Biodegradable fraction of organic carbon estimated in oxic and anoxic conditions

The biodegradability of water samples is usually estimated with bioassays under oxic conditions. In order to overcome some of the drawbacks linked to the incubation of the samples in aerobic batches, a new protocol is proposed and tested, which is based on an organic carbon (OC) balance after a 45 days incubation under anoxic conditions with excess nitrate. The biodegradable fractions of organic matter obtained with the anoxic protocol are slightly lower than those obtained under oxic conditions. Several possible reasons for a systematic underestimation of the biodegradable organic matter under anoxic conditions are evaluated and discussed: a reduced microbial metabolic potential, significantly reduced degradation rates for the slowly biodegradable organic matter, an additional production of refractory organic compounds during the incubation, or the inhibition of the recycling of the organic matter stored in bacterial biomass. Nevertheless, the 7% difference observed on the biodegradable total OC estimations keeps low enough so that the anoxic protocol can be proposed as a convenient alternative to the oxic one.