City of Ann Arbor’s Wastewater Treatment Plant: Frequently
Asked Questions
The City of Ann Arbor’s Wastewater Treatment Plant (WWTP) is responsible for the effective
collection, treatment and environmentally acceptable discharge of the wastewater
generated by the greater Ann Arbor community.
The Wastewater Treatment Service Unit (WWTSU), a department of the City of Ann Arbor, is
responsible for the operation and maintenance of the City’s WWTP and eight sewage lift
stations located around the City. The plant runs continuously, 24 hours a day, seven days a
week, and WWTSU staff are on duty at all times.
Quick facts:
The City of Ann Arbor’s WWTP is located at 49 South Dixboro Road, Ann Arbor, MI.
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WWTP services 110,000 residents of the City of Ann Arbor
The Plant also services an additional 40,000 residents from three surrounding
townships – Pittsfield Township, Scio Township and Ann Arbor Township.
The Ann Arbor WWTP receives and treats approximately 18.0 million gallons of
wastewater per day from the City and the three townships.
The WWTP processes about 330,000 gallons per day from industrial sources.
City of Ann Arbor’s Wastewater Treatment Plant: Frequently Asked Questions
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The current WWTP has a design capacity of 29.5 million gallons per day (MGD) and
consists of an older West Plant (constructed in the 1930s) and a newer East Plant
(constructed in the late 1970s).
The City’s sanitary sewer collection system is made of about 370 miles of pipes and
transmission mains, all feeding into the WWTP.
Sewer pipes range in size from 8” in some neighborhoods, up to 72” in areas closest
to the plant.
Q. What’s meant by the term “sanitary sewer collection system”? What does it include?
A. The sanitary sewer collection system or wastewater collection infrastructure refers to the
pipes, lift stations, and force mains that together, make up the City’s wastewater collection
system.
The City of Ann Arbor’s sanitary sewer collection system is a network of about 370 miles of
pipes all heading roughly northeast to the wastewater treatment plant. It helps to visualize
the sewer network like a tree; the leaves are houses, connected by small 4” service lines to
8-inch mains that run mostly beneath streets and then to 18-, 24- or 30-inch collectors,
leading to larger and larger pipes, as large as 72-inches closest to the plant.
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Most of the sanitary sewer collection system operates using gravity to move the waste
through the pipes to the treatment plant. For gravity to do its job, the pipe needs to drop
about half a foot per 100 feet of length, a slope of 0.5 percent, which is fast enough to keep
everything moving, but not so fast that the liquid races away from the solids. Bigger pipes-30 inches or larger--can slope even less. But they all must flow downhill, powered by gravity.
The system generally moves downhill, but pipes sometimes need to cross rises. So the city
has lift stations, where the contents of pipes are pumped to join other flows.1
Q. How is sewage “treated”? What happens to the stuff that leaves our toilets?
A. Wastewater includes human waste from toilets, but it also includes everything from
showers, sinks, and washing machines (plus all the things that people flush down toilets and
shouldn’t!) A high percentage of wastewater isn’t fecal material; it’s all the water we’ve used
to bathe, wash clothes, dishes, cars, etc. Some industrial processes also create wastewater
that gets added to our sanitary sewer system.
Wastewater treatment involves a series of physical, chemical and biological processes to
remove contaminants from wastewater and household sewage. The goal of wastewater
treatment is to create an environmentally safe liquid stream (or treated “effluent”) and a
solid waste (or treated “biosolids ”) that is disposed or reused as fertilizer.
Sewage treatment generally involves three stages, called primary, secondary and tertiary
treatment.
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Primary treatment consists of temporarily holding the sewage in a basin where heavy
solids can settle to the bottom while oil, grease and lighter solids float to the surface.
The settled and floating materials are removed and the remaining liquid may be
discharged or subjected to secondary treatment.
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Secondary treatment removes dissolved and suspended biological matter. Secondary
treatment is typically performed by water-borne micro-organisms in a managed habitat.
Secondary treatment may require a separation process to remove the micro-organisms
from the treated water prior to discharge or tertiary treatment.
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Tertiary treatment is sometimes defined as anything more than primary and secondary
treatment in order to allow reinjection into a highly sensitive or fragile ecosystems. At
the Ann Arbor WWTP, tertiary treatment consists of sand filtration to further reduce
solids (and phosphorus attached to solids) in the effluent.
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[http://www.scientificamerican.com/article/treating-sewage/]
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As a final step in the treatment process, water is sometimes disinfected chemically or
physically (for example, by lagoons and microfiltration) prior to discharge into a stream,
river, bay, lagoon or wetland or it can be used for the irrigation of a golf course, green way
or park. If it is sufficiently clean, it can also be used for groundwater recharge or agricultural
purposes. Ann Arbor’s WWTP disinfects with UV light after tertiary treatment.
Q. What happens to the fluids or effluent after the sewage has been treated?
A. Effluent is the treated wastewater that is released from the wastewater treatment plant.
In Ann Arbor, the effluent is discharged to the Huron River. The City has a National Pollutant
Discharge Elimination System (NPDES) permit from the Michigan Department of
Environmental Quality (MDEQ), which allows it to discharge effluent to the river, provided it
has been treated according to standards set by MDEQ and the Federal Clean Water Act.
Plant Capacity & Costs
Q. What is the plant’s current capacity? Is it enough to accommodate future growth?
A. The current average flow into the treatment plant is around 18 MGD. The annual average
daily design capacity of the City’s current wastewater treatment facility is 29.5 MGD. On an
average day about 60% of the plant’s capacity is used. The projected need in 2025 is 24.3
MGD or about 82% of the plant’s capacity. (These capacity numbers include both the East
and West plants.)
Q. Plant design flow capacity is 29.5 million gallons per day (MGD), while the peak hourly
flow is 70 million gallons per day (MGD.) What’s the difference between design flow and
peak hourly flow?
A. The plant was designed to reliably process an average of 29.5 million gallons per day,
however it has additional features, such as equalization basins that allow it to temporarily
store larger spikes in flow rates for treatment later. The design peak hourly flow is the largest
volume of flow that the plant can handle during a one-hour period. Therefore, the plant is
designed to be able to handle short peaks in flow rate that, if extrapolated to a daily rate,
would be the equivalent of about 70 million gallons in a day.
Q. Can the City’s Wastewater treatment plant be expanded?
A. No, the wastewater treatment plant footprint is constrained by its physical location. It is
surrounded by railroad tracks, a creek, and the river. Nor does the plant need to be
expanded – we are currently using about 60% of the plant’s full capacity. Water usage has
not increased in the last five years and future projections predict Ann Arbor will actually use
less water, not more in the future.
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Q. How does the City make sure the WWTP is functioning effectively and will continue to
have adequate capacity for the future?
A. The City of Ann Arbor commissioned a WWTP Facilities Master Plan in 2004. Performed by
Black & Veatch, this Master Plan reviewed the plant’s current age, infrastructure and
processes and compared those to projected needs (population growth or decline, asset
management.) The Master Plan made recommendations for repairs, renovations and
upgrades to keep the WWTP functioning effectively. You can see more details about the
WWTP Facilities Master Plan at this link: http://bit.ly/1p700ND.
Q. How is plant capacity impacted by the current renovations?
A. Let’s start with some details about plant 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 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
Ann Arbor Daily Flow of 24.3 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
treatment using ultra-violet (UV) light. The hydraulic capacity of the UV system is around 48
MGD or so. Doing the same analysis that Black & Veatch performed for the WWTP Facility
Master Plan but using 48 MGD as the amount “the plant can handle”, the storage is around
16 hours. As this estimate was based on a series of worst-case scenarios and restrictions,
treatment plant staff characterizes our storage as being from 16 hours to 1.4 days.
How construction at the WWTP impacts capacity:
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The projections in the Black & Veatch report did not have take into account the West
Plant being off line for replacement.
The flow estimate was based on a projection of growth that the area is 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 was out of service, the
estimated storage time would be reduced by about 1/3.
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Sanitary Sewer Overflows
Q. What is a Sanitary Sewer Overflow (SSO)?
A. Sanitary sewer overflows (SSOs) are discharges of raw or inadequately treated sewage
from municipal separate sanitary sewer systems, which are designed to carry domestic
sanitary sewage but not storm water. (Ann Arbor’s sewer system is separate; the stormwater
collection system has its own network of pipes, as does the sanitary sewer collection system.
In some older communities, storm and sanitary sewer is collected in the same pipe system.)
When caused by rainfall, it is also known as wet weather overflow. When an SSO occurs,
raw sewage may be released into basements, city streets, properties, rivers, and streams.
The main causes of SSOs are:
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Infiltration and/or inflow of excessive stormwater into sewer lines during heavy rainfall
Rupture or blockage of sewer lines
Malfunction of lift stations or electrical failure
Malfunction of treatment plant facilities or electrical failure
Human operator error at treatment plant facilities.
The City is required to report any SSOs that reach waters of the state to the MDEQ and to
the Washtenaw County Health Department. Sewage discharges into basement may also
occur, but there’s no requirement to report those events to the Michigan Department of
Environmental Quality under Section 324.3112(a) of the NREPA.
Q. How does stormwater get into the sanitary sewer system?
A. The role of the sanitary sewers is to transport wastewater from homes and businesses to
the treatment plant. Along the way, some stormwater enters the sewer pipes. Some
common sources of stormwater include:
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cracks or joints in pipes or manholes
cross connections to the storm sewers or drains
pick holes or vent holes in the manhole covers
footing drains connected to the sanitary sewer system. The 2001 SSO Prevention
Study Final Report identified that 70 to 90% of the total sanitary sewer flow – in some
portions of the system – was coming from footing drains during storm events.
Subsequent engineering studies found similar levels of stormwater from footing
drains.
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Q. How much does it cost to treat stormwater vs. sanitary sewage?
A. When any stormwater enters into the sanitary sewer system, unnecessary cost is incurred
because all the flow from the sanitary sewer pipes goes to the wastewater treatment
plant. By itself, there is no cost to treat stormwater at the pipe outlet, as this water is
collected by a separate storm sewer pipe system and ultimately discharged to the Huron
River without end-of-pipe treatment. Treatment and management of stormwater is handled
through other mechanisms such as source control, street sweeping, public education
programs, and stormwater collection system maintenance.
Stormwater does not make its way to the wastewater treatment plant unless it enters the
sanitary sewer system through defects in the sanitary sewer system or private property
sources like footing drains. The cost for treating sewage at the wastewater treatment plant is
approximately $1400 per million gallons.
Q. If we’re only using about 60% of the plant’s capacity, why are there ever sanitary sewer
overflows (SSOs)?
A. To understand how this situation can happen, it’s helpful to know a few facts:
SSOs include all sewage overflows, ranging from partially treated sewage released into the
Huron River when the plant gets overwhelmed, to small flows that surge out of the tops of
manholes.
Accumulations of grease or clogs from paper and plastic products can create blockages in
sewer pipes, causing backups. Tree roots can infiltrate sewer pipes, causing blockages and
even breaking pipes. Sewer pipes fit together simply, with a bell joint, and tiny root hairs find
their way to the nutrient-rich flow, then grow larger, eventually growing large enough to
shatter the vitreous clay pipe that forms so many service lines or dislodge a joint if the pipes
are cast iron.
One of the most common causes of SSOs are heavy rainfall events, which can cause massive
influx of stormwater into sewer lines. The combined flow of wastewater and stormwater
exceeds the capacity of the sewer system and sewage is released into local waterways to
prevent flooding in homes, businesses and streets.
The system does have ample capacity to handle the average daily flows from our community.
However, during more intense rainstorms, large amounts of rainwater enter the sanitary
sewer system through footing drains, manholes and cracks in the pipes.
Some SSOs occur because plant flow increases faster than plant operators can react to bring
equipment on-line that is not needed at the lower flow rates. The sewer system does not
have water towers or storage tanks to accommodate regular fluctuations in wastewater
volume (flow peaks at breakfast time and again in the evening between 5 and 10 pm), so it
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stores water in equalization tanks during periods of high flow and especially, storms. Wet
weather overflows can occur when the mixture of sewage and high volumes of stormwater
rushes through the system to the plant and exceeds its capacity before the equalization
basins can be made active.
Q. During the June 27, 2013 storm, there was an overflow at the treatment plant. How
many gallons? And what caused it?
A. The City estimates that the overflow was about 10,000 gallons. This was reported to the
MDEQ and the Washtenaw County Health Department. Press releases were also sent to local
media outlets.
On June 27, 2013, a sudden and heavy thunderstorm hit Ann Arbor. The wastewater
treatment plant flow volume tripled in half an hour, which was an unprecedented increase in
plant flow over such a short period of time. Plant personnel redirected much of the plant
flow into the retention and equalization facility and turned on additional equipment to
handle the increased flow. After the stormwater flooding at the plant subsided, plant staff
noticed debris on the ground, which was evidence of a sewage spill. All clean up procedures
were followed, required reports filed and notifications made.
Q. Is there a record of SSOs in Ann Arbor?
A. Yes, the City reports all SSOs to the MDEQ and the Washtenaw County Health Department
and keeps its own record. You can see the log of SSOs and their causes at this link:
http://bit.ly/1t2FkG9.
Maintenance and Repairs
Q. How is the sanitary sewer system maintained?
A. The City has developed a system maintenance plan, based on the characteristics of Ann
Arbor’s system and best practices among municipalities.
One method of ongoing preventative maintenance the City practices is televising its system.
Televising or “TVing” pipes involves running a small robot camera through the pipes to
identify cracks or blockages that could affect the sanitary sewer system operations.
Other maintenance measures include jetting rodding or cleaning pipes with high-pressure
water.
In addition to responding to problems, the City practices preventative maintenance, striving
to televise each pipe in the system once every seven years.
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Renovations and Upgrades
Q. Why is the Wastewater Treatment Plant under construction? What kind of work is being
done?
A. What we refer to as the WWTP is actually two plants - an older West Plant (constructed in
the 1930s) and a newer East Plant (constructed in the late 1970s). The West Plant is beyond
its useful life and was taken out of service in 2006 due to its dilapidated condition. The
current WWTP facilities capital improvement project involves demolishing the aged West
Plant, building new facilities and upgrading treatment processes at the East Plant.
The entire project will cost $120 million and is the largest capital improvement project in the
City’s history.
For details on the demolition and reconstruction, visit this link: http://bit.ly/1iQIRjN.
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