If you are referring to water ...

If you are referring to water from a settlement tank, then there is a residual BOD on which organisms such as Sphaerotilus natans can grow.  So it depends on the use of the water.  The water can be pumped through a filter containing AFM media (Dryden Aqua) but avoid sand because it will become colonised by bacteria, which AFM does not.  Depending on the use, the water can be chlorinated, brominated, chlorine dioxide treated and then re-used.  RO permeate will contain trace nutrients and bacteria will grow through membranes (don't believe sales promises that no bacteria can get through their membranes).  For a high purity water, we treated the water with UV immediately after the RO then fed a tank where it was chlorinated although chlorine dioxide could have been used in its place.  Although this is general advice, really the actual application would need to be investigated and recommendations made on the actual application.