Field Office Compliance - Assistance

Assistance ID - 113884
Des Moines Water Works - 310338211
9966 Maffitt Lake Dr Cumming, IA 50061
Dallas County

FO 5

Report
File Name File Type File Date Note
Comments
7/1/2020 Pickens, Aaron 1:42 PM (2 hours ago) to me, Taroon, Mark, Bob Interesting! Since my comment regarding a pumped sample was in a side email thread that I had with you Janet, I am repeating it here so that the others have it and it is part of the email chain of the discussion. As I said, Davenport has a pumped CFE sample as they have no other option. The installed the pumped sample a few years ago. They were using the average of all the filters at the time for compliance and I told them that was a no, no. They use Swan turbs which obviously may behave differently. But they are similar in the sense that they are a non-contact design with 360 degree reading. Davenport had an issue a few months ago where a power bump caused a false, elevated turb. Their operator used to work as a Swan technician and he provided quite a bit of data and explanation of the event. Here is part of his explanation provided at the time on the subject: " The max turbidity maxed out the analyzer at 2 NTU (shown in red below); however, this was due to the instruments sample hydraulics (instrument failure). If you recall two years ago when there was a lot of discussion going on with these Swan instruments, these are non-contact instruments, meaning the measuring optics never come into contact with the sample. With that being said, they are highly dependent on maintaining an exact level of sample inside the measuring chamber (controlled by the constant head apparatus) for an accurate measurement to occur. Since the nephelometric method measures the amount of 90 degree scattered light, with this particular design, the 90 degree angle of the detected scatter light changes as the sample level deviates from its calibrated position, due to diffraction of the light penetrating the surface. On our CFE Turbidity analyzer, sample is fed prior to the contact chamber by a submersible pump. During the power bump, the sample feed pump temporarily shut off, causing the sample level within the measuring chamber to drop. As the surface of the sample gets farther away from the optics (sample level dropping), a false positive NTU is obtained, causing the unit to read as max value. Too much flow (60+ L/Hr.) would do the exact opposite, providing a negative bias. As soon as power was re-established to the sample pump, the instruments sample chamber refilled;" Again, the Hach's may not behave the same but I think the bottom line is that the sample hydraulics can have a big impact on the readings. Similar to your event, a much higher reading showed up on the raw data from the instrument data storage and was not logged in the historian. Similar to DMWW the highest level was not captured in the 1 minute data as it was likely so brief in nature it fell between the logged data points. When I was dealing with Davenport's project a few years ago to install a pump for a true CFE sample, I asked PAI for some guidance on pumped samples. Here are some bullet points that Larry provided at that time. If at all possible, I encourage plants not to pump samples to turbidimeters because of turbulence (air entrainment) and pump O&M issues. But, sometimes options for gravity flow are very limited or not the best choice. This seems to be where Davenport is. Use of a sump pump could work, but I would keep it off the bottom of the basin to avoid pulling in settled particulates. I would make sure that there is easy access to the pump for inspection and O&M. · Other pumping options could also be considered. One issue I wonder about is the potential for corrosion of the sump pump impeller and flaking off of particles. To avoid this, the pump could be located outside of the basin with the suction line extending into the basin. But, I would avoid a long suction line in this situation. I don’t have any recommendations on the best type of pumps to use, but can check with a plant or two that pump their samples (I will get back with you on any useful feedback that I get). · With regard to transferring the sample, keep the sample line as short as possible. Would this line be buried from the clearwell to the high service pump building? To keep the detention time down, they can pump a larger flow rate than the turbidimeter requires and take a side stream from the sample line to feed the turbidimeter. Use a valve to control the flow to the turbidimeter that has good control and does not plug easily. I would avoid needle valves because they tend to plug easily. Ball valves seem to work well. · At the turbidimeter location, I would include a SST side tap and ball valve prior to the turbidimeter to allow the operator to easily take grab samples (I can send a picture if this would be helpful. Since the sample is being pumped, they may want to consider installing a bubble trap before the turbidimeter. If they do, make sure that it is not oversized. Our guidance on sample detention time for IFE and CFE is <= 1 minute, but they could push this some if they add a bubble trap. Hach sells a bubble trap that is ~ 1 inch diameter that would likely work. Hope this helps! Aaron JANET GASTINEAU
7/1/2020 Danley, Kyle 12:11 PM (28 minutes ago) to me MIT’s are taken each morning. Here are the results for Tuesday (6/29/20) UF #1 LRV 4.323 UF #2 LRV 4.254 UF #3 LRV 4.308 UF #4 LRV 4.257 HSP typically run continuously. We moved the finished sample to take it from the end of the clearwell several years ago. It has to be pumped with a very small sample pump to the instruments. Previously the sample was taken on the discharge side of the HS pumps and we could get spikes in NTU when HSP turned on or off. Since you asked about this, I did check the trend we have for pressures after the sample pump. Interestingly enough, the pressure did fluctuate (up 6 psi) at the sample pump the same time we had turbidity spikes. This is not normal. Therefore I think we found the issue. We switched to the 2nd sample pump last night and it has been steady on pressure. We turned off all HS pumps when we switch sample pumps and we did see a large spike in turbidity. Once turbidity was normal we turned HS pumps back on. This shows how a small pressure increase can affect these instruments greatly. We are replacing and cleaning tubing from the sample pump to the instruments today. We will also perform another verification on the turbidimeter. We downloaded the stored data off the SC200 this morning and discovered the highest recorded finished turbidity was 1.425 NTU at 2:41 PM. These values are stored every minute. I assume the spike was such a brief duration that it was not recorded on the minute. Happy to discuss more if you have any questions related to this NTU spike. I greatly appreciate the discussions how we handle these issues in the future as I am confident they will occur again. Instruments fail or cause minor pressure fluctuations that will cause very brief and false turbidity spikes that do not represent the water quality in the clearwell. JANET GASTINEAU
6/30/2020 Gastineau, Janet <[email protected]> 5:06 PM (6 minutes ago) to Kyle Just curious to know how the DIT's looked yesterday and what the LRV's were? Do HSPs run continuously or start and stop? Want to be sure the spikes don't line up with the pumps kicking on. On Tue, Jun 30, 2020 at 3:42 PM Danley, Kyle <[email protected]> wrote: Janet According to the log, a verification was done on this instrument yesterday (6/29/20). JANET GASTINEAU
6/30/2020 Called Kyle and let him know. JANET GASTINEAU
6/30/2020 Gastineau, Janet <[email protected]> 4:41 PM (30 minutes ago) to Bob, Aaron, Mark, Taroon, Anne Spike only happened on one of three meters, but it is their compliance CFE, which is discharge HSP. No IFE correlation. Yes, only one minute on the TU series meter. LRV and DITs all normal. Talked to Mark and he is similarly inclined to consider it a non-event/erroneous turbidity, which is what I will tell DMWW, along with how in general we are rethinking compliance monitoring/locations without revealing too much because no decisions have been made. JANET GASTINEAU
6/30/2020 On Tue, Jun 30, 2020 at 3:58 PM Gastineau, Janet <[email protected]> wrote: Unbelievable! DMWW had a 2.007 NTU turbidity spike at their Saylorville WTP about an hour ago. Turbidity was above 1 NTU for about a minute. No issues with the IFE turbidity, the meter on the combined IFE flows, or another meter prior to the clear well--Kyle described these as flat-lined (no spikes). Nothing out of the ordinary with source or treatment and chlorine residuals are fine. There was an operator on site that ran grabs after the spike; he was notified of the alarm/spike by the control center, but the event was pretty much over. The operator's grab results were slightly higher than the continuous instrument but lined up pretty well. According to the log, a verification was just done on this instrument yesterday. I'm inclined to let it go as it goes against what we know about turbidity and they have upstream data to support water quality/compliance. It's dissimilar to the event we didn't let go in December 2018 when there was no known plant process upset or operational explanation for the spike at the McMullen WTP--remember the "brown out" described by Hach? Corrupt data, etc. We gave that one a Tier 3 / 12 month PN. What do you think? JANET GASTINEAU
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