HETEROTROPHIC DENITRIFICATION - THE INFLU ENCE OF OXYGEN AND PH VALUE

A demonstration plant for biological heterotrophic water treatment of nitrate polluted groundwater has been operated in Coswig near Dresden since 1989. In this NEBIO tube reactor process the denitrification is achieved in a downstream fluidized bed with continuous regeneration of sintered polystyrene particles as inert carrier material. A nutrient consisting of ethanol and phosphate is dosed in the reactor influent. In the subsequent treatment stages the denitrified water is aerated, f iltered through a multi-layer and GAC filter, and is finally disinfect ed with chlorine gas. The influence of changing raw water quality (oxy gen content, pH value) on the process performance was examined. Increa sing oxygen concentration lowers the nitrate reduction potential and r ises the consumptive ratio DELTAC/DELTANO3-. The technology shows a hi gh removal performance of 270 g NO3- m-3 h-1 in the range of pH 6.2 to pH 7.3 which is typical for natural groundwaters. The degradation of nitrate is increasingly inhibited for pH values beyond 7.6. Nitrite pr oduction occurs significantly in high pH ranges. The results lead to f urther insight in the stoichiometry of heterotrophic denitrification. By expressing the stoichiometric equations for nitrate and oxygen resp iration as functions of oxygen and pH value it could be shown that the influent water quality has strong effects on the consumption of ethan ol. A kinetic model was developed to predict the reactor performance u nder changing raw water conditions. A two stage kinetic model was desi gned, regarding two main effects: biochemical degradation of oxygen, n itrate and ethanol and distribution of active biomass due to hydraulic properties of the tube reactor. This model may be helpful for reactor design for sites of various ground water qualities.