A NOVEL MODEL FOR SEWER VENTILATION

E M I L D I E T Z F U G L S A N G

D E PA R T M E N T O F C I V I L E N G I N E E R I N G

A A L B O R G U N I V E R S I T Y, D E N M A R K

Motivation – why are we interested in sewer ventilation?

Broken covers leed to discharge of air Corrosion of concrete and metal

Methodology - how to use CO2 as a natural tracer

Wastewater is always super saturated with CO2

For example at pH 7.5 water in equilibrium with atmospheric CO2:

• 0.2 mM CO2 -equivalents in the wastewater

BUT:

• Biological processes in wastewater produce 0.5-1mM CO2 –eq. per hour

• Furthermore, potable water is often CO2 -super saturated

Wastewater is always super saturated with CO2

𝐶𝑂𝐷+𝑂2→𝐶𝑂2

Sewer ventilation CO2 is continuously released from the water to the sewer gas and diluted by

the sewer ventilation

Alternating water levels can furthermore cause in- and outflow of sewer gas

Balancing production and release allows determination of ventilation

Methodology – in the field

• Length profile CO2 + pH

Inspected the sewer for openings to the atmosphere

Methods – computer simulations

• Full dynamic transient hydraulic model• Upper boundary conditions: intermittent operated pump station• Resulting in rapid water level changes in the pipe…

• WATS sewer process model• Full dynamic simulation of chemical and biological transformations

• Quasi stationary friction-drag gas flow model

Results two hours in the upstream and downstream pointMeasured Simulated

Length profile of CO2

• Large effect of pump starts at the top of the system

• Continuing release further downstream

Conclusions

Combining CO2 as a natural tracer and an in-sewer process model it is possible to predict levels and dynamics of ventilation in an intercepting sewer

QUESTIONS?