The groundwater of many areas is polluted with an excess of nitrate. Nitrate pollution is caused mostly by fertilizers or waste effluent from industrial or domestic sources. Because of their wide use of fertilizer, large agricultural areas such as the Great Plains or Midwest regions of the United States often have groundwaters polluted with nitrate. In order to provide safe drinking water, the excess amounts of nitrate have to be removed.
Too much nitrate in drinking water is poisonous to infants. Methemoglobinemia, commonly called "Blue-Baby disease," is caused by the lack of the infants' ability to metabolize the nitrate. The ability to metabolize nitrate is acquired with age, so it poses little threat to the majority of the population. However, because it can be fatal, the maximum safe concentration of nitrate in drinking water has been defined to be 10 mg/l by the Safe Drinking Water Act of 1974.
One way to remove nitrate from drinking water is to use denitrifying bacteria to convert the nitrate to dinitrogen gas. This method has been proven to remove nitrate from contaminated water in the laboratory, but is still in the experimental stages for industrial/commercial use.
To deal with the problem of nitrogen pollution in drinking water supplies, research was performed by the Dept. of Civil Engineering at the University of Colorado. In this study, researchers tested a packed tower biofilm process and investigated its effectiveness on a shallow groundwater source. The project involved the operation of pilot reactors in a laboratory setting for over a year, and the following performance factors were considered:
The process by which the toxic nitrate ion is removed from water and converted by bacteria to dinitrogen gas is called denitrification. Denitrification is an effective solution to nitrogen pollution, because the end product (dinitrogen gas) is a harmless one. The active bacteria in the process are facultative heterotrophes---facultative meaning that they can grow both with and without oxygen (though with oxygen is usually preferred), and heterotrophes meaning that they use an organic carbon source. These bacteria respire using either nitrate or oxygen as the terminal electron acceoptor, and require addition of a reduced carbon source to the water for purposes of energy production and cell synthesis. The Univ. of Colorado researchers chose acetic acid as the carbon source because it is non-toxic to humans, safer to handle than alternatives such as methanol, and available in mass quantities.
The fixed biofilm process that was tested in the lab was a feasible choice for denitrification because it avoided the problem of solids separation and was simple to operate. Each reactor consisted of a pair of clear acrylic cylinders connected in series. The cylinders were 2.6 meters tall and 15 cm in diameter, and they were both filled to 85 % volume capacity with a highly porous material. An upward influent flow of 1.2 L/min was maintained in the cylinders for the entire operation period, corresponding to a hydraulic loading of 66 (L/min)/M2 and a 40-minute detention time.
It was first necessary to formulate a model for determining the amount of acetic acid required for reaction of all nitrogen and oxygen in the influent water. Stoichiometric equations were derived by assuming that 65 % of the acetic acid is used for cell production when oxygen is the terminal electron acceptor, while 35 % is consumed when nitrogen is the terminal acceptor. The equations are as follows:
| 1.00NO3- + 0.88CH3COO- + H+ ----> 0.09C5H7NO2 + 0.46N2 + 0.42CO2 + 0.88HCO3- + 1.7H2O |
| 1.00O2 + 1.43CH3COO- + 0.26NO3- + 0.26H+ ----> 0.27C5H7NO2 + 0.10CO2 + 1.43HCO3- + 0.63H2O |
To test the proposed stoichiometry, two sets of acetate consumption data were gathered (and then plotted) when the reactor was removing 3 mg/L of influent oxygen and 11 mg/L of influent nitrogen. The plot looked like this:
Plot created from extrapolation of Cook et al. data
The darkened squares were measured one week prior to the open-square data, and the line represents the acetate consumption predicted from the previous equations. The researchers felt the agreement was sufficient to validate the hypothesized stoichiometry.
Plot created from extrapolation of Cook et al. data
Based on the lab study results, the researchers concluded that the packed tower biofilm denitrification process was a successful one. In addition to sufficiently denitrifying the water supply, the reactors effectively removed excess bacterial biomass, as well as some trihalomethane forming potential.
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