How Temperature Influences Dissolved Oxygen and Biological Activity in Wastewater Treatment Systems

For anyone who has spent time learning about, designing or even operating a wastewater treatment plant, each will have a “go to” book or resource as a trusted technical reference. For us , Wastewater Engineering: Treatment and Resource Recovery is one of those books. Not because it’s easy to read, but because it consistently explains why a system behaves the way it does.  Especially when reality doesn’t quite match expectations.

Temperature is a perfect example of one of those “why’s”.  It is a variable that is measured every day and yet, is rarely questioned.  Even though it quietly controls two of the most critical aspects of aerobic treatment: how much oxygen can physically be dissolved in water and how quickly will consume it. As the water temperature changes, these two effects move in opposite directions, shaping system performance in ways that operators routinely observe in the field and students must work through on paper.

A remark we often hear when called to troubleshoot a biological system is; “Nothing has changed, I’m having a hard time controlling my DO and my biomass is showing signs of instability, what’s going on?” Overlooking the temperature change in their biological system and how it drives the removal of contaminants is often the answer.

This article takes a look issues related to temperature and how it can affect the operational performance of a biological treatment system.

Temperature and Dissolved Oxygen Availability

Dissolved oxygen in an aeration basin is fundamentally governed by gas–liquid equilibrium. Under Henry’s Law, the concentration of oxygen that can dissolve in water is proportional to its partial pressure in the air above it. As temperature increases, the vapour pressure of water also increases, which reduces oxygen solubility and makes it more difficult for oxygen to remain in solution.

In practical terms, warmer water simply cannot hold as much dissolved oxygen as colder water. This limitation is physical, not operational. Regardless of blower capacity, diffuser condition, or mixing intensity, there is a thermodynamic upper limit to how much oxygen can be dissolved at a given temperature.

The chart above illustrates dissolved oxygen saturation as a function of temperature and provides a clear visualization of this effect. Within the temperature range typically encountered in wastewater treatment facilities, dissolved oxygen solubility decreases in a non-linear manner as water temperature rises.

As temperature increases, the maximum achievable dissolved oxygen concentration declines, establishing a physical limit on how much oxygen can be transferred or dissolved under given conditions.

For operators, this explains why maintaining the same DO setpoint becomes more challenging during warmer periods, even when aeration equipment is functioning properly. For students and engineers, it reinforces the importance of incorporating temperature corrections into oxygen transfer calculations and aeration system design.

Temperature and Biological Activity

Even though there is a reduction to the amount of dissolved oxygen that can be present in water as its temperature increases, there is an opposite effect that takes place as it relates to microbiological activity. Microbial growth, substrate utilization, endogenous respiration, and oxygen uptake are all upregulated as the water temperature starts to increase.  This response is best described using the Arrhenius Relationship.

Arrhenius Relationship

Using this equation, changes in microbiological activity can be quantified as temperature deviates from the standard reference of 20 °C. The Arrhenius relationship is therefore widely applied in wastewater treatment to describe how reaction kinetics including substrate degradation and oxygen consumption accelerate or slow as water temperature changes.

For readers interested in a deeper explanation of the Arrhenius principle and its derivation, a supplemental article is available at the link provided.

In practical terms, warmer conditions lead to faster microbial metabolism and higher oxygen consumption rates. Even when influent flow and loading remain constant, biomass will demand more oxygen as temperature rises. This increase in oxygen uptake is commonly observed during warm operating periods and often coincides with greater difficulty maintaining dissolved oxygen concentrations in the aeration basin.

What may initially appear to be “strong biology” or improved treatment performance can quickly evolve into oxygen stress if oxygen supply does not increase proportionally with demand.

Managing the Oxygen Balance: Supply, Demand, and System Stress

When the effect of temperature on both oxygen solubility and biological reaction kinetics is considered together, the following operational challenges become evident.

• Oxygen availability is getting limited based on its physical solubility limits.
• Oxygen demand is starting to increase due to an acceleration of microbiological activity.

Acting simultaneously, these factors narrow the operating margin that is needed to effectively operate a biological oxidation (aerobic) system.

From an operator’s perspective, this helps explain why systems often feel more stable and forgiving during cooler operating periods, yet become harder to control as air and water temperatures rise. It is not uncommon to approach operational limits in terms of oxygen uptake and dissolved oxygen concentration during warmer conditions particularly if loading increases and aeration capacity is already marginal.

While increasing aeration is typically the first response, it is not always the most effective or sustainable solution. In many cases, oxygen-related stress can be mitigated more efficiently by addressing oxygen demand upstream by reducing the contaminant load entering the biological system rather than relying solely on increasing oxygen supply.

Using this approach, some practical levers that operators can pull include:

• Optimizing upstream chemical treatment to reduce the amount of insoluble/particulate organics from getting into the biological system (reduce total BOD load)
• Stabilize you BOD loading so there are no spikes calling for excessive or peak oxygen demand (Increase or improve process water equalization)
• Managing nutrient availability where appropriate, to limit  excessive biomass growth and an unnecessary increase in oxygen consumption

It goes without saying, if you reduce the amount of biodegradable material that reaches the biological stage, oxygen demand can be moderated.  This in turn will restore balance without heavy reliance on your plant’s aeration capacity.

Restoring Balance Through Smarter Control

Temperature-related oxygen stress is not a failure of the biology or your equipment. Rather, it reflects the interaction of well-understood physical and biological principles, combined with operational variables that can influence system performance. Managing these effects requires balance. Adding more air is not always the root cause solution, nor is it always the most effective response.

This is where Aquasan’s expertise comes into play. We take pride in the knowledge and experience we have developed over the years and are ready to apply that expertise to address your operational needs. Whether it involves reducing organic loads by improving removal efficiency upstream or identifying opportunities to optimize biological performance, our focus remains on process stability and providing the flexibility required to successfully treat your water.

Aquasan can support your team with technical assistance and the insight needed to anticipate how temperature changes may impact your wastewater treatment system. Through regular site visits and comprehensive monitoring, we assess environmental and operating conditions and fine-tune applied chemistries to help maintain optimal performance. If seasonal temperature shifts have been challenging your system, we are ready to help.