Frequently
Asked Questions
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.
Question: 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.
Question: 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?
It is recommended to use prewashed, prepackaged silica sand for the
filters. In the United States, several sand and gravel companies offer
washed silica sand that can be purchased by the bag. 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 sterilized to reach optimal conditions for the filters. For sand
and gravel size specifications, click here.
Question: 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 filter?
A clear sight tube emerging from the filter shroud 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.
Question: 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.
Question: 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. They are 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-flow.
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 chemicals.
Question: 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.
Question: 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 undetectable levels.
Question: 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.
Question: 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.
