High Effluent Ammonia (Loss of Nitrification)

1Symptom

Effluent ammonia-N rises above its normal level (for example from below 0.5 mg/L to several mg/L), sometimes quickly. Effluent nitrate drops and pH may stay higher than usual because alkalinity is not being used.

2Check first

1. DO in the aerobic zones, especially the end of the basin. Nitrifiers need about 1.5 to 2.0 mg/L or more under load.
2. Alkalinity and pH in the effluent: below about 50 mg/L alkalinity or pH below 6.5 slows nitrification.
3. SRT: compare actual aerobic SRT with what the water temperature needs.
4. Temperature: a drop of a few degrees in fall can push a marginal SRT over the edge.
5. Ammonia load: centrate, septage, industrial discharges.
6. Signs of toxicity: loss of protozoa, high OUR swings, odd color.

3Possible causes

• Low DO.
• Low alkalinity or pH.
• SRT too short for the temperature, often after a wasting increase or solids loss in a storm.
• Cold water.
• Ammonia load increase (centrate, hauled waste).
• Inhibition or toxicity (metals, solvents, cyanide, high sulfide, some industrial chemicals).

4Next investigation

• Profile ammonia, nitrate, DO and pH along the basin to see where nitrification stops.
• Calculate aerobic SRT from actual records for the past two weeks.
• Run a nitrification rate test (ammonia uptake rate) on a mixed liquor sample if your lab can.

5Corrective actions

1. Raise DO and air in the aerobic zones.
2. Add alkalinity if residual is low (see the alkalinity calculator).
3. Cut wasting to rebuild SRT; remember nitrifiers double slowly in cold water.
4. Move centrate returns to low load periods or reduce dewatering hours temporarily.
5. If toxicity is suspected, find and stop the source and consider reseeding with mixed liquor from another plant.