Temperature is the most underestimated variable in biological treatment. Every 18°F rise roughly doubles the rate of most biological reactions — but also changes the balance between organisms competing in your system.
Most operators know to watch for seasonal shifts, but the timing matters as much as the response. Changes made two weeks too late cost more to correct than changes made on schedule.
Summer operations typically require increased wasting to maintain MCRT targets as biomass grows faster. Without adjustment, SVI climbs, clarifiers struggle, and filamentous organisms gain advantage.
Watch F/M ratio closely during temperature transitions. A stable F/M in spring can become overloaded biology by July if wasting rates aren’t adjusted upward to match growth rate.
Wasting in summer: Increase wasting rates incrementally — 10–15% increases every 1–2 weeks rather than large single adjustments. You’re chasing a moving target as temperatures rise, so make wasting a weekly check rather than a monthly review.
DO targets in summer: Higher temperature means lower oxygen saturation at the same partial pressure. Your blowers may be running the same as always while actual DO drops. Check DO against the temperature-corrected saturation curve, not just the probe reading. A reading of 2.0 mg/L at 82°F is a tighter margin than 2.0 mg/L at 59°F.
Filamentous risk: Warm temperatures favor low-DO filamentous organisms (Type 1701, Thiothrix) if DO dips below 1.0 mg/L in parts of the basin. In summer, run DO targets at the high end of your range (2.0–3.0 mg/L) and check for dead zones in the basin where mixing may be inadequate.
Denitrification bonus: Warmer temperatures also mean faster denitrification. If your permit requires nitrogen removal, summer is when your anoxic zones perform best. Use this to your advantage — you may be able to reduce recycle rates slightly and still hit TN limits.
Winter operations demand attention to minimum DO and extended MCRT to protect nitrification. Nitrifiers are slow growers — when wash-out risk increases due to cold temperatures, MCRT must be held high enough to maintain the population.
A useful rule of thumb: nitrification becomes unreliable below 50°F at typical MCRT targets. Extended MCRT and elevated DO setpoints compensate, but there’s a floor below which nitrification simply stops.
MCRT in winter: For every 9°F drop in temperature, target MCRT roughly 1.5–2× longer to maintain stable nitrification. A plant running 10-day MCRT at 68°F should target 15–20 days at 50°F. Reduce wasting to achieve this — but don’t stop wasting entirely. A bloated sludge inventory creates its own problems (poor settling, oxygen demand spikes) that are worse than the nitrification risk you’re trying to manage.
DO targets in winter: Paradoxically, oxygen saturation is higher at cold temperatures, so your blowers have more to work with. The problem isn’t saturation — it’s that nitrifiers need DO ≥ 2.0 mg/L to function and they compete poorly with heterotrophs when both are stressed. Hold DO at 2.0–3.0 mg/L minimum in the aeration basin during cold weather.
Effluent ammonia monitoring: Check effluent ammonia at least daily during the cold months. A step-change increase in effluent ammonia — not gradual, sudden — means you’ve lost some portion of your nitrifying population. Respond immediately: stop all wasting, raise DO, and check for any industrial discharges that may have affected the system.
Alkalinity support: Cold-weather nitrification consumes alkalinity faster relative to the rest of your chemistry because the process is running harder to compensate. Monitor effluent alkalinity and add sodium bicarbonate or lime if alkalinity drops below 80 mg/L as CaCO₃ — below that, pH will drop and nitrification will further destabilize.
Seasonal transitions — spring warming and fall cooling — are when most plants get into trouble. The biology doesn’t respond to a single change; it responds to cumulative conditions over days and weeks. By the time you see the effluent impact, you’re already behind.
Spring transition (cold → warm):
Start increasing wasting rates before the temperature rises, not after. Use historical data or a simple rule: when basin temperature crosses 59°F heading up, begin weekly wasting increases of 10%. You’re trying to stay ahead of the growth rate rather than chase it.
Watch SVI weekly starting in March (or whenever your temperatures historically begin rising). The first sign of trouble is SVI creeping above 150 — adjust wasting before it hits 200.
If your plant has a history of spring filamentous events, consider increasing DO targets to 2.5–3.0 mg/L during the transition and maintaining them until basin temperature stabilizes above 64°F.
Fall transition (warm → cold):
Reduce wasting rates before temperatures drop significantly. If you’re running a 10-day MCRT in summer, plan to be at 15 days before the basin temperature drops to 54°F. This gives the nitrifying population time to grow to the density it needs before cold conditions put extra stress on it.
The lag between surface temperature change and basin temperature change can be 2–4 weeks in covered or partially-covered basins. Don’t use air temperature as a proxy for mixed liquor temperature; measure the basin directly.
General rule for both transitions: Move slowly and measure more often. Make one adjustment, wait a week, measure the effect, then adjust again. Seasonal changes are not emergencies — but they become emergencies quickly if left unattended.