The comparison of growth and nutrient removal efficiency of Chlorella pyrenoidosa in settled and activated sewages

The microscopic green alga, Chlorella pyrenoidosa was grown in settled and activated sewage under two different culture systems, batch and semi-continuous. Good growth was obtained in both types of wastewater and the algal production was comparable to and even higher than that found in commercial Bristol medium. The semi-continuous culture supported more growth than the batch system. There was a close relationship between algal growth and the amount of nutrient removed from both settled and activated sewage. A more rapid drop in NH₄⁺N was found in settle rather than activated sewage. The NH₄⁺N of settled sewage dropped from its initial 27 to 5 mg litre^-1 in both culture systems. On the other hand, the NO₃^-N of activated sewage started to decrease from Day 2 onwards and the final NO₃^-N concentration was less than 1 mg litre^-1 (over 90% removal efficiency). The amount of total inorganic nitrogen being reduced due to algal culture was similar in both types of sewage. The changes of phosphate content followed the same trend in both sewages, the P concentration increased slightly in the first two days then decreased, especially in the semi-continuous cultures. The final ortho-P in the sewages treated by Chlorella in semi-continuous culture was less than 5 mg litre^-1 (about 62% reduction). Such removal efficiency was slightly lower than those reported in previous studies. In general, the semi-continuous algal culture appeared to be a more suitable and efficient way for wastewater treatment than the batch system. With respect to the total reduction of wastewater inorganic N and P by means of Chlorella cells, there was no significant difference between settled and activated sewage.