Sewer, pressurization, differential pressure monitoring, fully dynamic hydraulic modeling, air displacement modeling.

Transcription

1 Using Dynamic Hydraulic Modeling to Understand Sewer Headspace Dynamics A Case Study of Metro Vancouver s Highbury Interceptor Yuko Suda, P.Eng. Kerr Wood Leidal Associates Ltd A Still Creek Drive Burnaby, BC, V5C 6G9, Canada (604) ABSTRACT Metro Vancouver s Highbury Interceptor (HI) is a 6.1 km long 2,900 mm diameter combined sewer with significant odor and headspace pressurization issues identified along its length. During winter storms large amounts of air have been observed expelled from manholes and vents, resulting in howling noise. These events are significant enough that manhole covers have been lifted off and residents have reported observing heaving of the asphalt pavement around the interceptor manholes. Pressure monitoring found that two distinctly different mechanisms are influencing the air pressure within the sewer head space. A fully dynamic computer based hydraulic model in XP-SWMM revealed that the unique characteristics of the Highbury Interceptor profile resulted in the headspace in the sewer becoming completely isolated from upstream, downstream and tributary sewers under certain flow conditions. The results of the hydraulic model correlated well with the monitoring data, revealing that the extreme pressurization events occurred immediately following isolation of the headspace. An air displacement model was created, based on the hydraulic model, to develop the design parameters for an air extraction and odor control facility. KEYWORDS Sewer, pressurization, differential pressure monitoring, fully dynamic hydraulic modeling, air displacement modeling. INTRODUCTION The Highbury Interceptor (HI) is one of the principle trunk sewers in Metro Vancouver s (MV s) Vancouver Sewerage Area (VSA). It services the majority of the City of Vancouver and a portion of the City of Burnaby. The VSA is currently a combined sewerage network. In recent years the number of complaints related to significant odor and headspace pressurization issues along the length of the HI have increased. Large volumes of air have been observed expelled from manholes and vents during winter storms. These events are significant enough to result in loud howling sounds, manhole covers being lifted off, and residents having reported seeing heaving of the asphalt pavement around the interceptor manholes. SYSTEM DESCRIPTION The HI is 6.1 km long, starts at 1st Avenue in Vancouver, and travels south along Highbury Street. Figure 1 shows an aerial schematic of the HI. Three major interceptors enter the HI at the upstream end of the system; the English Bay Interceptor (EBI), the 8th Avenue Interceptor (8AI),

2 and the Spanish Bank Interceptor. The EBI is a 2,400 mm diameter pipe that runs along 1st Avenue. The 8AI is a 2,600 mm diameter pipe that enters the HI system at 8th Avenue and Highbury Street. Together EBI and 8AI service the majority of the north side of the City Vancouver and a portion of the City of Burnaby. The Spanish Banks Interceptor is a 1,200 mm pipe that services parts of the University of British Columbia Campus and the West Point Grey residential area. In addition, at the upstream end of the HI are two overflow siphons; the Alma- Discovery Street Overflow Siphons. From 4th Avenue, to approximately Marine Drive the HI is a tunnel, which consists of a combination of 2,950 mm dia. circular tunnel sections and 2,900 mm dia. Boston Horseshoe shaped (BHS) tunnel sections. The deepest portion of the tunnel is approximately 100 m below ground level. There are only two 300 mm diameter air vents (at 18th Avenue and 33rd Avenue) along the tunnel portion of the sewer. At Marine Drive, the HI flows southwest through the Musqueam Park and the Musqueam Indian Reserve. Inside the Musqueam Indian Reserve the HI crosses Musqueam Creek. At this point the HI becomes a partial siphon for approximately 18 m. On either side of the creek crossing are 450 mm diameter vents to atmosphere. The HI continues through the Musqueam Indian Reserve to the North Arm of the Fraser River, at which point it enters the Fraser River Siphon Chamber, which has three 300 mm diameter vents to atmosphere. The HI subsequently turns into a triple barrel siphon, crosses under the Fraser River, and enters the Iona Island Waste Water Treatment Plant (IIWWTP). The HI is a combined sewer system, and thus conveys both sanitary flows and storm flows. Therefore, the flows and air dynamics in the interceptor are affected by daily sanitary diurnal flow patterns and particularly by storm events. MONITORING In order to determine the headspace dynamics within the sewer a differential pressure monitoring program was carried out. The program consisted of two monitoring periods; first from June 25, 2010 to July 27, 2010 (summer program), and the second from September 29, 2010 to October 28, 2010 (fall program). The differential pressure monitors record the difference in pressure between the sewer interior and exterior atmospheric pressure. The differential pressure of a sewer reflects the headspace dynamics of the system with positive pressure corresponding to the release of air and odours to the atmosphere and negative pressure corresponding to air drawing in. The pressure monitor is capable of detecting differential pressures between + 50 mm and 50mm water column (W.C.), with a resolution of mm W.C. Monitors were placed at the following locations: 4 th Avenue Manhole; 33 rd Avenue Vent; Marine Drive Manhole; Musqueam Creek Crossing North Side Vent; Musqueam Creek Crossing South Side Vent; and Fraser River Siphon Chamber Vent.

3 Figure 1 - Layout of the Highbury Interceptor

4 Differential pressure monitoring along the interceptor revealed that the differential pressure in the sewer typically ranges from -2.5 to 5.0 mm of W.C; however, during some storm events pressure in the sewer increases rapidly, exceeding the differential pressure monitor s range of 50 mm of W.C. Pressures of this magnitude are considered significant and are rarely seen in sewer systems. The data revealed that the pressurization occurs abruptly, indicating a rapid change in displacement in the sewer. Conventional collection system air transport models did not explain this abrupt pressurization (KWL, 2011). Figure 2 shows the differential pressure monitor data for a storm that occurred from October 23-25, 2010, overlaid with the hourly rainfall data Differential Pressure (mm H2O) Hourly Rainfall (mm/hour) :00 A B C D 12:00 18:00 06:00 12:00 18:00 06:00 12:00 18:00 4th Avenue (0+348) 33rd Avenue (3+208) Musqueam Creek North (5+293) Musqueam Creek South (5+311) Fraser River (5+779) Rain Data Oct 26 Figure 2 - Winter Monitoring Differential Pressure Data The following observations are made for each of the time intervals labeled on Figure 2: Period A: This is during the dry weather period, before any rainfall. The graph shows that the 4 th Avenue monitor has a distinctly different pattern than any of the other monitors, indicating that its head space is influenced by a different system than the rest of the HI. This makes sense as the 8AI and the EBI are both upstream of the monitor and are influenced by their ventilation dynamics, rather than that of the HI. The four remaining monitors appear to have a similar pattern during normal dry weather flows. Period B: This period occurs during the first portion of the storm event. Up to this point the four monitors, mentioned above, have a similar pattern, however the pressures at 33 rd Avenue and Musqueam Creek north abruptly dip to below -50 mm of W.C, and the

5 pressures at Musqeuam Creek south and Fraser River increase. The pressures of the latter two monitors do not increase beyond 3.5 mm of W.C. likely due to the monitor location and vent configuration. Period C: As the initial storm subsides, the four above mentioned monitors converge again, and start showing a similar pattern. Period D: During the final storm, the pressures at 33 rd Avenue and Musqueam Creek north again drop, though not immediately, and the pressures at Musqeuam Creek south and Fraser River increase, again staying around 3.5 mm of W.C. Part way through the final storm the Fraser River monitor fluctuates significantly. This is due to the surcharging of the sewer at the monitoring location, rendering the data unusable. Period A reflects the type of pressure range and pattern that is expected due to air transport in a conventional collection system, however the abrupt pressurization during Periods B and D are not so easily explained. MODELLING In order to determine the cause of the rapid pressurization events a fully dynamic hydraulic model in XP-SWMM was developed. XP-SWMM was selected over other models because it is a dynamic, non-steady state model which solves the full St. Venant Dynamic equation, thus providing a more accurate hydraulic grade line (HGL) than a steady state model. The data was extracted and used to model the remaining volume above the wastewater (i.e. the sewer head space storage within the HI). This was critical to identifying and quantifying the air displacement in the sewer. Flow monitoring data from the same period as the differential pressure monitoring period was used to load, calibrate, and verify the model. The model revealed that the unique profile of the HI resulted in the headspace in the sewer becoming completely isolated from upstream, downstream and tributary sewers under certain flow conditions. This occurs during high sewage flow events, where surcharging cuts off the headspace at the upstream end of the system and the siphon at the Musquem Reserve near the downstream end prevents any entrapped air from escaping. Thus, as the sewage level increases with the storm event, a large amount of the trapped air can only be displaced through the few relatively small vents located along the sewer, resulting in high pressures and very high velocities through the vents. The results of the hydraulic model correlated well with the monitoring data, revealing that the extreme pressurization events occurred at the same time as the headspace being isolated. SEQUENCE OF EVENTS During dry weather flows air is forced into the HI by the EBI and the 8AI, as the two upstream pipes have a larger airflow capacity than the HI. The maximum air capacity of the HI is less than the incoming air, and in addition the headspace on the downstream side is blocked due to the Fraser River Siphon, resulting in generally positive pressures within the HI. However during storm events, as the flow rate increases from the two upstream tributaries, the sewage in the HI begins to backwater at the tie-in for the 8AI. The water level continues to rise

6 through the storm, decreasing the cross sectional area of the headspace in this area. This restriction, decreases the amount of air that can be supplied into the HI from the 8AI and the EBI, however the increase in the water velocities in the HI continue to pull air downstream. The vents along the interceptor cannot supply the required air due to the flapper style vents. Thus the HI quickly becomes starved for air, causing a sudden vacuum in the HI. When the water level at the 8AI connection finally reaches the crown of the pipe, no more air can be supplied resulting in the maximum negative differential pressure readings. At approximately the same time, when the 8AI becomes isolated, the water level at the partial siphon at Musqueam Creek reaches the crown of the siphon as well. The result of this is two air pockets forming in the HI, one to the north of the creek and another to the south. When this occurs, only the vents are connected to the headspace in the HI. Figure 3 shows the HGL profile in the HI at this instant. As the storm continues the water level continues to rise, and the headspace within the HI, continues to decrease. However with the two upstream tributaries now isolated from the headspace, and both the Musqueam Creek Partial Siphon and Fraser River Siphon, blocking the headspace on the downstream end, the only places that the air can vent is through the 18 th Avenue, 33 rd Avenue, Musqueam North and South, and Fraser River vents. Given the size and length of the HI, the amount of air that needs to be displaced is immense compared to the size of the vents. Thus, as the water level continues to rise the pressure in the HI increases significantly and a large amount of air is forced out of the vents at very high velocities. As the water level continues to rise and reach the crown of the pipe, each of the vents in turn becomes isolated from the headspace, forcing the air to be expelled out of fewer and fewer vents. CONCLUSION Based on the results from the hydraulic model, in conjunction with a collection system air transport model, a conceptual design and estimate for an air management and odor control facility was prepared. The facility included air pressure relief functionality to allow excess pressure peaks developed during storm events to be dissipated without danger to the public or property damage. Without a fully dynamic hydraulic model, the cause and magnitude of these pressurization events could not have been determined, and management of the peak air flows may not have been adequately addressed in the conceptual design for the odor control facility. REFERENCES Kerr Wood Leidal Associates Ltd. (KWL). Odour Control Strategy Development Highbury Interceptor, Final Report. Vancouver,

7 18 th Avenue Vent 33 rd Avenue Vent Fraser River Siphon Chamber and Vents 8AI Tie-in EBI Tie-in Musqueam Creek Partial Siphon and Vents Figure 3 - Screenshot of XP-SWMM.