Depending on the volume of ...

Depending on the volume of water to treat, you could possibly use biofiltration. Under anoxic conditions, bacteria can remove, "digest" the nitrates.
You would need enough residence time to do the appropriate removal.

Usually another vessel is added after the sediment filter and used as the biofilter. The biocidal treatment is done after the biofiltration.
To determine how much removal can be done, you would need to do preliminary bench work.This is, by far, the least expensive way to treat water.
 

If you need more info, feel free to ask.