Why Does Nitrification Performance Drop During Winter Operation?

Many wastewater treatment plants experience reduced nitrification efficiency during winter. Even well-operated systems may see higher ammonia levels as temperatures decrease.

The primary reason is that nitrifying bacteria are highly temperature-sensitive. When water temperatures fall below 15°C (59°F), the activity of ammonia-oxidizing and nitrite-oxidizing bacteria slows significantly, reducing ammonia removal rates.

Other factors can also contribute:

✅ Lower microbial activity – Biological reactions become slower in cold water.

✅ Oxygen limitations within biofilms – Although cold water holds more oxygen, nitrifying bacteria still require sufficient DO throughout the biofilm layer.

✅ Slower biofilm growth and recovery – Biomass lost due to operational disturbances takes longer to regenerate during winter.

✅ Alkalinity and pH fluctuations – Reduced buffering capacity can further inhibit nitrification performance.

For MBBR systems, carrier media plays a critical role in maintaining a stable nitrifying population. Media with high protected surface area and excellent biofilm retention can help improve process stability during cold-weather operation.

Winter Operation Tips

• Monitor ammonia and DO levels closely
• Maintain adequate aeration and mixing
• Ensure sufficient alkalinity for nitrification
• Avoid sudden hydraulic or organic load changes
• Optimize MBBR media filling ratios and reactor performance

Cold temperatures are unavoidable, but proactive operation and proper biofilm management can help maintain reliable nitrification performance throughout the winter season.

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