Memo_Bolton&Menk_8-30-19DBOLTON
& MENK
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MEMORANDUM
Date: August 30, 2019
To: Chris Hinze, Minnesota Pollution Control Agency
From: Jacob Humburg, P.E.
Ryan Goodman, P.E.
Subject: Scandia — Bliss Collector WWTF Nitrogen Mitigation
12224 Nicollet Avenue
Burnsville, MN 55337-1649
Ph: (952) 890-0509
Fax: (952) 890-8065
Bolton-Menk.com
Introduction
This memo has been written in response to the schedule included in the current State Disposal
System (SDS) Permit, issued by the Minnesota Pollution Control Agency (MPCA), for the Bliss
Wastewater Treatment Facility (WWTF) in Scandia, MN. The purpose of this memo is to
summarize the observations made over the current permit period and address the system's ability
to achieve compliance with the Minnesota Department of Health (MDH) drinking water standard
of 10 milligrams per liter (mg/L) for Nitrate Nitrogen within the downstream monitoring wells,
and to outline recommendations to the existing treatment system in order to achieve compliance.
The Bliss Collector WWTF was constructed in 1986 and is a soil -based subsurface sewage
treatment system (SSTS). It consists of a gravity and pressure collection system with two lift
stations, three septic tanks in series totaling 7,500 gallons, one lift station which doses the sand
filters, three 15,000 -square foot sand filters, one lift station which doses the drainfields, and three
drainfield trench with 1,210 feet of trench each. A figure of the Bliss treatment system is
attached. The facility treats wastewater from approximately 75 residences. Within the collection
system there are 70 individual septic tanks and 46 shared septic tank effluent pumping (STEP)
systems through which wastewater passes on its way to the WWTF. Each STEP station contains
a 15 -gallons per minute (gpm) pump. The current permit labels the system as a Class D facility
with a design flow of 19,800 gallons per day (gpd) and is effective from March 18, 2015 through
February 29, 2020.
H:\SCND\N13118082\2_Preliminary\C_Reports\2019 Bliss Nitrogen Final Report (Memo).doc
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Name: , Scandia - Bliss Collector WWTF Nitrogen Mitigation
Date: August 30, 2019
Page: 2
(;roundwater MonitorinL- Results
There are four groundwater wells surrounding the treatment system which are used to monitor
chloride, nitrogen, pH, and phosphorous in the area surrounding and downgradient of the system.
One of these, GW 004 (originally MW -3s), has consistently detected high nitrate + nitrite
nitrogen levels over the past number of years. In a Nitrogen Mitigation Plan submitted in 2016,
which was also required by the MPCA as part of the current SDS Permit, a table with nitrogen
concentration data in GW 004 was included for the years 2013-2015. Table 1, below, is an
extension of this original table which now includes data up to present time. Based on the data in
Table 1, it appears that nitrogen levels in GW 004 are increasing over time.
*Limit for Nitrate -Nitrogen is 10 mg/L.
**Total Nitrogen is the sum of TKN plus Nitrite/Nitrate-Nitrogen
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Table 1
Groundwater Monitoring Data: GW -004
Scandia Bliss Drainfield
Date
Ammonia
(mg/L as N) Nitrogen
Total Kjeldahl
Nitrogen (TKN)
Nitrite + Nitrate
Nitrogen In s N)1*
Total
Nitrogen**
Apr- 13
0.1
0.6
17.9
18.5
Jul -13
0.1
0.5
13.1
13.6
Oct -13
0.2
0.5
23.9
24.4
Apr -14
0.1
0.5
26.5
27.0
Jul -14
0.1
0.5
13.1
13.6
Oct -14
0.2
2.0
29
31.0
Apr- 15
0.0
0.0
27.4
27.4
Jul -15
0.0
0.0
13.4
13.4
Oct -15
0.0
0.0
21.5
21.5
Apr- 16
0.0
0.0
36.2
36.2
Jul -16
0.1
0.6
34.0
34.6
Oct -16
0.1
0.6
27.0
27.6
Apr -17
0.1
0.5
31.0
31.5
Jul -17
0.1
0.6
15.0
15.6
Oct -17
0.1
7.3
34.0
43.3
Apr- 18
0.1
0.5
39.0
39.5
Jul -18
0.1
0.6
24.0
24.6
Oct -18
0.1
0.5
34.0
34.5
Apr -19
0.6
0.5
46.0
46.5
Average
0.1
0.9
26.6
27.6
*Limit for Nitrate -Nitrogen is 10 mg/L.
**Total Nitrogen is the sum of TKN plus Nitrite/Nitrate-Nitrogen
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Name: Scandia — Bliss Collector W WTF Nitrogen Mitigation
Date: August 30, 2019
Page: 3
Investigation of Passible Sistem Issues
A number of investigations and inquiries have been made into the Bliss treatment system since
the 2015 permit was issued to try and detect and correct possible problems with the system's
operation. Flow to the three (3) drainfield cells is rotated regularly, with two in operation and
one resting at any given time. It was suspected by the City that perhaps Infiltration Trench A
might have been mostly responsible for the high readings in GW 004, but flow to this cell was
shut off for a period of three years with no discernable effect on the nitrogen readings.
In the summer of 2019 a number of minor problems were detected within the piping of the
system. It was recently discovered that all of the valves directing flow from the influent lift
station (LS 3) to the sand filters were in reverse configuration, meaning that those which were
thought to be closed were actually open, and vice versa. Instead of being spread between two
sand filter cells, flow was solely being directed to the one that was thought to be resting. This
condition somewhat obscures the actual flows moving through LS 3 in recent years, as well as
the amount of flow directed to each of the three filter cells. At the same time it was discovered
that when flow is being pumped by LS 3 to the sand filters, at least some of the flow is somehow
recirculating through the septic tanks located at the head of the treatment facility. These issues
are being systematically isolated and corrected, but at this time it is still unclear what affect, if
any, they may have had on the nitrogen levels entering the groundwater table.
Another possible contributor to the high nitrogen levels in GW 004 being considered is some
type of perched water table or similar condition in the area of GW 004 which could create a
direct conduit to the well from one or more of the drainfield cells, not allowing for any type of
additional treatment of the wastewater in the soil before it reaches the well. To this end, the City
opened dialogue with the MPCA about the possibility of replacing GW 004 with another
monitoring well in a slightly different location. After a protracted discussion process, it was
recently indicated to the City that it is unlikely this course of action will be approved.
Recommendations/Future Actions
As described in the previous section, a number of issues relating to the operation of the system
have recently been discovered and have either newly been remedied or are still in the process of
H:\SCND\N13118082\2_Preliminary\C_Reports\2019 Bliss Nitrogen Final Report (Memo)-doa
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Name: Scandia— Bliss Collector WWTF Nitrogen Mitigation
Date: August 30, 2019
Page: 4
being corrected. Furthermore, the possibility of constructing a new monitoring well to replace
GW 004 has just been ruled out.
Stemming from the recent events described above, the following actions are recommended as the
next steps to be taken:
1. Now that the sand filters will be operating as intended, determine whether there are leaks
within the piping system between LS 3 and the filters, and repair them if/as they are
discovered.
2. Continue to closely monitor the nitrate and nitrogen levels in both LS 4 and GW 004 to
determine if the recent and ongoing fixes and adjustments to operation of the facility have
any effects on effluent nitrogen.
3. Perform additional testing of the wastewater before and after treatment within the sand
filters to determine what effect, if any, the filters may have on nitrogen levels when
operating in the configuration for which the system was intended. This testing could
either confirm or eliminate the possibility of modifying/optimizing operation of the filters
to remove nitrogen to below the prescribed concentration. The proposed sampling
schedule is detailed below:
o Additional tests include the following, to be taken at WS 001 (LS 3, before sand
filters) and WS 002 (LS 4, after filters) once per week for six weeks in the fall of
2019:
■ BOD (Carbonaceous 05 Day)
• TSS
■ Nitrogen, Kjeldahl, Total
■ Nitrite Plus Nitrate, Total (as N)
• Nitrogen, Total
4. Draft a report/memo to the MPCA which details the results of steps 1-3 and provides
specific recommendations for additions to the facility for nitrogen removal, should they
ultimately be shown to be necessary.
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