ITEM 3.1_attach 4Technical Memorandum
Darkenwald's Riverbend Wastewater Co. LLC Proposed PUC Concept Plan
Amendment Review
To: Mr. Ron J. Wagner, Otsego City Engineer
From: Scott Schaefer, PE, AE2S
Date: July 11, 2017
Project No: P05409-2015-002
1 SUMMARY
The Darkenwald's Riverbend Wastewater Co. LLC currently owns the Riverbend Mobile Home
Park wastewater treatment facility (WWTF) located inside the city limits of Otsego, MN. The
Darkenwald's Riverbend Wastewater Co. LLC has changed their plan from the previous alternative
of proposing to install a new package treatment system to the "least cost" option of "Do Nothing"
as part of their proposed development amendments dated May 23, 2017. The "Do Nothing"
alternative does include some routine maintenance and replacement of components.
The revised plan further considers future alternatives including connection to the City system, a new
package treatment facility, or rehabilitation of the existing treatment system. Darkenwald's
consultant recommended the option for rehabilitation of the existing system when required without
defining a trigger for when rehabilitation will occur.
This Technical Memorandum (TM) reviews the proposed development concept plan amendment.
The findings of this TM are that the existing connections to the existing WWTF are already
approaching the BOD treatment capacity of an extended aeration activated sludge system on a
maximum month basis. Additional connections to the WWTF would jeopardize the treatment
performance of the system. Furthermore, without additional treatment (i.e. chemical phosphorus
removal), the proposed additional connections would likely violate the discharge. Additional
treatment would alter the economic evaluation. It is recommended that the WWTF be allowed to be
rehabilitated on the following conditions:
• No additional sewer connections are made to the existing WWTF
• Appropriate setbacks from the WWTF are established.
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2 CURRENT CONDITIONS
The private Riverbend Mobile Home Park WWTF was approved, in 1972, to treat an average flow
of 60,000 gallons per day (gpd). The permitted flow capacity was provided to specifically serve the
Riverbend Mobile Home Park. The permit makes no reference to the adjoining or any other
property.
2.1 EXISTING FACILITY
Updated flow information was included in the submittal including discharge monitoring report
information from 2014 to 2016. The existing treatment facility produced good results with average
effluent BOD and TSS below 10 mg/L, and the treatment analysis by Darkenwald's consultant
indicated repeatedly that these results justified continued use of the existing system. However,
during the review, no analysis was noted regarding the structural integrity of the —45 year-old
system. The specifications from the original design indicate a 1/4" thick steel plate was used for the
tanks along with six sacrificial anodes for corrosion protection. No design life was indicated for the
anodes. It is likely that the corrosion protection offered by the sacrificial anodes is expended or
nearly expended based upon this service life.
The existing system was designed as a "contact stabilization" system, which has different design
parameters compared to how the system has been historically operated as an extended aeration
activated sludge facility. The good effluent results are, at least, partly attributable to the existing
system being hydraulically under -loaded.
The existing flows indicate an overall average of 20,000 gallons per day (gpd) for the recent three-
year period. The maximum month was 33,000 gpd and the maximum day appears to have been
48,000 gallons per day. It should also be noted, the annual peak flow months consistently occurred
in January and February, which is different from most municipal systems. It is assumed that the cold
weather peal-, flows may be attributed to the nature of the facility's customers, who may need to let
their water run in the winter to avoid freezing issues.
The existing clarifier provides sufficient surface overflow rate and weir overflow rate relative to the
existing flows; however, the clarifier depth is problematic. The shallow clarifier has likely been
performing well due to the low overflow and weir loading rates despite the shallow depth. No
specific clarifier depth is indicated, ;but the maximum depth is likely 9.5 feet or less based upon the
listed depths of the aeration basin units. Based upon the listed clarifier surface area and volume, the
average depth is 6.96 feet. Ten State Standards requires a minimum side water depth of 12 feet for
activated sludge systems, although an exception can be considered for package plants with capacity
of less than 25,000 gpd with demonstrated "successful operating experience".
The average BOD loading to the facility was 51.2 pounds per day (ppd) with a maximum month
loading of 76.7 ppd over the 2014 to 2016 period. The maximum month to average BOD peaking
factor is 1.5 and nearly equal to the flow peaking factor, although the peaks are not concurrent. The
allowable BOD loading rate for an extended aeration activated sludge system is 15 lb-BOD/1,000
cubic feet of aeration basin. The Darkenwald's consultant and the MPCA permit both refer to the
existing facility as an extended aeration activated sludge system, which is appropriate considering
the capacity, operational characteristics, operational staffing, and instrumentation of the facility.
Note that the "original design capacity" was based on the higher allowable loading rate of a contact
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stabilization treatment system, although the system has been operated as an extended aeration
activated sludge system. The average BOD loading rate was 9.6 lb-BOD/1,000 cf for the period
analyzed, and the maximum month loading rate was 14.4 lb-BOD/1,000 cf. This indicates that the
existing system is already being loaded near the allowable BOD loading capacity of an extended
aeration activated sludge system.
The existing WWTF is located within a building. A cursory review indicates that the facility does
not meet current National Fire Protection Association (NFPA) 820 criteria.
3 PROPOSED CONDITIONS
Darkenwald's Riverbend Wastewater Co. LLC has proposed expanding the contributing
connections to the existing wastewater system. The wastewater treatment analysis indicated a
projected future annual average flow of 37,000 gpd based upon new construction with 170 additional
apartment units. The proposed concept plan amendment indicated an additional 192 apartment units.
The projected flow and/or number of units must be reconciled if the plan is to move forward.
Assuming a proportional incremental flow increase for the additional units, the average flow would
be approximately 39,200 gpd. Note that the basis for determining the additional flow per apartment
unit did not appear to be well documented.
Insufficient information was provided regarding the expected population of the expanded service to
the apartment complex. Additional BOD loading design was not provided by the Darkenwald's
consultant. The BOD characteristics of the expanded wastewater flow are assumed to be equal to
the existing BOD characteristics for evaluation purposes.
3.1 EVALUATION OF PROPOSED ALTERNATIVE
Continued use of the existing system while doubling the contributing flow is problematic. Historical
performancefor an under -loaded system is not necessarily indicative of future results when flow to
the system is significantly increased.
The existing system is already loaded near the allowable BOD loading rate under maximum month
conditions. The maximum month is appropriate for comparison due to calendar month compliance
periods. The proposed BOD loading would push the existing system over the allowable BOD loading
on an average basis and nearly double the allowable loading on maximum month flow basis, as
shown in the table below. Insufficient information was available to analyze the aeration system
capacity for the expanded BOD load; however, the existing blower/diffuser system is likely to be
insufficient for the proposed increase in BOD loading. Based upon the BOD loading rate, expansion
of the contributing flow and load to the existing treatment system should not be allowed.
*Limit of 15 lb-BOD/1,000 cf per Ten State Standards section 92.31
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The existing discharge permit allows up to 290 kg of phosphorus to be discharged during a rolling
12 -month average basis. The existing system averaged 162.6 kg/yr for the 2014-2016 period. The
proposed expansion would likely place additional stress on the treatment system resulting in reduced
phosphorus removal efficiency. When considering equivalent phosphorus removal efficiency, the
increased flow/load would be projected to increase the phosphorus discharge to 306 kg/yr and 325
kg/yr for the 170 apartment and 192 apartment flows, respectively. Both of these phosphorus
discharge rates are above the allowable 290 kg/yr phosphorus discharge. The Darkenwald's
consultant elected not to evaluate the impact of phosphorus treatment, which would be expected to
result in violation of the discharge permit. The provided analysis assumes chemical phosphorus
removal would be used, an appropriate phosphorus removal approach for this system. However, the
cost analysis did not consider the chemical costs, additional labor costs, or additional biosolids
disposal costs associated with chemical phosphorus removal. Expanded flows cannot be approved
for an analysis that does not address discharge permit violations.
The proposed rehabilitation is generally appropriate; however, the proposed coating of the treatment
system will be difficult due to the lack of redundancy of the system. Based upon rough estimates,
the cost included for hauling wastewater during the rehabilitation would likely provide one to three
weeks of rehabilitation time. This timeframe could be sufficient if a quick set coating is used, but
additional rehabilitation (diffuser replacement, etc.) will also need to be considered. The
construction phasing of any rehabilitation must be carefully planned, especially when no redundancy
is available.
The proposed rehabilitation recommends a _change to the disinfection technology to eliminate
chlorination/dechlorination and replace it with ultraviolet (LTV) disinfection. For this type of facility,
this process is appropriate.
The proposed rehabilitation does not appear to address the NFPA 820 issues.
3.2 DEVELOPMENT ADJACENT TO WWTF
The existing WWTF includes pre-treatment, extended aeration activated sludge treatment, chlorine
disinfection, de -chlorination, and aerated waste activated sludge (WAS) holding. The main
differences between the existing Darkenwald treatment system and the City's two WWTFs are the
flow capacity, the disinfection system, and the retention time in the solids holding. The odor risk of
the aerated WAS holding is less than the risk from the City's existing digested sludge holding, but
Ordinance 2017-05 was developed considering the City's future plan to modify the solids approach
to include aerated WAS holding, dewatering, and hauling. The City's planned future system is
comparable to the existing Darkenwald system. The minimum setbacks in Ordinance 2017-05
should be considered minimums for this proposed development. The setbacks were developed to the
property line or right-of-way. Additional setback requirements for the adjacent property should be
considered when establishing setbacks within this proposed development. The proposed concept
plan amendment lists 3,000 square feet as the area reserved for the WWTF. This proposed WWTF
footprint is inadequate to provide the appropriate setback.
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Considering the lack of compliance with current NFPA 820 criteria, additional setbacks should be
reviewed with the appropriate fire protection authority. Assuming that the NFPA 820 issues are not
addressed, the following minimum criteria should be considered:
• Setbacks to establish a non-flammable perimeter as recommended by the local fire protection
authority
• Appropriate access and turning radius for fire protection equipment specifically for the
As a condition of the proposed concept plan amendment, it is recommended that at least one of the
following conditions is met to address the tank corrosion issue:
• A full tank integrity inspection is performed by a qualified professional including a report
detailing the following:
o Estimated remaining service life
o Recommendations for follow up inspection frequency, which will become a binding
requirement
o Recommendations and timeline for rehabilitation
• Design, installation, and implementation of a groundwater monitoring well system to detect
tank leakage
o Nitrates and coliforms will be the primary indicators of contamination
o Sampling frequency will be a minimum of quarterly
o Elevated samples above background concentrations will require tank inspection
and/or rehabilitation
• Rehabilitation of the corrosion protection system including, at a minimum:
o Qualified inspection and rehabilitation of deficiencies or holidays in the steel tank
structure
o Corrosion protection coating including spark testing
o Renewal of sacrificial anode system
Considering the proposed co -location of a WWTF on the same parcel as residential housing,
setbacks must also consider noise abatement. State of Minnesota noise ordinances (Minnesota
Rules Chapter 7030) are generally arranged based upon land use classifications. Noise Area
Classification 1 is the most restrictive. Further information is available on the website,
https://www.pca.state.nin.us/sites/default/files/p-gen6-01.pdf. With the proposed co -location of
land use activities, one of the following is recommended for noise compliance:
• The entire parcel is required to meet Noise Area Classification 1 criteria
• A perimeter (setback, fence line, or similar) is established around the WWTF with Noise
Area Classification 1 criteria compliance at the perimeter
• Sub -division of the parcel with updated classifications to allow determination of noise
compliance
Note that compliance with the noise criteria may require noise reducing enclosure(s) for the
blower(s) or other equipment, upgrading HVAC systems to allow for continuous closure of the
WWTF building, or other necessary upgrades.
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