Influence of organic-N sources on an algal wastewater treatment system

Synthetic wastewater enriched with either peptone or urea as organic-N source was used to grow Chlorella pyrenoidosa under axenic and open conditions. Commercial Bristol medium was used for comparison. The growth rate constant in the peptone-rich wastewater was comparable to that in Bristol medium (k=0.30 d-1 vs. 0.31 d-1) and much higher than its open counterpart (k=0.16 d-1). Both axenic and open peptone cultures exhibited a N-limiting condition for growth as revealed from the inorganic N/P ratios of 0.40 and 1.63 on day 5, which correlated with the rapid removal of 99\% and 80\% of NH4-N, respectively. Better growth in axenic culture was supported by the simple amino acids of the nutritive peptone which was depleted due to bacterial consumption in the open culture. The growth rate constant in the urea set under open condition was slightly higher than that of the axenic culture (k=0.20 d-1 vs. 0.15 d-1), while both were significantly lower than that in Bristol cultures. Growth in the urea set was P-limited as reflected from the high inorganic N/P ratios in open and axenic cultures, with values of 30.6 and 44.9 on day 5, respectively. This appeared to correlate to the rapid removal of 99\% and 88\% of PO4-P under axenic and open conditions, respectively. Bacterial interaction through hydrolysing urea to NH4-N sustained a more neutral pH in open culture and accounted for a better growth, while the acidic pH ( < 4) in the axenic set definitely retarded growth. The results suggest that the degree of N and P uptake and their reduction from wastewater by Chlorella pyrenoidosa depend on the types of organic-N sources in wastewater. The presence of bacteria in the culture would also affect the microalgal growth and nutrient removal, and the bacterial effect seems to relate to the types of organic N available in the wastewater