Thursday, June 25, 2009

Sand Water Filters

Getting clean water at the HEAL Africa hospital is an intermittent affair. When all is operating as it should, city water pumps take water out of neighboring Lake Kivu and deliver it to paying customers by underground pipes, like most cities. The water is purified by chlorination, which is a very effective means of removing contaminants. However when the power goes out, as often happens, the pumps fail and the water stops.

HEAL Africa does have a backup, however. Large rainwater collection tanks dot the hospital campus, harvesting water from the roofs of buildings. This water allows the hospital to keep functioning, but only after the water has been purified. It cannot be used immediately because of the contaminants the water acquires while passing over the roof to the collection tank.

Currently, rainwater is purified by boiling before use. While this solution is very effective at producing safe water, it has several drawbacks. Firstly, it is difficult to scale up; boiling a few liters of water is easy - boiling hundreds (or thousands) of liters for an entire hospital is hard. Secondly, boiling water is energy intensive. It requires a large amount of charcoal, which fuels almost all stoves in Goma. The charcoal is expensive and its production is harmful to the local environment.

Due to these drawbacks, the hospital is seeking an improved solution for providing safe water during outages. There are many potential solutions, depending on the type of contaminants present. In this case, because the contaminants from the roof are all biological (there's no fertilizers or pesticides to worry about), slow sand filters can be utilized. These are extremely simple and effective, and should work well for HEAL.
The filters work by passing water through a bed of sand. As the water flows down, many contaminants are trapped in the sand. These contaminants then begin to form a "scum" layer on top of the sand. This scum layer is actually the most effective part of the filter - it digests other biological material as more water passes through the filter. Once the water emerges from the bottom, it is cleaned of all biological material (except viruses, which can fit through the sand and are not consumed by the scum layer. These will not be present in this case because the water is immidiately removed from the roof when it rains).

The filters have no moving parts, don't need to be refilled, and are low-maintenance. They do need occasional attention however to remove the scum layer when it becomes too thick. Because of this, the filters work best in an institutional setting where maintenance staff can be trained to care for the filters.

In July, we will be building a prototype filter for use in a small residential compound outside of town. Doing so will allow us to learn on a smaller scale before creating the larger filters needed for the main campus. I expect the filter to cost $100-$200 in materials, plus construction. Once complete, I will be performing a water purity test to verify the cleanliness of the water. Staff will continue to observe the filter before the next installation.

I'll be posting more pictures and updates about the actual installation as it proceeds. If you'd like to help support the prototype construction, please click on the link below. Thanks so much!

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