NOx is a generic term for the mono-nitrogen oxides NO and NO2.   NOx has a environmental impact as it contributes to fertilizing the environment. Air is a mixture of gases where the major parts are 78% nitrogen and 21% oxygen. The nitrogen can react with the oxygen forming NOx during special circumstances like in high temperature combustion. 

Injecting water into a gas turbine will reduce the combustion temperature and thereby reduce the formation of NOx. This injection water must have very high purity to assure that no scaling is formed in the turbine and that there is no wear on the turbine. Reversed Osmosis can be used to generate this type of very pure water using double pass technology.