OVERVIEW
We are 4 undergraduate students from the California Polytechnic State University; San Luis Obispo. As part of class, UNIV 392: Appropriate Technologies for the World's People – Design, we developed a slow sand filter which will be implemented in a village in Ghana.
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There is a need for clean, filtered water in Ghana. To solve this problem, we have made efforts to create a slow sand filter to be implemented in Ghana. A Slow Sand Filter has been used to prevent the spread of disease for over 150 years. Although these systems are slow, they are cost-effective and, thus, sustainable for low income countries. The system is set up so that the top layer of sand physically filters the water as well as biologically treats it through the use of biological organisms to remove pathogens. This complex biological layer is called Schmutzdecke.
WHY A SLOW SAND FILTER?
In determining if a slow sand filter would be an appropriate technology for the community of Agbokpa, we built a decision matrix that contained a variety of alternative systems with a number of different metrics, accompanied with a weight factor. Here, the slow sand filter achieves the highest score, followed closely with buying purified drinking water.
THE COMMUNITY
Agbokpa, Ghana
The community in which we are implementing our slow sand filter sits on the banks of Lake Volta, the largest reservoir in the world. Currently, community members drink directly out of the lake, which is often contaminated with bacteria and other pollutants that adversely affects their health.
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Our project seeks to address the need for better access to safe drinking water and improving the overall health of the people of Agbokpa.
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THE COMMUNITY
Agbokpa, Ghana vs California, USA
Ghana is 2 times smaller than California
California has 9.8 million more residents
Ghana's population density is 343 persons/sq mi
California's population desnity is 217 persons/sq mi
THE TECHNOLOGY
The Slow Sand Filter consists of a filter container, like a barrel, with an outflow pipe connecting the filter to a safe water storage container. The first layer is the Schmutzdecke, which is biologically active and clears the bacteria from the water. The second and third layers contain sand and gravel, starting with the smallest granules on top and the largest on the bottom. These layers filter out larger molecules and debris from the water and help to create physical pressure that moves the water through the filter.
WHAT IS SCHMUTZDECKE?
Schmutzdecke, German for "dirty layer," is a biologically active layer containing predatory bacteria and other pathogens that grow on top of the first layer of fine sand in a slow sand filter. This layer is the "cleaning agent" that removes harmful bacteria. This layer requires time to establish – usually about a month – and additional recurring maintenance to maintain the health of the schmutzdecke.
THE DESIGN PROCESS
The Drainage System
To begin, we designed and installed a drainage array from PVC pipe cut at varying lengths and attached to a central pipe with PVC cement. Each "arm" had holes drilled into them to facilitate the water drainage process. Once this array was complete, we drilled a hole into the side of the barrel to install and outflow pipe with a valve to toggle the water flow. Additionally, we fitted a rubber washer around the PVC pipe and the barrel to prevent leakage, and further sealed the pass-thru with hot glue.
THE DESIGN PROCESS
The Sand
Once the drain array was installed, we filled the barrel with gravel and sand. We laid gravel at the bottom, followed by a layer of coarse sand, and topped with a layer of fine sand. Between each layer, we washed the material by running a hose until the water exiting water was clear. This helped remove dust and other impurities from the sand. Once the sand washing was complete, we let the material dry, as the water from the hose contained chlorine, which will prevent the development of the schmutzdecke. By letting the sand dry, the chlorine will have evaporated.
THE DESIGN PROCESS
The Schmutzdecke
To build the biolayer, we filled the barrell with water from the nearby Aquaponics Clubs' tanks, saturating the sand with biologically active water. However, during our first attempt at building the biolayer, we realized there was a leak coming from the seal we built between the barrel wall and the output valve.
THE DESIGN PROCESS
The Leak
To fix the leak, we applied epoxy putty, which would create a watertight seal, to prevent future leaks. However, the leak prevented the filter from developing the necessary bacteria to operate as a slow sand filter. We were optimistic that the Aquaponics water had high levels of biological activity, which would hopefully grow the schmutzdecke faster.
THE DESIGN PROCESS
Filling the Filter
To properly develop a strong schmutzdecke, we were advised that we should avoid pouring a heavy stream of water into the barrel, as the concentrated flow can distrub the biolayer. Luckly, our team found a tub what fit the diameter of our barrel perfectly, and even had holes drilled in the bottom of it to disburse the flow of water.
THE FUTURE OF THE FILTER
By the end of the quarter, we were able to design and build a functional slow sand filter. We installed an effective drainage array, poured multiple layers of sand of varying textures, and were able to have water flow out of the system without leakage. However, due to the faulty seal during initial testing, we were unable to grow a strong schmutzdecke like we hoped due to time constraints. Even as the quarter comes to a close, we intend to maintain the filter to grow a functional schmutzdecke and compare the quality of water before and after filtration. This data will help inform the students and faculty who are installing a similarly designed slow sand filter in Agbokpa, Ghana this summer.
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Further testing will provide evidence for the viability of the slow sand filter abroad, though we are optimistic that it will be effective and affordable for all. Additionally, future projects should factor in extra time for the schmutzdecke to grow.
SEE THE WORK
