Alkalinity, acidity, buffer capacity during nitrogen elimination

Alkalinity (ANC = Acid Neutralizing Capacity) and BNC (Base Neutralizing Capacity) are important parameters of water, which can be easily measured experimentally. These characteristics are determined by titrating a sample with a strong acid or base to a selected equivalence point.

During nitrification/nitritation, alkalinity is being consumed. If the alkalinity falls below a minimum limit, then nitrogen oxidation is at first being slowed down and beyond that completely inhibited.

With the denitrification alkalinity is being recovered. During deammonification, correspondingly less alkalinity is consumed in a simultaneous process of nitritation and direct conversion.

Generally speaking, buffer capacity decreases with decreasing wastewater temperature in the aeration tank. The reason is the higher solubility of CO2, which is produced during biodegradation. As a result, calcium dissolves from the activated sludge floc and the stability of the floc is damaged. The result is increased suspended solids values in the effluent.

Some key figures related to ammonium elimination / buffer capacity / alkalinity / precipitant addition:
• Nitrification produces nitric acid. 0.14 mmol buffer capacity per mg NH4-N is consumed during nitrification
• During denitrification, buffer capacity is recovered: 0.07 mmol per mg NO3-N
• During deammonification, 0.258 mg / l NH4-N per mg / l alkalinity can be converted
• Precipitants (metal salts) releas H + and cause the pH to drop
• 3 moles of buffer capacity are consumed per mole of Fe3 + or Al3 +; e.g., 50 ml of FeCl3 per m3 of wastewater cause a consumption of buffercapacity of 0.5 mmol / l
• Alkaline precipitants provide buffer capacity
• The optimal pH range for nitrification is 7.2 to 8.5. Sufficient alkalinity is important for good removal performance and stable sludge flocs.

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