35-44 °C (95-111 °F) for ~1 week in a row. That was the temperature in Israel last week.
This extreme event gave us a great lesson regarding the effects of extreme weather on wastewater characteristics.
High temperatures can create several operational challenges, for example: septic influent loading leading to H₂S odors, GAOs outcompeting PAOs, and more.
But the issue that we saw across almost all of the plants we work with was a clear biomass change due to low dissolved oxygen (DO) from rising temperatures.
The mechanism in simple: Warmer water holds less oxygen - solubility decreases as temperatures climb, often falling below critical thresholds needed for aerobic treatment. This leads to incomplete organic matter breakdown and reduced treatment efficiency.
Also, in these extreme temperatures, microbial metabolism accelerates, creating a double impact of less available oxygen when demand increases most.
In some plants, we saw a dominance of Flexibacter, Spirochaetes, Spirillum, and Thiothrix in the biomass - all indicators of low oxygen conditions.
Spirillum (red) & Spirochaetes (yellow).
What did we recommend to overcome these days in Maji?
Increase aeration in advance to raise DO (theoretically, the blowers are designed to handle this, but practically, adjustments are often needed)
Monitor closely with both lab checks and real-time sensors (do not rely only on sensors) to track temperature and DO
Pay attention to mixing to prevent low DO zones
In some cases: Adjust sludge wasting to lower MLSS temporarily
Spirillum.