Nitrate removal from waste water generated at community urinals

We are starting a community sanitation project in schools in South Asia and want to ensure an environmental friendly, sustainable cycle for waste treatment for these community toilets. The waste water from the sample toilet in one school that we started contains a high amount of nitrate (but natural as students use the toilets mostly to urinate). We need to treat this waste and reduce the nitrate concentration from 20 mg/L to 10 mg/L before we dispose this water back into the ground (where we are building ground water recharge trenches). We are looking to get some inputs for designing a waste treatment system for this project. We will like to opt for simple biologically systems using bio material etc. Your inputs for selecting or designing the waste treatment systems will be very much helpful for us.