Rainwater Harvesting on a Superblock Scale

This blog first appeared here and was written by Dave Stark, International Sales & Design Specialist with Rainwater Management Systems.

The Minnesota United FC soccer teams’ first home game is scheduled for April 13, 2019, at the newly constructed Allianz Field. Located in the St. Paul Midway-Snelling neighborhood, the 346,000-sq.-ft. facility will offer seating for 19,400 fans. The area previously housed a city bus facility and redevelopment of the area, referred to as the superblock, encompasses a total of 34.5 acres. Redevelopment of the area surrounding the stadium will provide a mixed-use urban area with retail, office, residential buildings and pedestrian-friendly public green spaces close to public transportation. 

The largest of these green spaces, named the Great Lawn, is directly in front of the main soccer stadium entrance. Under it is a large-scale rainwater harvesting system with a holding capacity of 675,000 gal. The system was designed approximately 19 ft. below ground to account for slope on conveyance lines running from the stadium and surrounding buildings and provide freeze protection in Minnesota harsh winters. 

The project was a collaboration with various partnerships including the city of St. Paul, the Capitol Region Watershed District (CRWD), the St. Paul Port Authority, the design team, consultants, installers, system vendors and others. St. Paul and the CRWD are leaders in innovative stormwater treatment systems and have a program entitled, “Utilizing Rain as a Resource.” The premise of the program is to use rainwater rather than allowing stormwater pollution to enter surrounding lakes or the Mississippi River. 

St. Paul completed previous pilot projects, such as CHS St. Paul Saints Baseball Field, that used rainwater for indoor toilet flushing and turf irrigation. CRWD recently opened a new headquarters building with interactive rainwater harvesting in the main lobby, which will provide educational opportunities to better explain rainwater harvesting systems. 

Those projects helped inform the design and the city is implementing a method of managing stormwater called, “shared, stacked green infrastructure.” These systems attempt to serve multiple objectives, such as stormwater quality improvement and water supply while reducing runoff and the associated downstream erosion. 

Designing at the superblock scale presents major challenges for ownership and long-term maintenance, which the city is addressing through partnerships with their sewer department and hired contractors. 

Plumbing Code, Stormwater Focus

Similar to other water-rich states, Minnesota initially focused on the benefits of rainwater harvesting to provide stormwater control. However, the various portions of the state with poor groundwater quality and declining water tables prompted the use of alternative water sources. The first American Rainwater Catchment System Association (ARCSA) course was held in Minnesota in 2012 in response to increasing requests for using rainwater inside of facilities. 

In 2013, ARCSA/ASPE/ANSI 63-2013: Rainwater Catchment Systems was published and provided additional guidance as changes were underway in the land of 10,000 lakes. The Universal Plumbing Code replaced the Minnesota Plumbing Code, with Minnesota amendments, and took effect Jan. 23, 2016. Chapter 17, Non-potable Rainwater Catchment Systems, addresses the design, operation, and maintenance of rainwater harvesting systems. 

This significant change currently allows rainwater from roof surfaces and other above-ground impermeable surfaces to be captured, pre-treated, stored and distributed with the appropriate water treatment to meet end-use water-quality requirements recommended by the Minnesota Department of Health. The code applies to rainwater harvesting systems that utilize water inside of structures and for combined indoor and outdoor use applications. 

The city and project partners wanted a robust system capable of meeting end-use water-quality requirements of the plumbing code and protecting human health from spray irrigation while preserving the ability to transfer water to the surrounding buildings in the superblock for irrigation or other internal uses. 

Slightly more than 1 in. of precipitation is required to be retained and treated on site to meet current stormwater regulations provided by the Minnesota Pollution Control Agency. Before the redevelopment of the site, there was limited treatment of the stormwater runoff, including that from rooftop surfaces, feeding into the storm sewer. 

A core message in early design was to start stormwater management at the rooftop, integrating civil and mechanical engineering plans and preserving the largest number of options for the end-use of treated rainwater. This allowed rainwater harvesting and re-use to be utilized as the first method for stormwater management in the treatment train. 

Rainwater from the roof that is intercepted and treated serves as a pollution prevention strategy to offset other on-site infrastructure used for stormwater management. Various other stormwater best management practices — additional detention tanks for volume and rate control, and tree trenches that treat surface water — were used to meet the overall stormwater objectives for the site.

System Design from Roof to End Use 

The Allianz Field system was designed to serve the dual purposes of managing stormwater and offsetting the use of domestic water for irrigation. Rainwater is captured from the stadium and surrounding rooftop surfaces and conveyed to the storage tanks. The water will be used for outdoor irrigation in the surrounding open spaces.

The system includes a modulating valve to send water to future buildings in the superblock development. These buildings are yet to be designed but will likely include day tanks and final pressure distribution pumps, depending on the individual site irrigation or other end-use requirements. The city is hoping that having the stormwater infrastructure in place will make the site more attractive to developers. 

Water is pumped from a wet-well manhole adjacent to the storage tanks via duplex 15 horsepower submersible pumps to the below-ground vault that houses the treatment and control system on a pre-assembled skid. The pumps operate lead-lag, alternating on pressure drop, and include variable-frequency drives with pressure transducers. 

Level sensors are located in the primary storage tank and pump vaults along with low-level pump protection float switches. The final engineered design for the stadium called for a UL-listed rainwater system controller to monitor and integrate the various components. 

A below-ground conditioned and seasonally operated vault houses the water treatment system. It was sized to house the treatment, controls, monitoring and single-point power system for the entire catchment system. The vault includes a heater, ventilation and air-quality monitoring equipment, which is located below the frost line. It has hinged doors to allow ease of access for maintenance and equipment changes, and a sump pump to evacuate any groundwater infiltration to the storm sewer to protect electrical equipment. 

The catchment system includes digital flow meters, pressure sensors and water-quality instrumentation before and after the treatment system, including turbidity and pH probes. The Minnesota Plumbing Code requires annual testing at electronically monitored sites for turbidity (<1 NTU) and E-coli (<2.2 cfu/100ml) from a state-certified laboratory; pH, temperature and odor are to be recorded at the time of compliance testing. 

The water treatment equipment in the vault includes duplex back-flushing 50-micron filters, duplex 5-micron filters, carbon filtration, ultraviolet disinfection and an ozone recirculation system. A final duplex 7.5 horsepower booster pump skid provides the final design flow rate of 100 gal./minute at 100 psi. 

After treatment, a series of modulating and standard motorized valves directs treated rainwater to the irrigation system or the future outlot buildings in the superblock, ozone recirculation to the storage tank or to the storm sewer to ready the tank for incoming storms. Municipal backup water is provided if the rainwater level in the tank is low and is protected by reduced-pressure backflow devices. 

Innovative aspects of the system include the use of a centralized controller to produce data available to the building automation system in the soccer stadium and St. Paul’s SCADA system and to receive a signal from a weather forecasting software system for stormwater management. The software uses weather data and an algorithm to determine when to open a modulating valve on the system to draw the tank down to a defined level, based on the predicted intensity and volume of the incoming storm. 

If the existing irrigation uses have not lowered the water to the designated level, a modulating valve is opened and discharges the water after the initial filtration but before disinfection to the storm sewer. 

Lessons Learned 

The project missed opportunities to use treated rainwater inside the stadium. Possible uses included flushing toilets, washing down stadium seats and playing field irrigation, which would help to draw down the cistern for stormwater management. This approach was value-engineered early in the design process and likely had more to do with the fear that chemical treatment might have to be used and could affect the playing field grass. 

Some rainwater harvesting systems achieve disinfection with chlorine injection or aggressively treated water but most are applied with a five-step process including prefiltration, smoothing inlets, overflow siphons, floating intakes and a water treatment system designed to meet the end-use water-quality requirements. 

Indoor and outdoor uses need to be addressed in an integrated design for rainwater harvesting. To do this, realistic water demands, rates and pressures are needed for all end uses. Civil and mechanical designers need to collaborate on the catchment design and irrigation design should be completed early in the design process to have design flow rates and pressures relayed to the engineers. 

Systems designed for integrated year-round use, inside and outside of the facility, often have the best possibility of providing a cost-effective solution and offsetting other water main or stormwater infrastructure costs. Stormwater treatment devices, such as hydrodynamic separators, while being part of a treatment train to meet stormwater water-quality requirements, may not provide the treatment necessary to achieve strict water-quality compliance after treatment. 

If pretreatment devices allow high water bypass flow to enter a tank, designers should consider the resulting water quality impacts in the tank along with the associated tank-cleaning maintenance. The best rainwater harvesting-specific prefiltration devices do not allow high-flow water to bypass the filters and remove particulates greater than 1,200 microns as defined in ARCSA/ANSI Standard 63. This bypass flow is easily addressed with other stormwater infrastructure on site. 

The rainwater system vendor provided the Allianz Field system startup and training for the installing mechanical contractors, owners’ representatives, city and watershed staff before system shutdown in the fall of 2018. The system was winterized and will be re-started in the spring of 2019 when the first game will be played. 

Rainwater harvesting systems — whether implemented for water supply, stormwater control or a combination — are increasing across the nation. Designers, contractors, system vendors, and managers all benefit from the foundation set by ARCSA/ASPE/ANSI Standard 63. 

ARCSA now offers professional certification training thanks to a new partnership with the American Society of Sanitary Engineers (ASSE). Rainwater harvesting training workshops are now prerequisites for the ASSE Series 21000 Rainwater Catchment Systems Personnel Certifications and are combined with training at local UA training facilities and the installation of demonstration rainwater and greywater re-use systems.