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Frequently Asked Questions


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Frequently Asked Questions
Know 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 counterfeits.

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? Constant flow? 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.

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?
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 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?
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.

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, 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 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.



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