Statistical analysis of sewer odour based on 10-year complaint data

Sewer odour problems have been widely reported in cities worldwide such as San Francisco, Los Angeles, and Edmonton (Associated Engineering 2008; City & County of San Francisco 2009; City of Los Angeles 2011; Liang et al. 2019). This is primarily due to the fact that most of the current sewer design standards/guidelines only optimize the conveying of liquid phase (sewage) in sewers without consideration of sewer air movement in sewer headspace above the sewage (Edwini-Bonsu & Steffler 2004; Qian et al. 2018). Odorous compounds in the sewer air can release to the atmosphere at many parts of the sewer network such as manholes, catchbasins (for combined sewer system) and pump stations (Prez et al. 2013) and cause odour nuisances to nearby residents (Carrera et al. 2016; Sobhani et al. 2018).

Previous studies have been conducted on the different factors that cause air movement in sewers. These factors include the temperature difference and thus density difference between the sewer air and the air above the ground (Pescod & Price 1982; US EPA 1994), the sewage drag effect (Pescod & Price 1978, 1982; Parker & Ryan 2001; Edwini-Bonsu & Steffler 2004, 2006; Madsen et al. 2006), the sewer air pressure difference effect in various parts of sewer systems (Qian et al. 2016), and the sewage level fluctuation effect (Wang et al. 2012). For the sewer air pressure difference, drop structures that are used for dropping sewage to lower downstream pipes have been identified as one of the primary reasons (Corsi & Quigley 1996; Rajaratnam et al. 1997; Sorensen et al. 2000; Chanson 2007; Granata et al. 2011; Camino et al. 2014, Ma et al. 2016; Zhang et al. 2016; Guo et al. 2018; Wei et al. 2018). The generation and transport of the malodorous gases in sewer networks are rather complex and related to many physical, chemical and biological processes (Jiang et al. 2017).

Our current understanding on sewer odour and complaints is still limited, and one knowledge gap is the statistical correlations between sewer odour complaints and physical attributes of sewer systems. In the past, odour complaints have been merely used to examine the spatial distribution or as an index to evaluate the performance of odour mitigation measures (City of Los Angeles 2011). Extracting further information from odour complaint data such as their correlation to sewer properties has not been reported, to the authors' best knowledge.

The City of Edmonton, Alberta, Canada, has been collecting sewer odour complaints since 2003, with an average of 800–900 complaints each year. All these complaints data have been compiled into the city's GIS (geographical information system) database (Associated Engineering 2008). In this study, the 10 years of complaints data from 2008 to 2017 were analyzed. Temporal and spatial correlations between the complaint points and the attributes of the sewer system are presented. The study aims to reveal the possible major factors for the odour problem by constructing the statistical relationships between the complaint data and sewer physical properties.

In the past 15 years, thousands of complaints from residents have been recorded in Edmonton. For the first few years after the hotline was opened in 2003, the complaint numbers were relatively small. Considering that many residents may not have known about the hotline in the first few years, the 10 years' data from 2008 to 2017 (8,894 complaints in total) were used in this study. The original complaints information contained addresses but no specific coordinates. The addresses were converted to geographic coordinates using the geocoding function provided by the Google Maps Geocoding API and then compiled into the GIS and stored as layer files, as shown in Figure 1.

Edmonton has been using ArcGIS to manage its sewer systems for many years. According to the database, the total length of the city's sewer pipelines has reached approximately 6,450 km, and 3,600 km of them are sanitary and combined sewers. The total manhole number is around 80,000 and half of them connect to the sanitary and combined sewers. For each sewer pipe, the attributes in the GIS include coordinates of both upstream and downstream ends, upstream and downstream manhole IDs, pipe length, pipe material, pipe diameter, pipe age, slope, and pipe type. For each manhole, attributes include coordinates, manhole age and manhole depth.

represents the risk possibility of a parameter (e.g., manhole age) that contributes to the sewer odour complaints. Higher represents more risk or higher likelihood as an odour source.

The complaints' density map is presented as Figure 2. Darker colour represents higher density of complaints. Several odour hot spots in the city can be clearly identified. These hotspots are supposed to be related to the different sewer air movement capacity in pipes, forcing the foul to be vented out of the sewer system.

Figure 3 shows the monthly average number of complaints from 2008 to 2017. There are plenty of odour complaints (45–110) in every month from January to December. The number is smallest in May and largest in September. The monthly average temperature and precipitation data in each month are plotted in Figure 3 in order to find their relationships with complaint numbers.

Temperature is expected to be a key parameter, considering the generation of odour compounds such as H₂S is a biological process. The city, same as other places, has 7 months with monthly average daily mean temperature above 0 °C (from April to October). And ice and snow usually begins to melt in April and finishes by May. A large amount of snow water with temperature around 0 °C will enter the combined sewer pipes. Kaczor & Bugajski (2012) evaluated the effect of snowmelt inflow on sewage temperature in Lesser Poland Voivodeship and found that it had caused a decrease in average daily sewage temperature of 2–3 °C, and the biological processes in the sewer systems had been significantly inhibited. Considering the climate in Edmonton, the effect of the snowmelt inflow on sewage during the spring should be more substantial than in Voivodeship. This might explain why the complaints start to decrease from April and reach the lowest in May. After the snowmelt finishes in May, the complaints start to increase with the rising temperature in June and July.

Interestingly, the hottest month in Edmonton does not have the most complaints. This is likely due to the rainfall effect. As shown in Figure 3, the monthly rainfall reaches the highest 74.7 mm in July. The rainwater inflows and infiltrations through the manhole pickholes (holes in a manhole cover), cracks, catchbasins (for combined sewers) and other openings dilute the sulfate concentration in the sewage, which is the sulfur source of H₂S. Meanwhile, such inflows and infiltrations will flush the sewer pipes and likely remove the biofilms on pipe walls and sediments at the bottom, which generate odorous compounds such as H₂S. The effects of rainwater on reducing the odour concentration have been reported by previous field monitoring results in Edmonton in stormwater ponds (Ku et al. 2016) and sanitary sewer system (Guo et al. 2018). The rainfall sharply decreases after July, which possibly causes the next 3 months (August, September and October) to have the highest complaint numbers of the year.

In the coldest 5 months from November to March, there are still many complaints but with relatively constant numbers. Previous studies indicated that sulfide formation rates can be hampered in wastewaters at temperatures between 5 and 12 °C and stimulated above 15 °C (Carrera et al. 2016). In Edmonton, sewer trunks are buried deep (about 20 m below ground) and the sewage temperature is about 10 °C during the coldest months. This suggests that odorous compounds can be still generated despite the relatively cold temperature.

In summary, the snowmelt in spring and the rainwater in summer were found to be positive factors that can likely reduce the odour complaints. It implies that dosing some ice and snow into the sewer pipeline might be a potential temperature control method for inhibiting the biological processes that generate odorous compounds, thus reducing odour complaints. However, this will increase the sewage flow rate in the sewer system and increase the cost of the downstream wastewater treatment. Therefore, economic cost-benefit analysis is needed in the future.

A total of 14,651 sewer pipes and 7,302 manholes were searched out. More than 82% of all the complaints happened near the manholes within 50 m (the rest 18% are associated with manholes more than 50 m away). This is consistent with the field monitoring results conducted previously in the city. In the summer of 2016, an extensive field monitoring program was conducted on the sewer ventilation condition of a sanitary sewer system in a southern neighborhood of the city that has got frequent odour complaints from nearby communities (Guo et al. 2018). Continuous monitoring data on the H₂S level and air pressure in manholes along the studied sewer system were obtained. The field work found that the manhole pickholes were the most important odour release sources and played an important role for the city's odour problem.

There are nearly 40,000 manholes in the sanitary and combined sewer lines in the city, and the time from construction spans from 5 to 111 years. Table 1 shows the percentage in each age group. The 7,302 manholes searched out within 50 m of complaint locations were defined as the ‘risky’ manholes for odour sources. for each manhole age group was calculated. and manhole age have shown a certain positive correlation. When the manhole age is more than 50, according to Table 1. Even more, for manholes older than 60 years, ⁠, indicating they are 40–80% more likely to have odour complaints. Therefore, we can conclude that odour complaints are more likely to happen around older manholes. Since manholes were built at the same time as sewers, the above conclusion applies to sewer pipes. In addition to the possibly imperfect hydraulic conditions of the older manholes/pipes (e.g., more cracks, sedimentation and longer hydraulic retention time), the types of sewer system in the city are also related to more complaints around older sewers. Most of the city's older sewers were built as combined sewers and located in the central area with higher population, and thus they are associated with more odour complaints. As shown in Table 2, the combined pipes account for 26.76% of all pipes and reaches 1.48, while the sanitary pipes account for 73.24% and is only 0.82, suggesting combined sewers are more problematic in terms of odour complaints.

Compared to many other cities in the world, sewer pipes in Edmonton are buried relatively deep due to the city's terrain, soil geotechnical condition, and cold climate. Figure 4 shows the depth distribution of all manholes, where depth = ground elevation − invert elevation. In recent years, certain drainage structures such as dropshafts or drop manholes, which are widely used in Edmonton's drainage systems for dropping sewage from higher to lower elevations, have been recognized to have a significant impact on the city's odour problem (Ma et al. 2016; Zhang et al. 2016; Guo et al. 2018). When the sewage falls, the sewage flow becomes more turbulent and breaks into ligaments and even droplets, dragging plenty of air into the downstream sewer pipes. Meanwhile, a lot of odorous compounds such as may escape from the liquid phase to the sewer air space and release at the downstream manholes, which are pressurized due to the air entrained from the upstream. The characteristics of the drop height (drop height = upstream pipe invert elevation − manhole invert elevation) distribution of all manholes and risky manholes are presented and compared in Table 3.

The city has more than 1,000 drop manholes (some manholes have more than one drop) with drop height more than 1 m, which accounts for 9.78% of all manholes. It is found that R_i has a sudden increase (the value reaches as large as 1.84) when the drop height is larger than 10 m. Previous laboratory results by Ma et al. (2017) indicated that when the drop height exceeds 5 m, water flow will disintegrate into small drops and result in excessive air entrainment. Zhang et al. (2016) observed intensive sewage droplets in a drop manhole with drop height of 24.8 m in the field. Further experiments are needed to focus on drop structures with drop heights larger than 10 m.

Table 4 shows the characteristics of pipe materials for all pipes and risky pipes. Percentage of material refers to the ratio of the length of different materials to the total length of the sewers. It can be seen that more than 40% of the city's sewer pipes are clay pipes, followed by 24.74% concrete and 28.21% PVC. The value of is clay pipe > concrete pipe > PVC pipe. It should be noted that clay pipes were widely used in the early days in the city (the average pipe age is 59 years) and concrete pipes (the average pipe age is 48 years) and PVC pipes (the average pipe age is 19 years) were laid afterwards. Therefore, the effect of pipe material has somehow reflected the pipe's age effect.

Sewer pipe slope is one of the key factors to determine the hydraulic condition for sewage flow. The characteristics of the slope distribution are presented in Figure 5. Mashford et al. (2011) introduced two parameters of ΔSlope and ΔAngle and mentioned that they may cause turbulence in sewers and increase the conversion rate of sulfides from liquid phase to gas phase. ΔSlope = the slope difference between the upstream and the downstream pipes, while ΔAngle = the angle or direction change between the upstream and the downstream pipes. ΔSlope and ΔAngle were calculated by using the joint function and the field calculator in ArcGIS, and the results are presented in Figures 6 and 7 respectively. From Figures 6 and 7, all are close to 1. Therefore, no clear statistical relationship was found between the pipe slope, ΔSlope or ΔAngle and odour complaints.

In this study, statistical relationships between ten years' sewer odour complaint data in the studied city and physical parameters of sewer systems were analyzed based on the city's GIS system. Such analysis has not been reported to the authors' best knowledge. Detailed conclusions are as follows.

• (1)

  The snowmelt in spring and the precipitation in summer were believed to be positive factors that can reduce the odour complaints. It implies that dosing some ice and snow into the sewer pipeline might be a potential method for reducing odour complaints. Economic cost-benefit analysis is needed in the future to further evaluate its practicability.

• (2)

  Sewer odour complaints are more likely to happen around older manholes in the city, particularly with manhole age of over 50 years. The pipe material's influences on odour complaints also coincide with the age effect.

• (3)

  Odour complaints tend to concentrate around drop manholes, especially, when the manhole drop height is larger than 10 m.

• (4)

  A combined sewer system is twice as likely to have odour issue than the sanitary sewer system.

• (5)

  The pipe slope, ΔSlope and ΔAngle show no obvious influences on the odour problem in the city.

This simple study is useful for municipalities or utility companies in better understanding the odour complaint data and identifying potential causes and odour hot spots from a statistical point of view, which will guide the odour mitigation measures afterwards.