Results and conclusions for the field tests with a zero-valent iron filter
In the ZVI filter, nitrate in drainage water is being reduced to ammonium by the zero valent iron. The ammonium formed is subsequently retained by a zeolite filter unit.
The research of Copenhagen University shows that ideally all nitrate in the incoming waters can be converted to ammonium and retained by zeolite. In addition, we find that phosphorus is efficiently retained in the filter. Hence, the filter can produce water that is free from nitrate, ammonium and phosphorus.
When the zero-valent iron is being consumed (corroded), iron oxide precipitates are formed and also the zeolite is being saturated with ammonium.
Regeneration of zero-valent iron and zeolite
In a follow-up study we have tested procedures for regenerating the ZVI material and thus cleaning away the iron oxide precipitates to enable a higher flow and better performance of the ZVI filter.
In addition we have tested how the zeolite filter can be regenerated. Citric acid is found an efficient way to clean-up the ZVI filter unit, while sodium chloride solution can be used to regenerate the zeolite filter unit.
In a final stage we will try to precipitate the ammonium in the regenerate with phosphorus from the ZVI filter as struvite by adding magnesium. The goal is to produce a combined N and P fertilizer product.
Our studies show that the filter is very sensitive to oxygen in the inlet water, but also that the filter can treat both low and high concentrations of nitrate. The type of ZVI is also important for the performance of the filter and should also be further tested.It is also important to prevent the iron in the filter entering the zeolite filter as this reduces significantly its performance.
The results of our research allows us to conclude that the filter is particularly useful to treat smaller volumes of drainage waters and waste waters such as from green houses, but also from households and other point sources.
Watch the video about the zero-valent iron filter system