[PDF]BioSand Filters

Guidelines

Preparation of Media for the BioSand Water Filter

Three Layer System

April 30, 2007
Dr. David H. Manz, P. Eng., P. Ag.

The purpose of preparing and publishing these guidelines on the internet is to provide a readily
available technical reference to all those involved in BioSand Water Filter manufacture, funding,
use, evaluation or support.

Introduction

The three layer system was developed for use by individuals or organizations that produce a few
hundred household BioSand Water Filters in a central manufacturing facility per year. This
remains the most common type of manufacturing operation world wide. The three layer system
can be implemented with a minimum of capital investment, locally available materials and using
simple manual techniques. (See Guidelines for Preparation of Media for the BioSand Water
Filter Four Layer System for instructions pertaining to preparation of media when producing
larger numbers of filters (several hundred or thousands) per year.)

The three layer system requires a thorough understanding of the filtering procedure and how the
filter media affects the quality of the treated water. It is very important that the techniques
presented in this document be followed as close as possible. The apparent simplicity of the
three layer system seems to invite ad hoc modifications to the technique and filter performance
(and acceptance) suffers as a result.

Following is a description of the three layer system and how it may be implemented. Only
essential concepts are presented. Ultimately, local manufacturers must develop their own
techniques and procedures while adhering to the fundamentals expressed in these guidelines.

Detailed instruction and demonstration of filter media selection and preparation is provided in
workshops on the manufacture and use of the BioSand Water Filter and in shorter workshops
focussed specifically on filter media production.

Description of the Three Layer System

The three layer system requires the preparation of three types of media; the underdrain layer,
separation layer and one filtering layer.

Consider the following sketch of a cross-section of a filter bed in a BioSand Water Filter that
uses the three layer system.



Inlet of
standpipe.




Top of BioSand Water Filter.

Surface of water when the filter is paused
(stopped and allowed to drain normally).

Top of filter bed.

Filter media.
Separation layer.
Underdrain layer.



Figure 1. Cross-section of the filter bed of a typical BioSand Water Filter using the three
layer media system.



A description of each of the various layers follows:



1 . Underdrain layer : Allows vertical drainage through the filtering layer and allows filtered
water unrestricted access to filter standpipe. The thickness of the underdrain layer must
be sufficient to cover the inlet to the filter standpipe (located at the inside bottom of the
filter body) with 2 cm of underdrain material. The underdrain layer is composed of
particles ranging in size from 6.25mm to 12.5mm (1/4 to 1/2 inch) in diameter. The total
depth of the underdrain may be 8 cm or more. The thickness of the underdrain layer is
the same throughout the layer.

2. Separation layer : Prevents filter media from entering the underdrain layer and the
standpipe. The separation layer should be 3 cm in thickness. The separation layer is
composed of particles ranging in size from 3.125 mm to 6.25 mm (1/8 to 1/4 inch) in
diameter. The thickness of the separation layer is the same throughout the layer.

3. Filter media : Responsible for removal of particles and micro organisms including viruses,
bacteria, parasites and helminths. Flow of water through the filter is controlled by the
selection and preparation of the filter media. The largest particles comprising the filter
media must be less than 3.125 mm or 1/8 inch in diameter. The thickness of the filter
media should be no less than 40 cm. The thickness of the filter media is the same
throughout; that is, the top surface must be level after installation.



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Media Installation Instructions



The only way to evaluate prepared filter media is to install it in a full-scale BioSand Water Filter.
The following installation procedure is very similar to that used when installing filters for actual
use. Procedures for post-filtration disinfection are not included.

Media installation proceeds as follows:

1 . The first step is to determine the maximum depth of the media in the filter. If the filter
walls are made of plastic and are transparent it is possible to make all depth
measurements from outside the filter. If the filter body is not transparent, (concrete), then
it is necessary to fill the filter to the point where water is just starting to drip from the
standpipe. At this time the depth of water in the filter is equal to the depth of the media
plus the paused water depth. A piece of wood that is 1 or 2 cm in diameter and 10 cm
longer than the depth of the filter may be used to measure media depth. The wood is
placed on the bottom of the empty filter and held vertical. A second similar but shorter
piece of wood is placed across the top of the filter and the vertical piece is marked with a
pencil immediately above the horizontal piece. The distance from the bottom of the
vertical piece of wood to that mark will be equal to the depth of the inside of the filter
plus the thickness of the horizontal piece of wood.

2. Using a measuring cup or similar container add underdrain media such that it covers the
inlet to the standpipe with 2 cm of material. (Cups should be filled and levelled.) The
top of the underdrain layer must be perfectly level. Typically, your hand is used to level
the top of the layer. If your arm is too short a piece of wood can also be used. It is
normal for the water to become quite turbid and it will not be possible to see the media
through it. The measuring stick is now very useful. The depth of media required to
cover the inlet to the standpipe is noted and a second mark is added to the vertical stick a
distance equal to the depth of the underdrain layer below the first mark. The depth of the
media can be conveniently measured by placing the horizontal piece of wood across the
top of the filter and positioning the vertical piece against it. When the depth is exactly
correct the second mark and the top of the horizontal piece will be the same. Record how
many cups of underdrain media were used. The number of full and partial cups of media
will be used to determine how much underdrain media will be supplied with the filter.

3. Put the diffuser basin into position and add water until there is approximately 10 cm
standing above the underdrain material. (The diffuser basin is used to prevent the water
from disturbing the media when poured into the filter.) Remove diffuser basin.

4. Prepare to add the separating media. Add a second mark to the vertical piece of wood 3
cm below the first mark. Using the measuring cup carefully add 3 cm of separating
media. Again, the top of the separating layer must be perfectly level. Record how many
cups of separating media were used.

5. Put the diffuser basin into position and add another 10 cm or so of water. Remove
diffuser basin. There should never be more than 20 cm or so of water in the filter when
media is being added.

6. Place marks on the vertical piece of wood equal to the thicknesses of the filtering layer.



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7. Carefully measure the amount of filtering media required to bring the filter bed to the
mark that indicates the top of the layer. (Note that media should ALWAYS be added to
water and added quickly - almost dropped in. By adding media to water there is no
danger of trapping air in the media which can stop the flow of water through a process
known as air binding. By adding the media very quickly into no more than 20 cm of
water there is no danger the particles will separate with coarse falling into place first
followed by the fine particles, a process known as stratifying. The stratifying effect
greatly decreases the flow of water through the filter and is avoided using this procedure.)
Record the amount of media used to complete the second filtering layer. (Place a bucket
under the standpipe to capture any water that may dribble from the standpipe outlet
during media installation.)

8. Clean surface of the media by adding sufficient water to fill the portion of the filter
interior above the top of the filter media with 20 cm or so of water. Remove the diffuser
basin. Using your hand or a brush, vigorously stir up the water above the media surface.
Your fingers can penetrate the top of the media by a 1/2 cm but no more. The vigorous
agitating action will cause the top of the media layer to be release particulate material that
would otherwise plug it off. Once sure that all material is suspended that can be
suspended a cup is used to remove as much of the very turbid water from the filter as
possible. Wait a few seconds to allow the media to settle before starting this process.
Care should be taken not to remove any of the media by allowing the cup to scoop into
the media surface. The cleaning action can be repeated as often as required. Level the
top of the media as a last step of the cleaning process. The effect of this cleaning is to
maximize the flow of water through the filter bed that has just been installed.

9. Measure the flow rate through the filter by replacing the diffuser basin and adding
enough water to completely fill the filter. Using a calibrated measuring cup or a plastic
container such as an empty water or soda pop bottle, (any of the plastic bottles will work
though 500 ml and 1 litre bottles work the best), measure how long it takes to fill this
bottle with filtered water, which will still look very turbid. (Note that after three or more
20 litre buckets of water are added to the filter the water will flow very clear.) Calculate
the actual flow rate through the filter by dividing the volume filtered measured in litres
by the time required to produce the volume in minutes. Compare this flow to the
objective flow rate for the filter. The objective flow rate is calculated by multiplying the
surface area at the top of the media measured in square meters by 600 litres per hour per
square meter. For example if the surface area of the filter, at the top of the media, is 0. 1
square meter, the objective flow rate is 0.1 square meters times 600 litres per square
meter per hour equals 60 litres per hour or 1 litre per minute.

10. If the flow rate as measured is above or below the objective flow rate the procedure for
washing the filtering media must be adjusted. This is described later.

Selection of Crushed Rock for Media Preparation

The preferred source of material with which to produce the filter media is crushed rock from a
rock quarry. This material can be expected to be very clean and free from organic and microbial
contamination. There is usually an opportunity to acquire large quantities of raw materials
exhibiting consistent particle size distribution.



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Ideally, the material is very hard, a quartzite or granite. Very soft material such as crushed
limestone can also be used. Mudstones that contain oxidized iron or manganese should be
avoided as they will likely contaminate and color the water as it is filtered. A potential problem
with softer rocks, (like limestone and mudstone) is that some particle rubbing and grinding
always occurs during handling and transport resulting in the production of unwanted fines,.
From a consumer perspective the color of the rock should be as light as possible though any
color is actually satisfactory provided the color does not transfer to the water during the filtering
process.

Crushed rock is often available in a size already very close to that needed for the underdrain
layer and separation layers and may be purchased directly. Very little waste will result when
processed.

Crushed rock for the preparation of the filter media and separation layer is usually the finest
material in the quarry. Often it is considered waste material. The material used for the filtering
layer MUST contain particles varying in size from very fine, almost dust size, to larger than 1/8
inch or the opening of the filtering sieve. Material that is all dust simply won't work as the flow
through it will likely be too low or require considerable preparation to make work. Material that
does not contain any fines cannot work either because the flow rate through the media will be too
large.

Media delivered to the processing site should be protected from the weather and risk of
contamination from weather, animals or humans. This may require a roofed working area and a
concrete floor. Drying media outdoors will require a large concrete pad with good exposure to
the sun. The entire media processing area should be secured with appropriate fencing or walls
that also allow good access for trucks.

Every shipment of unprocessed media should be evaluated, using appropriate particle size
evaluation techniques, to insure that the media supplied is similar to previous shipments that
were used to establish media preparation procedures and filter bed design. Particle size
distributions can very considerably from shipment to shipment. However, it is reasonable to
assume that individual shipments are more-or-less homogenous and it is advisable to obtain the
largest shipments that can be managed.

If it is determined that the particle size distribution of a shipment is different from previous
deliveries, the media preparation procedure may need to be evaluated and changed if necessary.

Media Contamination

Clean (uncontaminated) media is free from toxins and any organic material, living or dead.
Media at risk of contamination with toxic substances by virtue of being near industrial activity or
affected by waste disposal activities should be completely avoided. Surface accessible deposits
of sands and gravels, including gravel pits, beaches and river banks, usually contain significant
organic material due to vegetation accumulation and human and animal activities. Only media



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obtained from rock quarries that produce material from solid rock formations can be expected to
be contamination free.

Media contamination occurs due to the presence of individual organic particulate material,
(pieces of plants, animals, insects, seeds, etc.), and organic material that was originally dissolved
or suspended in water and is now attached to individual rock particles. Organic material is food
for micro-organisms, a few of which might be pathogenic (disease causing). Standard practice
is to test for the presence of pathogen indicator micro-organisms such as fecal colliform bacteria
(found in the intestinal tracts of warm blooded animals and present in the many billions per gram
of human feces), Escherichia colliform or e-coli bacteria (dominant colliform bacteria found in
intestinal tracts of warm blooded animals) and total colliform bacteria that includes all forms of
colliform bacteria including those that naturally live in the soil or associated with other forms of
life. If colliform bacteria are present it is normal to find similar numbers of fecal colliform and
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