WATER FOR HUMANITY APPROPRIATE TECHNOLOGY:

THE SLOW-SAND BIO-FILTER

by Steven G. Herbert

          According to a United Nations report, 1.2 billion people in the world (one fifth the world population) lack access to safe drinking water. Twice that number lack proper sanitation facilities. To address this issue, in the year 2000 the UN established among its Millennium Development Goals the objective to cut these numbers in half by the year 2015. The American Society of Dowsers Water for Humanity Fund is proud to be a part of helping to reach this goal.

          Water for Humanity’s stated mission is to fund water resources development globally, in areas of critical need. However, it is unique among water resources development organizations in adding the aspect of dowsing, to locate water resources in a more efficient manner. Thus it improves water resources in terms of quantity, quality and accessibility while also accomplishing its second goal of demonstrating the practicality of dowsing and developing credibility for the art.

          The kinds of projects which the WFH Fund may fund include dug or drilled wells, rainwater harvesting systems, spring capitation projects, distribution of water filters, and building of composting latrines which conserve and protect ground water. WFH operates under the philosophy that all its projects must demonstrate that they are environmentally sustainable, technologically appropriate and culturally sensitive and respectful. Appropriate technology means that the technology proposed must be simple enough to be understandable to the beneficiaries, accepted by the community, repairable by locally trained appointed individuals, made of parts which are locally available or accessible, easy to use and maintain, and be affordable.

          In searching for appropriate technologies involving water purification, WFH came to focus on two in particular, the slow-sand bio-filter and the ceramic water filter. Both of these have the advantages of not involving chemical treatment, which can expose the user to carcinogens, nor needing firewood to boil, which adds the work of gathering and contributes to deforestation. In this article, we will focus on the slow-sand bio-filter.

          In 2006, I had visited the factory making the slow-sand bio-filters in Danli, Honduras, which is funded by Pure Water for the World and their Rotary Club associates. In Santa Barbara, Honduras, the next year, I was able to visit their other factory where David Putt of a group of Canadian volunteers was able to help me add to my knowledge. At both locations the slow-sand bio-filter is made of concrete, but a much lighter weight version is made of plastic and manufactured in the U.S. by HydrAid (www.HydrAid.org). In both versions, however, the design principles are the same. Water filters through a column of fine sand, and eventually also a biological layer which gradually develops on the top of the sand.

Left photo shows a concrete slow-sand bio-filter set up in a household. Center image shows the HydraAid plastic slow-sand bio-filter. On right is a home-made slow-sand bio-filter.

          Mr. Putt explained that these filters function on four different levels. The biological layer is the first, where predation of pathogens by beneficial micro-organisms takes place, and to some degree, also absorption of chemical contamination. The column of fine sand is the second level, which mechanically filters out pathogens. In the third level, where oxygen levels are reduced in the lower levels of the sand, more pathogens meet their demise. Lastly, electrostatic attraction between pathogens and sand particles further prevents passage. The effective pore size of the column of sand is 1 micron, David explained. Worms and protozoa (and their eggs and cysts) range about 30 microns in size and are 100% removed. Bacteria and viruses range 2 microns and smaller. With a filtration rate of one liter every 90 to 100 seconds, about 90% of bacteria and viruses are removed, all but the very smallest

          It should be noted that it is not practical or realistic to expect any purification method to remove 100% of everything. Rather, the goal is to remove as much as possible to bring the residual pathogen level down below the “minimum effective dose”. In other words, we seek to bring the level down far below the level in which it could make a person sick.

          The advantages of this filter is its low-maintenance, reliability, simple construction, being made from local materials and requiring no power. One bio-sand water filter is normally enough to serve a single household. It has been further proven to be appropriate technology in that it is valued by the beneficiaries, and its price is usually within reach of a typical household, and much more so when subsidized by an aid project. Since the biological layer grows, every few weeks the biological layer will need to be skimmed off, but if the water going in is pre-filtered through a clothe, the sand may never need to be changed. The only other precaution to take during daily use is to pour the water through a splash plate to avoid disturbing the biological layer.

          Rene Santos of Siguatepeque is owner and director of a center for sustainable agriculture which is my base when working in Honduras. In 2006, Rene and I had begun a project in seeing how we could home-build a slow-sand bio-filter. I found a 20 gallon food-grade barrel in the market, and it was Rene`s idea to construct a ferro-cement liner where the piping would run between inside of barrel and outside of ferro-cement. This space was then filled with ashes to insulate and Rene sealed it with a cap of cement.

At left, the home-made filter started with a food-grade plastic barrel. In center, Rene fabricated a liner of ferro-cement construction, beginning with chicken wire. Right photo shows the next step was to use a backing of cardboard and apply cement.

          From Santa Barbara in 2007 we were able to return with a prize of a big bag of sand, but I felt I could sieve it finer with brass sieves I brought with me, and trade off a slower filtration rate for a higher removal rate. So I spread the sand out to dry in the sun, and went to town to buy the screens I needed for gravel and pails to sieve into. On return, we screened gravel in four different sizes to layer it in the bottom 10 cm of the filter from coarsest on bottom to finest on top. We then were finally able to begin filling the filter with the fine sand, screened between a screen with 35 openings per inch (.5 mm) and another with 100 openings per inch (.2 mm). Obviously this was time-consuming, and represented the most labor-intensive part of the job.

In left image, we next put the PVC piping together to go between barrel and filter. In center, the completed liner and piping is fitted into the barrel. At right, the space between liner and barrel is filled with insulating ashes, then capped with cement.

          I wasn’t able to return again until 2009, at which time I had several minor projects to attend to. One was calculating the flow rate of our home-made slow-sand bio-filter. As Rene poured water in the top. I timed the filling of a one liter bottle at 2 minutes and 15 seconds. This translated to a flow rate of .45 liters per minute or 26.7 liters per hour, slower by comparison to the other versions we had seen. This was directly related to the size of the sand. Ours having the finest sand had the slower rate and an even more effective filtration.

On left, Steve screens the sand going into the liner. In center, we see the sand below the water level with a splash plate above. At left, the home-made slow-sand bio-filter is finished and in use.