Odour control of pounds of food industry wastewaters by controlled denitrification

The noxious odours during storage and treatment of food industry wastewaters are a major environmental problem today. Winery wastewaters clearly fall within this framework. Volatile fatty acids (VFA) form the major products resulting from the fermentation of carbon compounds (ethanol and sugars) in winery wastewaters, and are responsible for characteristic foul odours as a result of their low olfactory perception threshold. Natural evaporation in ponds, a rustic and economical technique, well adapted to discharge variations and seasonal production, is a widely used treatment method for winery and distillery wastewaters. The prevention of VFA formation by orienting the degradation of organic matter in wastewater into odourless products through anaerobic respiration with nitrate as the electron acceptor (denitrification) was studied in winery wastewaters to which nitrate had been added in the form of concentrated nitric acid or nitrate salts.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 (1 L) at different NO3/COD ratios and at full scale in natural evaporation ponds (2 x 7,000 m2) by measuring olfactory intensity. In the absence of nitrate, butyric acid (2.3 g.L-1), acetic acid (1.63 g L-1), propionic acid (1.56 g L-1), caproic acid (0.5 g L-1) and valeric acid (0.3 g.L-1) were produced from reconstituted winery wastewater. For a ratio of N03/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 (NITCALTM) to winery wastewaters (3,526 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.