Nina,
My experience with problematic closed loop cooling water systems treated with nitrite based inhibitors is that the dentrifying bacteria (once in the system) is almost impossible to erradicate without actually replacing all of the equipment. We have tried steaming out and biocide treatments, but i believe the bacteria takes safe harbor in subsurface pits or stagnant areas (deadleg) of the system and repopulates after the "sterilization" treatment.
Alternatively to a cleanout or replacement, I would recommend switching to a molybdenate-based inhibitor with is not utilized as food for the bacteria. This will halt the poplation growth of the bacteria colony and may even "starve them out." You would still need to perform some chemical cleaning to remove physical tubercules and deposits that could lead to under-deposit corrosion and pitting by simple "oxygen cell corrosion" (AKA under deposit corrosion).
As a side note, i have found that simple PCR or other bacteria sampling of the water is not sufficient to identify the severity of the contamination, as the planktonic bacteria is not actually causing the corrosion problem. Sessile bacteria which is actually causing the corrosion, remains on the pipe or in pits and is not able to be sampled in the bulk water. A more representative indicator of the step change in activity is comparing the nitrate levels, which should increase above baseline and the bacteria consumes nitrite and converts it to nitrate. If you see an uptick in nitrite makeup and and nitrate levels in the system, then it means the bacteria colony is growing and probably causing more widespread corrosion.
------------------------------
Orin Wakefield
Materials Engineer
Chevron Corporation; Oronite
------------------------------
Original Message:
Sent: 06-18-2020 02:44 PM
From: Nina Young
Subject: Denitrifying bacteria
Does anyone have experience with mitigating denitrifying bacteria in their cooling water system? We are unable to keep the temperature above 140°F, we do have low flow, and we are using a nitrite corrosion inhibitor. We are considering a system cleanout, but open to hearing what others have done in this situation. Also curious which biocide treatment was used for denitrifying bacteria. Thanks!
------------------------------
Nina Young
Chevron Phillips Chemical Company
------------------------------