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Spring conditions impacting wastewater treatment performance

Rainwater and snowmelt water entering wastewater plants, through I&I (Infiltration and Inflow) and for systems with combined sewers, can have a significant impact on wastewater treatment operations. While rain events are easy for operators to observe, the lag time that it takes for relevant quantities of storm water to enter their facility and the time it takes for this to significantly alter the concentration of contaminants into the system is not known. Additionally, at this time of year across North America increasing temperatures can be leading to snow melt providing additional complexity to this issue.

How this additional water flow impacts the performance of biology present in wastewater treatment plants can be a key insight to understanding fluctuating performance and imbalance events.

Using our installed network of SENTRY sensors we have been closely monitoring microbial activity across North America. Last week was particularly interesting as we watched decreasing microbial activity at the influent of WWTPs across Canada. Aggregated data from a range of wastewater treatemnt facilties identified a 75% drop in microbial activity that was directly correlated to a rise in daily high temperatures (snowmelt water) and rainfall.

Microbial activity drop-off in Canadian wastewater treatment plants.

Some key insights in monitoring microbial activity last week at WWTP influent locations:

- Average temperature increase of 5.4 C

- 8mm of rainfall fell at each location

- Average duration of impact of rain / snow melt infiltration was 22 hours

Based on these factors the average microbial activity dropped by 75%. The high microbial electron transfer (MET) before the rain and snow melt was > 200 MET with lower end values of ~ 50 MET recorded 5 hours after identified infilitration events. Biological recovery is being monitored where operators can clearly understand the time taken for biology to get back to typical levels.

These large fluctuations in incoming wastewater flow and concentrations require operators to carefully manage and maintain key process stages:

1. Clarifiers are maintained to stay within the appropriate flow velocities and avoid washout of their biology.

2. Critical Food:Mass ratio required to maintain optimal biological health and treatment performance.

With the SENTRY system installed, key municipalities in the Ontario and Atlantic Canada region were able to see the effect these events had in real time the hours after both known rain events, as well as the less obvious snow melts. This real time data provided operators the most lead time to understand the start time at their plant and the duration of their effects, giving maximum time for taking appropriate action. 

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