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Copy and Paste This Permanent Link:http://www.stormh2o.com/november-december-2008/unc-sustainable-projects.aspx

Additional Article Content

• Table One
• The Bell Tower Water Cistern

An aerial view of the UNC campus

The University of North Carolina (UNC) at Chapel Hill, the oldest public university in the nation, is faced with ever-increasing environmental regulatory requirements in a world where readily available energy and clean water are in decline and financial resources are finite. UNC has responded to these challenges by viewing them as opportunities to create innovative solutions to a complex array of issues using a holistic, sustainable approach. In particular, water in all of its forms—potable water, wastewater, stormwater, and reclaimed water (highly treated wastewater effluent)—is viewed as a single resource.

This innovative approach to water use has been developed and refined since the beginning of a $2 billion capital improvement program at the university in 2000. The program is adding 7.5 million square feet to the existing 13.5-million-square-foot, 740-acre central campus in a 10-year period.

Included among the improvements is the $231.5 million Bell Tower project, a mixed-use development that features one of the largest building projects ever completed on campus. Bell Tower also includes several new innovations in water use, which are being incorporated into the design as part of a full-scale pilot demonstration project for testing and optimization. The resulting sustainable design concepts will be standardized and implemented as part of the development of UNC’s new satellite campus, Carolina North. About 250 acres of development are anticipated at this new campus in the next 50 years.

Background
The university’s land holdings are located predominately within the corporate boundaries of the town of Chapel Hill, including Carolina North. UNC owns, maintains, and operates many of its own utilities:

• Stormwater. The university holds its own National Pollutant Discharge Elimination System (NPDES) Phase II municipal separate storm sewer system (MS4) stormwater permit, and operates as its own stormwater utility.
• Water and Wastewater. UNC is the largest water and sewer customer of the Orange Water and Sewer Authority (OWASA), representing more than 30% of the utility’s total demand. In 2004, UNC and OWASA jointly initiated design and construction of a reclaimed water distribution system on the campus to allow the university to reuse an average of 1 million gallons per day (gpd) of highly treated wastewater effluent for cooling tower makeup in support of the university’s 50,000 tons of central chilled water production facilities.
• Energy. UNC is a national leader in the utilization of district energy and combined heat and power (CHP) systems, which are a highly efficient, cost-effective, and environmentally responsible means of providing energy. The university’s three energy systems—chilled water, cogeneration, and electric distribution—are closely interrelated and operate together for optimal efficiency.

Innovation and Sustainability
The Bell Tower project is located in the heart of the UNC’s main campus. It includes a 710-car parking deck, a 25,000-ton chilled water plant, and a new Genome Science Laboratory Building (GSB). This building will provide approximately 210,000 square feet of modern classrooms, laboratories, and offices, including nine wet labs, four bioinformatics labs, and a 250-seat lecture hall.

The site integrates a number of features, including elevated pedestrian walkways to link adjacent buildings, new roadways, natural areas, and a fully integrated central park area. The park is underlain by a belowgrade, 360,000-gallon concrete stormwater detention structure and 350,000-gallon stone-filled cistern for storage and reuse of harvested roof water. The project also involves the redevelopment of an existing bituminous surface parking lot.

Key challenges confronting UNC as they initiated the Bell Tower project included:

• Stormwater management
• Limited potable water supply due to increasingly frequent and severe droughts
• Incorporation of sustainable environmental design principles
• Planning for long-term, sustainable asset management

Stormwater Management Issues
The university’s stormwater management philosophy and master plan are driven by its commitment to sustainable environmental management, town of Chapel Hill zoning requirements, state of North Carolina Jordan Lake Water Supply Nutrient Strategy Rules, and the university’s NPDES Phase II permit.

Key stormwater issues to be resolved prior to the construction of the Bell Tower project included the following:

Chapel Hill Zoning Requirements.
The university and the town  have recently negotiated new zoning regulations governing the central campus as a part of the permitting process for campus expansion. These regulations consist of an ambitious set of post-construction stormwater management criteria for maintaining or improving water quality, peak discharge attenuation, and total volume reduction. As a result, the Bell Tower project was designed to accomplish all of the following.

1. Stormwater best management practices (BMPs) were designed to achieve an average annual 85% total suspended solids (TSS) removal applied to the volume of post-development runoff resulting from the first 1 inch of precipitation.
2. The stormwater runoff rate leaving the post-developed site was designed so that it will not exceed the stormwater runoff rate leaving the predevelopment site for the local one-year, (3.00-inch), two-year (3.60-inch), 25-year (6.41-inch), and 50-year (7.21-inch) 24-hour storm events. BMPs were incorporated to draw down the captured runoff volume over two to five days or control one-year peak discharge (3.00-inch rainfall) to predevelopment levels.
3. The stormwater runoff volume leaving the post-developed Bell Tower site will not exceed the stormwater runoff volume leaving the predeveloped site (i.e., existing conditions) for the local two-year-frequency, 24-hour-duration storm event (3.60 inches).

UNC’s campus-wide strategy for meeting these requirements includes requiring each new facility to mitigate its own stormwater impacts, as well as installing complex regional BMPs on a watershed basis utilizing a treatment train approach.

Placement and construction of the four 10- by 10- by 120-foot precast concrete stormwater detention structures

Draft Jordan Water Supply Nutrient Strategy Rules.
The North Carolina Department of Environment and Natural Resources (NCDENR) recently issued total maximum daily load (TMDL) requirements for the Jordan Lake watershed. The TMDL requirements, which were primarily nitrogen driven, established aggressive nutrient reduction targets that require an innovative integrated stormwater management strategy for the Bell Tower project.

With the issuance of the NPDES Phase II permit, and the construction of numerous structural BMPs throughout the campus, UNC completed a comprehensive storm system infrastructure inventory, including field inspection and an enhanced GIS mapping system. In addition, permanent funding was allocated for such critical activities as stream monitoring and sampling, outfall inspections, and maintenance of the storm drain system. Maintenance schedules were also developed for all critical maintenance activities.

Bell Tower Existing Stormwater Conditions
The Bell Tower project is located immediately upstream of the confluence of two small streams that form the headwaters of Meeting of the Waters Creek. These two streams are predominantly situated in a closed piping system that joins together in a very large junction box located immediately upstream of Kenan Stadium, the university’s football facility. The small watersheds are 24 and 62 acres in size and are characteristic of the UNC sub-watersheds, with steep slopes and impervious cover that exceeds 75%. More importantly, the watersheds are extremely flashy, with time of concentrations being less than 10 minutes. Such watersheds are extremely sensitive to the intense, short-duration thunderstorms that typically occur in the humid southeastern United States in the summer and early fall.

The Bell Tower project site is located within the 24-acre sub-watershed. Existing buildings occupy much of the perimeter of the watershed, and a large 176,000-square-foot asphalt parking lot is situated in the center of the watershed. The rooftops of these buildings drain primarily to a centrally located small-diameter-pipe network beneath the asphalt parking lot. Although there are observable inefficiencies in the existing pipe network and inlet system, the current site conditions provide for a relatively rapid runoff condition that transfers peak flows as well as “first flush” pollutants downstream very quickly.

The GSB and the parking deck are two of the major building components of the Bell Tower development. The location of both of these facilities within the watershed required the relocation of most of the existing utilities, including stormwater infrastructure.

The parking deck creates a significant opportunity to reduce impervious area. The deck and GSB, as well as Kenan Stadium, will be connected by a new central park, a large green space that evolved from the removal of the existing parking lot and the desire to soften the campus landscapes. The central park rests on top of the stormwater management facilities that provide for both volume and peak flow reduction, water reuse, and, ultimately, significant water-quality improvements to the Meeting of the Waters Creek.

Stormwater Management System
The stormwater management facility is composed of five 120-foot-long, 10- by 10-foot precast box culverts that provide for stormwater detention. All new and existing stormwater surface water pipe and culvert systems are connected to the box culverts. The surface water that falls on adjacent building rooftops is segregated from the water that falls on the ground surfaces by a separate piping system.

The roof piping system is located in the same trench and on top of the surface drainage piping system. The roof water discharges into a stone bed that encapsulates the precast box culverts. The two sources of stormwater remain segregated within the stormwater management facility by a 50-mil PVC membrane. During a storm event, the larger surface water flow peaks (a source of downstream flooding) will be attenuated, while the cleaner roof water will be captured and reused as a water supply source for irrigation and toilet flushing. The entire stormwater management facility has the capacity to treat approximately 710,000 gallons of stormwater.

Peak Flow Attenuation
The design team participated in the original master planning efforts and, from that effort, learned of the significant bottleneck in the Meeting of the Waters Watershed in and around Kenan Stadium. The stadium was built in 1927 in a rather steep valley on top of the Meeting of the Waters Creek and still has some of the original storm drainage piping immediately beneath the field. The original stadium seated approximately 2,500. Today, the facility is one of the largest and most prominent NCAA Division I football complexes in the country and seats more than 60,000.

Cistern following placement of the geomembrane liner anchored to concrete structures

Of particular significance is the need to protect the unique turf and sub-drainage system that was installed in recent years to improve field drainage. Thus, any attenuation of peak flows by an upstream project was seen as a benefit to the downstream flooding concerns. The stormwater master plan efforts concluded that 55,000 cubic feet of storage should be incorporated into the Bell Tower project. A concrete underground detention system was originally designed that could be situated within the confining construction limits and beneath the central park. Within the precast box culvert system, peak flows from surface water are attenuated and temporarily stored. Using the US Army Corps of Engineers’ HMS model, peak flow reductions for the 1-, 2-, 10-, 25-, 50-, and 100-year design storms were determined in cubic feet per second (cfs), as shown in Table 1.

Stormwater Design Challenges
One of the biggest challenges the design team encountered was a need to consider the flooding concerns downstream, while also providing for the desired water-quality improvements. Local drought conditions during the design phase resulted in water restrictions on domestic water supply and, more importantly, a keen awareness by the general public regarding the use of potable water for irrigation of athletic facilities.

The proximity of the facility to Kenan Stadium made the concept of a cistern an attractive option. Furthermore, the design team had previous experience in other areas of the campus where a 500,000-gallon cistern at the Hooker Field Complex was used to irrigate other athletic fields. UNC Athletics had expressed satisfaction and appreciation for the availability of water from this cistern, particularly in light of the drought and water restrictions. However, a conventional cistern alone simply would not provide the assurances for flood mitigation downstream if sequential rainfall events occurred.

The design team began to explore constructability options, and the concept of a dual system evolved from comparing trucking costs for hauling soil with costs for hauling stone. Simply put, the project construction limits and excavation requirements for the detention facility required that all the soil be hauled offsite. To maximize the detention capacity and minimize the facility footprint, the culvert was to be placed as deep in the ground as possible. This resulted in approximately 10,000 cubic yards of material to be excavated and hauled offsite. From there, it was determined that approximately 350,000 gallons of storage would be available within the void space of No. 4 stone, using a 40% void ratio.

Rainwater Harvest Model
The long-term performance of the underground stormwater storage facility for the UNC Bell Tower project was evaluated by modeling a daily water balance for the facility. System inputs were estimated based on historic daily rainfall data, MEP water estimates, and an anticipated source of makeup water that would either be domestic water or reclaimed water. Water usage was evaluated based on predicted irrigation requirements for the Bell Tower project and recorded historical water usage for the Kenan Stadium Facility between 2003 and 2007.

A benefit cost analysis was prepared that considered the difference in the cost of No. 4 clean washed stone and an impervious liner installation versus the cost of properly compacted select backfill. The additional cost for stone and liner was estimated to be approximately $250,000, and would provide for a system volume of approximately 333,000 gallons.

Utilizing the Rainwater Harvest Model (NCSU BAE), a series of alternatives was evaluated to find the optimum size cistern relative to anticipated annual rainfall and available makeup water. Original modeling included the collection of condensate from the GSB and a makeup water source of 50 gallons per minute (gpm). Utilizing a water cost of $0.0044589/gallon, a seven-year payback was predicted by the Rainwater Harvest Model. Reducing the makeup water source to 10 gpm extended the predicted payback period to 11 years.

Volume Reduction
Volume reduction at the Bell Tower project is accomplished by capturing the rainwater from the available rooftops that serve the cistern. This includes the existing buildings as well as the new GSB. On an average annual basis, approximately 46 inches of rainfall are expected on these rooftops. Considering that 90% of the rain events are less than 1 inch, it is anticipated that the cistern will capture and reuse nearly one-half of the annual rainfall, or approximately 1.2 million gallons of water that fall on the rooftops connected to the cistern.

Interestingly, this significantly exceeded the required volume reduction for the Bell Tower project, because the required reduction under the current stormwater regulation determines the volume change based on a change in land cover utilizing the NRCS Curve Number Procedures. By converting the existing asphalt parking lot to green space (or lawn), there is no net change in the predicted volume of runoff.

Drought Spurs Reclaimed Water and Non-Potable Water Systems
Potable water for use by both the town and the university is supplied by OWASA. OWASA’s raw water is taken from two sources: the Cane Creek Reservoir, a 3-billion-gallon lake, and University Lake, a 450-million-gallon lake. A 200-million-gallon quarry reservoir west of Carrboro, NC, is also available as a backup water source. Both the Cane Creek and University Lake watersheds are located primarily in Orange County and have a combined area of approximately 64 square miles.

In 2002, the entire region, like much of North Carolina, experienced the most severe drought in nearly 100 years. Because UNC is the largest single customer of the water system, representing more than 30% of the utility’s total demand, the university immediately implemented aggressive potable water system conservation initiatives throughout the campus. These included a variety of measures, ranging from installing waterless urinals and dual-flush toilets to connecting research building distillation facilities to the chilled water system. In the end, however, using solely these measures and voluntary conservation to reduce demand by more than 15% on the university campus is extremely difficult.

The university’s five centralized chilled water plants and UNC Hospital’s two chilled water plants remained the largest users of potable water, with peak demands exceeding 1 million gallons per day. These peaks tend to occur in months when OWASA’s overall peak demand and susceptibility to drought conditions are at their worst.

In 2004, to effect a long-term, sustainable solution for reducing potable water demands from the expanding UNC campus, the university partnered with OWASA to design and build a completely new reclaimed water storage, transmission, and distribution system to convey highly treated wastewater effluent to the campus. The system will significantly reduce the potable water demand on campus and will provide the university with a reliable alternative source of non-potable water to meet the cooling needs of its medical, research, and high-performance computing facilities. The system also provides community-wide benefits by reducing drought risk to the entire system and extending the sufficiency of the local water supply, thereby deferring or eliminating the need to develop new water supply sources and treatment facilities.

Cistern following installation of stone, but prior to the placement of the upper membrane

The $14 million reclaimed water system was to be built in phases. The initial phase was expected to immediately reduce UNC’s potable water demand by 327,000 gpd. It will serve three chilled water facilities and is scheduled for completion by spring 2009.

However, in 2007, the drought worsened and OWASA went into stage 3 water restrictions, which prohibited the use of potable water for irrigation. This created significant problems for UNC Athletics and its various multimillion-dollar athletic fields. In light of these issues, UNC asked the design team to evaluate alternative solutions and the feasibility of serving the facilities with reclaimed water. While feasible, the reclaimed water distribution system was designed for relatively low pressures, because the water was originally to be used primarily for cooling tower makeup.

At about this same time, the design team was also proceeding with the stormwater detention system design for the Bell Tower project. Decisions had already been made to incorporate a roof water-harvesting cistern surrounding the proposed stormwater detention structure design. The cistern would store sufficient rainwater to reliably serve the irrigation needs of the adjacent Kenan Stadium.

After several joint brainstorming sessions with UNC, the concept of a non-potable water (NPW) system was developed. This concept takes advantage of the ability to harvest clean roof water from new and existing buildings and use it as the primary source of water. However, because roof water harvesting can be periodically unreliable during extended droughts, the NPW concept also incorporates an automatic reclaimed water makeup system to provide a reliable secondary source of water.

Sustainable Environmental Design Principles
The concept of using a combination of reclaimed water and NPW has further evolved to incorporate an independent, high-pressure NPW distribution system that will be extended to the GSB, the central park, and Kenan Stadium for a variety of additional uses. These include serving the existing high-pressure/high-rate irrigation system at Kenan Stadium and the landscape irrigation system and stream feature proposed for the central park, as well as for toilet flush water for the more than 1,000 new toilets and urinals planned for the new GSB and Kenan Stadium complexes.

UNC recently decided to proceed with phase two of the reclaimed water system, which will extend service to both the North Chiller (part of the Bell Tower project) and the Cobb Chiller plants. UNC also plans to include construction of the Bell Tower NPW system as well as one additional, independent NPW water system at the Hooker Field cistern to serve all athletic fields on the main campus.

Upon completion, the reclaimed water/NPW systems will immediately reduce UNC’s demand for OWASA potable water by an average of 1 million gallons per day, or about 10% of the average daily demand of the entire OWASA system. This volume reduction is expected to increase to 1.5 million gallons per day by the year 2028.

Benefits of Combining Roof-Harvested Water and Reclaimed Water
A number of benefits exist for using a variety of water sources to provide high-quality non-potable water:

• Drought Proofing. One of the most significant benefits of combining the roof water-harvesting system with reclaimed water is to provide a high-quality, reliable source of non-potable water that is completely “drought proof,” even if drought periods extend to multiple months or years.
• Sustainable Asset Management. The university currently operates 19 groundwater/roof water/stormwater storage cisterns on campus. Although various individual departments currently compete for the “free” water generated from these systems, operation and maintenance responsibilities have never been clearly defined. In addition, the university has never had a sustainable source of revenue to fund regular, ongoing stormwater/cistern system maintenance and repair activities. As such, the university has recently been forced to make more costly emergency repairs for key stormwater infrastructure components as a result of sudden failures.

The agreement between UNC and OWASA for construction of the reclaimed water system provided that UNC would fund the total cost of the system, including the improvements at OWASA’s wastewater treatment plant, and would purchase the reclaimed water at a price that reflects OWASA’s actual cost of service. This capital funding arrangement was key to making the system affordable because UNC’s costs of debt are lower than OWASA’s. The university’s reclaimed water rate to its customers, including both OWASA’s charges and the debt service on the system, will be below the potable water rate. For the chiller plants, the primary use, after adding in the increased cost of chemical treatment and the costs of using potable and reclaimed water, the costs are comparable. For uses that do not require additional chemical treatment, the cost of reclaimed water is less. Over time, OWASA’s potable water rate is anticipated to increase faster than its reclaimed rate to UNC, and the university’s savings will continue to grow.

UNC intends to incorporate all non-potable water systems (including reclaimed water, roof water, groundwater, and stormwater sources) into one non-potable water utility, and each customer will be billed at the established NPW rate (regardless of water source). The combined revenue stream will ensure that UNC has sufficient funds for ongoing system maintenance and operation activities, which will reduce the incidence of emergency repairs in the future.

Cistern following instalation of the top membrane to prevent contamination

Special Considerations for Using NPW as Toilet Flush Water
NCDENR regulates the use of reclaimed water for beneficial reuse in connection with a variety of non-potable applications, such as landscape irrigation, industrial cooling water, boiler makeup, odor scrubbers, street cleaning, and car washing. Although NCDENR regulations allow reclaimed water to be used as a source for urinal and toilet flush water inside public buildings, the agency does not review or issue permits for reclaimed water or non-potable water plumbing systems serving public buildings. NCDENR does, however, require that the building owner post signs at prominent locations at the point of use indicating “Reclaimed Water—Do Not Drink.”

The International Building Code governs non-potable “gray water” plumbing systems serving public buildings, and code compliance is the responsibility of the Department of Insurance (DOI). However, the current edition of the International Building Code does not adequately address requirements for non-potable or reclaimed water-plumbing systems. DOI expects that such systems will be better addressed in the next edition of the International Building Code.

Harvested roof water, unlike reclaimed wastewater, has not been subjected to a disinfection process and is therefore susceptible to contamination by pathogenic organisms. In keeping with the recommendations of Dr. Mark Sobsey of UNC’s School of Public Health, UNC will require that a 5-micron cartridge filtration system and an ultraviolet disinfection process be installed on the NPW service line inside each building prior to the first point of use.

Corrosivity is another factor to consider when using non-potable water for interior plumbing systems. By its very nature, roof water is naturally corrosive due to low pH, low dissolved salts, and high levels of dissolved oxygen. Therefore, copper piping, as well as fixtures and valves with brass/ bronze wetted parts, is much more susceptible to corrosion than CPVC or stainless steel pipe and components. UNC is currently in the process of drafting standard specifications for non-potable water plumbing systems that will serve any new UNC building.