115 E. Hickory Street + Suite 300 + Mankato, MN 56001507.387.6651 + www.is-grp.com

ARCHITECTURE + ENGINEERING + ENVIRONMENTAL + PLANNING

Woodchip BioreactorsA Best Management Practice for Agricultural Drainage

Figure 1: Faribault County, Minnesota CD 62 Bioreactor under construction

Meeting the need for improved crop production while also managing water quality and environmental regulations is an important balancing act that can be effectively accomplished through the use of agricultural Best Management Practices (BMPs). One practice that specifically targets removing nitrate from drainage tile are woodchip bioreactors. The work of a

bioreactor takes place underground, adjacent to field edges, often located so land is not taken out of production. Woodchips are placed in an underground trench, and tile water is diverted through the bioreactor to remove nitrate from subsurface drainage water. Water exiting the bioreactor is routed back into the underground tile with a much lower nitrogen concentration. Bioreactors serve as part of the treatment train to improve water quality and can be paired with other BMPs including alternative inlets to reduce the amount of sediment from surface water runoff. Located near the city of Blue Earth, in Faribault County, Minnesota, a 700-acre watershed that drains into the Blue Earth River realized the benefits of woodchip bioreactors and implemented a successful and innovative drainage project for the County.What do bioreactors do?

Remove nitrate from subsurface drainage water Woodchips serve as a substrate for bacteria to break down nitrate through

denitrification Contain microorganisms that consume carbon from the woodchips and ‘inhale’ or

absorb nitrate and ‘exhale’ or release nitrogen gas Provide anaerobic conditions that force microorganisms to ‘eat’ the nitrate versus

oxygen Create a process similar to that of a wastewater treatment digester

CHUCK BRANDEL PESenior Civil Engineer + PrincipalWoodchip bioreactors showcase how drainage improvements can be accomplished—often without taking land out of production—while also improving water quality. According to Chuck, “Bringing landowners together with drainage authorities and agencies is an important element to a successful project, and requires meaningful collaboration.” Landowners are concerned about their environmental impact and ISG works with them to provide drainage conditions that help to accomplish their production goals while also having a positive impact on water quality, habitat, and natural resources. Chuck’s significant expertise in agricultural

Page 2 of 6507.387.6651 + www.is-grp.com

drainage is widely recognized and has made him a frequently requested engineer on many drainage projects throughout the region.

FARIBAULT COUNTY DITCH 62The tile outlet of the system, labeled as CD 62, was in poor condition, damaged, and failing in areas. A petition for improvements was filed with the Drainage Authority. The Faribault County SWCD also applied for and was awarded over $190,000 in Clean Water grant funding to address water quality issues within the watershed. A landowner, who serves on the SWCD Board, had a grassed swale available for a conservation practice. With a need identified, a site located, and funding allocated, ISG designed the bioreactors into the drainage system. The design included three bioreactors in order to maximize the efficiency of nitrate reduction. Bioreactors are designed to hold water for a limited time. Once it is exceeded, the water starts decomposing and can create methylmercury. The design incorporated three bioreactors to meet capacity requirements while also meeting the time limitations for water storage. The system is designed to carry most of the base flow in the first bioreactor. After a rain event when more water is flowing, the water will fill up the first bioreactor and then move to the second bioreactor until it is full. Only when the first two reactors are full, does water enter the third bioreactor.

3 woodchip bioreactors are aligned parallel with 24” county tile Each bioreactor is 135’ long by 25’ wide by 5’ deep. Bioreactors aligned in parallel so the water from the CD 62 tile can feed all three

once capacity is met. This makes the bioreactors the most effective as more peak flow from the system can be treated

Utilizes water elevation control structures to divert water from 24” to 12” bioreactor tile line and into each bioreactor

Stormwater quality unit installed to remove sediment and debris prior to entering bioreactors

Page 3 of 6507.387.6651 + www.is-grp.com

Figure 2: Minnesota map, Faribault County Figure 3: Faribault County, Minnesota CD 62 Bioreactor following construction

Water Inlet + Outlet StructuresThe inlet is a water diversion structure that diverts water from the 24-inch CD 62 tile into the bioreactor tile line. This structure allows for large flows to bypass the bioreactor tile back into the 24” tile, protecting the drainage outlet of the CD 62 system. The bioreactor tile connects back into the 24” tile which outlets to an open channel tributary of the Blue Earth River.

Figure 4: Faribault County Minnesota CD 62 Stormwater Quality Unity

Page 4 of 6507.387.6651 + www.is-grp.com

Figure 5: Faribault County Minnesota CD 62 Bioreactor

Treatment NumbersEach of the three bioreactors contains a different make up of woodchip media. Monitoring will provide results on the most effective woodchip combination.Bioreactor #1Woodchip Properties

All MN hardwood woodchips Debarked and chipped Average diameter 2.5 inches Diameter range 1-4 inches

Nitrate Reductions Estimates Nitrate concentration reduction 55-70% Bed flow rate range 0.25-0.30 cfs Hydraulic retention time 6-7 hours Portion of peak flow treated 8-9% Annual nitrate removal 900-1100 pounds

Bioreactor #2Woodchip Properties

Mixture of MN hardwood and county ditch tree cleaning woodchips Mixture of debarked and chipped with fine particles/sawdust Average diameter 1.75 inches Diameter range 0.25-4 inches

Nitrate Reductions Estimates Nitrate concentration reduction 60-80% Bed flow rate range 0.20-0.25 cfs Hydraulic retention time 7-8 hours Portion of peak flow treated 6-7.5%

Page 5 of 6507.387.6651 + www.is-grp.com

Annual nitrate removal 600-850 poundsBioreactor #3Woodchip Properties

County ditch tree cleaning woodchips only Contains majority of fine particles/sawdust Average diameter 0.5 inches Diameter range 0.25- 2 inches

Nitrate Reductions Estimates Nitrate concentration reduction 70-95% Bed flow rate range 0.1-0.15 cfs Hydraulic retention time 8-9 hours Portion of peak flow treated 4-5% Annual nitrate removal 300-500 pounds

Figure 6: Faribault County Minnesota CD 62 Bioreactors

ANTICIPATED OUTCOMES 80-150 acre feet volume treated annually

1,875 cubic yards of underground treatment

72% anticipated annual nitrate concentration reduction

AGRICULTURAL DRAINAGE EXPERTISEWith over 40 years of firm experience that extends to clients nationwide, ISG’s agricultural drainage and water quality expertise is backed by a group of responsive and resourceful civil engineers, water resource specialists, environmental resource and compliance leaders, and construction administration professionals. These experts work as a water quality specialty team dedicated to providing sophisticated strategies and conservation drainage solutions.

WOODCHIP BIOREACTOR COST SUMMARYITEM ENGINEERS

ESTIMATEAVERAGE BID

COST AWARDED BID

COST Piping and control $ 54,530.00 $ 51,486.28 $ 53,050.00

Page 6 of 6507.387.6651 + www.is-grp.com

structures Woodchip bioreactor components

$ 72,700.00 $ 91,070.06 $ 56,675.00

Stormwater quality unit

$ 20,000.00 $ 19,177.44 $ 17,785.00

Monitoring piping $ 1,500.00 $ 6,066.67 $ 3,150.00 Erosion control + seeding

$ 12,450.00 $ 15,519.44 $ 10,650.00

Survey, engineering, construction administration

$ 35,500.00 $ 35,500.00 $ 35,500.00

TOTAL $ 196,680.00 $ 218,819.89 $ 176,810.00