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3.d.2 Bliss Drainfield Nitrogen Mitigation Memo UpdateBQ L__.TQN 8 M EE N K, Consulting Engineers & Surveyors " 12224 Nicollet Avenue • Burnsville, MN 55337 Phone (952) 890-0509 • Fax (952) 890-8065 www.bolton-menk.com MEMORANDUM Date: June 23, 2016 To: Honorable Randall Simonson, Mayor Neil Soltis, City of Scandia Jeff Anderson, City of Scandia From: Seth Peterson, P.E. Ryan Goodman, P.E. Subject: Scandia — Bliss Collector WWTF Nitrogen Mitigation Background I N G® The Bliss Collector Wastewater Treatment Facility (WWTF) is located in Scandia, MN. The system was constructed in 1986. It is a soil based subsurface sewage treatment system (SSTS) consisting of a gravity and pressure collection system with two lift stations, three septic tanks in series totaling 7,500 gallons, one lift station that feeds the sand filters, three 15,000 square foot sand filters, one lift station that feeds the drainfields and three drainfield trench cells at 1,210 feet of trench per cell. A figure of the Bliss system is attached. The facility treats wastewater from approximately 75 residential homes. There are 70 individual septic tanks and 46 shared STEP system through which wastewater passes before entering the WWTF. Each STEP station has a 15 gpm pump. The current permit regulates the system as a Class D WWTP with a design flow of 19,800 gallons per day (gpd) and is effective from March 18, 2015 through February 29, 2020. The current permit includes monitoring and testing for both phosphorous and nitrogen at various locations within the treatment system. Test results indicate the Bliss Drainfield is not effective in removal of nitrogen and high levels of nitrate nitrogen (above 10 mg/L) have been found in one of the monitoring wells. A recent compliance evaluation conducted by the MPCA indicated several deficiencies concerning monitoring data and the Nitrogen Mitigation Plan. Overall, the system is well DESIGNING FOR A BETTER TOMORROW Bolton & Menk is an equal opportunity employer ()J ON 0�' � maintained and is in good operating condition. However, several Discharge Monitoring Reports (DMR's) were submitted late and the Nitrogen Mitigation plan, which was required within one year of the permit issuance, was incomplete and lacked the detail necessary to comply with the current MPCA permit. Elevated nitrate nitrogen levels in one of the groundwater monitoring wells at the Bliss Collector WWTF warrants investigation of systems designed to consistently remove nitrate nitrogen to less than 10 mg/L. Groundwater Monitoring Results Groundwater testing results for ground water monitoring well GW -004 from April 2013 through October 2015 is shown in Table 1 below. It is evident that nitrate nitrogen levels exceed the MDH guideline of 10 mg/L and appear to be seasonally consistent. Currently, groundwater monitoring well GW -004 is the only monitoring well experiencing elevated nitrate nitrogen levels and this well is downgradient of the infiltration trenches. While some nitrification/denitrification and dilution may occur naturally in the soil, given the high total nitrogen concentration and lack of denitrification occurring in the system, some level of nitrogen treatment will likely be required. Several options for possible treatment methods to remove nitrate nitrogen are discussed below. Table 1 Groundwater Monitoring Data: GW -004 Scandia Bliss Drainfield Date Ammonia Nitrogen (mg/L as N) Total Kjeldahl Nitrogen (TKN) Nitrate Nitrogen Level (mg/L as N) Total Nitrogen** Apr -13 0.1 0.64 17.9 18.5 Jul -13 0.1 0.53 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 1 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 Average 0.1 0.5 20.6 21.2 *Limit for Nitrate -Nitrogen is 10 mg/L. **Total Nitrogen is the sum of TKN plus Nitrite -Nitrogen DESIGNING FOR A BETTER TOMORROW Bolton & Menk is an equal opportunity employer. ()J ON 0�' � Treatment Methods/Alternatives Nitrogen exists naturally in wastewater in two forms, organic nitrogen, and ammonium. Ammonium is created through the bacterial decomposition of organic nitrogen. Typically, nitrogen removal is accomplished by a two-stage process typically referred to as nitrification/denitrification. Nitrification is the conversion of ammonia into nitrate. This process is accomplished by nitrifying bacteria in an aerobic environment. Usually, Biochemical Oxygen Demand (BOD) is reduced in the aerobic environment simultaneously. Full conversion from ammonia to nitrate is dependent on several variables such as the number of nitrifying bacteria and oxygen levels. If the wastewater is placed in an anoxic tank (low dissolved oxygen levels), microorganisms convert the nitrate into inert nitrogen gas through denitrification. The nitrogen gas is dispersed into the atmosphere. Typically, external carbon sources, such as methanol, ethanol, or other proprietary carbon sources are added to assist in promoting denitrification. As noted earlier, nitrate nitrogen levels typically range from 15-30 mg/L in groundwater monitoring well GW -004. Ammonia levels tend to be relatively low when compared to nitrate nitrogen levels, indicating that there may be partial nitrification that occurs naturally in the system. However, the current MDH drinking water standard of 10 mg/L is exceeded for nitrate nitrogen. To help reduce total nitrogen levels, specifically nitrate nitrogen levels, it may prove advantageous to identify and evaluate systems that can promote both nitrification and denitrification. Both nitrification/denitrification and denitrification only systems will be evaluated to determine what options best suit the Bliss Drainfield to meet the MDH drinking water standard. Currently, the Bliss Drainfield sits on a 7.3 acre lot. Residential lots sit to the North, South, and East of the drainfield. Big Marine Lake is approximately 1,200 feet to the East of the drainfield. CSAH 15 (Manning Trail N) runs directly to the west of the drainfield. The main components for the nitrogen mitigation systems are described below in detail: A. Alternative 1: Denitrification Svstem To meet the MDH drinking water standard the system must consistently achieve a nitrate nitrogen level of less than 10 mg/L at either the end of discharge of the treatment facility or within downstream monitoring wells. The system will be designed to treat the design DESIGNING FOR A BETTER TOMORROW Bolton & Menk is an equal opportunity employer. ()J ON 0�' � flow of 19,800 gpd. A denitrification only system assumes the influent BOD and TSS are low and do not require extra treatment and the influent ammonia is nitrified. There are proprietary systems that can accomplish denitrification in fewer processes or simultaneously with other processes (such as simultaneous nitrification and denitrification). These systems use batch reactions and manipulate the microorganisms' environment to achieve nutrient removal to the highest level biologically possible. Generally, there will be four main components for a denitrification only system: • Denitrification reactor dosing tank • Denitrification reactor with chemical addition • Polishing tank • Drainfield dosing tank The typical layout of the tanks allows for the most efficient treatment for denitrification with the dosing tank at the head of the treatment train and the drainfield dosing tank at the end. Wastewater will flow by gravity through the denitrification and polishing tanks but is pumped into the drainfields from the dosing tank. The sizes of the tanks vary depending on the flow and level of treatment required. B. Alternative 2: Nitrification/Denitrification System Another type of system that can be used to treat wastewater high in nitrogen is a nitrification/denitrification system. This system uses the same denitrification process described above, but nitrification treatment is done first. Nitrification is an important step in total nitrogen removal. A typical nitrification/denitrification system includes the following: • Settling tank • Equalization basin • Nitrification reactor • Denitrification reactor dosing tank DESIGNING FOR A BETTER TOMORROW Bolton & Menk is an equal opportunity employer. ()J ON 0�' � • Denitrification reactor with chemical addition • Polishing tank • Drainfield dosing tank The sizes and numbers of tanks and reactors are dependent on the level of treatment necessary and the flow into the system. The systems are laid out with the settling tank at the head of the treatment train and the drainfield dosing tank at the end. The tanks are buried underground with access through hatches for maintenance and observation. It is important to note that a typical treatment system would consist of the above components for high nitrogen removal. There are other proprietary systems that are designed to nitrify and denitrify with various treatment methods. The above descriptions represent a basic layout for nitrogen removal that is efficient and keeps capital costs Cost Below are the estimated costs for the two systems described above. Table 2 provides a cost estimate for the denitrification system and Table 3 provides a cost estimate for the nitrification/denitrification system. Table 2 Cost Estimate for Denitrification System Item Cost Mobilization $20,000 Dosing Tank $15,000 Denitrification Reactor with Carbon Addition $575000 Polishing Tank $35,000 Drainfield Dosing Tank $725000 Chemical Feed Equipment $6,500 Installation $215,000 Land ?? Subtotal $420,500 Engineering & Contingency (25%) $105,000 Total Cost $525,500 DESIGNING FOR A BETTER TOMORROW Bolton & Menk is an equal opportunity employer. ()J ON 0�' � Table 3 Cost Estimate for Nitrification/Denitrification System Item Cost Mobilization $35,000 Influent Metering Manhole $20,000 Settling Tank $35,500 Equalization Tank $47,000 Nitrification/Aerobic Reactor $195,000 Dosing Tank $15,000 Denitrification Reactor with Carbon Addition $57,000 Polishing Tank $35,000 Drainfield Dosing Tank $72,000 Chemical Feed Equipment $6,500 Installation $250,000 Land ?? Subtotal $768,000 Engineering & Contingency 25% $192,000 Total Cost $960,000 Recommendation Based on the test results and nitrate -nitrogen concentrations above the MDH drinking water standard the City will need to install additional treatment at some point to meet the 10 mg/L MDH standard. This will require a significant capital cost as well as increase operation and maintenance costs. In addition, further investigation is needed to determine if improvements can be placed on the existing Bliss property. The City will need to continue to plan for improvements and evaluate options for treatment for the Bliss system. DESIGNING FOR A BETTER TOMORROW Bolton & Menk is an equal opportunity employer.