Review papers
Modeling air flow in sanitary sewer systems: A review

https://doi.org/10.1016/j.jher.2020.10.003Get rights and content

Abstract

Current designs of sanitary collection systems normally only consider the transport of wastewater without attention on the air movement in the sewer airspaces. Under anaerobic conditions, hydrogen sulfide (H2S) can be generated in the liquid phase in sewer systems. H2S is corrosive to concrete and steel structures and odorous or even toxic to human, which can cause corrosion and sewer odor issues. To develop a feasible sewer corrosion and odor control strategy, it is necessary to understand the mechanisms of air flow in sewer systems for developing practical tools to predict and control the air flow. This paper comprehensively reviewed previous efforts on modeling the air flow in sewer systems and provided recommendations on predicting the air flow for engineering applications. The air flow in a single pipe was firstly reviewed followed by the air flow in sewer structures as well as air flow models in sewer networks. Some other considerations such as temperature driven flow, transient water flow, and wind effect were also reviewed. The knowledge gaps were then identified, and recommendations on the further studies were provided.

Introduction

Sewer corrosion and odor is a world-wide issue in major cities such as San Francisco, Los Angeles, Seattle, Sydney, Edmonton etc. (WERF, 2007, City and County of San Francisco, 2009, City of Los Angeles, 2011, Wang et al., 2012, Guo et al., 2018). Under anaerobic conditions, hydrogen sulfide (H2S) can be generated in the liquid phase in sewers (Hvitved-Jacobsen et al., 2013, Jiang et al., 2017). H2S is corrosive to concrete and steel structures and odorous or even toxic to human, which can cause corrosion and sewer odor issues (Nielsen et al., 2012, Pan et al., 2020). Current designs of urban sewer systems normally only consider the transport of wastewater but with little attention on the air movement (Edwini-Bonsu and Steffler, 2004). The air in sewers can therefore move uncontrolled inside the entire sewer system. Fig. 1a shows the air movement in a conceptual sewer system (Qian, 2018). Air can enter the sewer system through manhole covers or other openings, and then moves in the system under the combined effect of the wastewater drag, pressure difference, temperature difference etc. With elevated air pressure in the system, the air in the sewer system tends to leak from manhole pickholes or other openings to the atmosphere.

The uncontrolled air can flow out of the manhole at a quite high velocity, e.g. over 40 m/s with the air pressure in the manhole at around 800 Pa in a specific location with a dropshaft in Edmonton (See Fig. 1b) (Qian et al., 2018). This velocity was relatively high given the high air pressure in this case. The air pressure was as high as 120 Pa at another location in Edmonton (Zobeyer et al., 2020), about 50 Pa in City of Los Angeles’ system (City of Los Angeles, 2011), and as high as 100 Pa in Lisbon, Portugal (Matos et al., 2019a). The releasing of the air can cause odor complaints in the neighborhood. Therefore, modeling the movement of air in sewer systems is critical to the municipalities for developing strategies with respect to sewer corrosion prevention and protection, and also for sewer odor control and mitigation.

Pomeroy (1945) started pioneer works explaining the reasons for ventilation of sewers and proposed mitigation methods including perforations in manhole covers, ventilation through house plumbing vents, and ventilation by forced draft. Pescod and Price (1982) conducted experiments on the air movement in a single sewer pipe and developed a series of curves of the air flow velocity versus the water flow and pipe parameters with only wastewater drag considered. USEPA (1994) reported a series of physical experiments on the air flow in a single pipe and proposed the relationship between air and water flow in a single pipe, and Odor and Corrosion Technology Consultants (1999) used a simple relationship describing the air flow in a single pipe. Koziel et al. (2001) conducted physical experiments and provided information between water and air flow in a pipe. Recognizing the effect of the pressure gradient in the sewer pipe on air flow, Edwini-Bonsu and Steffler, 2004, Edwini-Bonsu and Steffler, 2006a used numerical method to correlate the relationship between air and water flow considering the wastewater drag and air pressure gradient. More recently, Lowe (2017) proposed a relationship for air and water flow in a single sewer pipe using field measurements.

Another key component of researches is the air flow into and out of the sewer system through sewer structures as actual sewer systems contain components such as manholes, drop structures, ventilators, air jumpers, etc. For the air flow in manholes, the air flow through manhole pickholes can be simply estimated using the orifice equation (Edwini-Bonsu and Steffler, 2006b, Qian et al., 2018). Extensive studies have been done on the air flow in a dropshaft. Rajaratnam et al. (1997) physically obtained the curves between air flow and water flow in a dropshaft followed by the study of Jalil (2009). Granata et al., 2011, Granata et al., 2015 conducted physical experiments on the air entrainment in dropshaft with relatively small drop heights. Camino et al. (2015) conducted physical experiments on the air flow induced by a tall plunging type dropshaft. With respect to the pressure gradient induced in the dropshaft, Ma et al. (2016) developed a model for predicting the pressure gradient induced by the falling water in a dropshaft and compared it to lab experiments. To reduce the air pressurization and air demand in a dropshaft, various retrofitting techniques were studied such as with air circulation pipes (Ma et al., 2018) and an internal divider within the dropshaft (Wei et al., 2018). For the air flow in other sewer structures, Edwini-Bonsu and Steffler (2006b) proposed a parabolic equation describing the air flow in a ventilator, and Deering et al. (2006) reported the methods for estimating air flow in air jumpers. The effect of pump operation on the air presssure variation in sewer systems was also studied (Matos et al., 2019b and Qian et al., 2020)

With respect to modeling the air flow in sewer systems, the existence of the air pressure difference and wastewater drag along a pipe requires a drag coefficient at air–water interface and a friction coefficient for air flow at the pipe wall. Therefore, reliable estimation for these coefficients are important for developing the air flow model in sewer systems. WERF (2009) reported a field study for obtaining the drag coefficient. Bentzen et al. (2016) conducted physical experiments in a rectangular duct to determine the drag coefficient at the air–water interface. Qian et al. (2017) numerically assessed the effect of water drag and pressure gradient on air flow in a pipe and proposed an equation for estimating the drag coefficient.

Based on the air movement in a single pipe and in nodal structures, the air flow models for sewer networks were then developed. Edwini-Bonsu and Steffler (2006b) proposed a model for solving the air flow in a prototype system with some assumptions. Wang et al. (2012) proposed an air flow model with the field work results in Sydney, Australia. Qian et al. (2017) conducted analysis on the air pressure distribution in a sewer system and proposed a method to simplify the sewer network. Zhang et al., 2020, Zobeyer et al., 2020 developed air flow models which can be used in a large sewer system and be able to automatically build up the skeleton of sewer systems based on Geographic Information System (GIS) database.

As an important part of researches, fieldworks have been conducted for exploring the air movement in prototype sewer systems and for validating air flow models. WERF (2009) conducted a series of field measurement in the USA. City of Los Angeles (2011) monitored the air pressure variation in its sewer systems and assessed the performance of several air flow management facilities. Guo et al., 2018, Qian et al., 2018 monitored the pressure variation in a prototype sewer system in Edmonton, Alberta, Canada. Matos et al., 2019b, Matos et al., 2020 field monitored the pressure variation along a prototype gravity sewer pipe in Portugal. For the air flow in drop structures, Zhang et al. (2016) analyzed the field monitoring data for assessing the performance of a prototype dropshaft, and Ma et al. (2019) conducted further fieldwork analyzing the effect of air circulating pipes in a drop structure. Qian et al. (2020) field monitored the air pressure variation in the vinicity of pump stations in a prototype sanitary sewer system.

The above-mentioned studies focused on the major factors affecting air flow in sewer systems such as water drag, pressure differential, and friction force. Some other factors, e.g., temperature effect, water lever rise and fall, and wind condition have also been studied. USEPA (1994) proposed an equation for calculating the extra air flowing out of manholes induced by the temperature difference between the ambient air and sewer air. Olson et al. (1997) developed a model predicting the air flowrate using a thermodynamic method. Guo et al. (2018) reported that the unsteady water flow caused by the operation of pump stations can significantly affect the air pressure distribution in the system. The wind across a manhole may induce extra air flow out of the manhole and hence influence the air flow in entire system and this was studied by Monteith et al. (1997).

Several commercial software packages are available on sewer odor control in sewer systems but none on air flow modeling in sewer systems. Mike ECO lab is a software that predicts the generation of H2S. However, air flow is not simulated in the software. Mike (2017) thus stated that high H2S generation areas may not be high H2S concentration areas. The WATS model (Yongsiri et al., 1992, Vollertsen et al., 2011, Vollertsen et al., 2015) and SeweX (Sharma et al., 2008, Sharma et al., 2012, Liu et al., 2013) were also developed mainly for predicting the generation of H2S in sanitary sewer systems. Envirosuite (https://envirosuite.com/) is a software for modeling the odor transportation in the atmosphere but not in sewer pipes.

The objective of this paper is to comprehensively review the historical development, the physical mechanisms, and the key factors influencing the air flow in sewer systems and to provide recommendations on predicting the air flow for engineering applications. This paper firstly reviewed the literature on the current advancement of air flow models for a single pipe, followed by the air movement in sewer structures, then the air flow models in an entire sewer system, and finally various other factors. It should be noted that the air flow in building plumbing systems is not covered in this study.

Section snippets

Air flow in a single pipe

Modeling the air flow induced by flowing water in a single pipe has been studied for the past 30 years. As one of the early studies, Pescod and Price (1982) conducted experiments on the mechanism of water-dragged air movement in a sewer pipe using a UPVC pipe with a diameter of 300 mm and a length of 15 m, and both ends of the pipe were open to atmosphere. The air flow was measured using a thermistor sensing device (air velocity meter, AVM) for velocity profiles and carbon monoxide (CO) as a

Air flow in nodal structures

In a prototype sewer system, the pipes are usually connected by nodal structures such as manholes, drop structures, etc. Most nodal structures allow the air to move in and out of the sewer system (Edwini-Bonsu and Steffler, 2006b). Fig. 3 shows the schematic of a drop structure and a forced ventilator. Along with the regular manhole, the air flow in these nodes follows the continuity equation where the air flow into the node equals to the air flow out of the node. In addition, the drop

Air flow in a sewer network

As mentioned above, when dealing with the air movement in sewer systems, the sewer structures usually include links such as pipes, and nodes such as manholes, drop structures, ventilators, etc. The pressure in the system builds up mainly due to the wastewater drag or drop structures, and drives the air to move and hence changes the overall pressure distribution. Determining the air movement requires a systematic approach. Therefore, it is of importance to treat the air flow in the entire system

Temperature

The variation in air temperature and humidity can produce a buoyancy force acting on the air flow and induces ventilation condition in sewer headspace (Pescod and Price, 1982). Less dense headspace air tends to rise and flow out of the sewer, and more dense ambient air can sink and flow into the sewer. For the situation where warm headspace air is being released out of the sewer via pickholes, the buoyancy is caused by the temperature and humidity differentials between the air inside and

Conclusions and recommendations

Modeling air movement in sewer systems is of significant importance for sewer odor mitigation, corrosion control, and public safety. Understanding the mechanisms of air flow and how the factors affecting the air flow was the keys for a strategic plan of sewer air management. Over the past decades, significant research works have been done on developing air flow models for individual sewer structures and for sewer systems. The following is a brief summary on the conclusions, knowledge gaps and

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The writers gratefully acknowledge the financial support from the Natural Sciences and Engineering Research Council (NSERC) of Canada and the City of Edmonton/EPCOR.

References (60)

  • D.J. Dürrenmatt et al.

    Simulation of the wastewater temperature in sewers with TEMPEST

    Water Sci. Technol.

    (2008)
  • S. Edwini-Bonsu

    Air flow in sanitary sewer systems: a physical-based approach

    Department of Civil and Environmental Engineering

    (2004)
  • S. Edwini-Bonsu et al.

    Air flow in sanitary sewer conduits due to wastewater drag: a computational fluid dynamics approach

    J. Environ. Eng. Sci.

    (2004)
  • S. Edwini-Bonsu et al.

    Dynamics of air flow in sewer conduit headspace

    J. Hydraul. Eng.

    (2006)
  • S. Edwini-Bonsu et al.

    Modeling ventilation phenomenon in sanitary sewer systems: a system theoretic approach

    J. Hydraul. Eng.

    (2006)
  • S. Eftekharzadeh et al.

    Mathematical Modeling of Airflow in the City of Los Angeles Main Interceptor System

    Proc. Water Environ. Federation

    (2011)
  • S. Guo et al.

    Effects of Drop Structures and Pump Station on Sewer Air Pressure and Hydrogen Sulfide: Field Investigation

    J. Environ. Eng.

    (2018)
  • F. Granata et al.

    Hydraulics of Circular Drop Manholes

    J. Irrig. Drain Eng.

    (2011)
  • F. Granata et al.

    Air-water flows in circular drop manholes

    Urban Water J.

    (2015)
  • T. Hvitved-Jacobsen et al.

    Sewer Processes: Microbial and Chemical Process Engineering of Sewer Networks

    (2013)
  • L.H. Hentz et al.

    Ventilation and Odor Control for Sewers & Tunnels

    Ohio WEA Annual Conference, Mason, Ohio

    (2013)
  • A. Jalil

    Experimental and numerical study of plunging flow in vertical dropshafts

    Department of Civil and Environmental Engineering

    (2009)
  • G. Jiang et al.

    Odor emissions from domestic wastewater: A review

    Critical Rev. Environ. Sci. Technol.

    (2017)
  • J.A. Koziel et al.

    Gas-Liquid Mass Transfer Along Small Sewer Reaches

    J. Environ. Eng.

    (2001)
  • Y. Liu et al.

    Controlling chemical dosing for sulfide mitigation in sewer networks using a hybrid automata control strategy

    Water Sci. Technol.

    (2013)
  • S.A. Lowe

    Sewer Ventilation Modeling

    J. Water Manage. Model.

    (2017)
  • Y. Ma et al.

    Air Entrainment in a Tall Plunging Flow Dropshaft

    J. Hydraul. Eng.

    (2016)
  • H.I. Madsen et al.

    Gas Phase Transport in Gravity Sewers-A Methodology for Determination of Horizontal Gas Transport and Ventilation

    Water Environ. Res.

    (2006)
  • R.V. Matos et al.

    Understanding the effect of ventilation, intermittent pumping and seasonality in hydrogen sulfide and methane concentrations in a coastal sewerage system

    Environ. Sci. Pollut. Res.

    (2019)
  • R.V. Matos et al.

    Influence of Intermittence and Pressure Differentials in Hydrogen Sulfide Concentration in a Gravity Sewer

    Water

    (2019)
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