Air Flow in Sewers - Ohio Water Environment … Air_flow_in_sewers_5-10-2012_v02.pdf · than the...

Air Flow in Sewers

Approach to Design Sewers for both Air and Water

OWEA 2012 Collection Systems Specialty Conference

May 10, 2012

Foster A. McMasters, Jr., PE, BCEE

May 10, 2012

Air Flow Presentation Agenda

• Objective

• Background

• Approach to Calculating Air Flows

• Pressure Criteria to Control Odor Release

• Estimating Design Air Flow Rates

• Design Example 1 – Co-current Flow

• Design Example 2 – Counter-current Flow

May 10, 2012 Air Flow in Sewers Page 2

Objective A properly designed wastewater collection system must be

designed to satisfactorily convey two fluids -- air and water.

Proper sewer design can prevent discharge of the air at

unplanned locations, minimizing sewer odor complaints.

Sewers must be designed to convey both air and water

• Air flow within, as well as in and out of sewers is often neglected during

sewer design despite its importance in controlling the occurrence of

odor problems and the development of corrosion-inducing atmospheres

within the sewer.

• These issues have become more pronounced in the large, deep

interceptors used to convey and store wastewater to control overflows.

• Contributing factors:

– Dependence on hydraulic computer models

– Inappropriate friction factors used for design

– Lack of attention to obstructions to air flow

May 10, 2012 Page 4 Air Flow in Sewers

Plumbing code provides for both air and water

• Plumbing codes clearly define requirements for venting and trapping of

plumbing in homes and buildings

• The municipal wastewater collection system is an extension of the

building plumbing systems with:

– Wastewater discharging to the WWTP, and

– Air often discharging at unplanned locations

• Large interceptors are a particular concern

– limited number of manholes or shafts

– often allowed to become surcharged at their downstream end

– blockage to air flow results in a buildup of air pressure, escape of significant

air volumes at a few locations, and associated odor problems.


Background

VW

VA

x

W

Wastewater Related References

• Thistlethwayte, D. K. B. 1972. The

Control of Sulphides in Sewerage

Systems,

• Henry T. Falvey. 1980. Air-Water

Flow in Hydraulic Structures,

Engineering Monograph No. 41,

United Stages Department of the

Interior

• Pescod, M.B. and Price, A.C. 1982.

Major Factors in Sewer Ventilation.

Journal of the Water Pollution

Control Federation, 1982. Vol 54,

No.4.


From Pescod & Price (1982)

From EM No. 41 (1980)

Wastewater Related References

• Edwini-Bonsu, S and P.M. Steffler.

2004. Air Flow in Sanitary Sewer

Conduits Due to Wastewater Drag: a

Computational Fluid Dynamics

Approach, Journal of Environmental

Engineering and Science, Volume 3,

Number 5, August 2004, pp.

331342(12)

• Witherspoon, J., Easter, C., and

Apgar, D. 2009. Collection System

Ventilation Final Report. Water

Environment Research Foundation

Report ISBN 9781843392880.


Hydraulic Approach to Calculating Air Flows

VW

VA

x

W

Calculation Approach

• Headloss is calculated from air friction along the water

interface and along the conduit wall

• Two air flow conditions to evaluate

– Counter-current flow

– Co-current flow

• Calculate air flows and pressures over the full range of

wastewater flows for the project

VW

VA

VW

VA


The formula for counter-current flow is:

• DH / Dx = (fC / 4RHC) (VA2 / 2g) + (fW / 4RHW) (VA + VW)2 / 2g)

– Expanding gives:

– DH / Dx = (fC PC / 4A) (VA2 / 2g) + (fW W / 4A) (VW + VA)2 / 2g)

• DH = head loss (ft or m),

• Dx = distance (ft or m),

• VA = air velocity (ft/s or m/s),

• VW = water velocity (ft/s or m/s),

• PC = air contact perimeter with conduit, (ft or m)

• A = air cross section area (ft2 or m2),

• W = water surface width, (ft or m)

• g = acceleration of gravity (ft/s2 or m/s2),

• fC = air-conduit friction factor,

• fW = air-water friction factor

Calculation Approach for Counter-Current Flow


W

PC

VW

VA

The formula for co-current flow with VA less than VW is:

• DH / Dx = (fC / 4RHC) (VA2 / 2g) - (fW / 4RHW) (VW - VA)2 / 2g)


– DH / Dx = (fC PC / 4A) (VA2 / 2g) - (fW W / 4A) (VW - VA)2 / 2g)

Calculation Approach for Co-current Flow


VW

VA

W

PC

The formula for co-current flow with VA greater than VW is :

• DH / Dx = (fC / 4RHC) (VA2 / 2g) + (fW / 4RHW) (VW - VA)2 / 2g)


– DH / Dx = (fC PC / 4A) (VA2 / 2g) + (fW W / 4A) (VW - VA)2 / 2g)

– Applicable to fan driven systems

Calculation Approach for Co-current Flow


VW

VA

W

PC

Data Required to Characterize the Collection System

• Pipe invert, top of pipe, and ground elevations,

• Pipe slope, length, roughness coefficient,

• Access chamber locations and depths,

• Wastewater flows:

– typical day maximum, minimum, and average,

– peak wet weather flows,

• Air inlet and exhaust sizes and locations identified,

• Headspace obstructions identified.


Develop a spreadsheet model of the wastewater system:

• Each section between access chambers, pipe size changes, or pipe

slope changes is calculated separately,

• Wastewater depths and velocities calculated using the Manning

equation,

• Air pressures calculated based on collection system elevation and

pressure changes in the headspace based on the Darcy equation,

• Determine air pressure at each shaft and calculate the air inflow or

outflow using a conventional orifice equation,

• Concrete surface and water surface impacts calculated separately.

• Air flows are based on design basis air flows for each air exhaust

/intake location in the system.


Spreadsheet Headloss Calculation


• Wastewater = 2.8 m3/s (8.5 MGD)

• Air = 27,200 m3/hr (16,000 cfm)

Calculate each segment, each size change, and each

grade change


AS

11

AS

10

air water

Typical air pressure profile


Chainage (meters)

Ground Line Shaft Centerline

Sewer Wall 150.82 Invert Elevation

140

160

140

160

180 180

200 200

183.0

191.5

187.7

175.5 175.5

150.82 152.99

146.61 144.58

141.69

Shaft

9

Shaft

10

Shaft

11

Shaft

12

Shaft

13

Air

In

take

Air

Ou

t

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000

Ele

va

tion

(m

ete

rs)

120

100

120

100

112.32

123.99

Minimum Pressure Criteria Design Criteria to Control Odor Release

Negative Air Pressure Required to Prevent Escape of

Odorous Air

• “Rule of Thumb” value is typically -0.05 inches water pressure

• Deep shafts can require a lower negative pressure due to “stack

effect”

• The stack effect occurs as a result of the

air in the interceptor being of lower density

than the atmospheric air


Stack Effect Defined

• stack effect occurs when air in

the interceptor has a lower

density than atmospheric air,

• contrary to intuition, moist air has

a lower density than dry air at the

same temperature

• negative pressure at each shaft

needs to be greater than the

design inside to outside air

density difference times the shaft

height: DP > (Dr / r) H


Pressure head with outside air

in manhole

DP

Pressure Head with inside air in

manhole

Flow Out

Tunnel

H

(Dr/r) H

Atmosphere

Stack Effect Example

• Shaft depth is 44 m (144-ft)

• Outside air at -20oC (-4oF) and

25% relative humidity

• Sewer air at 13oC (55oF) and

100% relative humidity

• (Dr / r) = 0.14

• Required pressure is -0.29

inches water pressure

– 6 times the “rule-of-thumb”


Estimating Design Air Flow Rates

Starting Point to Select a Design Air Flow Rate

Thistlethwayte Recommendation

• 0.05 cubic feet per minute per square foot of sewage surface

• 0.914 cubic meters per hour per square meter of sewage surface

Natural flow condition to produce 0.05 cfm of air flow

• Generalized Case

– Pipe = 72-inch RCP

– Pipe wall friction factor = water surface friction factor

– Pipe Slope = 0.30%

– Wastewater Velocity at 50% full = 8.2 ft/s (2.5 m/s)

– Froude No is >1, so subcritical flow regime


Design Example No 1 -- Co-current Flow 3 m (9.8-ft) Diameter Trunk Sewer

YDSS Southeast Collector Trunk Sewer Project


Southeast Collector Trunk Sewer Project 15,000 m (49,200 ft) -- 3.0 m dia (9.84 ft) -- Tunnel construction


SeC Air Handling Design

• Central OCF Location

• Upstream air = 27,200 m3/hr (16,000 cfm)

• Downstream air = 31,000 m3/hr (18,200 cfm)

• Air Intakes at Shaft 13 & CDC Chamber

• Fans around Drop Shafts 4 & 6


Air Flow

Wastewater Flow

Southeast Collector Profile


Proposed OCF

(Shaft 9)

Air Drawn with Wastewater Flow

(within headspace of pipe)

Air Drawn Counter to Wastewater Flow


Air Bypass Piping and fan (to be confirmed) to allow air to pass submerged baffle drop structures and metering facility

Sh

aft

13

Sh

aft

12

Sh

aft

11

Sh

aft

10

Sh

aft

9

Sh

aft

8

Sh

aft

6

Sh

aft

5

Ma

int.

Sh

aft

Sh

aft

4

Ma

int.

Sh

aft

Sh

aft

2

Co

nn

ectio

n C

ha

mb

er

100 100

200 200

150 150

50 50

Distance (m)

Ele

vation (

mA

MS

L)

15+000 10+000 0+000 5+000

Air Intake Air Intake Air bypass piping and fans provided to allow

air to pass drop structures at high flow

Air Drawn Counter to Wastewater Flow


Air Drawn with Wastewater Flow


Southeast Collector (upstream section)

• Wastewater = 2.8 m3/s (8.5 MGD)

• Air = 27,200 m3/hr (16,000 cfm)


SeC Air Pressure Profile (Pressure = -0.25 to -0.45 in-water)


Southeast Collector (with “leaks” at access chambers)


Design Example No. 2 – Counter-current flow Parallel Trunk Sewers

Metro Vancouver South Surry Interceptor – Air Management and Odour Control

• Maintain negative pressure

• 64th Avenue to 152nd Street

• 10,100 meters (33,100 ft)

• Parallel Interceptors

– 1.4 m wide x 1.75 m high box

– 1.675 to 3.05 m diameter

• Relatively flat 0.03% slope

• Siphon at downstream end

• Two pumping stations

– Crescent Beach

– Cloverdale


South Surry Interceptors

Hwy 10 Grit Crescent

Beach JC

King George

Grit

Crescent Beach PS /

Central Trunk JC


Hydraulic Model Needed for more complex SSI System


• SWMM Model with dynamic wave routing

– Necessary to evaluate surcharge/backwater effects

• Typical Day and Peak Day flow patterns developed

– Crescent Beach Pump Station

– Cloverdale Pump Station

– Central Valley Trunk

Profiles Upstream of Crescent Beach Junction Chamber


King George Section (1.4 x 1.75 m box)

Trites Road Section (1.83 m & 3.05 m diameter)

Air Model Prepared for SSI System


SSI Air Management Recommendations


OCF at King George Grit Chamber Air Flow

Pressure

Downstream Design Air Flow 16,000 cfm

-3.0

in-water

Upstream Design Air Flow 7,000 cfm

-1.0

in-water

Total OCF Design Air Flow 23,000 cfm

• Locate Odour Control Facility at King George Grit Site

– Air dam required to separate downstream/upstream headspaces

• New Air Intake at Highway 10 Grit Chamber

– Crescent Beach Pump Station

• “Air Jumper” required around the Crescent Beach Junction Chamber

– King George Section planned to serve as “air jumper”

• Provide traps to block any large “air leak” locations (if any)

Conclusions

•All sewers should be designed to properly

convey both water and air

Thank You Please send examples from your system to the author at:

[email protected]

Air Flow in Sewers May 10, 2012

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