Effects of Hydraulic Retention Time on the Removal Efficiency of Pathogens in Algal Ponds

A semi-continuous flow system made of eight pilot ponds was set up and some algal population added as treatment agents. The specific objectives of the study were (i) to investigate the effects of hydraulic retention time on removal efficiency of E. coli in algal ponds, (ii) to assess the variation of physico-chemical conditions and (iii) to assess effects of hydraulic retention time on biological oxygen demand (BOD5) and suspended solids (SS) removal efficiencies in algal ponds. Each pilot pond consisted of a bucket connected to a small metal container and was fed with wastewater. The pilot ponds were located at Kimironko, in Gasabo District in the northern part of Kigali City of Rwanda. A range of hydraulic retention times (HRT) from 25 to 60 days were used with an interval of five days between two pilot ponds. Several samples of influent and effluent were collected over a period of 60 days and analysed in the laboratory using standard methods for SS, BOD5, and E. coli. Other parameters such as pH, dissolved oxygen (DO) and temperature were measured in the field. The results showed a positive correlation between pH and DO variations and an increase in hydraulic retention time in pilot ponds. The pond temperatures varied depending on the local weather. Removal efficiency of pathogens was satisfactory with the increase of HRT. Optimum removal (99.3%) of E. coli occurred within 50 days; whereas, biological oxygen demands BOD5 and SS removal decreased with an increase in HRT. From the results obtained, it was concluded that HRT is a key factor which enhances the extent of variation of physico-chemical conditions, inducing E. coli die-off in WSPs. Further research was recommended at pilot scale where a wide range HRT (> 60 days) and a variety of pond depths would be used.