A survey on the ratio of effluent algal BOD concentration in primary and secondary facultative ponds to influent raw BOD concentration

It is well known that algae play an important role in treatment of municipal wastewater with facultative ponds. The objective of the present study was to determine the ratio of effluent algal BOD concentration in facultative ponds to influent BOD concentration. This descriptive analytical study was carried out on a full-scale wastewater stabilization pond system located in Islamabad Gharb, northeast Iran. A total of 138 grab samples were collected from Primary and Secondary Facultative Ponds (PFPs and SFPs) twice per month over a period of one year and analyzed for algal cell number, chlorophyll a, and BOD5 concentrations. The mean ratios of effluent algal concentration of PFP5 and SFP7 to influent BOD5 of PFP5 were 1.11 +/- 0.53 and 1.59 +/- 0.66, respectively. Also, the mean ratios of effluent algal concentration of PFP6 and SFP8 to influent BOD5 of PFP6 were 1.20 +/- 0.63 and 1.57 +/- 0.68, respectively. Except PFP6, significant differences were found in the means of the respective ratio in PFP5, SFP7, and SFP8 between cold and warm seasons (p < 0.05). Although there was a significant difference in the means of the respective ratio between primary and secondary facultative ponds (p < 0.05), there was no significant difference between the same ponds in modules (p > 0.05). Lower effluent algal concentration to influent BOD5 ratio found in cold months indicates strong dependency of algal photosynthetic activity to sunlight and temperature. Also, the effluent algal concentration in secondary facultative ponds was higher than that in primary facultative ponds, since better condition is provided by secondary facultative ponds for algal-bacterial activity.