Ammonium conversion in liquid organic fertilisers

Liquid organic fertilisers allow growers to abandon the use of conventional de novo (mined or synthesised) fertilisers without major technological adaptions to the cultivation system. In prior experiments the conversion by aerobic substrate born bacteria of ammonium into nitrate was plant growth limiting. To improve the formulation of a commercial liquid organic fertiliser by increasing the nitrate concentration, a simple bench scale moving bed bioreactor of 20 L volume was constructed and used in continuous mode. The objective was to convert 80% or more of the organic N present in the original liquid organic fertiliser into nitrate. In the bioreactor we used naturally occurring aerobic nitrifying soil bacteria. Bacterial material from a soil sample was multiplied in vitro in selective media. The bioreactor was inoculated with ammonia and nitrite oxidising bacteria. Bacteria were maintained on a bio-carrier with a high specific surface area and a low specific mass. The bioreactor was aerated from below which allowed the bio-carrier to float and fertiliser solution could freely reach all bacterial mass. After initial inoculation, bacteria established themselves on the bio-carrier over a three week period after which accumulation of nitrite ended and stable production of nitrate as an only product of nitrification started. The nitrate production was monitored for several months during which it produced continuously. The bioreactor was supplied with fertiliser solution containing 6 mmol L-1 (total N, no nitrate). Over 90% of nitrogen supplied was converted to nitrate. Influent hydraulic retention time in the bioreactor was around 12.5 h. In conclusion, in line continuous conversion of liquid organic-N fertiliser into nitrate can be achieved for over 90% by a simple and reliable technique. The technique may be used by growers supported by the fertiliser industry or may be scaled to pre-treat commercial liquid organic fertilisers before marketing.