Enhancing the ventilation of sewers by using inhaled air in the vertical stack of a building

Anaerobic conditions in sewers can result in serious problems for the operation and management of sewer systems, including odor problems caused by hydrogen sulfide (H₂S), and the explosion risk of methane (CH₄) (Oviedo et al. 2012). Insufficient ventilation of collection system headspaces could lead to elevated H₂S-related corrosion of the concrete and steel in the sewage pipes. Ventilation of sewer systems is important for the maintenance of aerobic conditions in wastewater and to control odor and corrosion problems (Madsen et al. 2006; Saracevic et al. 2007).

Previous studies provided valuable information on ventilation systems (Jack et al. 2006). Municipalities, such as Edmonton and Los Angeles, have used forced ventilation, where sewer air is extracted with fans or blowers and is routed to air treatment facilities (Zhang et al. 2015). Ventilation equipment must be properly sized, and ventilation should always be maintained while the structure is accessible (Wierzchowski et al. 2009). This can make the cost of ventilation very high, and in some cases, ownership and responsibility for the operation, inspection, and maintenance of ventilation and sewer systems may not be clear (Hendershot 2015). Significant air entrainment processes are commonly observed in drop manholes (Granata et al. 2015), and the air could flow into sewers to enhance ventilation. The fundamental physics of air movement through sewer systems are also related to the ventilation of the sewers.

Five factors have been identified as major contributors to air movement in sewers (Pescod & Price 1982): sewage drag, air pressure differences in various parts of sewer systems, wind across ventilation stacks, rise or fall of the sewage level in sewer pipes, and temperature differences between sewer air and the air above ground. The influence of sewage drag has been studied numerically by Edwini-Bonsu and Steffler (Edwini-Bonsu & Steffler 2006). Pescod & Price (1982) developed theoretical models to assess the effects of wind speed and density. A dynamic ventilation model, developed by Wang et al. (2011), included pressure differences and sewage level fluctuations.

Inhaled air could be utilized to enhance the ventilation of sewers during drainage in the vertical stack of a building (Figure 1). Appliances discharge into the vertical stack of the building, which caused negative pressure in the upper floors and positive pressure in the lower floors. The pressure differential allowed the outer air to be inhaled and flow into the vertical stack in the shape of a bubble. A hydraulic jump was then generated in the connection, which caused the evolution of the air and positive pressure. Septic tanks are widely used in on-site wastewater treatment systems (Forquet & Dufresne 2015), and when the building was linked to a septic tank, the air ran directly out of the exhaust port of the septic tank. The septic tank, which was not used to its full potential, subsequently exploded, partially because of the ventilation. When the building was linked to the municipal pipe network, the air flowed directly into the sewer and the air pressure difference of the sewer system changed, ultimately enhancing the ventilation of the sewer.

This study focused on determining the quantity of air that could be used to enhance the ventilation of sewers and the effect of this ventilation. By detecting the pressure and air velocity of the ventilation cowl and the inspection wells, which are linked to the vertical stack, the direction of the inhaled air through the vertical stack of the building and the relationship between the inhaled airflow rate and the air used to enhance the ventilation of the sewers were confirmed. A model is also proposed as a way to calculate the airflow rate of the inhaled air according to the drainage height and the water flow rate of the sanitary ware. The model was constructed on the basis of dimensional methods and was validated using a different building. Finally, according to the model and the observed service condition of the sanitary ware in the building, it is easy to estimate the daily average quantity of inhaled air and the effect of this ventilation.

Field studies were carried out in building 1, which was the office building of the Municipal Environmental and Engineering Institute of Xi'an University of Architecture and Technology (Figure 2(a)). The wastewater connection of the building was linked to the municipal pipe network, and the wastewater flowed directly into the sewers, not into a septic tank. The office building (7 floors) has a single stack drainage system that contained three vertical drainage stacks (I, II and III). The horizontal fixture drain branch (branch pipe) was φ75 mm; the vertical stack pipe and the connection were φ125 mm; the diameter of the ventilation cowl was approximately φ125 mm. The height of each floor was 3 m, and the ventilation cowl was 1 m above the roof. The grade of the connection was 0.01, and the length was 2 m. The three vertical drainage stacks (I, II and III) separately served the women's lavatory, closet pans of the men's lavatory and urinals of the men's lavatory. There were four closet pans in every men's and women's lavatory. The normal wastewater flow of the urinals was too small to observe in the field data. The wastewater flow of the closet pan was 1.2 L/s. All of the drainage experiments were conducted on the closet pans of the men's lavatory. The water flowed through five inspection wells and then flowed into the main pipe of the school. The diameter of the pipe was φ300 mm, and the grade was 2‰. All of the opening holes to the inspection wells were 2 cm².

Model validation was conducted in building 2, which is the main building of Xi'an University of Architecture and Technology (Figure 2(b)). There are 10 floors in this building, and the height of each floor is 3 m. The ventilation cowl was 1 m above the roof, and the diameter of the ventilation cowl was approximately φ180 mm. There were three closet pans in every men's lavatory, and the wastewater flow of the closet pans was 1.2 L/s. The experiments were again conducted on the closet pans of the men's lavatory.

An outline of the experiment is shown in Table 1. The operating conditions were separated into three parts: Phase1, Phase 2 and Phase 3. Air velocity and air pressure were measured in the field survey. The air pressure and velocity were recorded whenever a sudden change occurred, and the measurements continued until the air pressure and velocity returned to their original state. The measurement sites were the ventilation cowl and the opening holes of the inspection wells.

A Testo-2 air velocity meter measured the instantaneous air velocity and average air velocity, and the sensitivity was 0.01 m/s. The Testo-2 also recorded the drainage time. A 521-1 differential gage measured the instantaneous air pressure and took a reading every five seconds to obtain the average air pressure; the sensitivity was 0.1 Pa. A M40 concentration meter was used to detect the concentrations of H₂S and CH₄; the sensitivity was 0.1 ppm.

The direction of inhaled air in the vertical drainage stack will determine whether inhaled air can enhance the ventilation of the sewers or not. If most of the inhaled air flowed upward into the environment after drainage, it could not enhance the ventilation of the sewer. In phase 1, the instantaneous air pressure and air velocity in the ventilation cowl were observed to confirm the direction of airflow in the vertical drainage stack, and the tendencies of the variation were almost identical. Figure 3 shows the air pressure and velocity when two closet pans drain at the same time from the seventh floor.

The air pressure reduced quickly and the air velocity increased rapidly within a very short time after drainage (Figure 3). The minimum air pressure coincided with the maximum air velocity. The air pressure then started to increase, and the air velocity began to decline, ending in their original states. After this, the air pressure and velocity remained in their original states. The total drainage time was approximately 50 s. These results demonstrated that as appliances discharged to the vertical drain stack, air was inhaled and flowed with water through the vertical drainage stack.

In phase 1, the average air pressure and air velocity in the ventilation cowl and opening holes were monitored to determine whether air flowed into the sewers or not. Figure 4 shows the average air pressure and air velocity of the ventilation cowl and opening holes under these conditions.

Along with the increases in the water rate, the absolute value of the average air velocity and air pressure increased (Figure 4). The conditions of the first and second inspection wells were identical because of the short distance between these two monitoring locations, but after this, the absolute value diminished gradually. The average air velocity and air pressure of the inspection wells were zero after the fourth inspection well. The results indicated that the condition of the opening holes changed with the variation of the ventilation cowl, so when appliances discharged to the vertical drain stack, inhaled air flowed with water into the vertical drainage stack and, ultimately, into the sewers.

Figure 5 shows the variation of the airflow rate in the connected sewers. The total air flowing into the sewers can be broken down into two parts: air flowing out of the sewer through the opening holes and air flowing through the headspace of the sewers. The total airflow rate was 16.19–21.72 L/s, and part of this air flowed out of the sewers. Therefore, the airflow rate started to decline and finally reached a steady state value, which accounted for 94% of the total air that flowed into the sewers. The tendencies of the variation under the three conditions were identical, indicating that little air flowed out of the sewer. Approximately 94% of the air flowed into the headspace of the sewer to enhance the ventilation.

The drainage of the building was intermittent and random, so the air flowing into the sewer was also intermittent. Inhaled air flowed with water into the vertical stack, and approximately 94% of the total airflow entered the sewer each time. This air enhance the ventilation in the sewer.

The airflow rate through the ventilation cowl determined the quantity of air that entered the vertical stack. The results of Phase 1 yielded the quantity of air that was delivered to the sewer with every water drainage. The airflow rate is a dominant component of the vertical drainage stack, and it plays a critical role in the subsequent operation of the vertical drainage stack, where the mechanism is assumed to be a quasi-fan machine (Cheng et al. 2005). The fan laws can be expressed using hydraulic parameters, such as the air density, pressure, and velocity. It was challenging to confirm the relationship between these three parameters. The relationship between the air velocity and air pressure was direct, so it was easy to generate. Using the dimensional method, the air pressure was expressed in terms of the air density, the water flow rate and the height of the drainage floor. Hence, the airflow rate could also be presented using these variables.

In phase 2, the average air velocity and air pressure of the ventilation cowl were measured (Figure 6). Figure 6 shows that as the water flow rate and drainage height increased, the average air velocity increased and the average air pressure decreased. The variation tendency of the average air pressure was more obvious than the average air velocity because the pressure was more sensitive. However, a small variation in the average air velocity would result in a large change in the airflow rate.

The relationship between the average air velocity and average air pressure is shown in Figure 7. The R² value was found to be rather high (0.9642), indicating that there is a good linear relationship.

According to the drainage height and the water flow rate of the sanitary ware, the total airflow rate of the inhaled air discharging into the vertical drainage stack could be calculated. Approximately 94% of the total air flows into the sewer, so the flow rate of the air, which was used to enhance the ventilation, can be determined.

In phase 2, the model of the airflow rate was validated using an office building. When the four closet pans on the same floor, between the second floor and the seventh floor, started discharging to the vertical drain stack, the airflow rate was measured. The measured airflow rate was plotted against the modelled airflow rate, as shown in Figure 9. The measured data were plotted against themselves to form a perfect-fit line. The model predictions were then compared to this perfect-fit line.

In Figure 9, the R² value is 0.9864 and the absolute and relative errors of the airflow rate are 0.59 L/s and 3.9%, respectively. This indicates that the trends produced by the model-predicted values followed the tends observed in the observational data. Therefore, we can use the model to express the airflow rate in this office building.

In phase 3, the model of the airflow rate was validated again using building 2. Using the air velocity and diameter of the ventilation cowl, the measured airflow rate was obtained. According to the actual parameters of the model, the modelled airflow rate was increased. The measured airflow rate versus the modelled airflow rate is presented in Figure 10. As with Figure 9, the measured data are plotted against themselves to form a perfect-fit line. The model predictions were then compared relative to this perfect-fit line.

In Figure 10, the R² value is 0.974 and the absolute and relative errors of airflow rate are 2.25 L/s and 10.5%, respectively. This model, built on the basis of the dimensional method, has provided a way to calculate the airflow rate and is applicable to a different building.

Using the model and the observed service condition of the sanitary ware in building 1, the daily average quantity of the inhaled air was easily estimated, and approximately 94% of the inhaled air flowed into the headspace of the sewer. Using this information, the average daily quantity of air used to enhance the ventilation of the sewer could also be estimated.

By observing the drainage frequency of the closet pan on different floors for a week, the total frequency of drainage from each floor was 382, 368, 359, 346, 331, 314 and 283 for floors one through seven, respectively. These observations were carried out every day from 8:00 to 22:00. According to Equation (6), the airflow rate of the drainage could be calculated using the total drainage time of approximately 50 s. Using the airflow rate associated with drainage, the total drainage time and drainage frequency, the total quantity of the inhaled air in a week was calculated to be 1,116.86 m³. The daily average quantity of the inhaled air was approximately 159.551 m³/d, so the daily average quantity of the air that was able to enhance the ventilation of the sewer was approximately 149.978 m³/d.

The quantity of the inhaled air was compared with the amount of oxygen injected into the sewers to reflect the effect of ventilation. A study conducted on the Tugun-Elanora wastewater system injected oxygen to reduce emissions of hydrogen sulfide (Park et al. 2014). The amount of oxygen injected was 440 kgO₂/d, which is equivalent to 1,550 m³/d of air. The total quantity of inhaled air in a week was approximately 1,116.86 m³, so after a few days, the effect of inhaled air ventilation on the vertical stack of the building would be equivalent to injecting oxygen. The effect of ventilation was tested by measuring the concentrations of the H₂S and CH₄ in the five inspect wells. These concentrations were always zero, indicating that the ventilation of the sewer was enhanced by the inhaled air in the vertical stack of the building. These results suggest that the use of inhaled air in the vertical stack of the building is an effective and persistent way to enhance the ventilation of sewers.

• 1.

  When appliances discharge to the vertical drain stack, outside air could be inhaled and flow toward the sewer with water. Approximately 94% of the air will flow into the headspace of the sewer to enhance the natural ventilation.

• 2.

  A model that was built on the basis of dimensional method was proposed as a new way to calculate the airflow rate of the inhaled air using the drainage height and water flow rate of the sanitary ware. Model validation was conducted in a different building, and the relative error of the airflow rate was 10.5%.

• 3.

  Using the airflow rate of the drainage, total drainage time and drainage frequency, the daily average quantity of inhaled air was estimated to be approximately 159.551 m³/d, so the daily average quantity of the air used to enhance the ventilation of the sewer was approximately 149.978 m³/d.

• 4.

  The concentrations of the H₂S and CH₄ in the sewers were zero, indicating that the ventilation of the sewer was enhanced by the use of inhaled air in the vertical stack of the building. This shows that inhaled air is an effective and persistent way to enhance the ventilation of sewers.

This research was supported by National Natural Science Foundation of China (No. 51778523).