The blue-green algae Microcystis aeruginosa and Phormidium tenue were grown in nutrient-limited continuous cultures and together in mixed continuous cultures under various N: P supply ratios and temperatures. The Droop model accurately described the growth of these two algae. Competitive abilities for P and N distinctively differed among these two algae. In competition experiments, A4. aeruginosa was the superior competitor under conditions of low N : P supply ratio and high temperature, whereas P. tenue was superior under conditions of high N : P supply ratio and low temperature, which corresponded to field observations of the algal community in Lake Kasumigaura. Both laboratory and field observations suggest that the N : P supply ratio and temperature are significant environmental factors affecting the relative abundances of A4. aeruginosa and 1'. tenue.In Lake Kasumigaura, Microcystis has generated dense blooms in summer since the 1970s. In recent years, the N: P ratio of lake water has increased. This increase has been caused by the discharge of nitrogen in untreated wastewater, nitrogen loading from agricultural fields, the spread of phosphorus-free detergents, and other factors. Microcystis has simultaneously decreased and been replaced by the filamentous blue-green algae Phormidium and Oscillatoria in summer. Previous research in Lake Kasumigaura attributed this change to the increase in the N: P ratio (Takamura et al. 1992). In the lakes, Microcystis dominates in only summer, whereas filamentous blue-green algae such as Phormidium and Oscillatoria dominate in all seasons, so that temperature also seems to affect their composition. It has not been clear how the N : P ratio and temperature affect the relative abundance of Microcystis and filamentous blue-green algae.The hypothesis that the N : P ratio caused changes in species composition involves competition for these nutrients. It is therefore necessary to study the competitive ability for phosphorus and nitrogen of each algae in pure culture along a temperature gradient. It is well known that algal growth rates depend on external and internal nutrient concentrations. Monod's equation (1950) describes the relationship between algal growth rate and dissolved nutrient concentrations. Many researchers have reported on this relationship for a variety of nutrients and algal species (e.g. Tilman 198 1; Grover 1989). Droop's equation (1973) describes the relationships between algal growth rate and cell quota (Q), the total amount of nutrient per cell. The relationship between i Present address: Department of Brewing and Fermentation, Tokyo University of Agriculture, l-l-1, Sakuragaoka, Setagaya-ku, Tokyo 156, Japan growth rate and Q has been reported for many algae (e.g. Gotham and Rhee 198 la, b;Grover 1991;Olsen 1989;Rhee and Gotham 1980;Shimizu et al. 1992). Besides nutrients, temperature affects algal growth distinctively. Reynolds (1984) provided a review of the effect of temperature on algal growi:h rates for many phytoplankton species.It is als...

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Biodegradabilities of chelating agents were tested with activated sludge. Ethylenediaminetetraacetic acid (EDTA) remained intact in the effluent even after acclimation for 100 days, but propanediamine-N,N'-disuccinic acid (PDDS) and nitrilotriacetic acid (NTA) were biodegraded after acclimation for 5 and 23 days, respectively. Optical isomers of ethylenediamine-N,N'-disuccinic acid (EDDS) had different biodegradabilities: SS- and RS-isomers were susceptible to biodegradation, but the RR-isomer was resistant. SS-isomer was degraded even by activated sludge without acclimation.

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One of the major problems connected with the biological phosphorus removal process is that the phosphorus absorbed by the activated sludge may be released if the sludge is handled incorrectly, and this makes it difficult to treated sewage efficiently. Therefore, phosphorus in the return water from the sludge treatment process must be carefully controlled and reduced. Lime and ferric chloride were added to fix phosphorus in the digestion tank. Phosphorus fixation improved with the addition of lime. The digestion ratio reached a maximum at about 0.8 mol/mol to total phosphorus (T-P) with lime addition and decreased thereafter. Lime or ferric chloride was added to the supernatant to fix phosphorus. Ferric chloride removed suspended solids (SS) and phosphorus more efficiently than lime. A combination of these chemicals gave the most favourable results: a phosphorus fixation ratio of 90% or more was attained at a dosage of about 1 mol/mol. Magnesium ammonium phosphate (MAP) was efficiently formed by adding magnesium to the supernatant, and by increasing the system pH to at least 8 by the addition of sodium hydroxide or by aeration. The phosphorus removal ratio in pilot plant experiments was 70%, and granulated MAP was produced.