Edmonton International Airport. Engineered Wetlands. EIA s traffic grew over the past ten years, and glycol usage volumes increased significantly over

Transcription

1 Edmonton International Airport Engineered Wetlands EIA s traffic grew over the past ten years, and glycol usage volumes increased significantly over the past five years. Since 2000, a constructed wetland system has been successfully used but its capacity was inadequate to handle additional flows of glycol-contaminated runoff associated with airport expansion and increased airplane de-icing operations. The low treatment rate constructed wetlands was upgraded to a relatively high rate engineered wetlands. With only a $2.5 million construction budget, two of the six trains were upgraded in The upgrade was conducted within the existing footprint of the old wetland system and uses one-third of the original system footprint. Existing materials on site were reused thereby reducing sourcing and trucking of new materials to site. The system has been designed and constructed to withstand a cold climate where snow is on the ground for six months of the year and storm water only flows from April to November. Proper aeration tubing specification was necessary to assure the reliable operation and performance of the oxygen transfer system. This system has been utilized in other applications such as submerged attached growth reactors, which is an emerging technology. Biological commissioning occurred in the spring of The new system treated the glycol contaminated storm water pond within a 60 day period while consistently producing effluent that is a reliable treatment system which is simple to operate and readily expandable. The successes of this first Canadian application of Engineered Wetlands technology could assist other airports, both Canadian and International, in adopting this approach for handling glycol-contaminated storm water. The technology could also have application to other types of facilities requiring treatment of high strength, soluble organic contaminants. July 1, 2013 Lisa Dechaine, P.Eng., MBA PM Environmental Specialist Edmonton Airports

2 ENGINEERED WETLANDS Edmonton International Airport 2013 ACI-NA Environmental Achievement Awards

3 ENGINEERED WETLANDS Edmonton International Airport 1.0 INTRODUCTION The Edmonton International Airport (EIA) needed to upgrade the storm water management system as an element of its Expansion 2012 project. EIA s traffic grew over the past ten years, and glycol usage volumes increased significantly over the past five years. Since the Expansion project focused on creating facilities capable of handling 9 million passengers per year and the corporate focus is attracting those passengers, we expect continued growth in aircraft operations. Since 2000, EIA had successfully utilized a constructed wetland system but its capacity was inadequate to handle additional flows of glycol-contaminated runoff associated with airport expansion and increased airplane de-icing operations. To add to the challenge, spring snow events and summer precipitation along with the associated increase in glycol contaminated stormwater could overtax the limited onsite storage capacity if the storm water could not be treated more quickly. 2.0 CURRENT STORM WATER APPROACH EIA s surface runoff for the majority of the operational areas (including main apron, runways, taxiways, deicing area and developed area north of the Terminal) is directed to two storm water ponds. The catchment area for this system is 1000 acres. Glycol-contaminated run-off is directed to a pond referred to as the Gun Club Pond for subsequent treatment by the wetlands, then released to the nearby Whitemud Creek. It has a capacity of cubic meters. Uncontaminated storm water is directed to a separate pond referred to as the Retention Pond for subsequent direct discharge to Whitemud Creek. It has a capacity of approximately cubic meters. Both ponds have wiring systems to deter birds. Figure 1 provides an aerial view of the storage ponds and the original Constructed Wetland system. 1

4 Figure 1: Storage Ponds and Original Constructed Wetland System 3.0 CONSTRUCTED WETLANDS In 2000, the constructed wetlands treatment facility was commissioned by Edmonton Airports under license by Alberta Environment. The original system consisted of six wetland trains. These were horizontal, subsurface flow wetlands, each train comprising two 43 m x 43 m (area) x 600 mm (gravel depth) cells operating in series. An influent lift station pumped contaminated stormwater through pipes and distributed the flow across the head end of each of the first (or primary) cells and continued, by gravity, through each of the second (or secondary) cells. The gravel in each cell provided a surface upon which bacteria would grow and consume glycol and any other organic contaminants as the water slowly traveled horizontally through the gravel beds. The gravel beds supported natural vegetation, mainly cattails, that assisted in the treatment through passive oxygen transfer from the roots of the vegetation into the water-filled gravel. While cutting edge in its day, the system is now considered a rather crude design, as its treatment capacity was limited by natural oxygen transfer rates and nutrient deficiencies. As a consequence, the system typically required four months of operation to fully process the contaminated water from the Gun Club Pond. 2

5 Figure 2: Constructed Wetlands 4.0 NEW SYSTEM DESIGN To meet our growing treatment needs, we commissioned an engineering consulting firm to provide concepts and ultimately a final design for a treatment system that would meet our budget of $2.5 million. While the consulting firm created multiple concepts, they could not provide a final design that satisfied our treatment expectations with the budget set. Edmonton Airports released that firm and approached another bidder on the project, who has worked on aspects of storm water management at EIA in the past. Associated Engineering brought in Naturally Wallace Consulting to provide specialist process engineering services on the wetland design. Their treatment system designs have been successfully implemented at London Heathrow and in Buffalo, NY. The consulting team developed a concept for a higher rate treatment system that would enable the full treatment of 3

6 contaminated runoff in a thirty day period each spring after being shut down during the winter. The new system concept was comprised of two wetland trains each with two cells operating in series, using the base footprint and some of the features of the former constructed wetlands. Significant alterations to the existing trains were enabled greater capacity and improved treatment. 5.0 DESIGN FEATURES These are the following significant system design features and considerations. The concepts are illustrated in Figure 3. Operating Season: The treatment system lays dormant and typically covered with snow from November until April. Glycol only appears in our storm system during spring. The system is planned to be in operation for about sixty days per year, at the most. Then the treatment system lays dormant until the following spring and our storm system drains in a controlled manner to the creek, bypassing the treatment system. Conversion from Constructed to Engineered: Two of the six Constructed Wetlands trains were converted to Engineered Wetlands trains. The new Engineered Wetland trains provide more than double the flow capacity of the Constructed Wetland trains in one-third the space. To minimize construction cost, gravel from other unconverted cells was utilized in the new configuration. Continued utilization of the existing influent lift station: To achieve the design capacity, the existing two fixed speed pumps were replaced with higher capacity, variable speed submersible pumps. Reliable Hydraulic Design: A hydraulic control structure with weirs automatically splits the pumped influent flow between the two operating trains. The control structure incorporates provisions for a third weir should Edmonton Airports wish to increase the system capacity further in the future. 4

7 Figure 3: Engineered Wetlands Primary Cell Flow Distribution: Influent flow is distributed evenly over the surface of the gravel in each primary cell through parallel distribution pipes with flow control orifices. Vertical flow distribution rather than horizontal flow distribution prevents overloading the initial portion of the gravel bed which would otherwise become anaerobic with reduced treatment performance and increased clogging potential. Primary Cell Effluent Flow Collection: The effluent flow is controlled by parallel pipes spaced uniformly over the bottoms of the primary cells and flows to the recirculation pump well and then through a level control manhole to the secondary cells. The level control manhole allows us to adjust the water level in the primary cell. Secondary Cell Flow Control: Flow through the secondary cells flows horizontally through a gravel bed for further polishing and exits at another level control manhole. Enhanced System Controls: Control of the new pumps is managed by setting the desired flow rate measured by a new magnetic flow- meter and a Programmable Logic Controller. Flow rates are based on influent Chemical Oxygen Demand to keep the organic loading on the wetland cells within the design limits. 5

8 Figure 4: Piping Systems Simplified Nutrient Dosing: As the system requires nutrient pre-treatment dosing to ensure an adequate food source for the system s bacteria, batch nutrient addition infrastructure was incorporated into the Gun Club Pond, through a floating pipe distribution system. Aeration System: Two 56 kw positive displacement blowers supply the necessary air to aeration tubing fully covering the bottom of the primary cells and the initial portion (first 10 m of the 43 m) of the secondary cells. The air distribution system on the bottom of each primary cell uniformly distributes air to 25 mm diameter high density polyethylene tubing which incorporates orifices for efficient oxygen transfer. A total of linear meters of the tubing is installed in the two primary cells and an additional 10% is installed at the head of each secondary cell to aid in aerobic polishing of the final effluent before discharge. The tubing is similar to drip irrigation tubing. Recirculation System: Each primary cell incorporates a low head recirculation pump which can recycle primary cell effluent, returning it through a separate pipe/orifice distribution system on the surface of each primary cell. The pumps are used during startup to speed bacterial acclimation during spring at the beginning of each operating year. 6

9 The recirculation pumps also provide the opportunity to re-treat and/or dilute high strength influent flows at the beginning of the year as well. Winter Freeze Protection: The diffused aeration system is capable of withstanding freezethaw cycles, however a small 3.7 kw blower was also include that supplies air through the aeration system during-sub-zero conditions to maintain the tubing free of ice and enable quick spring start-up. Figure 5: Aeration Piping in Primary Cell 7

10 Table 1 summarizes the main design features of the new system compared to the original system. Table 1 Wetland Design Feature Comparison Component Original Constructed Wetland New Engineered Wetland Typical influent BOD mg/l mg/l Nutrient addition None A liquid blend of Nitrogen and Phosphorus is pre-mixed by chemical supplier and batch-applied to the Gun Club Pond through a floating pipe distribution. Influent lift station capacity Approx 2000 m 3 /d >4,000 m 3 /d Design organic loading rate (primary cell) N/A (horizontal flow) 250 g BOD/m 2 /d Treatment time days days Number of wetland trains 6 2 Number of cells per train 2 2 Cell media Gravel Gravel Influent flow distribution Aboveground pipes Hydraulic flow splitter structure with 2 weirs and provisions for an additional weir for future train. Primary cell Approx 1850 m 2 (43 m x 43 m) area, 600 mm gravel depth, horizontal flow Approx 1850 m 2 (43 m x 43 m) area, 900 mm gravel depth, vertical flow (influent distribution pipes on surface of gravel and effluent collection pipes on bottom of gravel) Secondary cells (following primary) Aeration system Treatment cell approx 1850 m 2 (43 m x 43 m) area, 600 mm depth, horizontal flow None (limited oxygen may be transferred through emergent vegetation, e.g. cattails, roots) Polishing cell approx 1850 m 2 (43 m x 43 m) area, 300 mm gravel depth, horizontal/surface flow Two positive displacement air blowers delivering compressed air to aeration tubing uniformly at the bottom of the primary cells and for the initial 10% of the secondary cells. Recirculation system None Recirculates primary cell effluent to enables faster spring start-up 8

11 6.0 REGULATORY LIMITS Since our former system was permitted provincial authority, our new system also had to be permitted accordingly. Alberta Environment set the following discharge limits for the new system (based on the arithmetic mean of weekly samples) for the treated storm water discharged from the Engineered Wetlands: Glycol - <100 ppm Nitrate Nitrogen - <10.0 ppm Ammonia Nitrogen - <1.0 ppm Total Phosphorous - <1.0 ppm The new limits include parameters that corresponds to the supplementary nutrients added, which could results in excess nutrient concentrations in the effluent. The previous effluent discharge parameter was only a <100 mg/l glycol limit. 7.0 CONSTRUCTION TEAM Project construction commenced in July 2011, under the direction of a construction manager, Stewart Olson Dominion Construction Limited. Nelson Environmental Inc. was the construction contractor responsible for installation. Associated Engineering provided field oversight during the installation of key system components, as Naturally Wallace focused on operation details. 9

12 8.0 COMMISSIONING There were two stages to commissioning, hydraulic and biological, which had to occur separately due to the seasons. The initial hydraulic commissioning of the Engineered Wetland System was completed late 2011 water and air flowed through it and could be controlled as expected. Biological commissioning was completed in the spring of The Gun Club pond was treated with nutrients to test the nutrient mixing system and to achieve half of the theoretical nitrogen and phosphorous concentrations required for optimum aerobic biological utilization of the organics from the pond. Higher nutrient doses were considered but a lower does was selected to prevent a possible exceedance of the approved effluent nutrient concentrations. As we gain operating experience with nutrient dosing, the nutrient application will be adjusted in future years to further optimize system performance. Water from the Gun club pond was pumped into the primary cells and the aeration system was turned on for the two wetland trains. After filling, the recirculation pumps at each primary cell were activated. It was expected that bacterial acclimation to the glycol in the primary cells would take a couple of weeks before the effluent quality met limits. Sampling confirmed that a two-week conditioning period was justified before continuous treatment could begin. Subsequently, an influent pump was started at low speed, with continuous flow through the Engineered Wetland cells. The effluent quality was monitored and the influent flow rate was gradually increased while ensuring full compliance with the approval limits. The effluent quality consistently was well below the approval limits during continuous flow operations: Highest glycol concentration in system was 311 ppm in mid-may After conditioning period was over, continuous treatment started June 1: o Flow rates ranged between 1200 to 1500 cubic meters per day or about one third of the possible operating rate o Glycol levels in effluent were non-detectable 10

13 o Inlet BOD levels ranged from 100 to 200 ppm, with outlet levels being below 10 ppm This in part is attributable to our conservative system operation as we gain experience and assurance that the system would reliably meet the approval limits. In future years, the conditioning period is expected to be reduced to as little as one week as the beds will already be seeded with glycol-consuming organisms. 9.0 PROJECT BENEFITS Features and benefits of the Engineered Wetlands project are summarized below: Environmental Benefit This project enables EIA s continued expansion by providing the necessary facilities to properly treat increased quantities of glycol-contaminated storm water. Additionally it helps to maintain and even enhance the aesthetic and environmental value in the downstream Whitemud Creek watershed. The new facilities provide reliable assurance of environmental protection, well in compliance with the approval limits. The upgrade was conducted within the existing footprint of the old wetland system. The system uses less space than the previous system, Future capacity upgrades can still take place in the existing footprint, thereby minimizing or eliminating future requirements of Greenfield space. The upgrade reused existing materials on site, thereby reducing sourcing and trucking of new materials to site. Gravel from the adjacent constructed wetland cells was used to supplement gravel requirements for the new engineered wetland cells. 11

14 Increasing the treatment rate allows the glycol-contaminated storage pond to be drained quicker which creates more storm water storage capacity in the summer without having to construct more ponds. Fewer ponds mean fewer wildlife management issues. Innovation The new Engineered Wetland system for treating glycol-contaminated storm water is the first of its type in Canada. The system has seen limited use at other airports such as New York s Buffalo International Airport and London s Heathrow Airport. The system has been designed and constructed to withstand a cold climate where snow is on the ground for six months of the year and storm water only flows from April to November. The design allowed for draining the feed piping, the hydraulic control structure, and the primary and secondary cells. A small blower is also used to provide continual aeration at minimal power usage to prevent freezing of the aeration tubing in the winter and enable quick spring start-up. Batch nutrient addition to a nutrient-deficient storage pond is a novel approach which assists in simplifying both design and operation, improves operating reliability, and reduces capital and operating costs. Proper aeration tubing specification was necessary to assure the reliable operation and performance of the oxygen transfer system. This system has been utilized in other applications such as submerged attached growth reactors, which is an emerging technology. Effective Implementation The robust design provides Edmonton Airports with a reliable treatment system which is simple to operate and readily expandable. The initial year of operating data demonstrate that the system surpassed its treatment targets. The project was completed on a very tight schedule as limited time was available between decommissioning of the existing system and construction of the new system. All this had to happen in six months. 12

15 Widespread Applicability The successes of this first Canadian application of Engineered Wetlands technology could assist other airports, both Canadian and International, in adopting this approach for handling glycolcontaminated storm water. The technology could also have application to other types of facilities requiring treatment of high strength, soluble organic contaminants. Cost Effectiveness The design maximized the use of the existing infrastructure, helping to achieve the $2.5 million construction budget. The new system provides EIA with a higher capacity and more efficient treatment system within one-third of the original system footprint. The limited capital budget for this project constrained the available options and required ongoing value engineering review throughout the design and construction. This innovative design required innovative thinking as previous experience to draw upon was very limited. 13