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So You’ve Had an Excursion
Josh Jondro – Chemical Engineer
Wastewater Management Team An excursion is an instance in which there was wastewater discharged to the sewer system outside the acceptable pH range, 5.5-12 s.u. for MWRA customers. Usually an excursion is found when it is recorded on an effluent chart recorder. Before panicking, here’s what you need to know and do to fix the problem.
1. Is the system still in excursion? If the system has recovered, review steps 2 and 3. If the system has not recovered, skip to 4.
2. Determine if there was flow during the time in which the pH was outside the acceptable range. Effluent chart recorders that chart both pH and flow are a great source for this information. If there was no flow, there was no excursion. During no flow periods, bacterial growth can result in low pH readings. This condition will clear once flow resumes.
3. Was there an exceptionally high flow during the excursion? Active, gravity flowing pH systems have a maximum flow above which they cannot neutralize the wastewater. Find out why there was such a high discharge flow rate. It’s possible that something failed upstream (ie leaking valve, broken pipe, etc.).
4. If there was flow consistent with what is usually recorded, start looking into the pH system components. The key components to look at are listed below.
a. Are the pH sensors clean and calibrated? Remove the sensors from service, clean, and calibrate them.
b. Is the mixer running and agitating the tank appropriately? A non-mixed tank will not efficiently neutralize wastewater. Possible reasons for mixer failure are a blown fuse, mixer motor failure, gearbox failure, mixer shaft or prop failure.
c. Is the chemical delivery system working as required? First, check the chemical level and ensure there is plenty of acid and base. Then, check the metering pumps by actuating them. This will show if the pumps are energized and primed. Make sure after checking pump prime that the pump is put back into the auto mode for treatment.
5. If nothing significant has been detected yet, start looking into the pH setpoints established in the pH analyzer/controller. Are the setpoints aggressive enough to treat the wastewater at the flow rates recorded? Have there been any changes in upstream processes that are making the wastewater more difficult to neutralize?
An excursion isn’t catastrophic, but it needs to be addressed and put in the Operators Daily Log. The list above should help you determine the problem so it can be addressed. Isolating the problem is the most difficult part of getting to the final repair.
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