Low Alkalinity

1Symptom

Effluent alkalinity has dropped below about 50 to 70 mg/L as CaCO3 and pH is drifting down, sometimes below 6.5. Nitrification may slow, floc may weaken, and effluent turbidity can increase.

2Check first

1. Measure influent and effluent alkalinity on the same day.
2. Calculate expected consumption: 7.14 mg/L per mg/L ammonia nitrified.
3. Check how much denitrification is happening (it returns about half).
4. Check chemical additions that consume alkalinity, such as alum or ferric for phosphorus removal.

3Possible causes

• Full nitrification of a soft, low alkalinity influent.
• Little or no denitrification.
• Alum or ferric chloride dosing: about 0.5 mg/L alkalinity is consumed per mg/L of dry alum, and about 0.9 mg/L per mg/L of ferric chloride (as FeCl3).
• Infiltration of soft water or rain diluting alkalinity.

4Next investigation

• Do an alkalinity balance across the plant.
• Evaluate whether an anoxic zone or cycling aeration could recover alkalinity.

5Corrective actions

1. Improve denitrification (anoxic zone, internal recycle).
2. Add supplemental alkalinity: magnesium hydroxide, sodium hydroxide, lime, soda ash or sodium bicarbonate. Use the alkalinity calculator to size the feed.
3. Reduce metal salt dose if phosphorus allows.