There are a number of ...

There are a number of possible cheaper alternatives depending on the severity of the concentrations, treatment process and end-use. Given enough residence time and sufficient aeration, nitrate and nitrite levels should naturally go down due to the de-nitrification process of bacteria. The most cost-effective way to do this is implementing fine and ultrafine bubble technology which can be powered by solar or diesel/grid systems or a combination of both. You could also explore the possibility of a membrane bio-reactor (MBR) if the residence time and footprint of your process is small. This essentially concentrates the process in a tank known as a 'bio-reactor' with membrane filtration in-tank as a tertiary treatment process. You could also explore media filtration, especially the use of zeolitic material which is known to be a good absorber of nitrates and nitrites. 

If you are happy to share details of your unique issue, I can work with you to find the most cost-effective solution.

Please email me at andrew@waterandoilsolutions.com.au to speak further.