before you begin.
Slow sand filtration is proven to be
very reliable if done properly and carefully. That said, we have seen
more than a few "slow sand filters" that have not followed the rules,
sometimes with serious consequences.
Question: How do slow sand filters work?
Slow sand filtration is a biological process that cleans water much the
way the sandy bed of a river cleans and recharges an aquifer. A column
of water passes through a three-foot layer of fine sand at the rate of
0.1 gallons (0.38 liters) per minute per square foot or less. On the
top of the sand, an intense layer of microbes naturally develops. This
layer lives by consuming whatever is passing through in the water. In a
slow sand filter, this layer, called the schmutzdecke,
is responsible for removing up to 99.99% of all bacteria, viruses,
Giardia, Cryptosporidium, and parasites through predation. As the water
passes through the deeper layers, other processes such as
sedimentation, mechanical filtration, and electrical attraction remove
still more. The result is that slow sand filters may be the best stand
alone water filters known.
You can build your own, but... Though
SSF is a simple technology, the rules are very, very important and it
is vital to you and others that you do the research and stick to them,
as we do. We at BFFI have spent almost 20 years and much academic
research and testing to arrive at our "simple" solutions.
Unfortunately, we have seen the unhappy results of under-researched
Question:Are the do it yourself filters you see on various internet sites reliable?
Most of the do it yourself filters on the internet use cheap plastic
barrels for the filters instead of using more expensive tanks better
suited for slow sand filtration. The result is insufficient head to
provide long filter runs. When you have to clean the filter more often,
you actually sacrifice pathogen removal efficiency. Not providing
carefully calculated flow control also effects efficiency.
Unfortunately, we have seen situations where a "slow sand filter"
was assembled using paint cans with little consideration for sound
design and the result was a user came down with Cholera. That's why
Blue Future never scrimps on materials, design, or testing. The devil
is in the details, folks. Be skeptical!
Question:Is there a mimimum diameter for
slow sand filters? Probably. If filter tanks are
too small in diameter, the ratio of filter bed to side wall surface
area reaches a point where slipstreaming may occur down the sidwall,
avoiding the biolayer entirely. As a rule of thumb, Blue Future Filters
are never less than 18" diameter for safety. We know of "filters" that
have been deployed without concern for diameter or flow rate that have
resulted in cases of Cholera.
Question: Is filtration rate important?
Very! Too high filtration rate and breakthrough of pathogens is
possible. That is why the accepted maximum
filtration rate is .1 gpm/ft2 for sand surface. Beware of
systems that purport to be "slow sand filters" that use much higher
filtration rates to sell a smaller footprint. These are not
true slow sand filters. Slow sand filters work best when operated under
constant flow conditions using storage to buffer demand.
Do these systems work with grey water?
Although slow sand filters are designed for surface water sources such
as streams, ponds or shallow wells, they can be used for grey water
systems with some modifications.
Do these systems work with rainwater?
Yes, with some recommended modifications. Rainwater catchment systems
tend to use intermittent source water that is acidic (low pH) and lacks
nutrients. We recommend using as much storage as possible for both the
raw rainwater and the filtered water, as well as recirculation of the
filtered water through the slow sand filter to prevent anaerobic
conditions and protect the biolayer. There are currently some
innovative solutions for rainwater storage that we can also recommend,
such as stormwater detention systems and underground dikes. We also
recommend adding a layer of calcite to the top of the sand to both
adjust the pH and add nutrient content to the source water. We can also
provide nutrient mix to help promote biological activity.
Question: Does any kind of sand work?
Despite what some people say, yes, virtually any clean silica sand will
work, rough or spherical grains as long as filtration rate is
determined by influent or effluent control and not by the media
itself. It is recommended to use prewashed, prepackaged
silica sand for the filters if available. In the United States, several
sand and gravel companies offer washed silica sand that can be
purchased by the bag or super sack. If sand is to be found through
other means, either at a commercial sand yard or (as often is necessary
in remote locations) from a river bed, it can be sifted and cleaned to
reach optimal conditions for the filters. For sand and gravel size
How often do I have to change the sand?
The sand may never need to be replaced; however, at some point after 10
years, it may be necessary to remove the sand and wash or replace it.
In that event, you will need to drain the filter, dig the sand and
gravel out, and both the sand and the filter may be cleaned with fresh
water. Once clean, the filter is refilled with media and restarted.
Question: How do I clean the filter?
The filter is cleaned by a process called wet harrowing
(raking the sand bed). This simple process involves opening the lid of
the filter vessel and stirring the top layer of the sand vigorously
down two inches into the surface of the sand. Once stirred, the water
will contain large amounts of silt and other fine material, which is
then drained off from above the sand through a harrowing valve. Once
the water runs clear, the harrowing valve is then closed and the filter
quickly returned to normal operation.
Question: How do I know when to clean the
Blue Future filters moniter headloss through the filter, cheaper
filters do not. A clear sight tube emerging from the filter tells you
the condition of the filter. This tube, called a piezometer, indicates
the pressure loss in the filter as material builds up and is filtered
out of the source water. As the filter begins to clog from buildup of
suspended solids, the water level in the tube will drop until you can
no longer see water in the tube, even though the filter vessels are
full. This condition indicates it is time to clean the filter. Our
community sized filters also use a site tube for easy comparison of
supernatant level and headloss.
What do I do with the waste water?
Depending on the preferences of the customer, waste water disposal can
be as simple or as comprehensive as desired. The water from the
harrowing can be put back into the groundwater through trenches lined
with rock; channeled into a centrifugal separator; or sent to a
distiller where the distilled water can be reclaimed and the left-over
solid waste can be disposed of as toxic waste.
Are the filters approved?
Yes. Slow sand filtration is one of only four federally approved
technologies that meet the Surface Water Treatment Rule and the
Enhanced Surface Water Treatment Rule, and Blue Future Filters adheres
stricly to the guidelines as established. SSF is recognized
as a superior technology by the USEPA (United States Environmental
Protection Agency), the World Health Organization, and various state
agencies, including California, and they enjoy widespread use in the
U.S., Europe, and developing countries. In addition, Blue Future
Filters uses NSF-approved tanks and components for its filters to meet
individual state, national and international water-quality requirements
and recommendations. For links to federal and international guidelines
and regulations on slow sand filtration, visit the Blackburn
& Assoc. links page.
Question: Do these systems require power?
Slow sand filters, like all of the systems designed by Blue Future
Filters, have no electricity requirements, but instead work on simple
gravity head to operate.
Question: What ongoing costs will I have?
The only ongoing costs with the filters is maintenance. These systems
do not require electricity, replaceable cartridges or expensive
Do I backwash these filters?
Slow sand filters are non-pressurized systems that use a process called
harrowing to clean them. Iron-removing filters are also non-pressurized
and use a process called "downwashing," which is simply opening a valve
at the bottom of the filter and allowing the water to drain out.
Do slow sand filters only work on surface water?
Slow sand filters work well with surface water or water under the
influence of surface water, including shallow wells of fewer than 30
feet. Slow sand filters have also have shown results in removing
arsenic from well water, however, as well as iron and suspended solids.
Blue Future Filters also offers a line of iron-removing filters (ME and
FE models) for treatment of ground water drawn from greater depths.
These iron filters will remove hydrogen sulfide, iron, and manganese to
Do slow sand filters work with high-turbidity (cloudy) water?
Slow sand filters work optimally when treating murky or cloudy water
with up to 10-20 NTUs (nephlometric turbidity units) in turbidity,
removing most of this suspended material in the process. For higher
turbidity levels, prefiltration is recommended. Blue Future Filters
manufactures gravel up-flow roughing filters that can work in concert
or stand-alone as a preliminary step in treating especially
high-turbidity water, causing a 50% to 80% reduction in turbidity.
Will the slow sand filter remove pesticides and industrial byproducts
from the water?
With the inclusion of granular activated carbon (GAC) in the filter-bed
media, a slow sand filter meets European standards for pesticide
removal and removes a number of industrial chemicals, including
petroleum byproducts like VOC (volatile organic carbon) and
chlorine-disinfectant byproducts (THM, trihallomethanes).
Question: I see in the installation
instructions that there isn't a screen or cloth between the sand and
the gravel layers. Does this mean the sand will wash out?
One of the essential operating principals in a slow sand filter is the
low loading rate. The 0.1 g/ft2/minute maximum loading rate that is
specified for all of Blue Future Filters slow sand filters means that
the water passes through the filter at a slower rate than the settling
velocity of the sand. Therefore, the sand stays suspended above the
gravel and does not pass through. If the flow rate is increased, then
the sand would wash out of the filter. Additionally, besides being
unnecessary, adding a screen or fabric between the sand and the gravel
creates a problem area that potentially can get clogged. Operators
would then have to dig out all the sand to clean the screen or fabric.