Field monitoring and design optimization of dropshafts with air circulation pipes

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Abstract

Dropshafts with air circulation using an airshaft and horizontal air circulation pipes have been implemented for reducing downstream sewer air pressure and preventing sewer odor emission. There is a need to optimize the design for improved effectiveness and cost-saving. A one-week field monitoring program on a dropshaft with air circulation pipes was conducted in Edmonton, Canada, in order to validate the air flow model and assess the performance of the dropshaft with different air circulation pipe configurations. The air flow model was based on the upstream/downstream conditions of the dropshaft which were obtained from the field monitoring program. This study showed that the horizontal air circulation pipes on the top positions were more efficient in re-circulating air and depressurizing the downstream than the bottom ones, and increasing the size of the top horizontal air circulation pipes could significantly improve their effectiveness. This study provides guidance for designing or retrofitting dropshafts with air circulation pipes.

Introduction

Dropshafts are commonly used in urban drainage systems to convey water from shallow sewers to deep tunnels. They usually have a considerable drop height of dozens of meters. Plunging flow dropshaft is the most common type, which simply directs water to freely fall in a vertical shaft. The plunging flow dropshaft can entrain a significant amount of air into the sewer system, in some cases reaching over a hundred times the water flow rate (Zhang et al., 2016). According to Camino et al. (2015), after the impingement of the inflow on the dropshaft wall, a portion of water fell clinging to the dropshaft wall and the rest moved freely inside the dropshaft. The free-falling water would break up into small water drops after a certain falling distance (Ma et al., 2016a). The water drops significantly increased the water–air interaction area and enhanced the drag force imposed on the air phase. The momentum transfer from the water drops to the air phase was considered as the primary mechanism of substantial air entrainment in the dropshaft (Ma et al., 2016b). The dropshaft can also pressurize the downstream sewer headspace (Edwini-Bonsu and Steffler, 2006), and cause severe downstream odour emission (Guo et al., 2018, Qian et al., 2018). Based on the field monitoring program reported in Guo et al. (2018), the air pressure downstream of a 17-m-high dropshaft reached over 800 Pa, which could induce an air velocity of about 40 m/s when the air was emitted to the ground.

To lower the risk of sewer odour issue, various types of dropshaft have been proposed, like dropshafts with internal dividers (Anderson and Dahlin, 1968, Margevicius et al., 2009, Wei et al., 2018), vortex dropshafts (Jain, 1988, Yu and Lee, 2009), the stacked drop manhole (Camino et al., 2010) and the baffle drop structure (Odgaard et al., 2013). These dropshafts were reported to entrain less air than the common plunging flow dropshaft, but their construction could be more complicated and are not applicable for retrofitting of existing dropshafts.

Some existing dropshafts have been retrofitted with air circulation pipes in Edmonton, Canada, in an effort to mitigate sewer odour issues. An airshaft was built downstream the dropshaft and connected to the dropshaft via several horizontal air circulation pipes, with which air could be circulated between the dropshaft and the airshaft so that less air would be transported downstream. The field monitoring program on two of the retrofitted dropshafts in Edmonton from 2006 to 2011 was reported in Zhang et al. (2016), which mainly focused on the upstream/downstream air pressure and the air flow in the air circulation pipes. The air circulation pipes were found to be effective in reducing the downstream air pressure of the dropshaft. However, no direct measurement of air flow rate in the upstream trunk had been conducted, which was important for studying the air entrainment of the dropshaft. Ma et al. (2017) conducted a physical model study on the dropshaft connected to the airshaft with different numbers of horizontal pipes and developed a prediction method for air flow in such dropshafts. From their study, the airshaft was found to be useful in reducing the air entrainment while the horizontal circulation pipes had various contributions. It is understood that the scale effect needs to be considered when extrapolating the results of laboratory works to prototypes. However, air/water two-phase flow is complicated and the laboratory results are usually difficult to be scaled up to the prototypes. Field monitoring on a full-scale dropshaft is needed for the validation of the results obtained from the laboratory experiments.

Currently, the City of Edmonton, Canada, is considering retrofitting close to 800 dropshafts for sewer odour mitigation. The existing modification of dropshafts with air circulation pipes can cost over one million Canadian dollars each. Design optimization for a configuration that is more efficient in reducing the air demand and the downstream pressure, as well as more cost-effective, is needed. The design limitation for such a modification is that the vertical airshaft is always requested for air circulation. Thus, the optimization is mainly focused on the location, the number and the size of the horizontal air circulation pipes.

In this study, a one-week field monitoring program was conducted on a dropshaft with air circulation pipes in Edmonton. The air entrainment and the downstream air pressure were monitored when the dropshaft was connected with different numbers of horizontal air circulation pipes. Additionally, an air flow model was established and validated with the field data. With the air flow model, the performances of the dropshaft with different configurations of air circulation pipes were predicted and assessed. Combining the field monitoring results and the model prediction, the design optimization of dropshafts with air circulation pipes was explored.

Section snippets

Field monitoring program

The plan view and side view of the study dropshaft in a sanitary sewer system, its air circulation pipes and several upstream and downstream manholes are illustrated in Fig. 1. The dropshaft has a drop height of H = 24.8 m and a diameter of Dds = 1.2 m with the inlet pipe buried about 9.3 m below the ground. An airshaft with the diameter of Das = 1.2 m is in parallel to the dropshaft and connected to the dropshaft via five horizontal pipes of 0.6 m diameter and 2.7 m length for air circulation.

Field monitoring results and discussions

The field conditions of the horizontal air circulation pipes are shown in Fig. 3, as extracted from the videos taken in the field. As the video of pipe #2 does not show a good view of the pipe, it is not included in Fig. 3. From Fig. 3, much sludge can be observed in the horizontal pipes, especially in the pipe #5. Observed from the videos, many tiny drops were flying towards the camera in the pipes #4 and #5, which indicated that the air flow in the pipes #4 and #5 was from the dropshaft to

Air flow modeling for the dropshaft with air circulation pipes

Ma et al. (2017) developed an air flow model for the dropshaft with air circulation pipes, where the measured air pressure in the airshaft manhole should be one of the inputs. An improved model without this restriction is developed in this study, which can be used to predict the performance of dropshaft with different configurations of air circulation pipes.

To establish the air flow model, boundary conditions of the study dropshaft need to be determined first. As reported in Ma et al. (2017),

Conclusions

Dropshafts are being retrofitted to allow air to recirculate internally using air circulation pipes in Edmonton, Canada, for sewer odour mitigation. Its design optimization was explored based on the field monitoring program as well as the model study in this paper. The field monitoring program checked the air entrainment and the downstream air pressure of a prototype dropshaft when it was connected with different numbers of horizontal air circulation pipes. The field monitoring results showed

Acknowledgments

The writers gratefully acknowledge financial support from the Natural Sciences and Engineering Research Council (NSERC) of Canada and the City of Edmonton. The authors also would like to thank Perry Fedun for his technical assistance.

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    Citation Excerpt :

    In prototype systems, the drop may not be high enough (e.g. less than 5 m) for the water to break into droplets, which reduces the drag effect of the water flow on the air and therefore using Eq. (13) may cause an overestimation on the pressurization. Qian et al. (2018) showed that Eq. (13) overestimated the air pressure gradient along a prototype dropshaft by 2 times the measured values while Ma et al. (2019) showed consistent results in the field measurements with the lab experiment by replacing the 6 m/s droplet terminal velocity with 8 m/s and assuming a linear pressure gradient at the first 10 m drop. Therefore, careful calibration is needed for different dropshafts when applying Eq. (13).

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