Breakthrough of indicator organisms from slow sand filters as part of a drinking water production system for Sub-Saharan Africa
Sub-Saharan Africa has the lowest proportion of population in the world with access to potable water, so that there is a dire need for low-cost, low-energy robust treatment technologies for drinking water. Constructed wetlands followed by optimized slow sand filtration has the potential for improving raw surface water quality to an acceptable level. A laboratory study examined the removal of Enterococcus durans and environmental coliforms with associated heterotrophic bacteria from slow sand columns operating with different sand sizes and flow conditions. E. durans removal far exceeded 90% in most systems, with better performance from a sand column with finer sand size (0.425 mm) and continuous flow. A column with 0.425 mm sand also performed better than a column with 0.6 mm when both were operated intermittently. Removal of environmental coliforms followed the same trends as observed with E. durans, but with roughly four times the overall breakthrough. Correction of the column breakthrough data to remove the effect of decay observed in control columns demonstrated that, in most cases, E. durans removal was accomplished by more than static loss of culturability. In the case of the environmental coliforms, corrected breakthrough was still below 100%, but much higher than with the E. durans, suggesting that extrapolation of results from a single species could produce erroneous estimates of removal of other organisms.