Biosand Filters

OVERVIEW OF THE BIOSAND FILTER

What is the biosand filter?

The biosand filter is a modified form of the traditional slow sand filter in such a way that the filters can be built on a smaller scale and can be operated intermittently. These modifications make the biosand filter suitable for household or small group use. The biosand filter can be produced locally anywhere in the world using materials that are readily available.

The biosand filter should be used as part of a multi-barrier approach which is the best way to reduce the health risk of drinking unsafe water. Barriers which protect water from pathogens can occur in each of the following steps:

Step 1 – Protecting the water source

Step 2 – Sedimentation

Step 3 – Filtration (e.g. biosand filter)

Step 4 – Disinfection

Step 5 – Safely storing water after treatment

Separating Gravel Layer – 6 mm (1/4”) size – 5 cm deep

Prevents sand from plugging under drain gravel.

Diffuser Plate

Protects the biological layer from damage when water is poured into the filter.

Filter Lid

Prevents contaminants from entering the filter.

Sand Layer – 40 to 50 cm deep

Traps organic and inorganic material at the top of the filter media.

Outlet Pipe – 6 mm (1/4”) inner diameter (I.D.)

Conducts water from filter base to outside.

Underdrain Gravel Layer – 12 mm (1/2”) size – 5 cm deep

Promotes flow of water into outlet pipe.

Sand Layer

• Ideally obtained from clean, crushed rock.
• Screened through 0.7 mm (24 mesh) wire sieve of perforated metal sheet.
• Washed to ensure an Effective Size (ES) of 0.10 to 0.25 mm (prefer 0.15 to 0.20 mm) and Uniformity Coefficient (UC) of 1.5 to 2.5 (prefer <2).

Concrete Filter Body

• Mix concrete (by hand or with mixer).
• 1 part normal (ordinary or general use) cement (approximately 15 kg [33 lb])
• 1 part clean gravel 6 mm (1/4”)
• 1 part clean gravel 12 mm (1/2”)
• 2 parts clean sand
• Weight when empty – 72 kg (170 lb).
• Weight when full of sand and water - 160 kg (350 lb).

Diffuser Plate

• Required to prevent the disturbance of the sand layer when water is poured into the filter.
• Can be made of various materials that are suitable to be submerged in water such as heavy plastic, acrylic, plexiglass, or galvanized metal.
• 100 holes, no larger than 0.3 cm (1/8”) diameter, are drilled or punched in the material on a 2.5 cm x 2.5 cm (1” x 1”) grid.
• If arsenic removal is desired, the diffuser must be made in a box shape and filled with 5 kg (11 lb) non-galvanized less than 2.5 cm (1”) long iron nails. The hole diameter can be made larger 0.6 cm (1/4”) if excessive iron clogging occurs.

Lid

• Tightly fitting lid prevents contamination of water and unwanted pests.
• Can be made from various materials, usually wood or galvanized metal.

Mold Design

The steel mold used for the biosand filter is designed to produce a good final product, while being easy to use. With good care and maintenance, this mold should be suitable for several years of filter construction. The mold design has gone through eight generations of improvements, but there may still be revisions that would add value.

How does the filter work?

A bucket of contaminated water is poured into the top of the biosand filter. The water simply flows through the filter and is collected in another storage container at the base of the spout. A biological layer (often called the biolayer) of slime, sediment and microorganism develops at the sand surface. Pathogens and suspended material are removed through various physical and biological processes that occur in the biolayer and sand. When water is flowing through the filter, oxygen is supplied to the biolayer by the dissolved oxygen in the water. During pause times, when the water is not flowing, the oxygen is obtained by diffusion from the air. If the standing water layer is kept shallow, enough oxygen is able to pass through to the microorganisms to keep them alive and effective.