U.S. Patent 6,966,980

How the SSF/x Works

Slow sand filtration is primarily a biological process that treats water much the way a river bank does. A column of water slowly passes through a three-foot layer of fine sand. At the top of the sand, an intense layer of micro-organisms naturally develops. This layer lives on whatever is passing through in the water. Also called the schmutzdecke, this layer is responsible for removing up to 99.99% of all bacteria, viruses, Giardia, and Cryptosporidium 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. Slow sand filters are recognized as a superior technology by the USEPA and the World Health Organization, and enjoy widespread use in the U.S., Europe, and developing countries.

The patented SSF/x slow sand filter (U.S. Patent # 6,966,980) is for use in treating drinking water for homes or rural villages, with features providing simple on-site assembly, preset maximum flow, easy monitoring of pressure head loss, protection of piping, and a wet-harrowing system for cleaning.

The filter employs a novel hydraulic scheme which places most pipe and plumbing within the filter vessel, and also uses a device that combines flow control, pressure measurement, freeze protection, and anti-siphon features. This in combination with a relatively shallow supernatant level provides for consistent reliable operation, easy maintenance, and reasonably long filter runs as deemed very desirable in remote rural locations.

System requirements

Location
The SSF/x, which has a 2.5' X 2.5' footprint, should be located on a concrete slab, prepared and compacted earth site, or a floor capable of supporting 3,000 pounds. The best location is between the raw water source and a storage tank; however, many other possible arrangements are possible.

Storage
The SSF/x has a maximum output of 450 gallons per day at a constant flow rate of 0.25 gallons per minute. The filter is designed to run continuously at this slow rate, and therefore requires storage of the filtered water to accommodate peak usage. Any size tank is appropriate as long as it is at least one day's use. For remote communities, the SSF/x will supply 5 gallons of safe drinking water for 90 people per day.

Raw Water Quality
Slow sand filters are very good at removing turbidity (cloudiness) and bacteria. However, too much turbidity may cause the filter to clog prematurely. We recommend a maximum turbidity of the raw water at no more than 20 NTU for continuous use. The filter can tolerate higher turbidities for short periods, however. If your raw water is very cloudy and has substantial suspended solids, you should install a sediment tank and/or a roughing filter ahead of the SSF/x.

Installation

Plumbing connections
There are three plumbing connections: the inlet, the outlet, and the harrowing outlet. The inlet is the uppermost fitting on the filter. This is where the raw water enters the filter. The outlet is from the 1/2" fitting located a little more than half way up the filter. Install the supplied outlet fitting by carefully screwing it in to the 1/2" fitting on the tank making sure that the clear tube (the piezometer) is pointing up, as shown. Connect with appropriate fittings to the female threads on the outlet fitting and connect directly to your storage tank. The other fitting located about halfway up is the harrowing outlet. It is 1" threaded. Install the supplied 1" ball valve into this fitting. Connect piping with appropriate fittings to the valve and to wherever waste water is to be discharged. This may be into the sewer system or a gravel sump or other suitable waste scheme. This completes plumbing installation.

Loading the filter
Before you load the filter, fill the SSF/x with water to check for leaky fittings due to transport. Tighten any loose bulkhead fittings, etc. Drain water from filter through the exterior mounted drain fitting at bottom edge of the filter vessel. Now media loading can be initiated. Start by adding gravel, about 2 cubic feet or enough to cover the cross shaped underdrain assembly by at least 2 inches. Then add sand (about 9 cubic feet) up to but not above the harrowing fitting located 36" up from the bottom. Smooth the top of the sand level. This completes filter loading.

Start-up
Make sure that you do not operate the SSF/x without installing the outlet fitting. This fitting has a built in flow control device that is critical to the effective operation of the filter. No other regulating valve is necessary. Initial filling of the filter with water should be accomplished by filling slowly. It is a good idea to run the first water through the filter to waste because it may have dust from the sand and because the filter needs some time to ripen. It is recommended that safety chlorination disinfection of the distribution lines and tanks be initiated prior to using the system. Do this by adding chlorine as bleach or powder to your storage tank. Alternately, you can add chlorine to the SSF/x and flush it thoroughly prior to putting the sand in it. The precise amount of bleach to add is dependent upon storage volume and organic materials contamination of tanks, pipes, and fittings. The best way may be to add incrementally until a residual of free chlorine is established in the distribution piping.

A ripening period is necessary for full performance of the biological layer of the filter to be achieved. This is variable depending on raw water quality, season, and temperature, and may take from a few hours to a week or more. Water testing for microbial quality of finished water is recommended prior to use for drinking water.

Maintenance
The 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. 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 vessel is full. This condition indicates it is time to service the filter.

Wet harrowing
The filter is returned to proper operating condition by wet harrowing. Open the harrowing valve by rotating the handle one quarter turn. Refer to the photo and Figure 2 for location of this valve. This will begin to drain the standing water above the sand. Open the lid of the filter vessel and stir the top layer of the sand vigorously down two inches into the surface of the sand. Continue to stir the sand until the water has drained off from above the sand. You will notice large amounts of silt and other fine material are being drained away. Once water runs clear, shut the lid on the filter, re-close the harrowing valve, and return the filter to normal operation. It is important to not let the top layer of the sand be exposed to air for long. This can damage the biological layer and affect filter performance.

Troubleshooting
The SSF/x is remarkably simple to install and operate, and there are not a lot of things to go wrong. However, the following chart should be helpful.

Media Specifications
Gravel
2 cubic feet of 3/8 inch pea gravel

Sand
9 cubic feet of 0.35 mm silica sand (often called 30 mesh).
Uniformity coefficient = 1.5-2

Sand Sieve Analysis
sieve mm cum. passing
#16 1.18
#20 0.85 100 +- 0
#30 0.6 92 +- 1
#40 0.43 34 +- 2
#50 0.3 7 +- 1
#70 0.21 1 +- 0
#100 0.15

Technical assistance
Voice (360) 756-0071, Fax (360) 543-5606, email hb@bluefuturefilters.com

Warranty Statement

Blue Future Filters, Inc. (BFFI) provides warranty coverage to the purchaser that all products are reliable and free from known defects. BFFI will, without charge, repair or replace defective parts that fail due to manufacturing defect. BFFI will provide warranty coverage for filter tanks for a period of up to five (5) years from the purchase of the filter-system. BFFI will provide warranty coverage for all valves and fittings for a period of one (1) year from purchase of the filter-system. Although BFFI provides technical support before, during and after installation, BFFI will not be held responsible or extend a warranty for operational failure or interruption that may be due to incorrect installation or maintenance. BFFI will not provide warranty for damage to system components caused during or after installation, or through failure to follow installation instructions. Given the variability in each water system and in source-water conditions, BFFI will not provide warranty for filtration performance based on water-quality parameters (including climatic changes, raw-water quality, turbidity, etc.).