Biological Nitrification and Denitrification | Activated Sludge Process | Wastewater Treatment Plant

Biological Nitrification and Denitrification

Nitrogen is removed from wastewater in two steps: biological conversion of ammonia nitrogen to nitrate nitrogen, followed by denitrification via biological conversion of the nitrate to nitrogen gas.

The majority of the organic nitrogen that enters a wastewater treatment plant is in the form of ammonia, which is formed during the hydrolysis process as the material travels through the sewer pipes. The largest and most common source of organic nitrogen is faeces and urea dumped into sewers.

Ammonia to nitrate conversion is an aerobic biological process. This procedure necessitates the oxidation of ammonium to nitrite, followed by the secondary oxidation of nitrite to nitrate. Nitrosomonas bacteria are responsible for the first conversion, while Nitrobacter bacteria are responsible for the second. The two reactions happen at the same time and progress quickly to the nitrate stage. As a result, nitrite concentrations in wastewater entering the treatment plant are generally low.

Both of these bacteria are strict aerobes, which means they require free oxygen to convert. Furthermore, the nitrification process necessitates a lengthy detention period, a low food-to-microorganism (F/M) ratio, and a lengthy mean cell residence time. Because the process generates various acids, an appropriate pH with adequate alkaline buffering is required for the process to proceed reliably. Although nitrification has been reported at pH values ranging from 6.5 to 7.0, the optimal pH for these reactions is between 7.5 and 8.5. For every mg/L of ammonium nitrogen oxidized, approximately 7.1 mg/L of alkalinity (as CaCO3) is required.

Temperature is involved in these reactions, but it has not been shown to have a significant impact on the rate of reaction. Nitrification appears to peak between 30°C and 35°C, but then rapidly declines after about 40°C. Nitrification appears to proceed slowly down to a temperature of about 10°C, but it rapidly declines below that temperature to zero. Denitrification takes place over a similar temperature range, with the reaction rate increasing as the temperature rises.

Denitrification is the process by which nitrate is converted to nitrogen gas by facultative, heterotrophic bacteria. The oxygen attached to the nitrate (NO3) serves as an energy source for the facultative bacteria. This happens when the DO concentration is so low that it effectively shuts down aerobic bacterial activity, resulting in “anoxic” conditions. Instead of going through the trouble of dissociating the nitrates, facultative bacteria use the DO in aerobic conditions. They will break down the nitrates and allow the nitrogen gas to escape from the liquid under anoxic or anaerobic conditions.

Denitrification occurs at pH levels ranging from 7.0 to 8.5. It also produces alkalinity as a byproduct, which offsets about half of the alkalinity required for the nitrification process. There must also be a sufficient source of available carbon for the denitrifying bacteria, as nitrates provide none. Although the wastewater contains enough carbon, some supplemental carbon additions, such as methanol or acetic acid, may be required in situations where ammonium levels are unusually high or when denitrification occurs after secondary treatment. Secondary treatment tends to use the available organic carbon in the wastewater that enters the plant, and the carbon augmentation stage is required to provide enough carbon for the denitrifiers to work effectively.

The majority of nitrification/denitrification reactions are two-stage, allowing for complete oxidation of ammonia to nitrate in an aerobic reactor, followed by denitrification in an anoxic reactor. Recycling a certain percentage of the material from the anoxic reactor to the aerobic reactor provides enough detention time to achieve good results. The total detention time required is determined by the ammonium concentration, water temperature, the presence of a suitable carbon source, and the alkalinity buffering capacity present.