6.5. SR 05-16-2005
Item # 6.5.
MEMORANDUM
TO: Mayor and City Council
FROM: Terry Maurer, City Engineer 4
DATE: May 16, 2005
SUBJECT: Wastewater Treatment Plant Master Plan
Attached to this memo is the three-page executive summary from the Wastewater Treatment
Plant (WWTP) Master Plan. A public hearing on the Plan is scheduled for Monday
evening's City Council meeting. I will have a PowerPoint presentation which will quickly go
through the components of the study including proposed improvements and cash flow
analysis for ultimate expansion to 3.5 millions per day of sewage treatment capacity. Once
the public hearing is concluded, the City Council should consider adopting the Plan. Once
adopted by the City Council, the Plan will be finalized including the section regarding public
involvement and public hearing and then will be sent to the Minnesota Pollution Control
Agency (MPCA) for their review and approval. The Plan cannot be reviewed by the MPCA
without a public hearing. Once the plan is approved by the MPCA, it will provide the
framework by which all future expansion projects at the WWTP are reviewed by the MPCA.
S:\EngineerImprovProj\2005 Improvement Projects\ WWfP Headwooo-Digester\05 1605 cc memo.doc
1.0 EXECUTIVE SUMMARY
The City of Elk River is a quickly growing municipality located northwest of Minneapolis,
Minnesota. Wastewater generated by the City is treated through processes that include
preliminary, primary, secondary treatment, and disinfection. Secondary treatment is
accomplished using a trickling filter with provisions for upgrading to activated sludge.
Treated wastewater is discharged to the Mississippi River. The Minnesota Pollution
Control Agency (MPCA) is the regulatory agency responsible for Elk River's National
Pollution Discharge Elimination System (NPDES) permit. The MPCA has classified the
Mississippi River as an Outstanding Resource Value Water (ORVW) at the reach to
which the City of Elk River discharges. Stricter standards apply to ORVW's to protect
the receiving water. The MPCA requires evaluation of alternative discharge and
advanced treatment options to ensure protection of the Mississippi River at this reach.
Mass loading to the river, with respect to organics and suspended solids, has been fixed
since 1984. Effluent loading for five-day, carbonaceous biochemical oxygen demand
(CBODs) is fixed at 216 pounds per day and total suspended solids (TSS) is fixed at 260
pounds per day. More advanced treatment is needed to meet the mass loading
requirements. New permit conditions have been suggested as highly probable by the
MPCA. Phosphorus concentrations in the effluent are to be reduced to less than 1
mgP/L, and mercury concentrations are to be reduced to less than 6.9 ng/L.
A facility plan has been prepared to assess the wastewater treatment processes,
analyze the sanitary sewer collection system, and provide recommendations for
upgrades to meet future flows, loadings and permit requirements for operation through
the year 2025. The existing plant was evaluated to determine the condition of the
equipment and tanks. Wastewater flow was characterized and future flows and loadings
were projected. Three areas of development have been identified for trunk sanitary
sewer expansions, and new flows from those areas have been identified. Based on
projections, the plant must be capable of treating an average wet weather flow of 3.7
million gallons per day (mgd) with a peak hourly flow of 7.0 mgd. Organic loading to the
facility was projected based on typical residential, commercial and government
contributions. Industries are major contributors of organics to municipal wastewaters,
and a typical industrial contribution was incorporated into the wastewater projections.
A number of upgrades to the facility are needed to ensure successful treatment of
increasing wastewater flows in accordance with the anticipated permit requirements.
Improvements are needed for preliminary treatment, primary treatment, secondary
treatment, and disinfection. Additionally, a phosphorus removal system is necessary to
meet the new 1 mgP/L, and filtration is required to polish the treated wastewater to meet
strict requirements for CBODs, TSS, phosphorus, and mercury.
The MPCA mandates a nondegradation review to be performed for treatment facilities
that discharge to an ORVW. Alternatives have been evaluated as part of the
nondegradation review for Elk River. Advanced treatment processes are a practical
option that can be implemented at the treatment facility at a consequential expense.
Specifically, sand or membrane filtration are options that would polish the wastewater to
meet the requirements for an ORVW. An analysis shows that Elk River wastewater,
treated to secondary standards, would have little effect on the Mississippi River with
respect to CBODs and TSS due to the high dilution provided by the river. Phosphorus
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and mercury are critical contaminants that could affect the river, however, and a
requirement for some form of treatment is reasonable.
Expansion of the existing facility is necessary to meet higher flow rates with higher
organic loads. Pretreatment processes are in the process of being upgraded with new
bar screens and a new grit separator. As part of the pretreatment upgrade, a chemical
phosphorus removal system is being added to the facility. Primary treatment is to be
expanded to meet new flow demands with the addition of two new primary clarifier
settling tanks equal in size to the existing tanks.
Expansion of secondary treatment is critical to removing soluble organics from the
wastewater. The trickling filters are effective while operating in series mode, but need to
be switched to parallel mode to handle flow rates in excess of 2.9 mgd. In parallel
mode, the trickling filters can accommodate a maximum flow of 5.8 mgd, which is less
than the projected peak hourly flow rate of 7.0 mgd. Two models have been applied to
evaluate removal of organics from the wastewater. The results show that the trickling
filters would not meet the strict fixed loading requirements at flows as low as 2.0 mgd.
An activated sludge system would improve the systems performance considerably. The
facility has previously had upgrades in preparation for the addition of an activated sludge
system. An aeration tank and blower building have been constructed. Additional
aeration tanks would be needed to provide redundancy and to meet higher flow and
loading demands.
To complete the activated sludge system, a new secondary clarifier would be needed.
Solids loading to the secondary clarifiers would increase dramatically with an activated
sludge system, and an additional clarifier tank is needed for adequate settling. The new
settling tank would be equal in size to the three existing, operational tanks.
Two final steps to treatment are needed to meet permit requirements. As mentioned,
filtration is advised to polish the wastewater and reduce CBODs, TSS, phosphorus and
mercury concentrations to acceptable levels. Additionally, the existing UV disinfection
system is sized for an average flow of 1.1 mgd. Another channel is available for
installation of an additional train of UV disinfection bulbs. The existing train of UV bulbs
would need to be replaced with a higher intensity system; depending on the type and
efficacy of the previous filtration step.
Solids generated during wastewater treatment must be handled. Digestion facilities
would need to be added to meet new solids production requirements. An additional
primary digester, secondary digester, and digested sludge storage tank are needed to
adequately reduce, stabilize and store solids.
To meet space requirements for the expansion of the treatment facility, it is
recommended that the City investigate the possibility of land acquisition to the south of
the existing site currently owned by Great River Energy. A new main building with a full
Supervisory Control and Data Acquisition (SCADA) system and improved laboratory and
office space could then be built south of the existing parking lot. The existing main
building would be demolished to create space for treatment processes. Part of the
existing garage space could be preserved in the existing main building, however.
System hydraulics would be simplified by locating treatment processes in the same area
as existing processes, and costs for new treatment processes could be reduced
considerably.
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The SCADA system is useful to improve control of the facility. Complex processes
would be automated, and operators would be immediately notified of problems occurring
in the facility and at satellite lift stations through alarm signals transmitted to the main
control station. Operation of the facility would be better documented through the SCADA
system, because flows and alarms would be recorded.
Based on the recommended facility improvements, the cost of upgrading the treatment
facility is $17,350,000. This cost includes the expense of upgrading the facility's
headworks, adding a phosphorus removal system and renovating the existing secondary
digester; which is scheduled for construction in mid May. A detailed breakdown of the
cost can be found in Chapter 19.
Funding for upgrading the facility may be available from the State Revolving Fund
(SRF). Strict procedures must be followed for submitting the facility plan, environmental
assessment worksheet and permit application in a timely manner. Additionally, the City
must formally request funding for upgrades from the Public Facilities Authority (PFA).
A preliminary schedule for plan and implementation of the proposed facility plan has
been prepared. Based on need, it is recommended that the facility plan be implemented
in four major phases beginning with the current headworks upgrade. Phased upgrades
have been scheduled as needed for the facility to continue to meet anticipated permit
requirements as a function of flow and organic loading projections. The facility would be
completely upgraded by 2011. A full detailed schedule is shown in Chapter 23.
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