City of Ann Arbor SSWWEP - Citizens Advisory Committee Coordination - 2.16 -C- How often does the plant get at capacity during these large rain events?

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Joe Conen on 14 Jan, 2014:

Charlie, Robert, Nick:

This is an excerpt of a followup I provided to Charlie by email on November 19. In summary, the answer above is not sufficient.

"The specific link only gets you to a home page with other links, including one that allows a search of all state discharges. Based on my search of that link, the most recent discharges had nothing to do with rain or the treatment plant, they were blockages in lines. It appears to me that this answer made the leap that overflows only occur when capacity is exceeded. Based on my reading about past overflows, it appears that operational issues lead to overflows even when capacity is not exceeded (From the summaries of the overflows on June 27: "Plant flow went from 18 MGD to over 50 MGD within a 30-minute period, and peaked at over 65 MGD. This nearly instantaneous change in flow occurred faster than staff could react."; "Sewage overflow due to heavy rains and tree root blockage in sanitary sewer line located near 237 Pineview Court. Dilute raw sewage released to storm sewer and Huron River."). If we are to adequately address the storm flow impacts on overflows, a more rigorous answer is necessary. Note that reviewing the information from the DEQ site search, I found no events attributed to capacity of the plant being exceeded (looked at 3 years). I am certain that the city has the system flowrates during these event and can relate that to the total plant capacity."

Lori Byron on 5 Feb, 2014:

[CITY]

The attached excerpt from the WWTP Facilities Master Plan prepared by Black & Veatch in 2003 contains design parameters of the plant. By definition, any sustained flow over one of these design flows could be considered as exceeding the plant’s capacity. However, in these instances the consequences may be insignificant because the plant is able to contain the flow and treat all wastewater to acceptable discharge standards. In instances when the plant is not able to do so, it is typically because plant flow increases faster than plant operators can react to bring equipment on-line that is not needed at the lower flow rates, which can result in a sanitary sewer overflow event. As part of the SSWWEP study, we’ll be looking at the peak flows amounts which that plant experienced during recent storm events.

Staff is working on getting the full 2003 WWTP Facilities Master Plan uploaded to Basecamp. (It's an extremely large file.)

Joe Conen on 5 Feb, 2014:

Does the plant log events which exceed these limits? If so, what is the
recent history (last several years)? Please provide summaries of events when any of these limits
were exceeded. I am also interested in learning more about situations described above: ”...typically because plant flow increases faster than plant operators can react to bring equipment on-line that is not needed at the lower flow rates….”
Thanks
Joe

Lori Byron on 15 Mar, 2014:

[CITY]

Attached is the listing of SSOs from the WWTP.

[CITY]

Here is some additional information about plant capacity from the 2004 WWTP Facility Master Plan and current capacity:

The capacity of the retention and equalization facility is a function of the intensity and duration of a given storm event. With a total retention and equalization volume of 16.76 million gallons (including chlorine contact volume), it is possible to formulate how many days of storage is available based on a given peak day flow. Assuming the plant can handle incoming flows of 2.5-times the 2025 AADF of 24.3 MGD (or 60.75 mgd) and the 2025 peak hour flow of 72.7 MGD, a total of 11.95 MGD would need to be diverted to the flow Equalization and Retention Facility (72.7 mgd – 60.75 mgd). At this rate of diversion the peak hour design flow of 72.7 mgd could be sustained for approximately 1.4 days (16.7 mg/11.95 mgd) before the capacity of the equalization facility is exceeded.

Since the WWTP Master Plan was developed, the disinfection process was changed to ultra-violet (UV) light. The hydraulic capacity of the UV system is around 48 MGD or so. Doing the same analysis that B&V performed for the WWTP Facility Master Plan but using 48 MGD as the amount “the plant can handle”, the storage is around 16 hours. Treatment plant staff consider this as a worst-case scenario, and would characterize our storage as being from 16 hours to 1.4 days.

Michelle Lovasz on 16 Mar, 2014:

I’m sorry Lori, I didn’t get that. Perhaps some could extrapolate the
answer but I didn’t. How many events a year is the plant “at capacity” or
beyond? 2013? 2012? 2011? Could you re-pose the question to the City?
Thanks.

Jim Osborn on 16 Mar, 2014:

It appears that the facility lost capacity when it switched to ultraviolet light from chlorine. Is this a true statement? If so, why was this done, and why was the chlorine equipment not kept to augment the ultraviolet during peak rain days.

We have two problems here:

1) the plant being overwhelmed at times during peak rain events

2) individual sewer pipes in certain neighborhoods being overwhelmed during peak rain events.

Something such as a FDD may need to be done to fix #2, but not #1, if capacity was reduced due to a sterilization change.

Michelle Lovasz on 16 Mar, 2014:

I did get that Jim, but Joe's question was:

"Does the plant log events which exceed these limits? If so, what is the recent history (last several years)? Please provide summaries of events when any of these limits were exceeded."

What Joe is asking for is the plant's reports, so that we are not left guessing.

Lori Byron on 16 Mar, 2014:

My apologies, it looks like I forgot to include the SSOs attachment in the response above. Thanks, Michelle.

[CITY]

Attached is the listing of SSOs from the WWTP.

Joe Conen on 19 Mar, 2014:

Lori,

This question relates to your post on Feb 15. Does the discussion of the capacities relate to the current situation with the West plant down for refurbishment, or is it the combined system, assuming both plants are in service? How does the answer change given the status of the West plant?

Thanks

Joe

Lori Byron on 20 Mar, 2014:

Thanks for the question, Joe. I'll share it with the City to get clarification. Tonight's presentation will also touch on WWTP capacity to handle rain events.

Lori Byron on 1 Apr, 2014:

[CITY]

Earlier in this thread, Joe asked "Does the discussion of the capacities relate to the current situation with the West plant down for refurbishment, or is it the combined system, assuming both plants are in service? How does the answer change given the status of the West plant?"

Here is the clarification from plant operators:

The projections would not have taken into account the west plant being off line for replacement.
The flow estimate was based on a projection of growth that we are not near.
The west plant would be available if and when the 2025 projection is met
If the flows where at the projected rate and the west plant is out of service I would expect the time estimate to be reduced by about 1/3.

Jim Osborn on 3 Apr, 2014:

On Sunday, March 16 I asked, " appears that the facility lost capacity when it switched to ultraviolet light from chlorine. Is this a true statement? If so, why was this done, and why was the chlorine equipment not kept to augment the ultraviolet during peak rain days?"

It was not answered.

I would still like to have an answer to MY two questions. I do believe that Basecamp is much less busy than it was a month ago, so perhaps now this question can be addressed.

Jim

Lori Byron on 3 Apr, 2014:

Thanks for the nudge, Jim. I'll follow up on those, too.

Lori Byron on 24 Apr, 2014:

Jim, here's the response to your question about the WWTP's switch from chlorine to ultraviolet light:

The WWTP has the ability to fully treat 48 million gallons of wastewater per day on an average annual basis, including disinfection using the ultra violet (UV) light system. This does not mean 48 million gallons per day is the maximum flow that could be sent through the plant. Both prior to and following the conversion from chlorine to UV disinfection, the WWTP might have to bypass some of the treatment processes for a prolonged high flow, depending on specific plant conditions (e.g., storage available, equipment out of service, duration and volume of flow entering the plant, etc.). The decision to eliminate the chlorine disinfection system was based on the risks to human health and the environment that would result if one or more of the one-ton chlorine cylinders ruptured and leaked.

City of Ann Arbor SSWWEP - Citizens Advisory Committee Coordination — City of Ann Arbor

2.16 -C- How often does the plant get at capacity during these large rain events?

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