Ventilation in Sewers Quantified from Measurements of...
Transcript of Ventilation in Sewers Quantified from Measurements of...
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Ventilation in Sewers Quantified
from Measurements of CO2
Presenter: Emil D. Fuglsang
Co-authors: Asbjørn H. Nielsen, Jes Vollertsen
Department of Civil Engineering
Aalborg University, Denmark
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Conveyer of water
Conveyer of gas
Biological and chemical reactor
WHAT IS A SEWER?
Gas flow
Water flow
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Odor problems
- As long as the smelly sewer gas stays in the sewer – why
care?
Asset corrosion
- Corrosive hydrogen sulfide attacks concrete and metals
Safety issues
- Hydrogen sulfide is highly toxic
WHY STUDY VENTILATION?
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Sewer gas is transported by means of:
- Drag effects
- Wind at surface
- Barometric pressure gradients
- Temperature gradients
- Changes in water level – pumps starting and stopping
WHAT DO WE KNOW ABOUT SEWER
VENTILATION?
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Measuring methods typically used:
- Anemometers
- Tracer gas experiments
- Pressure transducers
- Physical models
Ventilation is a dynamic
phenomenon changing in time and
space
Tracer measurements and
velocities are labour intensive and
do seldom give us the full
dynamics of the process
HOW IS VENTILATION MEASURED?
Madsen H I, Hvitved-Jacobsen T, Vollertsen J (2006).
Gas Phase Transport in Gravity Sewers – A methodology for determination of
horizontal gas transport and ventilation.
Water Environment Research, 78(11), 2203-2209
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We test if CO2 can be used as a naturally occuring tracer –
overcomming those issues
OBJECTIVES
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CONCEPTUAL IDEA
High CO2 concentrations in water
Caused by biodegradation and
potable water CO2 contents
Easily measurable
Low CO2 concentrations in atmosphere
Constant
CO2 is released at a known rate from
water to gas
Release rates depend on hydraulic
conditions in the pipe
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CONCEPTUAL IDEA
In a completely closed system the partial
pressure of CO2 is the same in the water
as in the gas
In a completely ventilated system, the
partial pressure of CO2 in the sewer gas
is the same as in the atmosphere
In a partly open system, the CO2
concentration in the gas tells us how fast
the sewer gas is ventilated out
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We have tested this at an intercepting sewer in Denmark
PROJECT SITE
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Installation of equipment in the sewer
MEASUREMENT CAMPAIGN
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Data from January 2012
RESULTS
Time Jan 13-14 2012
17:00 21:00 01:00 05:00 09:00
pH
[-]
7.0
7.5
8.0
8.5
9.0
Concentr
ation o
f C
O2 [ppm
]
0
2000
4000
6000
8000
10000
12000
14000
pH
CO2
Background CO2
Equilibrium concentration
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CONCEPTUAL IDEA
Known
Known
Known
Calculated
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WHAT WILL THE RETENTION TIME BE?
Calculated sewer gas retention time [hours]
0 1 2 3 4 5 6
Pro
ba
bili
ty [%
]
10
30
50
70
90Jan 13-14, 2012
Feb 3-4, 2012
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To release CO2 from the water to the air, pH must be below ~8
A sufficient amount of alkalinity must be present in the
wastewater
LIMITATIONS OF THE METHOD
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A new approach to the sewer air ventilation has been
suggested
In this case study sewer-air retention times around 1.5-2.5
hours were found
The method should be addaptable to other systems as long as
the pH is not too high and when a certain amount of alkalinity
is present
CONCLUSIONS
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Questions?