NC0024112_Report_20230327

City of Thomasville
Hamby Creek WWTP
Laboratory and Pretreatment
March 27, 2023
Ms. Diana Yitbarek
NPDES Municipal Permitting
NC Department of Environmental Quality
Division of Water Resources
1617 Mail Service Center
Raleigh, NC 27699-1617
Subject: Representative Study Data Results
Dear Ms. Yitbarek,
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PUBLIC UTILITIES
A representative study of dissolved oxygen, pH, and fecal content was required as a part of
the latest NPDES permit for the Hamby Creek WWTP, Permit number NCO024112. This
study was between our Outfall (00 113) and the sample point within the WWTP property
typically reported from as Outfall 001. Please find attached the spread sheets used for
collecting and analyzing the dissolved oxygen, pH, and fecal data collected for our study
performed from September 1, 2022 — January 9, 2023.
For each parameter analysis included a t-test: Two sample Assuming Equal Variances and an
F-test: Two Sample for Variances. For the dissolved oxygen and fecal data, both tests showed
a P value greater than 0.05, indicating that there was no significant difference between Outfall
001 and 00 1 B. For the pH, the t-test had a P value less than 0.05 indicating significant
difference, while the F-test did not indicate significant differences.
The statistical difference noted in the t-Test for pH may have two possible reasons. First,
temperature is known to have an effect on pH. Temperature at Outfall 001 was recorded but
was not required for Outfall 00113, and thus was not recorded. It was noted that as samples
warmed while waiting for the bench pH meter to stabilize the pH would continue to slowly
rise. The bench meter was used for Outfall 001 samples, while a portable pH meter was used
for Outfall 001B samples to be sure that samples were analyzed within a close enough time
frame to be considered essentially the same sample. Further testing would have to be done to
determine if traveling underground for a long distance altered the temperature enough to
induce the significant difference in pH noted. Secondly, it has been recently noted that the pH
probe used for the bench meter is losing it's performance and is in the process of being
replaced. While it is still passing calibration and meeting the criteria for secondary standard
checks, it is stretching the limits of acceptance, and was doing so during the study period. It's
slow decline in performance may be a contributing factor to the differences noted. If
necessary, an extension of testing of pH could be done to ascertain if either of these issues
contributed to the t-test failure. Additionally, it is noted that the average difference between
the two sampling points was only 0.24 SU. This small amount is unlikely to affect pH
compliance as our pH typically ranges from 6.5 — 7.5. If anything, testing at Outfall 001 for
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Thomasville, NC 27361 Fax: [phone]

pH is more likely to catch a non-compliance since Outfall 00 1 B seemed to have a slightly
high bias.
The F-test statistical analysis on each data set showed no significant difference and the t-test
showed no significant difference in the DO and fecal data. The pH data may be affected by
other factors that contributed to the failure of the t-test. Thus, the City believes that there is no
significant difference between Outfall 001 and Outfall 00 1 B. This coupled with the fact that
Outfall 00 1 B cannot be consistently sampled due to flooding of the creek during heavy rain
events, make Outfall 001 an acceptable alternative to Outfall 00113.
Please let us know if you agree with this determination or would like us to further examine the
pH issue. I look forward to hearing from you on this soon.
Sincerely,
/01 Wl-
Misty Conder
Laboratory Supervisor
Pretreatment Coordinator