Prevention of volatile fatty acids production and limitation of odours from winery wastewaters by denitrification

The effect of the addition of nitrate to winery wastewaters to control the formation of VFA in order to prevent odours during storage and treatment was studied in batch bioreactors at different NO3/chemical oxygen demand (COD) ratios and at full scale in natural evaporation ponds (2×7000 m2) by measuring olfactory intensity. In the absence of nitrate, butyric acid (2304 mg L−1), acetic acid (1633 mg L−1), propionic acid (1558 mg L−1), caproic acid (499 mg L−1) and valeric acid (298 mg L−1) were produced from reconstituted winery wastewater. For a ratio of NO3/COD=0.4 g g−1, caproic and valeric acids were not formed. The production of butyric and propionic acids was reduced by 93.3% and 72.5%, respectively, at a ratio of NO3/COD=0.8, and by 97.4% and 100% at a ratio of NO3/COD=1.2 g g−1. Nitrate delayed and decreased butyric acid formation in relation to the oxidoreduction potential. Studies in ponds showed that the addition of concentrated calcium nitrate (NITCAL™) to winery wastewaters (3526 m3) in a ratio of NO3/COD=0.8 inhibited VFA production, with COD elimination (94%) and total nitrate degradation, and no final nitrite accumulation. On the contrary, in ponds not treated with nitrate, malodorous VFA (from propionic to heptanoïc acids) represented up to 60% of the COD. Olfactory intensity measurements in relation to the butanol scale of VFA solutions and the ponds revealed the pervasive role of VFA in the odour of the untreated pond as well as the clear decrease in the intensity and not unpleasant odour of the winery wastewater pond enriched in nitrates. The results obtained at full scale underscored the feasibility and safety of the calcium nitrate treatment as opposed to concentrated nitric acid.