Slow Sand Filters
Slow sand filters are the oldest type of municipal water filtration system.
They generally consist of a bed of sand placed over layered coarse sand
and pea gravel (necessary to retain the sand). Water flows downward
through the sand to the gravel layer, and a water collection system is
used to convey the filtered water from the gravel. Because biological
mineralization (i.e., conversion to carbon dioxide and water) of organic
matter occurs within the filters, they are also called “biological sand
filters”. Large continuous-flow systems can be constructed for small
communities, and small “point of use” systems can be constructed for
household use. Point-of-use slow-sand-filters are operated as “batch”
systems, in which the raw water is added on top of the sand layer
generally once or twice each day. Although these filters can significantly
reduce the number of micro-organism in the water, it is critical that the
filtered water be treated with a disinfectant, such as chlorine and UV
light, to inactivate any micro-organisms (bacteria and protozoa) that
Figure 1. A Slow Sand Filter Unit
do pass through the sand. These filters also significantly reduce
at the Buga Elementary School
dissolved organic matter and suspended particle concentrations.
Removal of suspended particles (which cause the water to be turbid or opaque) is very important, as
chemical disinfectants and UV light are much less efficient in inactivating micro-organisms that are encased
within particles. After several weeks of operation, a mat of biological material should form on the top sand
surface of the filter: This biological mat in normal and actually helps the filtration process, as much of the
particles and micro-organisms are removed by this mat, allowing the sand layer to act more efficiently. To
develop this layer, the standing water level needs to be maintained at 2-5 cm above the sand surface.
The slow-sand-filters constructed by Purdue University are made from readily available 5-gallon plastic
pails. A new design feature is a porous plate in the bottom of each filter, replacing the traditional gravel
layers. One slow-sand filtration unit consists of a stack of two 5 gallon pails (two, for adequate total depth
of filtration – about 10 inches of sand in each pail). In each pail, food-grade plastic tubing conveys the
filtered water from the porous plate up to a hole on the side of each pail, by gravity where it is either
conveyed (from the top pail) to the bottom pail, or (from the bottom
pail) to the collection pail. The level of water above the sand is
controlled by the location of the hole on the side of each pail, which is
placed several centimeters above the top of the sand layer. At a
specific time each day (i.e., morning), 5-10 Liters of raw water are
gentle poured into the top pail of each unit (2 units are provided to
each school) using a small “Raw Water Pail”. The filtered water is
collected in a “Clean Water Pail”. After the water stops flowing into
the “Clean Water Pail”, it should be disinfected.
Figure 2. Filtered water (left) and
unfiltered water (right) at Graciano.
If necessary, an additional 5-10 Liters of water may be able to be
filtered through each unit after 8 hours. Pouring more water through
the filters may decrease performance. It is also important to add water each day, or at least every other
day, even at a lower volume (2.5 Liters), as this maintains the dissolved oxygen concentration in the filter,
which is necessary to keep the “good” micro-organisms in the filter alive.