Factors influencing nitrification and denitrification

Nitrification and denitrification in wastewater treatment allows the removal of ammoniacal nitrogen, which is one of the causes of eutrophication of watercourses along with phosphorus. Like all reactions, certain operating conditions must be respected in order to optimize them.

Nitrification

One of the conditions to check for nitrification is the pH of the wastewater. Indeed, the optimal pH is located at 7.5. It is at this pH that nitrogen is most readily available to the various microorganisms involved in nitrification. Outside this range, the rate of reaction can greatly decrease. For example, the rate of reaction at a pH of 6 is about 10% to 20% of the rate of reaction at a pH of 7. The nitrification reaction can even be completely inhibited if the pH drops below 5.5. It is important not to go below a pH of 7.5 since there is a consumption of alkalinity during the oxidation reaction of ammonia to nitrite. On the other hand, if the pH is too high, the oxidation reaction of nitrite to nitrate is less efficient.

Temperature is also a factor that influences nitrification. Ideally, the ideal temperature is between 28 and 32°C. Since nitrification takes place with the help of microorganisms, the temperature of the wastewater should not fall below 4°C or rise above 45°C. It is outside this temperature range that the reaction slows down significantly as the microorganisms start to slow down.

A very important factor to consider is the presence of dissolved oxygen in the water. Since the microorganisms involved in nitrification are aerobic s, i.e. they need oxygen to live, the dissolved oxygen concentration must be about 3 mg/L in order to obtain optimal reaction rates. Below 1.5 mg/L, the rate of nitrification decreases greatly until it is completely inhibited for a DO of 0.5 mg/L.

Microorganisms are sensitive to many chemical contaminants that may be present in wastewater. To name a few, there are chemical solvents, amines, phenols, ethers and metals. For example, only 0.1 mg/L of copper is required to inhibit nitrification. These contaminants can cause a decrease in the rate of reaction of the microorganisms but can also kill the microorganisms if the concentration becomes too high. It is therefore important to check for the presence of contaminants that are toxic to microorganisms in the water to be treated.

Denitrification

Denitrification, unlike nitrification, is not really affected by pH. Although the optimum pH for denitrification is between 7 and 8, the rate of reaction is only slightly reduced if the wastewater pH is not in this range as bicarbonate, an alkaline agent, is produced during the denitrification reaction.

Like nitrification, denitrification uses microorganisms to reduce nitrate to nitrogen gas. These microorganisms, like those used for nitrification, have an optimal temperature range between 28 and 32°C and slow down significantly for temperatures above 45°C and below 4°C.

The presence of dissolved oxygen is also an important factor in denitrification. Since the microorganisms involved in denitrification are anaerobic s, i.e. they live in an oxygen-free environment, the presence of oxygen is harmful to these microorganisms. Dissolved oxygen in the water can therefore slow down and even inhibit denitrification. A concentration of 0.2 mg/L dissolved oxygen can inhibit the denitrification reaction. However, nitrification/denitrification can be accomplished in a hybrid type system, allowing both aerobic and anaerobic sites in the same environment.

At Aquasan, we specialize in wastewater treatment and can advise you on the most appropriate coagulants and polymers to use in your treatment. We provide expertise and creative solutions to optimize your nitrification and denitrification processes.