Do consider slow sand ...

Do consider slow sand filtration as your treatment option if you need a simple system and you are using river water that could have pathogenic organisms and some turbidity, but not too high (< 100 NTU).  You may also want to dose the storage reservoirs with some chlorine periodically (say 5ppm). 

If you already have an existing mechanical or hydraulic treatment system, you should indicate what these processes are - there are many operational guides on the internet but often quite complex.