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Memo_Bolton&Menk_8-30-19DBOLTON & MENK Real People. Real Solutions. 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 Balton & Menk is an equal 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 H;\SCND\N13118062\2_Preliminary\C_Reports\2019 Bliss Nitrogen Final Report (Memo).do" Bolton & Menk is an equal tip 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 H;\SCND\N13118062\2_Preliminary\C_Reports\2019 Bliss Nitrogen Final Report (Memo).do" Bolton & Menk is an equal tip 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 Bolton & Menk is an equal opportunity employer. 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. H:\SCND\N13118082\2_Preliminary\C_Reports\2019 Bliss Nitrogen Final Report (Memo).do" Balton & Menk is an equal opportunity employer.