I found more than one (old) websites (in German) (http://www.teichratgeber.de/wodkafilter_bau01.html and http://www.ohligers.de/Nitratfilter1.htm and there're probably more) which consider a biological nitrate filter a cheap and easy way of denitrification. Other research is difficult because it takes places in non-Q&A forums which are overly painful to read. I found 3 references to the instructions above.
My research concludes that one needs a redox sonde for about 150 €, a dosage pump for another 60 € (and you probably want to spend double for some basic quality) and a potentially complex chain of filter chambers (not expensive, but not free neither) in order to avoid bacteria duff. The danger of the filter producing extremely smelly sulfuric products is omnipresent and makes it difficult to leave the filter unattended during vacation - or requires electronical regulation hardware which will cost a multiple of the basic setup described above.
A anion exchange granulate-based filter for a 600 l aquarium cost 150 € (or 500 € for 2500 l) and requires recharging every x weeks in boiling water for half an hour. The granulate lasts about 10 years; seporax, the filter media in biological filter, will certainly last longer, but some wearout will happen to it over the time, I assume. The filter shouldn't stop running, but the granulate isn't damaged as long as water flows through it, no matter how much nitrate it contains or whether it's charged or not.
I don't see the point. Am I missing something? Did the conditions/prices change over time (the second link estimates the cost at 800 DM for a 700 l aquarium which is ~400€)? Or did I just hit two pages based on superficial research?