Temperature Effects

1Symptom

Process performance changes with the season. In fall and winter, nitrification slows and ammonia creeps up; in summer, DO is harder to hold and rising sludge and odors increase. Some filaments and foams (M. parvicella) favor cold water.

2Check first

1. Trend mixed liquor temperature daily and compare against SRT.
2. Check whether the SRT target was adjusted for the season.
3. Note DO saturation changes: about 9.1 mg/L at 20 C versus about 7.6 mg/L at 30 C at sea level.
4. Check microscopy for seasonal filaments.

3Possible causes

• Nitrifier growth rate roughly halves for every 8 to 10 C drop.
• Higher temperature lowers oxygen solubility and raises oxygen demand.
• Warm water speeds denitrification in clarifiers (rising sludge).
• Cold water increases viscosity and slows settling slightly.

4Next investigation

• Set a minimum aerobic SRT for the coldest month with a safety factor (for example 1.5 to 2.0).
• Build inventory before cold weather arrives rather than after ammonia rises.

5Corrective actions

1. In fall, reduce wasting gradually several weeks ahead of the temperature drop.
2. In summer, watch DO, clarifier blankets and RAS rate closely.
3. Use the SRT and inventory calculators to plan the MLSS increase and check clarifier capacity with the state point tool.