Comparison Between Ethanol & Methanol as Carbon Sources for Denitrification

The success of recirculating aquarium and waste disposal systems depends partly on their ability to convert metabolic fish waste. Nitrogenous fish waste in these systems is eliminated through nitrifying and denitrifying biological filters. Although nitrifying filters are relatively easy to manage, the biological balance in denitrifying filters is far more delicate. Through the process of denitrification, nitrite (NO2-) and nitrate (NO3-) are reduced to elemental nitrogen (N2). The process is driven by anaerobic micro-organisms, which require donors from either organic or inorganic sources. Both ethanol and methanol can be used as sources for the denitrification process.

  1. Denitrifiers

    • Denitrifiers are facultative anaerobes which use nitrate as a final electron acceptor in oxygen-poor environments. Heterotrophic denitrifiers, which utilize organic carbon compounds as a source of electrons, are the most commonly encountered denitrifiers in nature. In environments that are low in dissolved carbon, autotrophic denitrifiers use other materials, such as methanol and ethanol as their electron source.

    Methanol

    • Methanol was originally the first choice as a carbon augmentation source to remove nitrogen, with 2.0 to 2.5 parts methanol being required for each part of nitrate that was denitrified. In terms of chemical cost per quantity of nitrogen removed, methane was initially the least expensive choice, and was the most commonly used carbon donor for denitrification at wastewater treatment plants. Methanol also results in low biomass yields, so more methanol was used to reduce the nitrate, than to produce biomass. Ethanol, due to a number of important criteria, is steadily surpassing methanol as a substitute for carbon.

    Ethanol

    • Methanol has typically been added to denitrifying filters as an alternative source of carbon. Ethanol, which is also used for the denitrification of sewage, is replacing methanol however, because it is an easy to obtain and relatively inexpensive product. Ethanol is a very suitable replacement carbon source and the rate of denitrification in the presence of ethanol is two to three times greater than in the presence of methanol. Ethanol metabolizes easier than methanol, which results in higher denitrification rates, particularly at lower temperatures, at which methanol is less easily utilized.

    Similarities

    • An oxygen-deficient environment must be established and the dissolved oxygen levels in this area must be close to zero for either methanol or ethanol to be used as a substitute for carbon. As regards biological filtration in aquariums, both ethanol and methanol can be utilized as a carbon source in the denitrfication chamber or filter.