Nitrification and Denitrification: Why Total Nitrogen Misses the Limit
Biological nitrogen removal in wastewater treatment depends on two groups of bacteria with very different living requirements: the nitrifiers, which are obligate autotrophs using the energy released by ammonia oxidation for their own metabolism and are sensitive to their environment with low growth yields, and the denitrifiers, mostly facultative heterotrophs that use the energy released by nitrate and nitrite reduction when molecular oxygen is absent and fall back on organic matter decomposition when it is present.
Nitrogen in wastewater may also be present as organic nitrogen. Wastewater containing organic nitrogen is typically hard to biodegrade, high in nitrogen and low in carbon, and complex in composition. Whether that organic nitrogen is effectively broken down determines how well the whole system performs, so the degradation of organic nitrogen and the removal of the ammonia it releases are the main difficulty in treating this kind of effluent.
Nitrification does not remove nitrogen at all: it only converts ammonia to nitrite and nitrate. Final removal depends on denitrification, which requires anoxic conditions with dissolved oxygen below 0.2 mg/L — above that, denitrification stops. Because the internal recycle liquor carries more than 2 mg/L of dissolved oxygen, it competes with nitrate for carbon in the anoxic tank and directly affects nitrogen removal. Denitrification also needs organic carbon as the electron donor for nitrate and nitrite reduction; in practice a BOD5/TKN ratio above 4 gives satisfactory removal.
Why Total Nitrogen Misses the Limit: Nine Common Causes
1. The organic nitrogen in the raw water was never analysed. The remedy is to characterise the raw water properly and to provide an organic-nitrogen conversion step such as anaerobic or hydrolysis treatment.
2. Carbon source shortage. If the influent carbon is insufficient and no external carbon is dosed, denitrification lacks electron donor and total nitrogen cannot be removed completely. The remedy is to calculate the carbon dose precisely and to choose the right carbon source.
3. Internal recycle ratio too high. An excessive nitrified-liquor recycle carries too much dissolved oxygen into the anoxic tank and prevents denitrification. The remedy is to adjust the recycle ratio, optimise the internal arrangement of the nitrification tank and provide a deoxygenation zone.
4. Tank volumes not properly calculated. The volumes of the nitrification and denitrification tanks must be sized accurately for the nitrogen load.
5. Aeration demand underestimated. If the blower sizing for the nitrification tank ignores the oxygen demand of ammonia, ammonia is never fully converted to nitrate. The oxygen requirement for biological ammonia oxidation must be calculated separately and precisely.
6. Foaming in the nitrification tank. Excessive foam is controlled with defoaming or spray equipment.
7. Temperature effects. Denitrification slows as temperature falls, and insulation or heating must be provided. Nitrification rate rises with temperature and peaks around 30–35 °C; below 15 °C the denitrification rate falls markedly, and at about 5 °C it virtually stops.
8. Sludge age. An unsuitable sludge age upsets the balance between nitrifiers and the rest of the biomass; the remedy is to adjust the sludge wasting rate.
9. pH in the nitrification tank. Nitrification consumes alkalinity and depresses pH, so dosing equipment and automatic pH control should be installed.
Carbon Sources for Denitrification
From a biological standpoint the organic carbon available to denitrifiers falls into three classes. The first is the organic matter already in the wastewater — organic acids, alcohols and carbohydrates. The second is external carbon, added when the BOD5/TKN ratio is too low; methanol is the usual choice in engineering practice because it breaks down to carbon dioxide and water and leaves no persistent intermediates. The third is internal carbon, released when micro-organisms in the activated sludge die and lyse; its denitrification rate is very low, only about one tenth of the other two.
A useful measure of whether carbon is adequate is the BOD/TKN ratio, normally 3–4. The more abundant the organic matter, the faster the reaction; below 3, external carbon is needed to achieve the target. The three practical sources are therefore the wastewater's own constituents (organic acids, starch, carbohydrates), added carbon such as acetic acid, propionic acid or methanol — often available as a nearby industrial by-product — and endogenous carbon from sludge lysis.
Alkalinity and the Case for Pre-Denitrification
Denitrification recovers alkalinity. Reducing 1 kg of nitrate nitrogen to nitrogen gas theoretically releases 3.57 kg of alkalinity, and each 1 kg of BOD5 removed produces about 0.3 kg of alkalinity. Nitrification, by contrast, consumes it: oxidising 1 kg of ammonia nitrogen requires about 7.14 kg of alkalinity expressed as calcium carbonate, and the bio-oxidation of ammonia also consumes a great deal of oxygen — roughly 4.6 kg of O2 per kg of ammonia nitrogen.
This is why engineering practice places the anoxic stage ahead of the aerobic stage: pre-denitrification, the A/O process. Placing denitrification first lets the organic matter in the raw wastewater serve as the electron donor, and the alkalinity recovered in denitrification offsets about half of what nitrification consumes, greatly reducing chemical dosing and operating cost across the A/O system.
There is an oxygen benefit as well. The oxygen demand of a pre-denitrification system covers oxidation of the carbonaceous matter plus nitrification of the nitrogenous matter, but because denitrification takes place first, part of the influent BOD is consumed there as carbon source and the oxygen bound in nitrate is used, saving 2.86 kg of oxygen demand for every kilogram of nitrate nitrogen denitrified. The higher the denitrification rate, the greater the saving and the less aeration power required.
Nitrogen removal in this configuration is improved by raising the recycle ratio, but the ratio should not be pushed too far: dissolved oxygen carried in the recycled mixed liquor disturbs the anoxic condition of the denitrification stage, and pumping costs rise.
Design Data Worth Remembering
Per kilogram of nitrate nitrogen reduced to nitrogen gas: 3.57 kg of alkalinity recovered, 2.86 kg of oxygen demand saved, and 2.47 kg of methanol consumed (equivalent to about 3.7 kg of COD). Per kilogram of BOD5 removed: about 0.3 kg of alkalinity produced.
Because nitrification produces a large quantity of protons, adequate alkalinity must be present for the reaction to proceed, and because the cell yield of nitrifiers is very low it is difficult to maintain a high biomass concentration, especially in winter. Both facts argue for careful pH control and conservative design of the nitrification stage.
