NC0020184_Correspondence_20210722

Strickland, Bev
From: Scheringer, Stephanie <[e-mail]>
Sent: Thursday, July 22, 2021 3:25 PM
To: Tuvia, Ori A
Cc: Brothers, Maurice; Shellenbarger, David
Subject: [External] Long Creek Effluent BOD Issue
Follow Up Flag: Follow up
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Ori,
Advising that Long Creek WWTP was noncompliant with the weekly BOD limit for the week of 7/12 — 7/16. The weekly
limit is 7.5 mg/L and the Effluent BOD average for that week is 7.7 mg/L. The frequency of tertiary filter backwashing
was reduced due to a safety issue with the chlorine gas. This reduced frequency of filter backwashing is believed to be
the cause of the noncompliance.
The chlorine feed system uses effluent reuse water to create a vacuum that pulls chlorine gas from the tank. The gas
mixes with water and the solution is fed to the chlorine contact basin. Effluent reuse water is also used to backwash the
tertiary filters. When the effluent flow is relatively low (6 - 6.5 mgd) and the filters are being backwashed, the plant
effluent flow temporarily ceases since all the effluent water is being used for backwashing. Under this condition there is
not enough water pressure in the chlorine feed system to maintain a high vacuum level. Typically, this is not
problematic with filter backwashing, proper chlorination or chlorine safety. Once filter backwashing is finished, the
effluent flow increases/resumes, and the chlorine feed rate automatically increases proportionally to the flow. A good
chlorine residual is maintained in the chlorine contact chamber under these conditions.
However, for several days the plant was experiencing chlorine gas leaks each time a filter was backwashed. Accordingly,
the frequency of washing filters was reduced until this significant safety issue could be resolved. The root problem was
a pinhole in the diaphragm inside the chlorine regulator. This pinhole was allowing chlorine gas to leak out of the tank
when filters were being backwashed. Since the chlorine vacuum pressure is reduced when filters are backwashed at
relatively low plant flow, the lower vacuum level was allowing chlorine gas to leak through the pinhole in the diaphragm
regulator.
Gastonia had a vendor inspect the chlorine system last week. The vendor identified the pinhole and replaced the
diaphragm. Since that time, the backwashing process has been operating properly and no chlorine leaks have occurred.
An additional challenge with backwashing filters is that filters can only be backwashed when the instantaneous effluent
flow is greater than 6 mgd. The plant does not have a water utility connection (well water is used for domestic
purposes) and plant flow has been low due to the dry weather. During this time period, the effluent flow was below 6
mgd instantaneously for portions of the day, further restricting the times of day that filter backwashing could be
performed.
Staff is working now to perform extra filter backwashes to get the filters properly and fully cleaned.
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The current average of all completed Effluent BOD data in July is 5.3 mg/L. It is possible the plant will exceed the
monthly average limit of 5 mg/L. If the monthly limit is exceeded, we will let you know. If additional information is
needed at this time, please let me know.
Thank you,
Stephanie Scheringer
Division Manager Wastewater Treatment
Two Rivers Utilities
City of Gastonia
704-866-6726
OUR MISSION: To provide fair, competent, responsive, cost-effective services at the highest level.
The City of Gastonia is committed to creating and maintaining a work environment that is inclusive, equitable and welcoming. We
value diversity and promise to honor your experiences, perspective and unique identity.
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