FERTILIZER PRODUCTION BY DIGESTATE NITRIFICATION
Deshai Botheju1*, Øystein Svalhei2, Bjørn Rydtun2, Jo-Ela Johansen2,Trond Knapp Haraldsen3 and Rune Bakke1
1Telemark University College, Norway, 2BioTek AS, Norway, 3Bioforsk, Soil and Environment Division, Norway.
(free)
Effluents from anaerobically digested wet organic wastes can be upgraded to a commercial
fertilizer by increasing its concentration. Direct evaporation is not appropriate as it would lead
to a considerable loss of ammonia at the relatively high pH typical to these digestates.
Nitrification of the digestate prior to evaporation is suggested and evaluated through a series
of experiments based on anaerobic digestate from a large scale biogas plant in Norway.
The digestate is successfully nitrified to achieve above 75% NH4-N conversions without any
addition of extra alkalinity. pH of the digestate is reduced below 5.0 due to nitrification and
therefore stabilising the remaining ammonical nitrogen in NH4+form. Consequently, the
partially nitrified digestate is stable and can be concentrated by evaporation. The aesthetic
quality of the digestate is greatly improved by nitrification partly due to the improved
sedimentation characteristics. More importantly, nitrification aided removing a greater part of
the toxic heavy metals present in the raw digestate so that the final product can be
categorized as a high ranking organic fertilizer. This study points out that the effluents from
anaerobic digesters operating on wet organic wastes can successfully be converted into a
high quality commercial grade liquid fertilizer through partial nitrification.
KEY WORDS
Anaerobic digestion, Fertilizer production, Heavy metals, Nitrification, Nutrients
