitrate and phosphate from the wastewater. R. sphaeroides could remove all the above nutrients except nitrate, but the rate of removal was relatively low. The rate of nutrients removal by C. sorokiniana was higher, but the organism could not remove propionate; S. platensis could efficiently remove nitrate, ammonia and phosphate, but none of the organic acids. A mixed culture of R. sphaeroides and C. sorokiniana was therefore used for simultaneous removal of organic acids, nitrate, ammonia and phosphate. The optimum ratio of the cells depended on the composition of the wastewater. Introduction The initial anaerobic treatment of high strength or- ganic wastewater results in an effluent with high concentrations of various organic acids, phosphate, ammonia, nitrate and other low molecular weight sub- stances. Since high concentrations of organic acids are inhibitory to many aerobic microorganisms, the con- ventional activated sludge method is often not efficient for treating undiluted high strength organic wastewa- ter. The high cost of treatment and the environmental problems associated with the huge amounts of sludge from the conventional activated sludge method have led to renewed interest in the development of altern- ative wastewater treatment methods, particularly for high strength organic wastewater. Many photosynthetic microorganisms are able to metabolize various types of organic acids, nitro- gen and phosphate and their potential application in wastewater treatment has been investigated by many authors (Shelef et al., 1969; Dor, 1975; Doran & Boyle, 1979; Sasaki et al., 1988; Hashimoto & Fur- ukawa, 1989; Travieso et al., 1996). A major ad- vantage of using photosynthetic microorganisms for wastewater treatment is the possibility of combining wastewater treatment with production of useful meta-