• Patrick Kiely

Microbes love the Summer! Monitoring seasonal impacts on biological wastewater treatment!

Executive Summary:

Municipal wastewater systems are challenged by seasonal temperature variability because of its associated impact on biological activity and organic treatment. SENTRY was installed in the primary influent and plant effluent of a 5 MGD municipal wastewater treatment plant in Atlantic Canada for a one-year period. Microbial activity (MET) was measured in the influent and effluent wastewater and compared to the facilities biological oxygen demand (BOD) treatment performance. Microbial activity in the influent was found to have a greater than two-fold range, with lowest activity recorded in Winter into early Spring (~ 100 MET) and highest activity in the late Summer (> 200 MET). The variability in MET was demonstrated to be descriptive of the facility’s organic removal performance, and a clear link between higher measured microbial activity (MET) and superior effluent wastewater quality as a result of improved treatment efficiency was evident.

SENTRY is validated as a real-time microbial monitoring platform that can accurately track wastewater treatment plant performance and predict process imbalance. The data clearly conveys the impact of long-term trends and seasonal temperature variability on a facilities performance, providing actionable information for process improvement and optimization.


Modern wastewater treatment is a biological process that leverages microbial metabolism to degrade and consolidate contaminants. Microbes are the keystone of the process, and the maintenance of an active population is critical for ensuring effective and efficient treatment. Microbial activity is rarely stable and is impacted by many parameters such as temperature, toxicity, and nutrient availability; resulting in process efficiency and effluent quality variability.

The common suite of measured parameters (flow, suspended solids, turbidity, COD and BOD) offer little insight into microbial activity. Microbial performance is deduced from the treatment performance and failure or imbalance is typically identified or understood when it is too late. Operators then must scramble to perform biological tests that are labor intensive and slow to provide actionable data such as oxygen uptake rate (OUR), additional BOD, and toxicity testing. These investigations are time consuming and costly. The availability of a tool to both alert operators of potential problems in activity and to understand longer-term impacts on plant performance is of great industry value.

Municipal wastewater systems in temperate climates, especially if outdoors, are challenged by seasonal temperature variability, with water temperature in the summer exceeding 20 °C while the temperature can drop below 10 °C in the winter. Perhaps not widely understood, but this change in temperature results in winter activity being 50% - 33% of summer activity and translates into longer hydraulic retention times being required to achieve the same effluent quality.


SENTRY was installed in the primary influent and plant effluent of an Atlantic Canada municipal wastewater treatment plant for a one-year period. The treatment performance, operational parameters, and MET were recorded. The year of data was separated into Warm season (May 15 – Nov 14) and cold season based on water temperatures which were identified to vary between 10 °C and 22 °C. 

SENTRY sensors installed for wastewater treatment plant biological monitoring

Figure 1 Atlantic Canada Wastewater Treatment Facility and microbial sensors installed at (b) effluent and (a) influent locations.

The MET (activity) data reflects the treatment performance results, with the warm season having 50% higher average activity (Figure 2) in the primary influent and 90% less activity in the plant effluent (Figure 3). In the influent the higher activity is attributable to superior environmental conditions (temperature) resulting in elevated metabolism, with a potentially modest bump from higher organic carbon concentration. In the effluent the lower MET readings are as a result of organic carbon limitation. 

MET variation through the year

Figure 2 Seasonal MET activity in the primary influent. Shown is much higher typical activity in the warm season (extreme low outliers are storm or influent events). MET is much lower in the cold season, particularly in February – April which are the coldest periods. Data missing for June-July.

MET data has the unique power of being real-time and high frequency (every minute). SENTRY’s real-time data highlights that it is not just the average activity that changes in the effluent quality but also the variability, with the potential for intermittent periods failing to meet treatment expectations. This is visible in Figure 2 where the cold season has many high outlier values in the effluent that reflect intermittent process conditions where treatment is less effective than the average.  SENTRY demonstrates that this system is less robust in the winter and informs the operators of periods when the process is stressed and operations require elevated attention, as well as providing information for improved maintenance schedules by helping operators select periods when the plant is known to be running optimally. 

MET variation on the WWTP effluent location

Figure 3 Box plot of monthly MET values of the plant's effluent. The box plots demonstrate much higher MET in the cold months, specifically March and April. The high outliers (dots) and wider quartiles (larger rectangles) represent higher variability towards higher effluent MET values and represent greater effluent organic carbon concentrations.

The data shows that despite higher organic carbon concentrations (BOD) and mass loading in the influent during the warm season, the effluent during the warm season had significantly lower BOD concentration and mass export. This translates into higher organic carbon removal performance ({[influent] –[effluent]} / [influent] * 100 ) in the warm season (98%) compared to the cold season (95%) and 50% higher BOD mass export in the winter compared to the summer.

Table 1 Comparison between warm and cold seasons’ operational and treatment parameters. Similar flows were recorded in 2008 but the warm season has influent higher in organic carbon. Despite the higher strength and loading, effluent quality in the summer is more consistent and of higher quality, demonstrating the impact of microbial activity variability on the treatment performance.


Great WWTP design incorporates seasonal variability and plans accordingly so the plant always achieves a high effluent quality. Over time, changes in demands will require operators and infrastructure to adapt. Insight into the health and variability of the microbial activity supports improved infrastructure investment decisions.

SENTRY provides novel insight into the biological process’ microbial metabolism. SENTRY is validated as a real-time microbial monitoring platform that can accurately describe and project wastewater treatment plant performance and clearly outline the impact of long-term trends and seasonal temperature variability. 

The platform allows operators to have their finger on the “pulse” of the biological treatment process by continually monitoring the population’s response to short and long-term trends. This data stream informs operators when their system has spare capacity and when conditions are such that they need to be more alert. This information can be leveraged for resource allocation and maintenance, targeted sampling and peace-of-mind compliance monitoring. SENTRY is a plug-and-play technology with minimal maintenance requirements and is commercially ready to help your operators meet and exceed plant treatment performance expectations while improving efficiency.

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