A plastic filter can be made from any plastic barrel that has the required height below the outlet for the sand (minimum 50cm), gravel (say 10cm) and water level (maximum 5cm). Like in the other sand filters, the outlet has to be placed above the sand level in order that water rests above the sand during pause time.
Probably the best way to make the outlet is by perforating a PVC pipe and laying it internally at the base. The base of this pipe is perforated and will be surrounded by gravel so sand should not enter it. This pipe is bent or glued to elbows which allows it to rise and then exit the barrel through a hole (see photo). To bend the pipe to avoid using elbows and glue, you will need to plug one end of the pipe (e.g. with a piece of plastic bag), then fill the pipe with sand and plug the other end, before heating the section to be bent over a fire while continuously turning the pipe. When the pipe starts to go limp, this is the time you can bend it. Pour water on the hot pipe in order to fix the position.
To allow the pipe to exit the barrel without water leakage, you should use a male and female-threaded adaptor. These are plastic connectors that are threaded on one side, but can be glued on the other side. The hole in the barrel where the pipe should exit can be cut, but to get an exact hole it is easier to heat a metal pipe which has the exact diameter as the PVC pipe – when it is hot, it can be used to bore a hole through the plastic wall of the barrel. Before attaching the connectors, you should cut 2 rubber washers from car or tractor inner tube which can fit over the male thread on the male-threaded connector. Push the male-threaded connector through the hole in the wall with one rubber washer on either side, then fix using the female-threaded connector on the other side. After this is done tightly, you can now glue the internal and external parts of the outlet pipe to these connectors. Note: don’t glue beforehand since you need to screw the connectors tight first.
You will still need a diffuser plate to diffuse the flow of water entering the filter. One idea (if you have an existing lid from the container you made the filter with) is to use a bucket perforated in the lower section (see photo – credits: Joseph Mankidy). As the bucket will be tapered, if you cut the hole in the original lid correctly then it can sit snugly there without falling through.
There are also some mass-produced slow sand filters on the market. Hydraid produces a plastic filter and can ship it with or without filter media – 2,250 units can fit into a 40′ container.