The effect of carbon source and dose on removal of nitrate from drinking water supplies using expanded bed reactors

The objectives of this research were to: (1) assess in detail the impact of the electron donor on biological denitrification in an expanded-bed reactor using methanol and ethanol as electron donors, (2) assess in detail the feasibility of a strategy of minimizing effluent nitrate, nitrite and electron donor concentrations by controlling the amount of electron donor added, and (3) assess the nature of the organics in the effluent from biological denitrification using methanol and ethanol as they relate to the formation of trihalomethanes.; The methods used to achieve the objectives included the construction of fluidized bed reactors for biological denitrification using filter sand as a support medium for biofilm formation. Nitrate, buffered mineral media and electron donors were fed to the reactors in various COD:N ratios over a period of two years. The effluent was analyzed for nitrate, nitrite, electron donor and total organic carbon. Certain samples were filtered through sand or granular activated carbon, chlorinated and then tested for trihalomethane (THM) formation.; The findings included that COD:N feed ratios can determine effluent nitrate, nitrite, electron donor and soluble microbial product (SMP) concentrations as well as effluent THM formation potential. Models to describe these results were developed. At COD:N ratios of 4.0 and above, nitrate was found to be the limiting substrate. Below COD:N ratios of 4.0, the electron donor was the limiting substrate. It was found that controlling biological denitrification effluent nitrate concentrations by varying the electron donor concentration could create effluent nitrite problems. In the region where the electron donor is limiting, nitrite was found to be present in an inverse relationship to the COD:N ratio. It was found that granular activated carbon filtration significantly reduced THM formation from the biological denitrification effluent and there was no apparent advantage between methanol and ethanol as biological denitrification substrates in the generation of THM. Methanol and ethanol were found to be relatively insignificant substrates for THM formation relative to SMP.