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Home My WebLink AboutNOV-2023-LV-0394 response letter.pdfUNIONCOUNT Y WATER Mr. Andrew H. Pitner, P.G Regional Supervisor Division of Water Resources, NCDEQ 610 East Center Avenue, Suite 301 Mooresville, NC 28115 July 17, 2023 Dear Mr. Pitner, This letter is in response to NOV-2023-LV-0394, outlining the exceedance in the Weekly Average CBOD5 that we experienced at our Twelve Mile Creek WRF. The NOV states that this violation occurred in the month of April, however it was reported and occurred in the month of March during the week of March 26`h — April 1 ` 2023. This exceedance was a result of an outlier data point on March 27`h of 20.3 mg/L. After investigating, we found multiple potential contributing factors: • Possible Laboratory Error- Lab analyzed influent samples prior to effluent samples which may have led to cross contamination • New Effluent Tubing- tubing installed on March 25 may have been contaminated • Filter pannel failure- Found 1 of 1,764 pannels ruptured • Solids Buildup in the UV channels Please see the attached copy of a letter from the Twelve Mile Creek Supervisor (ORC) for a detailed explination of the issues and methods used to correct them. In the months following the violation we have constistently seen CBODS levels below pen -nit limits. • April — 4.83 mg/L • May — 4.15 mg/L If you have any questions or concerns please contact me at 704-296-4215. Sincerely, Andrew Neff, PE Water & Wastewater Division Director CC: Hyong YI, Public Works Administrator Josh Brooks, Assistant Division Director, Water & Wastewater Jonathan Jordan, Water Reclamation Facility Superintendent Union County Water 500 North Main Street, Suite 400 Monroe, NC 28112 T 704.296.4210 unioncountync.gov/water U UNION COUNTY WATER In reference to the March 2023 weekly and monthly CBOD violation received by 12-mile creek WRF the following observations and actions took place on the following days. On April 4th 2023 1 received an email from Statesville analytical informing me that 12-mile creek WRF had received elevated readings for CBOD on the influent and effluent samples that were submitted on the 27th of April. The influent sample result was 575 PPM and the effluent sample result was 20.3 PPM. Plant staff immediately started looking at process control data for that day which all appeared to be within the normal range of what we see at the plant. I made a call to Roberto Scheller to report the issue on April 4th at 1600 and explained to him what we were seeing with the process control data compared to the analysis data we were seeing from Statesville analytical. During the conversation Roberto advised we look into several different things including: Air demand for the day which we received the elevated CBOD readings (During the 24-hour period the composite sampler was pulled we did see some slightly elevated air demands in the aeration but we were also seeing elevated flow due to higher flows caused by rain and I&I). During the conversation we also discussed elevated CBOD reading we had received late in 2022 which were also not supported by process control data and other parameters. During this period in 2022 we split CBOD samples to another lab to ensure accuracy and repeatability. On April 5th I set up a call with Statesville analytical to discuss the elevated readings at 12 mile and also some additional elevated readings that were received from another Union County plant in a different basin. During that call Statesville analytical said they normally run influent samples before effluent which was a concern for me. Crystal little at Statesville stated they would start running effluent samples first going forward and that she would QA/QC the documentation for the samples in question. Pace analytical was contacted on April 5th also to quote some additional CBOD sampling for 12 mile in case we continued seeing an issue. Plant staff started looking over plant processes closely to see if anything was out of place which could cause a discharge of CBOD like this. On April 6th I called Wes Bell with NCDEQ to discuss the elevated CBOD readings further and to ask if he had heard of any issues with Statesville analytical from any of the plants in his area or if he had any recommendations on next steps for 12 mile. Wes agreed with Roberto that we need to look over our process control data closely and see if anything is out of the ordinary for the 27th. Wes also suggested splitting samples with another lab to ensure accuracy and said that they would also QA/QC the data once the DMR was submitted. On April 10th all plant staff was involved with a thorough inspection of the plant to determine if we had any issues that have been overlooked. During this inspection it was decided that we should take down the disc filters one at a time for a thorough inspection and also take down the UV channels and drain so they could be inspected. Also, on this day 12 mile started splitting the influent and effluent CBOD sample to both Statesville and Pace labs for comparison. On April 11th operations decided to increase DO setpoints from 1.5 MGL up 1.8 MGL on the back side of all basins in service as a precaution and to ensure enough air was present for treatment. Disc U COUNTY WATER filter #1 was taken out of service and cleaned and inspected. During this inspection we found 1 filter panel damaged and it was replaced the same day and put back into service. We also drained the 2 ultraviolet channels and all debris and solids were washed down and removed. Neither of these maintenance items appeared to be something that would cause the CBOD issue. On April 12th disc filter #2 was taken out of service and inspected. No issues were found with this filter but it was cleaned completely and put back into service. All spray nozzles were removed and clean nozzles were installed as part of the maintenance. On the following day the same service was done to #3-disc filter and no issues were found at this time. On April 14th while inspecting the effluent sampler it was noted that the length of the suction tubing may be allowing it to pull water from the bottom of the effluent weir box which could be allowing stale water or solids to be pulled into the composite sampler. The tubing was replaced on the sampler and a temporary pipe was put into place so that the suction tubing will be pulling from the center of the weir box in moving fresh water all of the time. We also contacted CEM on this day to calibrate our solids analyzer used for process control. We wanted to ensure our mixed liquor levels were truly at the level we believed them to be. This calibration has been completed at this time and no issues were found. As of June 14th, the 12-mile staff never found a definitive answer to the March 27th elevated influent and effluent CBOD readings. No further process control changes have been made for CBOD removal at this time. Since this incident we have received some elevated readings on CBOD in the 5-7 PPM from Statesville analytical but for the month of April and May the averages are below permit limits. Since April 10th 12 mile has been splitting CBOD samples to Pace and Statesville 4 days a week. The effluent results show on average that Pace is about half of what Statesville's results are. With this data in hand I believe this isolated incident could have been a lab error or sampling handling issue possibly. With no supporting process control data and lab results which are in normal range for COD, solids and Fecal for the same day I cannot determine an operational or mechanical cause for this violation. 12 Mile operations will continue to split sampling as long as it is economically feasible but will probably be using a different lab in the future for at least this particular analysis. We will continue to monitor this issue closely and make operational adjustments if needed. 12 Mile WRF Supervisor Keith Purgason 6/14/23