The ammonia concentrations in 1.25 and 2.3 metre deep waste stabilization ponds at Extrabes N.B. Brazil receiving domestic sewage were largely independent of the surface organic loadings on the deep ponds at least above loading rates of 200 kg BOD5 ha−1 d−1, but varied with time between 14 and 29 mg L−1 (NH3-N). During daylight, pH values reached about 9.0 (or sometimes higher) in the surface water layers as a result of algal photosynthesis. Laboratory growth studies showed that a pond isolate of Chlorella was more tolerant to ammonia than the Euglena species and grew in media containing l0mM ammonia at a pH of 9.0 and a temperature of 25°C when 40% of the total ammonia concentration was in the toxic unionised NH3 form. This tolerance would appear to be inherent since the Chlorella was isolated from ponds containing ammonia concentrations at 1 to 2mM and was subsequently maintained routinely in the laboratory at such ammonia levels. The differential sensitivity to ammonia exhibited by pond algae is discussed in terms of waste stabilization pond performance.

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