We have an evaporative cooling system here at the power plant that uses city water. The water is pumped up and cascades down across a filter media cooling the intake air to the gas turbine. This is a recirculating system but a certain amount of the water is blown down continuously to a floor drain. The water goes through the floor drain system to an oil-water separator then to a collection sump. The water is then pumped out to the city sewer system. There are pH monitors in the discharge to the city sewer.
When ever the evaporative cooling system is in operation we have a problem with the discharge pH meters going low on pH causing alarms. It only happens when the pumps are off and the water is sitting stagnant in the wet loop where the pH meter probes are located. Over time the pH will drop from around 7.5 down to a reading of around 6 which brings in the alarm. If we recirculate the sump water through the line the pH will go back up to match the sump pH but once the pumps are turned off the pH will start to slowly drop again till the alarm comes back in. Again, this only happens when the evaporative cooling systems are in service.
My question is could the drop of pH be caused by CO2 being dissolved into the water as the air is being draw across it in the filter media then coming out of solution and turning to carbonic acid as the water sits stagnant in the pipe where the pH probes are located? It is definitely not an instrumentation error but we are at a loss to explain why this happens.
Thank you for your help.