most inexpensive way for ammonia-nitrogen removal in small wastewater plant?

Answer 1

You MIGHT try putting some limestone gravel in it, and letting the rocks turn to salts.

Answer 2

charcoal, Carbon and limestone, layered, then the water passes through it, to be filtered. (Sort of the way a cleaning filter works in an aquarium. Fish produce ammonium in the water. The charcoal carbon filters will filter out and neutralize the ammonium from the water.