Odour and Air Management Studies: Key Tool in Determining Effective Odour Control

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1 Odour and Air Management Studies: Key Tool in Determining Effective Odour Control Solutions Yuko Suda, P.Eng., Kerr Wood Leidal Associates Ltd. Ted Steele, P.Eng., Kerr Wood Leidal Associates Ltd A Still Creek Drive, Burnaby, BC Chris Hunniford, P.E., OCTC, A V&A Company 8220 Jones Road, Suite 500, Houston, TX ABSTRACT Sewer odours from pump stations, force main discharge points, and manholes have been a costly problem for many municipalities. Understanding the air and sewer dynamics within the sanitary sewer system is essential to identifying the cause of odour in sewer systems, and to developing cost-effective odour control solutions. Local pressurization of the air space above the sewage in a sewer results in odourous air being expelled at manholes and vents. The conventional solution has been to either seal manholes at these locations or install carbon scrubbers. This, however, increases the pressurization within the overall system and causes air to be expelled elsewhere, potentially shifting the odour problem to a new location. This reactive solution wastes millions of dollars for the municipal wastewater industry because it compounds rather than solves the problem. Completing an odour and air management study is critical in determining the cause of the odour complaints from the sewer system. Once the source or cause is identified there are a number of effective alternative options to address these issues. One such solution is to install active odour control facilities (OCF). Often one or two appropriately sized and well located OCFs are sufficient to address the odour issues for entire sewage areas. The Highbury Interceptor in Metro Vancouver is an example of this. Numerous carbon scrubbers had been installed to address resident complaints; however this method did not resolve the local issues, and was proving to be a costly exercise. An odour and air management study determined the root cause of the odour emissions, and proposed a solution that would eliminate the problems, and reduce the overall operating cost. INTRODUCTION AND BACKGROUND Due to the combination of organic waste material and bacteria in the sewer, generation of hydrogen sulphide and volatile organic compounds (VOCs) are practically unavoidable, especially in large collection systems where detention times can be significant. These odourous compounds exist within the sewer head space to varying degrees, however as long as the air stays within the sewer system no complaints will result. Odourous air is released from the sewer systems as a direct consequence of pressurization in the headspace above the sewage. There are a number of physical mechanisms that result in this pressurization, however the primary mechanism is air movement in the sewer from the effects of friction drag.

2 AIR MOVEMENT IN SEWERS The primary force for air movement in gravity systems is the friction between the sewer headspace air and the moving wastewater below. As the wastewater flows through the pipe, the air-water interface imparts a friction drag force on the air, and thus dragging the air along with the water. The only resistance to air movement in a sewer pipe is friction between the air and the stationary wall of the sewer. Given this set of simple boundary conditions, it is possible to generate a velocity gradient profile for air movement in sewer, similar to that illustrated in Figure 1. As might be anticipated, the velocity of air is at a maximum near the surface of the water and decreases as it gets near the pipe walls. Figure 1. Idealized Air Velocity Contours for a Sewer at Half Full (in Percent of Water Velocity). The friction factor between the water and the air varies depending on the behaviour of the water. More turbulent and rough water surfaces will result in an increase in friction factor, and. conversely, slower moving smooth water surfaces will result in a lower friction factor. The How rate of the air that is conveyed is proportional to the velocity of the air in the headspace and the cross sectional area of the head space. PRESSURIZATION IN SEWER HEAD SPACE Pressurization in the sewer headspace can occur when there are abrupt changes in the rate of air flow in the sewer. Generally, this results from a high flow rate of air from one section of the sewer colliding with air in a subsequent section of pipe that has a lower flow rate. There are two basic cases that results in this change of air flow rates; change in pipe slope and restriction in the air head space. A change in the pipe slope results in a direct change in velocity of sewer. A steeper section of pipe will result in a faster wastewater flow, and thus impart a higher friction drag on the air in the headspace. When the slope of the pipe is less steep, the resulting air velocity will be less as well.

3 Thus when the pipe slope decreases from one section to another, a potential for pressurization exists. A decrease in the cross sectional area of the headspace will result in a decrease of the air flow rate that the pipe can convey as well. There are a number of physical and hydraulic conditions which causes a decrease in the cross sectional area of the headspace including decrease in pipe diameter, increase in water depth (either from a backwatering effect or a sag in the pipe), and a partial or complete inverted siphon (a complete inverted siphon will result in zero head space, and thus zero air flow beyond this point). When the area of pressurization coincides with a manhole or a vent, sewer air will be expelled at that point, which may result in odour complaints. This previous discussion has focused on the simple condition of a pipe flowing partially full, however in a complex system many other factors can contribute to odour ventilation problems. These factors are briefly listed below. A change in barometric pressure (i.e. atmospheric pressure) will cause air movement into or out of sewers depending upon the relative pressures inside and outside the sewers. A decrease in pipe diameter in the downstream direction limits the air carrying capacity and causes a localised pressure zone. A change in ambient temperature (and therefore a change in air density) causes the convection of air into and out of sewers depending upon temperature gradient direction. Opposing or perpendicular flows entering a junction structure or sewer can cause a temporary back up of air resulting in local air pressurization. Strong winds blowing over the top of a manhole or an open standpipe can draw air by eduction out of the sewer. Ventilation effects of sewers are more pronounced when there is limited access to the sewer. Fewer connections to a sewer increases the ventilation effect by limiting the locations through which pressure can be relieved. This is predominantly a deep sanitary sewer phenomenon, but can occur in moderately sized sanitary sewers and deep combined sewers if manholes and tributaries are limited, widely-spaced or sealed for odour control. Ventilation effects can be severe at inverted siphons, full-flowing or surcharged sewers and pump stations. In these situations, all airflow stops although the upstream sewer continues to drag air. This creates high pressures in the sewers which can cause significant odour release from pump stations and sewers, force air upstream in service connections to release on the roof and even blow water out of residential plumbing traps under severe conditions. CONVENTIONAL SOLUTIONS Conventional solutions to these types of problems have been to either bolt the problem manholes down or to place a passive activated carbon scrubber on, in, or adjacent to the manhole. However doing this without due consideration for the cause of the odour coming out of the sewer can lead to more complaints rather than solve the problem.

4 By bolting down the manhole the zone of pressurization is now extended further than it was previously since pressures can no longer be relieved through that manhole. This means that the zone of pressurization may extend to the next set of manholes, sewer laterals or roof vents, effectively moving the problem to another location. The result is odour complaints in an area that previously did not have a problem. Although passive carbon scrubbers allow air to be vented from the sewer they still increase the local pressurization due to the head loss through the carbon system, thus still potentially moving the problem to a new area. Both of these methods require that odour complaints be systematically addressed until there are no more complaints associated with the sewers. This can be an unpredictable exercise and cost much more than initially expected. ODOUR AND AIR MANAGEMENT STUDY An odour and air management study would determine the source or cause of the odour complaints and determine feasible cost-effective solutions. An odour and air management study may include the following components: Monitoring program - which may consist of monitoring hydrogen sulphide concentrations, sampling VOC concentrations, sampling compounds in the wastewater, and monitoring differential pressure (pressure between the atmosphere and the sewer headspace). Ventilation modelling - the model will be used to perform an analysis of the ventilation dynamics within the sewer system to determine the areas of pressurization and the associated air flow rates. Hydraulic modelling - a dynamic hydraulic model will be used to determine whether any displacement effects take place in the sewer. As the water volume in the sewer system increases a similar amount of air must be expelled from the system, either through the vents or through the local sewer systems. Air Management and Odour Control Strategy - based on the above work, an evaluation will be conducted to develop the most effective strategy for mitigating odour emissions. The selection of the preferred strategy will take into consideration such factors as cost, feasibility of implementation, environmental impacts (i.e., presence of hazardous chemicals/by-products, noise, etc.), and overall treatment effectiveness. ACTIVE ODOUR CONTOL FACILITIES An active odour control facility is one solution that can address local pressurizations within the sewer. An active odour control facility draws air from the sewer using a fan, treats it, and releases it to the atmosphere. There are many technologies available that can be used to treat the air, such as biofilters, carbon adsorbers, chemical scrubbers etc. By drawing air in from the sewer with a blower and treating it, an area of negative pressure, also call a zone of influence, can be created in the sewer system around the odour control facility. This area of negative pressure will prevent air from escaping into the atmosphere. Based on the sizing of the facility and the geometry of the local sewer system, a zone of influence extending several kilometres is possible. The advantage of this system is that a single facility can address

5 odour issues in an entire neighbourhood. Although the initial capital cost of an OCF can be high, the overall life cycle cost can be much less compared to multiple carbon scrubbers throughout the system. CASE STUDY - HIGHBURY INTERCEPTOR, METRO VANCOUVER The Highbury Interceptor, owned and operated by Metro Vancouver, is one of the principle trunk sewers in the Vancouver Sewerage Area (VSA), and services the majority of the City of Vancouver and a portion of the City of Burnaby. The sewer is a 6 km long 2,900 mm diameter combined sewer with significant odour and headspace pressurization issues identified along its length. In recent years the number of complaints regarding odour from the interceptor vents have increased. In addition to problems from venting foul air, 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 blown off and residents have reported seeing heaving of the asphalt pavement around the interceptor manholes. Ventilation modelling and hydraulic modelling were carried out to simulate the ventilation dynamics within the sewer. Differential pressure monitoring, hydrogen sulphide monitoring and air phase sampling along the sewer was carried out during dry weather periods and wet weather periods as well. The differential pressure data indicated that periods of significant positive pressure occur throughout the Highbury Interceptor. As a result, the potential for odour emissions exists at every point of release along the sewer. While the air pressure within the sewer is relatively moderate under average daily flow conditions, during some storm events pressure in the sewer increases rapidly, exceeding the differential pressure monitor's range. Pressures of this magnitude are considered significant and are rarely seen in sewer systems. Based on the ventilation modelling the peak friction drag airflow condition at the downstream endof the sewer was estimated to be 283 m3/min (10,000 cfm). Since the downstream end of the Highbury Interceptor is a siphon, no air can be conveyed beyond this point, resulting in a significant area of pressurization. In addition to this the dynamic hydraulic 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. This occurs during high sewage flow events, where a backwatering effect cuts off the headspace at the upstream end of the system, and a siphon at 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 small vents located along the sewer, resulting in high pressures and 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. The model calculates that a typical storm can displace as much as 200 m3/min of air. The team's recommendation for the sewer sytsem was to install three well placed active odour control systems along the HI to address both the odour complaints and the pressurization events.

6 The main odour control facility, with a treatment capacity of 283 m3/min, would have a zone of influence of approximately 4.6 km. CONCLUSION Although the Highbury Interceptor would be considered a special case example it illustrates that without a well planned odour and air management study that included both ventilation modelling and hydraulic modelling, the cause ofodours being released into the atmosphere and the extreme pressurization events would not have been identified. The study also identified that one well placed active odour control facility will likely be able to address pressurization events along 80% of its length. Conducting an odour and air management study is key in determining the root cause of odour complaints in a neighbourhood, and developing cost effective solutions. Without a comprehensive study, municipalities can only react to odour complaints and will not be able to develop a cost effective solution that can solve the problem for entire sewerage areas.