Last year and the year before, we did some work on five gallon slow sand filters.
We set up 3 of them, ran them for a year and a half, and tested the output water. We found that they just barely worked at all. Several issues became apparent. First, the buckets are easily distorted by the weight of the sand. Because of this, if a filter bucket is moved or bumped; the biofilm will be broken and contaminated water will break through to the output. Second, the inside of a new plastic 5 gallon bucket is usually very smooth. This prevents a good seal from forming between the inside surface of the bucket and the sand. The result is that contaminated water slips down the side between the smooth surface of the bucket and the sand. Third; in order to facilitate effective filtration, an extremely slow flow rate must be maintained – much like that of a dripping faucet. Any faster, and the water does not have enough contact time at the top of the sand where the bacterial action takes place.
In the most dire of situations, one of these 5 gallon filters would be better than nothing if all the above issues are taken into consideration. Make sure the inside of the bucket is roughed up considerably, and make sure the flow rate is very, very ,very slow; about 1 litre per hour. Do not disturb the 5 gallon filter container once it is running. Use very fine sand, around .15 mm effective size. Be sure the water put into the filter is non-turbid. Muddy water will NOT work in one of these filters. If you only have access to five gallon containers, consider using two 5 gallon filters in series. That is put water through one, and then through a second filter of the same size. If you use 2 filters like this, consider using a coarser sand in the first filter; around .35 mm effective size; and then .15 mm effective size in the second filter. Most importantly, know that this will work only marginally at best.