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09.c3 2022 Bliss Nitrogen Facility Evaluation Report FINAL H:\SCND\N13118082\2_Preliminary\C_Reports\2022 Bliss Nitrogen Facility Evaluation Report FINAL.docx 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 Date: August 31, 2022 Page: 2 H:\SCND\N13118082\2_Preliminary\C_Reports\2022 Bliss Nitrogen Facility Evaluation Report FINAL.docx 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 Page: 3 H:\SCND\N13118082\2_Preliminary\C_Reports\2022 Bliss Nitrogen Facility Evaluation Report FINAL.docx 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 Date: August 31, 2022 Page: 4 H:\SCND\N13118082\2_Preliminary\C_Reports\2022 Bliss Nitrogen Facility Evaluation Report FINAL.docx 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 Date: August 31, 2022 Page: 5 H:\SCND\N13118082\2_Preliminary\C_Reports\2022 Bliss Nitrogen Facility Evaluation Report FINAL.docx 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 Name: Scandia – Bliss Collector WWTF Nitrogen Mitigation, Facility Evaluation Report Date: August 31, 2022 Page: 6 H:\SCND\N13118082\2_Preliminary\C_Reports\2022 Bliss Nitrogen Facility Evaluation Report FINAL.docx 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 Date: August 31, 2022 Page: 7 H:\SCND\N13118082\2_Preliminary\C_Reports\2022 Bliss Nitrogen Facility Evaluation Report FINAL.docx 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 Name: Scandia – Bliss Collector WWTF Nitrogen Mitigation, Facility Evaluation Report Date: August 31, 2022 Page: 8 H:\SCND\N13118082\2_Preliminary\C_Reports\2022 Bliss Nitrogen Facility Evaluation Report FINAL.docx 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