I am working on an adding air scour to an existing rapid sand filter system at a water treatment plant. Because of the pipe configuration, I have very limited horizontal room where the pipe turns into the filter walls.  Airflow will be controlled by a butterfly valve at each filter cell. 

Is a check valve necessary after the control valve? If so what type is typically used? I was considering a swing check because it has low air flow restriction but has to be mounted horizontally and takes a lot of room. 

(I have a lot of vertical room in the pipe but several of the check valve types I have looked at can't be mounted in a vertical pipe with downward flow.) Looking for suggestions for short low flow restriction horizontal check valve or vertical check with downward flow and low flow restriction.

Check valves aren't typically installed in the air piping except at the blower.  The butterfly valve will suffice.

First make sure your underdrain is designed for air scour.  If not, you could cause big trouble in the filter.  For retrofits, there was one manufacturer that was making air scour grids to install at the bottom of the sand layer.

There can be various piping arrangements; but, you will need to loop the air header to an elevation higher than the filter overflow. This will prevent any water from flowing back from the filter to the blower.

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