In the event of an upset in an activated sludge plant, what is the most efficient first step to identify the cause?

To identify the cause of an upset in an activated sludge plant, looking at the plant data for the previous three weeks is the most efficient first step. This approach provides a comprehensive historical context by showing trends in performance, operational anomalies, and changes in influent characteristics that may have led to the current issue.

By analyzing plant data over a time frame, operators can pinpoint any deviations from normal operational baselines, assess how treatment performance has varied, and correlate this information with any known incidents, such as heavy rainfall or industrial discharges. This initial overview allows for quicker diagnosis since it helps in identifying patterns or seasonal variations that may contribute to the upset.

The other options, while they could provide useful data, are more focused actions that may not provide the broad context needed for a quick assessment of the situation. Running specific tests might reveal current conditions but might not indicate how those conditions have been evolving, and checking the collection system or effluent parameters may miss broader trends observable only through historical data. Thus, reviewing past data is prioritized as it facilitates a quicker understanding of the problem's root cause.