Biological Nitrogen Potential (BNP): A New Methodology to Estimate Nitrogen Transformations During Anaerobic Digestion of Organic Substrates

In this paper, nitrogen transformations were studied on 13 substrates from agro-industrial, agricultural or urban sources using a new method. This method was developed to determine the kinetics and balance of nitrogen transformation during anaerobic digestion with high inoculum-to-substrate ratio while avoiding instabilities with nutrients addition. Inoculum was washed to reduce ammonium content and carbonates were added to adjust buffer capacity. Results show that kinetics of nitrogen mineralisation depends mainly on initial substrate. Nitrogen was quickly transformed before stabilisation (from 2 to 8 days). Four types of nitrogen transformation were identified: (a) rapid mineralisation (transformation of organic nitrogen (Nₒᵣg) into ammonium) for blood, (b) moderate mineralisation, for sludge, green waste and food waste, (c) limited protein hydrolysis (no transformation of nitrogen forms) for slurry and manure and, (d) immobilisation of nitrogen (transformation of ammonium into Nₒᵣg (biomass growth)) for fruits, straw and greases. Biological nitrogen potential (BNP) ranged from − 7.1 to 22.8 gN kgWW⁻¹ and showed a good correlation (R² = 0.80) with Nₒᵣg from substrates. On co-digestion tests, no significant inhibitions of mineralisation nor synergies between substrates were assessed allowing to calculate BNP value of the mix as the weighting average of the BNP values of each added substrate. Examples of calculation of nitrogen form on digestate show that do not use BNP may induce to under or overestimations of the final NH₄⁺ content. A comparison of this methodology with an already published methodology shows an excellent correlation but a systematic difference of 7.2 gN kgWW⁻¹, which seems to be explained by a better capacity of the methodology developed in this study to describe immobilisation phenomena.