Biological Denitrification of High-Nitrate Wastewaters: a Comparison Between Three Electron Donors

Wastewaters discharged by several industrial activities, such as synthetic fibers, mineral processing, fertilizers, metal finishing, and ammunitions and explosives industries, have an high-salinity content and are characterized by a very high concentration of nitrates (more than 3 g/L). The treatment of these wastewaters generally deals in an anoxic biological process performed in activated sludge reactors (ASR). Due to the practical absence of an organic component, the treatment involves the addition of an external source of carbon, as electron donor for denitrification reactions. In addition, explosives industries wastewaters are characterized by low pH (2-3), since nitrates are generally discharged as nitric acid, and this induces a further difficulty in reactor operation, due to the extreme sensitivity of denitrifying biomass to pH conditions. In this paper the results of an experimentation performed in a laboratory scale anoxic ASR treating a highnitrate wastewater, simulating the explosives and ammunitions industries wastewaters, are presented and discussed. Three different carbon sources (methanol, acetic acid and sucrose) were compared, and the conditions to achieve the maximum removal of nitrogen were assessed. The ratio C:N to be maintained in the reactor to optimize cell growth and denitrification rate was also investigated.