09.c3 2022 Bliss Nitrogen Facility Evaluation Report FINAL
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MEMORANDUM
Date: September 1, 2022
To: Laura Geyen, Minnesota Pollution Control Agency
Randy Thorson, P.E., Minnesota Pollution Control Agency
From: Jacob Humburg, P.E.
Ryan Goodman, P.E.
Subject: Scandia – Bliss Collector WWTF Nitrogen Mitigation, Facility Evaluation Report
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 Facility
Evaluation Report is to summarize the effect of operational changes and upgrades on the
WWTF’s nitrogen treatment capacity, as well as 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 within the downstream monitoring wells .
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 for sand filter dosing,
three 15,000-square foot sand filters, one lift station for infiltration trench dosing , and three
infiltration trenches with 1,210 feet of trench each. A map of the Bliss WWTF is attached in
Appendix A.
The facility treats wastewater from approximately 75 residences. W ithin the collection system ,
there are approximately 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) pum p. The current permit labels the system as a Class D
Name: Scandia – Bliss Collector WWTF Nitrogen Mitigation, Facility Evaluation Report
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facility with a design flow of 19,800 gallons per day (gpd). The current permit is effective from
September 1, 2021 through August 31, 2026 .
Groundwater Monitoring Results
There are four groundwater wells surrounding the treatment system which ar e used to monitor
chloride, nitrogen, pH, phosphorous , specific conductance, and water temperature in the area
surrounding and downgradient of the system . One of these, GW 004 (originally MW -3s), has
consistently detected high nitrate + nitrite levels over the past 10 years. In the Nitrogen
Mitigation Plan submitted in 2016, a table showing nitrogen water 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.
Table 1
Groundwater Monitoring Data: GW-004
Scandia Bliss WWTF Infiltration Trench B
Date
Ammonia
Nitrogen
(mg/L as N)
Total Kjeldahl
Nitrogen (TKN,
mg/L as N)
Nitrate +
Nitrite*
(mg/L as N)
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 0.8 13.1 13.9
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 41.3
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
Name: Scandia – Bliss Collector WWTF Nitrogen Mitigation, Facility Evaluation Report
Date: August 31, 2022
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Jul-19 0.6 0.6 11 11.6
Oct-19 0.1 0.6 25 25.6
Apr-20 0.1 0.8 35 35.8
Jul-20 0.1 0.7 36 36.7
Oct-20 0.1 0.5 35 35.5
Apr-21 0 0 53.1 53.1
Jul-21 0.1 0.5 5.8 6.3
Oct-21 0.1 0.5 7.6 8.1
Apr-22 0.1 0.5 13.7 14.2
Jul-22 -*** -*** 23.4 -***
Average 0.1 0.8 25.9 26.8
*Limit for nitrate-nitrogen is 10 mg/L.
**Total nitrogen is the sum of TKN plus nitrite/nitrate-nitrogen
***Data for ammonia and TKN was not available at the time of publishing this memorandum
A plot of nitrogen concentrations was also created to aid in the understanding of temporal change
in nitrogen at GW004.
Figure 1: Total nitrogen concentrations at groundwater monitoring well GW004 from April
2013 to July 2022. The red line represents the MDH limit of 10 mg/L nitrate.
0
10
20
30
40
50
60
4/1/20128/14/201312/27/20145/10/20169/22/20172/4/20196/18/202010/31/20213/15/2023Total Nitrogen (mg/L)
Name: Scandia – Bliss Collector WWTF Nitrogen Mitigation, Facility Evaluation Report
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Based on the data in Table 1, the nitrogen levels in GW 004 have increased over time from 2013
– Spring 2021. After this date, it appears the nitrogen levels somewhat decreased, though they
rebounded in Spring 2022 to exceed the MDH nitrate limit of 10 mg/L.
Operational Changes and Upgrades
In addition to the operational changes and upgrades as described in the August 30, 2019
Memorandum, the City continues to take steps to optimize the operation of the Bliss System. In
the summer of 2020, a series of restrictor plates were added to the splitter boxes of the
drainfields to try and force the incoming flow to be more evenly spread throughout more of the
drainfield laterals. This was done because investigation revealed that the ground was accepting
flow at a faster rate than was likely anticipated by the system designers, and only one or two of
the laterals in each drainfield ha d actually been in use. It is theorized that this could possibly be
because the sand filters are so effective at removing the BOD and TSS from the wastewater, and
as a result no real biomat has been built up under the drainfield trenches. While excessive
biomat buildup is frequently the ultimate cause of failure for this type of system, in this case it
could actually provide additional treatment by slowing the wastewater down as it enter s the
ground and possibly allowing for more biological activity . In any case, the installation of these
laterals allowed for effluent to be properly dispersed across the full drainfield area, rather than
concentrating flow within the first one or two trenc hes. While no immediate change in GW004
nitrogen levels was detected, the total nitrogen at GW004 did drop from 53.1 to 6.3 over the
summer of 2021. It was not apparent whether this was due to the recent modifications or other
environmental factors.
In addition to the work described above, the City also installed flow meters on the effluent lines
directed to infiltration trench A, B, and C, to assist with tracking the proportion of effluent
distributed to each trench. These meters were installed in February 2022, and were named
WS004, WS005, and WS006, respectively. The meters will be integrated into the City’s
SCADA system. A plot of the data received from these meters from March – July 2022 is shown
in Figure 2 below:
Name: Scandia – Bliss Collector WWTF Nitrogen Mitigation, Facility Evaluation Report
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Figure 2: Flow data for effluent entering Infiltration Trench A (WS004), B (WS005), and C
(WS006).
As shown in Figure 2, Infiltration Trench A has not been used since March 24th. This was done
to allow for Trench A to rest while Infiltration Trenches B and C were used. Infiltration Trench
C has received the majority of effluent flow. Monitoring well GW004 is located near Infiltration
Trench B. A relationship between the flow entering Trench B and observed nitrate
concentrations at this well could not be investigated due to the lack of nitrogen sampling
occurrences since the flow meters were installed (2 data points).
A summary of all operation changes and upgrades to the Bliss treatment system is shown below
in Table 2.
0
5000
10000
15000
20000
25000
2/18/20223/10/20223/30/20224/19/20225/9/20225/29/20226/18/20227/8/20227/28/2022Flow (gpd)WS004 WS005 WS006
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Table 2
Operational Changes and Upgrades Timeline
Scandia Bliss WWTF
Date Action
October 1986 System constructed
2016-2019 Rotation of infiltration trenches implemented
Summer 2019 Piping inspection: discovered all valves are in reverse
configuration & portion of flow is recirculated to head of
system.
Issues corrected
Summer 2020 Modifications made to the drop-boxes in infiltration trenches to
ensure even effluent distribution across the length of the trench
February 2022 Flow meters installed on the effluent flow entering infiltration
trenches A, B, and C
March 2022 New pumps installed at Lift Stations 3 and 4 which distribute
flow, respectively, to the sand filters and drainfields.
Expected Future System Changes
The Bliss WWTF currently only serves residential customers. This is not expected to change in
the near future, as no nearby land is zoned for industry an d the City has not been approach by
any Significant Industrial Users (SIUs).
Approximately 75 customers are currently connected to the Bliss System. There are no plans to
add significant numbers of additional customers to the system in the near future, a nd it is
expected that the system’s influent flows will remain relatively constant.
Analysis of System Ability to Meet Nitr ate Standard
To try and gauge the ability of the sand filters to remove nitrogen from the wastewater stream ,
additional recent samples were taken from station WS001, located upstream of the sand filters,
and WS002, located just downstream of the filters. This data is summarized below in Table 3:
Name: Scandia – Bliss Collector WWTF Nitrogen Mitigation, Facility Evaluation Report
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Table 3
Sand Filtration Nitrogen Removal
Scandia Bliss WWTF
Date Analysis Results Percent Removal WS001 WS002
8/10/2022
TKN (mg/L) 75.7 0.85 -
Nitrate + Nitrite (mg/L) 0 32.6 -
Total Nitrogen (mg/L) 75.7 33.5 56%
8/15/2022
TKN (mg/L) 76.1 0 -
Nitrate + Nitrite (mg/L) 0.059 44.6 -
Total Nitrogen (mg/L) 76.2 44.6 41%
8/23/2022
TKN (mg/L) 70.3 1.3 -
Nitrate + Nitrite (mg/L) 0.058 54.2 -
Total Nitrogen (mg/L) 70.4 55.5 21%
This preliminary data shows that the sand filters are converting incoming nitrogen from organic
forms (TKN) into nitrate. It also shows that the sand filters can potentially remove 21-56% of
the total nitrogen from the wastewater.
As discussed in the pre vious section, efforts made to improve the operation of the WWTF in
regard to nitrogen treatment has no t resulted in consistent GW004 total nitrogen concentrations
below 10 mg/L. However, a number of studies have shown that passing wastewater through
sand filters of this type more than once can enhance total nitrogen removal. Given this and the
recently-gathered data shown in Table 3 , further study will be needed to determine if this could
be an appropriate treatment solution for the Bliss WWTF.
At this time though, it is assumed that significant improvements to the system will be needed to
lower the Total Nitrogen concentration in GW004 to less than 10 mg/L. Possible upgrades have
been discussed previously, in the City’s Nitrogen Mitigation Plan. The recommended
alternatives are the conversion of the system to either a denitrification system, or a combined
nitrification/denitrification system. Details on these systems can be found in the City’s Nitrogen
Mitigation Plan, section “Treatment Methods/Alternatives”. As the City continues to monitor
nitrogen levels in the system and at GW004, the decision between the two treatment alternatives
will be reexamined to determine the best option for the Bliss WWTF.
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 been remedied. Additional monitoring systems have
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also been installed to gather data on system performance, including flow meters for each
drainfield. In addition to continued system monitoring, the following actions are recommended:
1. Collect further samples of influent wastewater at WS001 and effluent from the sand
filters at WS002 to determine how much nitrogen the sand filters are able to remove.
2. After additional data is acquired, complete a Facility Plan examining which system
upgrades are necessary and prud ent to allow the system to meet the nitrogen standard .
3. Obtain a modified permit from the MPCA for this upgraded system.
4. Submit plans and specifications to for the proposed system upgrades to the MPCA for
approval.
5. Construct and begin operation of the upgraded system by the Fall of 2028.
Schedule of Proposed Upgrades
The proposed schedule for recommended upgrades is presented in Table 3.
Table 4
Improvements Implementation Schedule
Scandia Bliss WWTF
Item Date
Submit Facility Plan/Engineering Report August 2023
Submit Permit Modification Application August 2024
Submit Plans and Specifications for System Improvements August 2024
Begin Construction of Improvements August 2025
Submit Additional Technical Documents and As -Builts 1 Year After Operational Initiation