Evaluating water treatment options

Part of EWB-USA’s goal in any of its projects is to deliver a robust engineering solution that is affordable and sustainable, often by utilizing local materials and methods. Therefore, we were able to eliminate some disinfection methods off the bat.

UV Disinfection was not feasible due to initial cost of purchase and installation, and due to operating cost, both of which were prohibitive. Additionally, the lamps in a UV disinfection system require periodic replacement and may not be locally available in Usalama. A power supply would be required for operation, yet electricity is currently not available in many portions of the community. Finally, the possibility of elevated turbidity (cloudiness) in Usalama’s water, especially during the rainy season, could reduce the effectiveness of UV light, which is scattered and dissipated by suspended solids in water.


An example of a UV disinfection unit. Source: inspectapedia.com

- Upstream chemical dosing was another option, and would supplement current efforts by the Kibwezi-Mtito Andei Water and Sanitation Company (KIMAWASCO) to dose the uphill Umani Springs water supply. However, dosing the water supply several kilometers upstream of the distribution kiosks introduces risk. Underuse of chlorine could compromise water quality at all downstream take-offs, and even if the dose was correct, a temporary decrease in flow through the system could amplify the effect of slow degradation of the chlorine and allow any potential leaks in the piping network to re-contaminate the water sitting still in the pipes. Preventing this by regularly testing the chlorine residual in Usalama’s water would be expensive and laborious.

 


KIMAWASCO has an existing doser which is meant to chlorinate the water, but water quality test results indicate that it is not effective.

 

– Using a venturi-style doser to inject chlorine at the new water tank is a novel, electricity-free solution, but can be inaccurate when flow into the tank is as varied as is Usalama’s; water shortages during the dry season cause the flow to decrease appreciably. A venturi doser uses pressure differentials caused by flow rate to dose liquid disinfectant, but was not feasible for our usage in this situation.


A Venturi, pictured below, sits between the Raw water and Chlorinated water in the image above. The pressure drop in the Venturi sucks the chlorine into the water.

 

The team settled on point-of-distribution dosing of sodium hypochlorite (known as bleach when dissolved in water). By allowing each kiosk operator to dose each villager’s individual jerrican when distributing their water, this system gives Usalama full control of the disinfection process. The kiosk operators place a pre-determined amount of diluted hypochlorite liquid stock solution into each jerrican BEFORE filling it with water. Filling the jerrican with water agitates the mixture, adequately distributing the chlorine throughout the jerrican, and each villager’s walk back to their home is enough contact time for the disinfection process to remove the vast majority of waterborne contaminants.


Each kiosk is staffed by an operator who collects money, dispenses water, and now also doses jerrican with hypochlorite solution.

 

-Patrick Farnham

January 15, 2015

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