Cold Weather, Ice Cover and Controlled Discharge
Cold-climate lagoon operation is where a lagoon operator's judgement is genuinely tested, and
it is the part of the job that no mechanical-plant study material covers.
What cold does to a lagoon
- Biological rates fall. Both bacterial treatment and algal photosynthesis slow markedly as
temperature drops, so the same load receives much less treatment.
- Ammonia removal stops first. Nitrification is more temperature-sensitive than carbonaceous
removal, so a pond that removes ammonia well in summer may remove almost none in winter. This
is the most common cold-weather permit problem.
- Ice cover cuts off the oxygen supply. Ice and, critically, snow on the ice block light,
so photosynthesis stops, and ice blocks surface reaeration. A facultative pond under snow-
covered ice has essentially no oxygen source and