It is also possible to make bio-sand filters from plastic drums or even old fuel barrels. This can be a good option in areas where it is not practical to produce concrete filters, for instance because cement and gravel are difficult to obtain.
When making drum filters, keep in mind that it is important to keep a sand column of at least 50cm within the filter, to ensure that proper filtration takes place.
Fuel drum filters
In previous field programmes, the sand column in filters made from old fuel drums was kept at 55cm, underlain by 5cm of fine gravel and 5cm of coarse gravel. This allowed for sufficient sand depth coupled with enough water storage space above the outlet level. To make one drum filter using the above sand column dimensions, while allowing for a 5cm layer of standing water above the sand layer during pause time, required the following tools and materials:
- Oil drum x 1
- 75cm sections of straight 3/4″ galvanised iron plumbing, threaded both ends x 1
- Elbows 3/4″ x 4
- Nipples 3/4″ x 3
- PTFE plumbing tape roll x 1
- Steel or plastic tray or bowl (diffuser plate) perforated with 2mm holes
- 2mm galvanised binding wire
- 5 sq.cm piece of plastic or metal mosquito mesh
- Pipe wrench x 2
To make a filter using a fuel drum, the bottom of the drum needs to be cut off first of all – this will act later as a lid. The edge must be filed, after which the inside must be washed well using washing powder.
Every old fuel drum normally has 2 openings – a large one (2″) and a smaller one (3/4″). The larger one should normally be watertight since it has an o-ring, but this needs to be checked since the rubber may be expired. If it is, seal the thread using plumbing tape.
The galvanised parts are put together as shown in the photo (right), and attached to the drum using the smaller 3/4″ fuel outlet. One problem occuring with drum filters is when insufficient plumbing tape is used, resulting in leaking joints. So ensure you use enough plumbing tape, especially at the join of the galvanised pipe and the drum.
Tip: at this stage, it is much easier to assemble the pipe in two parts – the first part consists of the nipple-elbow-nipple that attaches to the drum itself. You should put these 3 parts together first, and then screw it onto the drum. Secondly, you should put the rest of the parts together first, and then attach it to the piece already on the drum – to do this requires a windmill-type action. Putting the pipes together in this way avoids overtorquing the threads and is relatively quick.
The small section of mosquito mesh should be placed inside the drum over the outlet hole, to avoid any gravel pieces from blocking it. The drum should be placed out of direct sunlight, and a small hole needs to be dug into the ground to accommodate the u-bend of the galvanised pipe where it exits the drum. The filter then needs to be filled with coarse and fine gravel, and then sand.
Note: it is exceedingly important to wash the sand and gravel prior to placing it in a filter. The waste water should run clear before the media is used. The drum also needs to be half-filled with water after the gravel is placed inside, so that air pockets do not form in the sand layer.
The diffuser plate should be made from rustproof material. Previous versions have used aluminium dinner trays and plastic bowls. The important thing is that there are sides to the diffuser plate that avoid water splashing onto the sand layer. This plate is then suspended above but close to the water layer using the galvanised binding wire. This wire is attached at 3 points on the plate, and bent over the edge of the drum, where it sits easily and can be removed quickly.
Plastic drum filters
Plastic drum filters are made in a similar way, but often plastic pipe is used instead of galvanised plumbing. It is also often internal rather than external, and exits the filter at the pause time water level. Here it can connect to a threaded plastic connection that in turn connects to a metal flange fitting.
One common problem with plastic filters is that the pipe needs to penetrate the side wall, requiring this well-made flange that needs to be properly bolted to the plastic.