Modeling Denitrification of a Waste Stream in a Packed-Bed Bioreactor
Wastewaters containing nitrates (e.g., those from septic tank sewage systems, agricultural runoff, and some food-related industries) can present serious pollution hazards for aquatic environments containing marine life. These wastewaters often represent a primary source of nutrients for the growth of those microorganisms that are the root cause of eutrophication of surface waters. A classic approach to the problem of removal of nitrogenous compounds from waste streams involves aerobic nitrification of the waste stream, followed by anerobic denitrification. In this process facultative heterotrophic bacteria reduce nitrate ions to nitrogen
6 NO _3^{-}+5 CH _3 OH \rightarrow 3 N _2+5 CO _2+7 H _2 O +6 OH ^{-}
A. L. Parker, L. J. Sikora, and R. R. Hughes (20) studied biological denitrification of wastewater streams in packed beds at temperatures from 5 to 20°C. For steady state microbe populations and feed nitrate concentrations of less than 100 mg/L, the reaction above obeys a rate expression of the form
r=\mu^* Y C_{ NO _3}
where μ^∗ is the rate constant for pseudo first-order removal of nitrates, Y the biochemical yield coefficient, and C_{NO_{3}} the nitrate concentration.
Residence time distribution studies by these researchers indicated that for a tube packed with 3-mm glass beads, an appropriate n-CSTR model to use involves a number of stirred tanks that is in the high teens. Consequently, the authors concluded that the extent of longitudinal dispersion was relatively small. Indeed, it is not unreasonable to assume that a PFBR model might provide good estimates of the conversion to be expected in packed beds of this type. If a similar packed bed is operating at steady state at a temperature for which the μY product is 0.309 h^{−1} and for which the inlet concentration of nitrate ions is 80 mg/L, what space time is necessary to reduce this concentration to 20 mg/L?