Treatment wetlands As ...

Treatment wetlands

As suggested by others, the 20 mg/l nitrate you find might not be the biggest problem but in stead urea and ammonia. So I suggest you first analyze the water more thoroughly before starting to look for the best solution.

We have been treating toilet water with much success using treatment wetlands. The most interesting system for sanitation purposes might be the French Phragmifiltre® approach as this system can decompose solids that are present in the toilet waste water on top of the wetland surface and turn them in to compost. The remainder of the waste water will percolate into the gravel filter and the solluble pollutants will be decomposed there by bacteria that layer on the gravel surface. This is an aerobic stage that will mainly break down organic matter and turn the urea and ammonia into nitrate. After this stage, you will probably end up with much higher nitrate concentrations than the 20 mg/l you mention since all the ammonia will have been nitrified.

So you need a second stage to remove the nitrates that have formed in the first stage. This step is called denitrification. This can be done in a second treatment wetland stage that is anaerobic. Surface flow wetlands are a simple solution here where the plant biomass will be the carbon source you need for denitrification. This system however requires quite some space. If space is not available, you can also turn towards filters with wood chips as filter material.