Control Denitrification & Oxygen Levels

Denitrification is triggered whenever the oxygen content in a bed of EN media falls below 1 mg/L while in the presence of organic material (“carbon”) that can serve as an energy source for heterotrophic bacteria. Some heterotrophs only denitrify while others have the ability to switch from oxygen as an electron acceptor, or in the absence of oxygen to nitrate (NO3). Whereas oxygen is converted to carbon dioxide, nitrate is converted to inert nitrogen gas (N2) through a few enzymatic steps. This process is probably unavoidable at some level within a heavily loaded bead filter and certainly occurs spontaneously in PolyGeyser sludge storage basins when thick (carbon-rich) sludge accumulations inhibit oxygen transfer. The behavior of the bead bed is controlled by the conditions it experiences. The same filter can be flipped back and forth from aerobic nitrification and denitrification in a relatively short time frame just by controlling oxygen levels. In many cases, the bottom of bead bed will nitrify while the upper portion (void of oxygen) will denitrify simultaneously.

Encouraging denitrification in the fixed film bead bed generally requires that the carbon source is introduced while the oxygen supply is driven down. The latter can be accomplished by purging the inflowing water with nitrogen gas, or by using the oxygen-depleted effluent from an aerobic filter as the influent to the denitrification unit. In fact, the denitrifying bed itself will usually finish the job of oxygen removal. It is important to note that marginal oxygen levels (near 1 mg/L) can cause incomplete denitrification resulting in nitrite accumulations, an undesirable outcome.

Carbon source enhancement is usually accomplished by the introduction of a soluble carbon source into the wastewater ahead of the denitrifying bed. Common carbon sources for denitrifying beds include methanol, ethanol, sugar, and molasses.

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