When constructing a slow sand water filter, make sure the sand used is not contaminated.
Make sure the sand source provides the correct type of sand.
Running some chlorinated water through the filter before starting it running with unfiltered water is a good idea. If the sand is contaminated with bacteria, the filter may not produce clean water. Running five gallons of chlorinated water through the filter initially, will kill any residual bacteria present in the sand. Do not use chlorinated water after the first decontamination run, just the first time the filter starts, that’s it. After that the filter will build up a biolayer on the top of the sand where it is supposed to be.