A Tracer Study of Headspace Ventilation in a Collector Sewer

7/30/2019 A Tracer Study of Headspace Ventilation in a Collector Sewer

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Parker and Ryan

582 Journal of the Air & Waste Management Association Volume 51April 2001

ISSN 1047-3289J. Air & Waste Manage. Assoc.51:582-592

Copyright 2001 Air & Waste Management Association

TECHNICAL PAPER

A Tracer Study of Headspace Ventilation in a Collector Sewer

Wayne J. Parker

Department of Civil and Environmental Engineering, Carleton University, Ottawa, Ontario, Canada

Helen Ryan

Industrial Wastes Section, Regional Municipality of OttawaCarleton, Ottawa, Ontario, Canada

ABSTRACT

A field-scale tracer test was conducted to evaluate in-situ

ventilation rates in a major collector sewer. The sewer

under study was ~11 km long and ranged from 0.61 to

2.1 m in diameter. For the purposes of the tracer testing,the collector was divided into four reaches, each of which

was tested individually. The tracer test involved injecting

a measured volume of CO gas into a manhole over a short

time period. CO concentrations were then measured in

the collector headspace at selected manholes along the

length of the reach.

The technique employed successfully measured av-

erage headspace velocities over extended lengths of the

collector. In a section that had a relatively stagnant

headspace, ~1.1 km of sewer could be evaluated, with sub-

stantial tracer loss attributed to losses to manholes. In a

section of the sewer with elevated headspace velocities, asection ~7.0 km long was successfully tested with one in-

jection of tracer gas. The velocities observed in the collec-

tor varied substantially with time and location in the

collector. The lowest velocities measured were in the up-

stream sections, with a minimum observed value of 3.8

m/min. The highest velocities were observed in the down-

stream sections, with a maximum value of 31.5 m/min.

The presence of a substantial drop structure appeared to

reduce the headspace velocity in the upstream reach. In

general, there was an increasing trend in gas-phase flows

with distance along the length of the collector. Flows at

IMPLICATIONS

Ventilation in sewer systems impacts on gas-phase con-

centrations of volatile substances as well as on emis-

sions of these substances from sewer systems. The

study described here consisted of tracer analysis of

headspace ventilation in a full-scale collector sewer and

is the first study of its kind. The results of this study pro-

vide a range of ventilation rates that can be used as guid-

ance when modeling the fate of volatile substances in

sewer systems.

the discharge end of the collector were almost 2 orders of

magnitude greater than those at the beginning.

INTRODUCTION

The Regional Municipality of OttawaCarleton is conduct-ing an ongoing study of the ventilation patterns within

its collector sewer systems. The information collected in

this study will subsequently be employed to assess the

fate of volatile organic compounds and odor-bearing com-

pounds within the collector system. A number of studies

describing the behavior of these substances in sewer en-

vironments have been reported.1-5 In these studies, the

parameter consistently recognized as an unknown is the

degree of headspace ventilation that occurs under natu-

ral conditions. Five factors that have been identified as

contributing to sewer ventilation include wind across ven-

tilation stacks, wastewater drag, the rise and fall of waste-water levels, temperature differentials, and barometric

pressure fluctuations.6

Pilot and field-scale tests were performed to assess the

impact of wind and wastewater drag on sewer ventilation.6

In the pilot study, it was found that the water velocity and

the relative depth of water in the sewer impacted upon air

movement. The average air velocity was found to increase

with the product of the water surface width and the liquid

velocity divided by the hydraulic radius of the headspace.

In experiments with a 300-mm diameter pipe, the average

velocity approached a limiting value of 0.2 m/sec. The field

studies focused primarily upon exchange of headspace gaswith the atmosphere through educt stacks and did not fo-

cus directly on movement of the headspace gas.

Field testing of headspace ventilation in two sewer

reaches with lengths ranging from 464 to 803 m has been

reported.7 In this study, the use of continuous injection

of sulfur hexafluoride (SF6) and slug injection of CO was

evaluated, with the latter determined to be superior when

losses of tracer to manholes occurred. In a sewer with a

diameter of 0.9 m and a slope of 0.2%, the headspace

velocities were found to vary from 0.09 to 0.17 m/sec.


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Parker and Ryan

Volume 51April 2001 Journal of the Air & Waste Management Association 583

The studies of Pescod and Price6 and Monteith et al.7

do not include all of the sewer structures and conditions

that may be present in large collector sewer systems. For

example, the presence of drop structures and hot-water

discharges in a sewer system may impact upon the migra-

tion of sewer gases. An approach for estimating ventila-

tion in full-scale systems based on pressure measurements

and the use of empirical correlations that relate velocity

to pressure drop has been reported.8 However, limited

measured data are available that describe ventilation pat-

terns in full-scale sewer systems; hence, in this project,

the ventilation patterns in a major collector sewer were

evaluated. A volume of tracer gas was injected into se-

lected manholes along the length of the collector and was

continuously monitored at downstream manholes. All

manholes were maintained in place to ensure that the

sewer was functioning in a natural state. Two rounds of

testing were completed to investigate ventilation under

warm-weather conditions.

DESCRIPTION OF COLLECTOR SEWER

The sewer evaluated in this study collected sewage from a

mixture of residential and small industrial sources and

drained into a second collector through a drop structure

with a drop height of ~5.8 m. The collector had a length of

~11 km and varied in diameter from 0.61 to 2.4 m. The

slope of the collector varied considerably over the length of

the sewer, with minimum and maximum slopes of 0.001

and 0.9%, respectively. The physical dimensions of the col-

lector are summarized in Table 1. An inverted siphon was

located 1717 m from the upstream end of the collector. Thesiphon was constantly submerged and, hence, acted as a

permanent block for headspace movement past this point.

Two drop structures were located 3496 and 3755 m

from the upstream end of the collector. Both structures

were located in the 1.4-m diameter portion of the collec-

tor and had drop heights of 7.1 and 1.3 m, respectively.

The collector had a number of small connections enter-

ing into it along its length. For the purposes of this re-

port, only the major city connections will be documented.

The characteristics and locations of the major connec-

tions are presented in Table 2.

SEWER VENTILATION TESTING

A tracer technique was employed to characterize the ven-

tilation patterns in the sewer. In this technique, a volume

of CO gas was introduced into the sewer at a manhole

and was monitored at selected manholes downstream of

the injection point. Selective monitoring was performed

to determine whether the sewer gas moved countercur-

rent to the wastewater movement. Under none of the cir-

cumstances evaluated in this study was the tracer gas

found to move countercurrent to the wastewater flow.

The CO gas was injected into the sewer at a controlled

flow rate from a cylinder of pure gas via a pressure regula-

tor, a rotameter, and a length of 3-mm inside diameter

flexible copper tubing. Sampling was conducted by con-

tinuously pulling gas from the headspace of the sewer via

this copper tubing and then continuously analyzing the

gas for CO with an Industrial Scientific Model 410 gas

detector. The gas was drawn with the detectors Industrial

Scientific SPF400 on-board pump at a pumping rate of

~23 cm3/sec. All injection and sampling tubing was in-

stalled several days before the tests, and the lines were

cleaned with compressed air just prior to the test. In all

cases, the copper tubing was passed through the pick holes

in the manholes so that the ventilation testing could be

performed with all manholes in place. The CO gas was

introduced and withdrawn at an elevation approximatelyin the middle of the sewer headspace. The injection pip-

ing was bent to direct the CO gas downstream, while the

sampling tubing was bent to collect gas from upstream.

Because it was anticipated that CO gas would be lost

from the tracer due to entrapment in manholes and out-

gassing from some manholes, the sewer was divided into

four reaches for testing. The first reach extended from the

beginning of the collector to the siphon. A summary of

Table 1. Summary of collector physical characteristics.

Segment Diameter (m) Incremental Cumulative Range of

Length (m) Distance (m) Slopes (%)

1 0.61 1189 1189 0.0070.67

2 0.69 643 1832 0.090.94

3 0.76 532 2364 0.0010.77

4 1.4 518 2882 0.040.48

5 1.2 1765 4647 0.0010.83

6 1.4 2788 7435 0.0010.65

7 1.5 1294 8729 0.0060.32

8 2.0 1511 10,240 0.0010.44

9 2.1 808 11,048 0.0010.70

Table 2. Summary of major connections to collector.

Cumulative Diameter (cm) Cumulative Diameter (cm)Distance (m) Distance (m)

0 30, 45 1798 75

361 25, 25 3291 37.5

749 25 3496 30

1111 30 3755 37.5

1313 20 3760 30

1491 22.5 3969 22.5

1533 22.5 6417 61

1575 37.5


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the reaches and sampling locations is presented in Table 3.

Differing quantities of CO gas were injected into the

reaches depending upon the diameter of the reach tested.

Table 4 presents a summary of the quantities injected into

these reaches.

In addition to monitoring CO concentrations, the

headspace temperature was monitored at the sampling

locations over the duration of the test. Wastewater flow

data were collected from the in-sewer monitoring station

located at the discharge of the collector. Information on

atmospheric conditions including temperature, wind

speed, and barometric pressure was obtained from the local

weather office for the testing period.

RESULTS

The results of the tracer testing consisted of CO con-

centrations measured at each monitoring location as a

function of time. Figure 1 presents typical response

curves obtained from the three sampling locations in

the third reach for the first round of testing. The shapes

of these curves are indicative of a dispersed plug flow

regime in the sewer headspace. At all three sampling

locations, the CO concentration increased rapidly to a

peak value and then decreased at a slower rate with some

tailing of the curve.

In all of the runs, the CO peak height consistently

decreased in magnitude as it progressed through a reach.

For example, in the first run conducted on the third reach,

the peak concentrations were 759, 171, and 111 ppm at

the three sampling locations. There was less of a trend in

peak widths, with corresponding values of 63, 32, and 35

min, respectively, for this run. Peak widths were deter-

mined as the time from which detectable concentrations

were measured at the monitoring location until the time

that the gas could no longer be detected. In other runs,

the peak width increased in some cases and decreased in

others. The shape of the response curve observed in the

sewer headspace would be determined by longitudinal

dispersion in the sewer reaches, exchange of sewer

headspace with manholes, and gas-liquid mass transfer

of the tracer. This assumes that migration of the tracer is

dominated by advective processes and that diffusion can

be neglected. Given the relatively high velocities measured

in this study, this assumption is valid.

If dispersion were the only mechanism affecting the

shape of the tracer response curve, then the mass of CO

would be conserved; that is, there would not be any loss

of CO from the sewer. Exchange with manholes would

likely result in the loss of some of the CO from the sewer,

thereby tending to reduce the total area under the con-

centration versus time curve. In addition, CO gas thatmigrated into a manhole as the bulk of the CO passed by

could then reenter the sewer headspace and thereby tend

to extend the duration of the CO peak. Entry of air into

the sewer along a reach would also change the tracer re-

sponse curve by diluting the tracer concentration.

When tracer gas transferred into the liquid phase, it

would then migrate with the liquid phase. If there were a

velocity differential between the two phases, the tracer

could reenter the gas phase from the liquid. To assess the

potential for this latter mechanism, a dynamic model

considering the gas-liquid mass transfer of CO for a sewer

reach flowing half-full with a diameter of 1 m and a lengthof 0.5 km was assembled. A dispersed plug flow regime

was employed to represent the mixing of the headspace

in the sewer. A mass transfer coefficient for CO was calcu-

lated using the correlation of Parkhurst and Pomeroy,9

which was corrected for the diffusivity of CO, and a

Henrys Law coefficient of 5 104 atm/mol fraction10 was

employed. In this test reach, it was found that less than

2% of the tracer gas transferred from the gas to the liquid

phase, and there was negligible reentry of CO into the

gas phase. Hence, for the purposes of this study, it was

Table 3. Summary of reaches tested.

Reach Start/End Sample Location Distance from

Distances (m) Distances (m) Injection (m)

1 38 118 80

1651 573 535

1189 1151

1651 1613

2 1794 2329 535

3722 3722 19283 3722 4765 1043

7563 6379 2657

7563 3841

4 7563 8620 1057

11,048 9882 2319

11,048 3485

Table 4. Summary of CO gas injection.

Run Reach CO InjectionVol. Flow Mass Flow Duration Volume (L)

(L/min) (g/min) (min)

1 1 10 9.6 5 50

2 10 9.6 5 50

3 30 28.8 5 150

4 30 28.8 10 300

2 2 10 9.6 5 50

1 10 9.6 5 50

3, 4 30 28.8 5 150


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assumed that migration of tracer in the liquid phase had

a minimal effect on the shape of the gas-phase tracer curve.

Traditionally, tracer mass recovery is employed as a

measure of quality control for a tracer study. However,

these tracers are commonly employed in closed systems

with constant volumetric flow rates. In the system under

study, velocities and flow rates varied with distance along

the sewer. The values subsequently reported were aver-

aged over the length of each of the subreaches (length of

collector between an injection and sampling point or be-tween two sampling points) studied and do not reflect

values at any specific location. Therefore, it was not pos-

sible to accurately calculate a mass flow rate of tracer at

any of the monitoring locations. And because an inde-

pendent measure of flow rate at each monitoring point

was not available, there was insufficient information to

perform a formal tracer mass recovery analysis. Subsequent

discussions of tracer losses refer to changes in the area

under the concentrationtime diagram and do not repre-

sent a true tracer recovery analysis.

The tracer technique employed here was effective

in quantifying the travel times for headspace gas overrelatively long lengths of collector. In the second run, a

single injection was effectively employed to character-

ize the travel times in both reaches 3 and 4, constituting

a combined length of ~7 km of sewer. This effectiveness

was primarily due to the relatively high rate of move-

ment of sewer gas in these reaches, which appeared to

result in small tracer losses during the test. More diffi-

culty was encountered in reach 1, where the headspace

velocities were low and substantial tracer loss was observed.

In this reach, only the first 1100 m of the reach could be

evaluated with one injection. The average time required

for the CO to move through a subreach was estimated as

the first moment of the peak area around the origin, as

follows:

(1)

where tavg

is the average travel time for CO tracer (min); ti

is the time of individual CO concentration measurements

(min); Ci is the CO concentration (ppm); and n is the to-tal number of concentration measurements.

The average velocity of the gas in each subreach was

then calculated as the subreach length divided by the

average travel time in the subreach. It should be noted

that these velocities were averaged over a considerable

length of sewer and in several of the subreaches encom-

passing more than one size of pipe. The entrance of tribu-

tary connections between sampling locations also

impacted upon the localized velocities in that additional

liquid flow entered the sewer at these points. The addi-

tional liquid flow increased the velocity in the collector

sewer and, hence, increased the liquid drag on the gasphase. In addition, it is likely that collector sewers intro-

duce additional gas flows pulled from the tributary sewer

along with the liquid flow. Thus, local velocities at a point

would likely have deviated from the average values mea-

sured in this study. The estimated values of the average

velocities calculated from the data collected in this study

are summarized in Table 5.

Examining Table 5 reveals that the velocities observed

in the collector varied substantially with time and location

in the collector. Reach 1 demonstrated the lowest velocities

Figure 1. CO concentrations in reach 3, run 1.


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in this portion of the study. The low velocities observed

in this section resulted in considerable loss of tracer gas

as it progressed through the system. The tracer loss was

apparent from the substantially diminished size of the

CO peak observed at the monitoring sites. When the CO

tracer moved slowly through the collector, there would

be an increased opportunity for a fraction of the gas to

migrate into each manhole it passed. In both runs, the

concentrations measured after the first three subreaches

had diminished to such an extent that it was not possible

to measure concentrations in the last subreach, so it was

not possible to estimate a velocity there. Reach 1 wasbounded by connections to city sewers on the upstream

end and the siphon on the downstream end. In addition,

this reach had the smallest diameter, with predominantly

0.61- and 0.69-m diameter pipe. The combination of the

obstruction posed by the siphon and the increased friction

exerted by the small-diameter pipe was likely responsible

for the low velocities. In general, it would appear that the

headspace in this section was relatively stagnant as com-

pared with the other reaches examined in this study.

The velocities observed in reach 2 were generally

higher than those in reach 1, likely partially due to the

absence of a confining structure at the downstream endof reach 2. In both tests of this reach, the velocity in

subreach 1 was noticeably higher than the velocity in

subreach 2, despite the fact that the upstream end of

subreach 1 was effectively sealed by the presence of the

siphon. The decrease in velocity in subreach 2 may have

been a result of the presence of a very substantial drop

structure (drop height = 7.1 m) located in this subreach.

Drop structures such as these can cause substantial

in-gassing at that location and pressurize the bottom of

the drop.8 This pressurization may have reduced the air

flow through the drop structure; however, because the

tracer passed through the drop structure, apparently this

pressurization did not completely stop it. In the second

test, the velocity in subreach 1 was almost twice that ob-

served in the first test, indicating a substantial day-to-day

variability in ventilation within this subreach.

The velocities observed in reaches 3 and 4 were gen-

erally higher than those observed in reach 2. The ve-

locities increased and decreased somewhat randomly

and appeared to have reached a plateau at a velocity of

~25 m/min. Considerable variability was observed be-

tween the two test runs in some of the subreaches. The

largest difference was for subreach 1 of reach 3, where

the velocities differed by a factor of almost 3. Consid-

erable differences between tests were also observed for

subreaches 1 and 2 of reach 4, with a factor of almost 2

for the latter.

The temperature of the sewer headspace was mea-

sured at each sampling location as an indicator of the

discharge of warm streams into the sewer that might

impact on gas movement. Table 6 summarizes the mini-

mum, mean, and maximum temperatures measured at

each sampling location. Several observations can be made

with respect to the sewer headspace temperatures. In

general, at each sampling location the headspace tem-

peratures did not substantially change over the duration

of a test. This is apparent from the relatively narrow

ranges about the mean values (typically 1 C). In addi-

tion, similar trends in temperature were observed along

the length of the reaches for the two test runs. In fact,

there were relatively small differences between the aver-age temperatures at each sampling location for the two

test runs, despite an interval of ~10 days between the

test runs. This would suggest that the phenomena im-

pacting the headspace temperatures were consistent over

the period of the testing.

In general, the headspace temperature increased along

the length of the collector from the upstream to the down-

stream end. There was one notable exception to this pat-

tern around the second sampling location of reach 2. In

this case, the temperature increased dramatically from the

upstream sampling location (5.7 and 3.3 C on the first

and second test runs, respectively). The temperature thendecreased over the next section, with decreases of 6.7 and

4.4 C, respectively. The increased temperature at the sec-

ond sampling location of reach 2 was also indicated by a

substantial amount of vapor emitted from this manhole

when the cover was removed for equipment installation.

The increased temperature there may have been due to

discharges of sewage with elevated temperatures into either

this manhole or manholes located immediately upstream.

The decrease in temperature between this manhole and the

downstream sections is presumably due to dilution of the

Table 5. Summary of average velocities.

Reach Subreach Average Velocity (m/min)

Run 1 Run 2

1 1 4.0 5.8

2 4.0 3.6

3 4.0 12.9

4 NA NA

2 1 16.9 28.8

2 11.3 8.2

3 1 8.7 23.4

2 30.1 22.8

3 24.8 25.9

4 1 28.8 17.7

2 31.5 16.8

3 22.7 24.2

Note:NA = gas velocities too slow to measure.


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air stream with cooler air from other tributary sewers as

well as heat transfer from the gas to the cooler liquid phase

in the collector.

Data on the ambient temperature, atmospheric pres-

sure, and wind speed were obtained from the Ottawa

weather station for the days that the testing was con-

ducted. The minimum, mean, and maximum values for

the parameters over the test period are summarized in

Table 7. The atmospheric conditions tended to vary some-

what from day to day as the testing progressed. The

greatest contrast in conditions was between day 2 andday 3day 2 was warm and calm, while day 3 was cool

and windy. Unfortunately, the reaches tested on these two

days were different, and therefore a comparison of the

impact of these contrasting conditions on ventilation rates

could not be performed. In addition to the variation be-

tween test days, the ambient temperature and wind speed

changed throughout each day. Temperature followed an

increasing trend with time each day, while the wind speed

tended to vary randomly with time. By contrast, the at-

mospheric pressure tended to remain relatively constant

during each test day. Hence, it is not expected that fluc-

tuations in atmospheric pressure would have had any sig-

nificant impact on the movement of gas in the sewer

headspace.

The headspace velocities observed in this study ap-peared to plateau at ~25 m/min. In a pilot study,6 a pla-

teau of ~12 m/min was observed. The results of this

study clearly indicate that headspace velocities in full-

scale systems can exceed those observed in the previous

Table 6. Summary of sewer headspace temperatures.

Reach Subreach Temperature (C)

Minimum Mean Maximum

Run 1 Run 2 Run 1 Run 2 Run 1 Run 2

1 1 NA 14.9 NA 15.0 NA 15.1

2 NA NA NA NA NA NA

3 10.0 13.7 11.0 13.8 13.6 14.3

4 NA NA NA NA NA NA

2 1 13.3 14.4 13.4 14.4 13.6 14.5

2 18.9 16.9 19.1 17.7 19.6 19.0

3 1 12.1 13.1 12.4 13.3 12.6 13.4

2 12.5 14.4 12.7 15.0 13.0 15.8

3 12.9 17.7 14.3 16.2 15.4 17.6

4 1 14.6 17.0 14.7 17.1 14.7 17.2

2 15.8 NA 16.0 NA 16.1 NA

3 15.4 NA 15.4 NA 15.4 NA

Note:NA = not available.

Table 7. Summary of atmospheric conditions.

Parameter Value Day 1 Day 2 Day 3 Day 4 Day 5

Run 1 Run 1 Run 1 Run 2 Run 2

Reach 1 Reach 3 Reach 4 Reach 1 Reach 3

Reach 2 Run 2 Reach 4

Reach 2

Temp. (C) Min 5.0 7.9 10.0 8.5 8.8

Mean 10.8 16.6 11.5 12.0 14.6

Max 16.8 22.0 12.4 15.2 19.2

Atm. press. (mbar) Min 1017.5 1023.5 998.2 1001.0 1008.4

Mean 1018.3 1024.7 999.5 1004.2 1010.4

Max 1019.2 1025.6 1000.9 1005.8 1011.2

Wind speed (km/hr) Min 16.7 7.4 17.8 11.1 13.0

Mean 19.3 9.8 36.5 21.9 19.3

Max 24.1 16.7 55.5 27.8 31.5


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pilot study.6 Correlations between the headspace veloc-

ity and the following lumped parameters have been

proposed6

(2)

(3)

where Wis the width of the water surface; Vis the mean

water surface velocity; L is the perimeter length of the

headspace; andR is the hydraulic radius of the headspace.

In both of these parameters, the driving force for gas

movement is the product of the water surface velocity

and the width of the surface. The resistance to gas-phase

movement is primarily due to frictional forces exerted by

the sewer walls. In these parameters, the unwetted pe-

rimeter (eq 2) and the hydraulic radius (eq 3) are repre-

sentative of this resistance. Hence, the velocity of gas

movement should increase with the liquid drag force and

decrease with the frictional drag.

The correlations of Pescod and Price6 were determined

for a single pilot-scale sewer reach and have not been

evaluated with full-scale data. Therefore, the observed gas-

phase velocities were plotted against the above param-

eters, which were computed for each subreach.

The values ofW, L, andR were computed as the aver-

age values for each subreach in a fashion similar to that

employed to estimate the headspace cross-sectional ar-

eas, which is subsequently described. The resulting plots

are presented in Figures 2 and 3 for eqs 2 and 3, respec-tively. From these plots, it is apparent that there was a

poor correlation between the observed velocities and the

computed parameters. These poor correlations may have

had several causes, including

(1) errors in estimation of the wastewater flows in

the subreaches required to estimate the param-

eters in eqs 2 and 3;

(2) the averaging of sewer properties and flows over

the lengths of the subreaches; and

(3) the presence of structures in the sewer, such as

drops and manholes, that influence gas move-

ment that were not present in the sewer tested

by Pescod and Price.6

In conclusion, it is not likely that the correlations of Pescod

and Price6 can be employed with distance-averaged in-

puts to estimate gas velocities in large systems as employed

in this study.

Drag exerted by wastewater on the headspace gas will

likely influence the movement of the gas. In this study,

wastewater flow rates were monitored at a station located

near the discharge of the collector. In all cases, a diurnal

flow pattern was observed, with little change in the pat-

tern of wastewater flows observed during the testing pe-

riod. The minimum flow tended to vary between test days;

however, these flows occurred very early in the morning

prior to the testing and were therefore expected to have

little impact upon the gas movement measured by the

tracer tests. The average flow observed at the collector

discharge over the five test days was 0.71 m3/sec.

Quantification of the volumetric gas flow rate re-

quired that the cross-sectional area of the sewer occu-

pied by wastewater be determined so that the gas-phase

cross-sectional area could be calculated. In addition, thewastewater flow conditions, such as velocity and inter-

facial surface area, would impact upon the drag exerted

Figure 2. Headspace velocity vs. WV/L.


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by the wastewater on the headspace gas. To investigate

the impact of these parameters on the gas-phase flow, it

was necessary to estimate the liquid flow conditions.

Ideally, measured values for the flow of wastewater in

each of the subreaches examined would be employed

for this purpose. However, this parameter could not be

measured at all of the monitoring locations while the

sewer was closed for the gas-phase testing, so these data

were not obtained.

As previously indicated, wastewater flow data were avail-

able at the discharge of the collector, and it was decided to

use these data to partition the flow to each of the majortributaries into the sewer. The wastewater flow in each

subreach was calculated by accumulating the tributary flows

along the length of the collector. For the purposes of this

study, the flows from the tributary sewers were assumed to

be proportional to the areas of the contributing sewers.

Hence, the total flow measured at the collector discharge

was partitioned to each tributary sewer by multiplying it by

the ratio of the cross-sectional area of the tributary sewer to

the sum of all of the tributary cross-sectional areas. Because

the flow characteristics did not change substantially between

the testing days and the tests were conducted over an ex-

tended period of time each day, the average total flow of0.71 m3/sec was employed to estimate the wastewater flows.

The estimated wastewater flows in each of the subreaches

are summarized in Table 8.

The headspace volumetric flow rates in the collector

were calculated as the product of the gas velocity and the

cross-sectional area of the headspace in the sewer. The

headspace cross-sectional area available for gas flow de-

pends on the liquid flow in the sewer. With a change in

the liquid flow in a reach, the elevation of the liquid will

vary, thereby changing the headspace cross-sectional area.

In addition, a change in slope will also change the liquid

depth in the sewer, thereby influencing the headspace

cross-sectional area.

In this study, average velocities of the headspace gas

were determined for a number of subreaches of the sewer.

These velocities were averaged over the range of pipe di-

ameters and headspace cross-sectional areas present in the

subreaches. Therefore, to determine the average volumet-

ric flow rate in each subreach, an average headspace cross-

sectional area was calculated, as follows:

(1) The subreach was divided into segments with re-

spect to consistent pipe diameter.

(2) The liquid flow in each segment was estimated as

the sum of the flow entering the segment in the

collector sewer and any tributary connections.

(3) The average slope of each segment was calculated

as the weighted average of the individual lengths

Figure 3. Headspace velocity vs. WV/R.

Table 8. Wastewater flows estimated in subreaches.

Reach Subreach Estimated Flow (m3/sec)

1 1 0.09

2 0.133 0.18

4 0.26

2 1 0.44

2 0.58

3 1 0.60

2 0.71

3 0.71

4 1 0.71

2 0.71

3 0.71


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Parker and Ryan


within the segment as follows:

(4)

whereLiis the distance between manholes; S

iis

the slope of the collector between manholes; and

n is the number of pipe length in the segment.

(4) The depth of liquid in the segment was calcu-

lated using Mannings equation and was then em-

ployed to calculate the cross-sectional area

occupied by wastewater.

(5) The cross-sectional area of the headspace was cal-

culated by reducing the cross-sectional area of

the collector by the area of the cross-sectional

segment occupied by wastewater.

(6) The average cross-sectional area of the subreach

was calculated as

(5)

where xiis the length of the segment with a con-

stant diameter; Ai

is the cross-sectional area of

the segment; and n is the number of segments of

constant diameter.

Table 9 summarizes the average volumetric flow rates

estimated for each of the subreaches. In interpreting Table

9, it must recognized that there was considerable uncertaintyin the wastewater flows employed to estimate the headspace

cross-sectional areas and, hence, the volumetric flow rates

in the subreaches. This uncertainty was due to the potential

errors associated with partitioning the total wastewater flow

to each of the tributaries. There was a greater potential for

error in estimation of the upstream flows than of the down-

stream values because all of the significant tributaries were

located ahead of the second subreach of reach 3. In general,

there was an increasing trend in gas-phase flows with dis-

tance along the length of the collector. Flows at the discharge

end of the collector were almost 2 orders of magnitude greater

than those at the beginning.

Figures 4 and 5 present a comparison of the measured

velocities and the calculated flows with distance along

the reach. These figures suggest that while the headspace

velocities tended to vary somewhat erratically and pla-

teau at the downstream end of the reach, the flow tended

to increase more steadily along the reach. The difference

between the two parameters results from the varying

headspace cross-sectional area along the length of the col-

lector. For example, the diameter of the collector in subreach

2 of reach 2 (1.4 and 1.2 m) increased substantially from

that of the prior reach (0.76 m), and the headspace areas

therefore increased from 0.16 to 0.96 m2. Over this

subreach, the velocity of the headspace decreased on both

days, while the flow increased. Hence, as the gas moved

along the collector from a small cross-section to a large

cross-section, the velocity was reduced but the total flow

continued on and actually increased.

In the first test run, the volumetric flow decreased in

subreach 1 of reach 3 (Case 1) and along subreach 3 of

reach 4 (Case 2), while in the second run, the flow de-

creased in subreach 1 of reach 4 (Case 3). The flow may

have decreased in Case 1 because testing of this subreachwas conducted on a different day than that of the up-

stream subreach. However, in Cases 2 and 3, the reaches

of interest were tested in conjunction with the upstream

and downstream reaches. These reaches were both down-

stream of all major tributary sewers, and therefore the

estimation of wastewater depth and, thus, the gas-phase

cross-sectional area should have been relatively accurate.

The decrease in flow along these subreaches would sug-

gest that there was significant out-gassing from the col-

lector at these locations. The cause of this apparent

out-gassing is unclear because it was not replicated on

both days for any of the subreaches in which it was ob-served. The lack of repeatability would suggest that it was

not caused by a physical structure but rather by some other

time-varying mechanism, such as wind-induced draft out

of a manhole. The exact cause of the reduced flow with

distance in the sewer could not be identified conclusively

with the results obtained from the tracer testing.

CONCLUSIONS AND RECOMMENDATIONS

The CO technique appeared to function well for quanti-

fying the movement of headspace gas in a large collector

Table 9. Summary of estimated volumetric flow rates.

Reach Subreach Flows (m3/min)

Run 1 Run 2

1 1 0.6 1.1

2 0.6 0.5

3 0.6 1.84 NA NA

2 1 2.7 4.6

2 10.8 7.9

3 1 4.4 11.9

2 25.6 19.4

3 25.8 26.9

4 1 28.8 17.7

2 81.9 43.7

3 68.1 72.6

Note:NA = not able to estimate gas velocity.


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Parker and Ryan


Figure 4. Headspace velocity and volumetric flow vs. distancerun 1.

Figure 5. Headspace velocity and volumetric flow vs. distancerun 2.


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Parker and Ryan

592 Journal of the Air & Waste Management Association Volume 51 April 2001

sewer. In reaches where headspace movement was rela-

tively fast (2030 m/min), longer reaches (7 km) could be

assessed with one injection. Where movement was slow

(313 m/min), a shorter distance (~1 km) could be as-

sessed with one injection.

The velocities observed in the collector varied substan-

tially with time and location in the collector. Reach 1,

bounded by the upstream end of the collector and the si-

phon, demonstrated the lowest velocities in this portion

of the study. The velocities observed in reaches 2, 3, and 4

(8.231.5 m/min) were generally higher than those ob-

served in reach 1 (3.612.9 m/min). The presence of a sub-

stantial drop structure appeared to reduce the headspace

velocity in the upstream reach. Downstream of this struc-

ture, the velocities increased and decreased somewhat ran-

domly and appeared to have reached a plateau at a velocity

of ~25 m/min. Employing the correlations of Pescod and

Price6 with distance-averaged inputs did not accurately pre-

dict the gas velocities observed in this collector.

While the headspace velocities tended to vary some-

what erratically and plateau at the downstream end of

the reach, the headspace volumetric flow tended to in-

crease more steadily along the reach. In isolated cases,

the volumetric flow rate appeared to decrease in certain

subreaches, suggesting that out-gassing occurred in these

instances. However, these occurrences were not repeated

in both runs at any location, suggesting that they were

not caused by a physical structure but rather by some time-

varying mechanism, such as wind-induced out-gassing.

REFERENCES

1. Jensen, N.A.; Hvitved-Jacobsen, T. Method for Measurements ofReaeration in Gravity Sewers Using Radio-Tracers; J.Water Pollut.Control Fed.1991, 63, 758-767.

2. Zytner, R.G.; Madani-Isfahani, M.; Corsi; R.L. Oxygen Uptake andVOC Emissions at Enclosed Drop Structures;Water Environ. Res.1997,69, 286-294.

About the Authors

Wayne Parker is an associate professor in the Department

of Civil and Environmental Engineering at Carleton Univer-

sity in Ottawa, Ontario. Helen Ryan is supervisor of the In-

dustrial Wastes Section of the Regional Municipality of Ot-

tawaCarleton in Ottawa, Ontario. Correspondence should

be directed to Dr. Wayne Parker, Department of Civil andEnvironmental Engineering, Carleton University, 1125 Colo-

nel By Drive, Ottawa, Ontario, K1S 5B6; or e-mail:

[email protected].

3. Corsi, R.L.; Quigley, C.J.; Melcer, H.; Bell, J. Aromatic VOC Emissionsfrom a Municipal Sewer Interceptor; Water Sci. Technol.1995, 31, 137-146.

4. Corsi, R.L.; Birkett, S.; Melcer, H.; Bell, J. Control of VOC Emissionsfrom Sewers: A Multi-Parameter Assessment; Water Sci. Technol. 1995,31, 147-158.

5. Quigley, C.J.; Corsi, R.L. Emissions of VOCs from a Municipal Sewer;J. Air & Waste Manage. Assoc.1995, 45, 395-403.

6. Pescod, M.B.; Price, A.C. Major Factors in Sewer Ventilation;J.Wa-ter Pollut. Control Fed.1982, 54, 385-397.

7. Monteith, H.; Bell, J.; Harvey, T. Assessment of Factors Controlling

Sewer Ventilation Rates. In Control of Odors and VOC Emissions, Pro-ceedings of the Water Environment Federation Specialty Conference,Houston, TX, April 2023, 1997.

8. Sorenson, H.; Joyce, J.; Day, D.; Fallara, T.C. Odor Control for LargeDiameter Deep Sewer TunnelsThe City of Columbus, Ohio. In Odorsand VOC Emissions 2000, Proceedings of the Water Environment Fed-erations Specialty Conference, Cincinnati, OH, April 1619, 2000.

9. Parkhurst, J.D.; Pomeroy, R.D. Oxygen Absorption in Streams;J. Environ. Eng. 1972, 98, 101.

10. Metcalf; Eddy. Wastewater Engineering: Treatment, Disposal and Reuse,3rd ed.; McGraw-Hill: New York, 1991.

Documents

A Tracer Study of Headspace Ventilation in a Collector Sewer