A study of fecal coli form removal in Dandora waste stabilization ponds

Abstract

The study investigates fecal coli form removal both in laboratory and full scale waste stabilization ponds.

Although it is widely recognized that waste stabilization ponds are very effective in fecal bacterial removal, there is little published quantitative information about the degree of fecal bacteria removal in ponds in Kenya.

In the study, fecal coli forms were enumerated using the membrane filtration method. Studies from laboratory scale waste stabilization ponds showed that the die-off rate constant was slower in the ponds with lower initial coli form counts, and that the die-off rate constant decreases down the pond series and with increased in retention time.

Dandora treatment works which is the biggest pond installation in Kenya was used in the full scale study.

It was revealed that the works is currently overloaded. The estimated overall retention time is 25.6 days, while the total design retention time is 49 days. The mean overall percentage removal is 99.84%. The die-off rate constants in the ponds ranged from 0.54 d-1 to 0.98 d-1. However, it was noted that even with over 99% fecal coli form removal, the number remaining is alarmingly high.

An operation curve for estimating fecal coli form in the final effluent at Dandora Treatment Works when the retention time and raw sewage fecal coli form counts are known is presented.