May 07 1

Collection and Conveyance Systems

CEE484May 2, 2007

This topic deals with appropriate technology for cluster or

decentralized wastewater treatment systems

Need for collection and conveyance• Source

– From home directly• Gravity flow• Pressure pumping• Vacuum pumping

– From septic tank• Gravity• Pressure pumping

• Destination– Cluster systems– Decentralized wastewater treatment

It takes work to move the liquid• Water flows downhill - Gravity system

– A gravity system is the conventional approach– Used for most all municipal systems– Gravity flow is interrupted with pump stations to lift

water back up to higher level • Push it – Pressure system• Pull it – vacuum system

• Work is ft-lb• Rate is Horsepower – 1.0 Hp = 550 ft-lb/sec• 1 Hp = 0.746 kW

We evaluate work to be done by applying energy equation

0.85) to (0.50 range 85%-50 in - fraction a as efficiency pumpηlb/ft 62.4 water,of gtspecific wγ

/sft flow,Q

where)lb/sec(-t 550

)Qγ(h Hp Pump

ft water,of weightunit per done be to workor head pumph

hh2gV

zγ

ph2gV

zγp

3

3

pump

pump

minorfriction

22

22

pump

21

11

21

==

=

=

=

++++=+++ ∑ ∑−

ηf

How much installed Hp for our 360 gal/day home examplefor lift to gravity line sewer system

• Gravity line assumptions– Assume pump lift needed to overcome pipe

energy losses and elevation changes = 50 ft – pump efficiency of 60%.

• Wastewater flowrate and time to fill sump basin– Sump volume = 30 gal– peak flow rate = 2.5 times average rate for a

45 min period• Flowrate = 2.5(360 gal/day)(1 day/1440 min)

= 0.625 gal/min • Time to fill sump = 30 gal/0.625 gal/min = 48

min

Pump rate = 10 Pump rate = 10 –– 15 gal/min15 gal/minUse 15 gal/min Use 15 gal/min

Rate of work = Mass rate times liftRate of work = Mass rate times lift(15 gal/min)(8.34lb/gal)(50ft)(min/60sec) = (15 gal/min)(8.34lb/gal)(50ft)(min/60sec) = 104.3 ft lb/sec104.3 ft lb/sec1 Hp = 550 ft lb/sec1 Hp = 550 ft lb/sec

Motor Hp = 104.3 ft lb/sec divided by 550 ft Motor Hp = 104.3 ft lb/sec divided by 550 ft lb/sec per Hp divided by 0.60lb/sec per Hp divided by 0.60

Hp = 0.31 ( 1/4 to Hp = 0.31 ( 1/4 to ½½ Hp pump) Hp pump)

At pump station collecting flow from At pump station collecting flow from many homes, pump sizes can bemany homes, pump sizes can be

relatively large relatively large –– (20 to 200 to 500 Hp)(20 to 200 to 500 Hp)

Elements of gravity flow system• House connection pipe – 4 in. diameter• House hold pipe connects to laterals (6-8 in.)• Laterals connect to main lines• Manholes every 300 to 500 ft and at direction

changes or large elevation changes• located under pavement• below frost line• Will also get infiltration and inflows – higher

flowrates• follow grade where possible -necessary slope

is required to maintain velocitiesmin. velocity = 2.0 ft/sec, Max < 5 ft/sec

Gravity pipe design considerations

• Design flows– Flows vary during 24 hours – Maximum and minimum– Future hook ups and flow changes

• Pipe slope• Pipe diameter• Minimal velocity (>2.0 ft/sec for untreated

wastewater)- check this at minimal flows– At low flows solids can settle and cause

greater odor problems– Similar problem with small fraction of hook

ups in a collection system

Gravity pipe design considerations

• Maximum velocity (<5.0-8.0 ft/sec) check at maximum flows

• Table 6-5 in book gives slope and velocity for different flows and pipe sizes. Prefer grade of <1.0% (1ft/100ft)– (done at max flow – not minimum)

Hazen-Williams equation for full depth flow in PVC pipe

/LhS and SD0.43CQ

,SDC55.0V

L0.542.63

hw

0.540.63hw

==

== VAQ

V = velocity, ft/s, Q = flow, ft3/s, D=diameter, ftChw=coefficient, 120 to 150S = pipe slope which equals pipe friction loss per unit

pipe length, ft/ftNeed to use Manning Equation for pipe flow at partial depth

Manning equation commonly used for flow in channels and pipes with flow at fraction of

full depth

)s)(0.462/n)(D(Q

DQ4

AQ Velocity

S(1.486/n)RV

EquationManning

8/3

2

0.500.667

=

==

=

π

Determine Maximum flowrate estimation for gravity pipe design

Peak flow rate, gal/min = 0.50(#EDUs) Eq. 6-1, pg351

EDU = equivalent dwelling unitExample:

Town of Buckeye (near Phoenix, AZ) census report shows 3.2 people/dwelling unit. Assuming a per capita average daily wastewater flowrate of 68 gal/cap-d, what is the peak/average daily flow ratio using the above equation for a 200 unit development? Using table 6-5, what pipe size and slope do you recommend for pipe flowing full?How much does pipe drop in 0.5 mile distance?

Did we leave anything important out in this calculation?

What is the velocity in the pipe at a minimal flowrate of 0.30 times the average daily

flowrate? (note: 7.48 gal/ft3)

Home collection system can be all gravity flow or gravity flow plus pump station

Pump station

To decentralized treatment facility

Gravity flow in lateral

Pump it from the home to a pressure sewer line

What if we pump from home into pressure collection system instead of gravity flow?

• Home grinder pump– Small sump basin at home– Pump has chopping blades to reduce solids

size and allow smaller pipes for transport– Pipe is watertight and under pressure– Can serve more than 1 home– Called low pressure sewers (up to 60 psi)– O.5 to 2.0 Hp pumps (9-14 gal/min)– Check valve prevents back flow– 1.5 to 8 in pipe versus 4 to 20 in for gravity

• Vacuum sewers– Wastewater collected in sump– At certain level valve opens to the vacuum

system – Flow goes to vacuum station

• May be lifted to next pipe– Pipe system is watertight – Can serve more than 1 home– About 20 ft of maximum head– Pipe is smaller compared to gravity flow

Or instead we use suction

Advantages of grinder pump and vacuum sewer systems

• Grinder pump system can deal with almost any topography – uphill

• Vacuum pump does not need slope but best with flat terrain

• Smaller pipe diameter can be used– Less installation cost and quicker

• More shallow installation• Less sulfide odor problem as little settling out in

line• Others?

Systems for septic tank effluent

• Existing septic tanks• Go to cluster or decentralized treatment system• Go to cluster drainage field• Connect to centralized collection and treatment

system• Septic tanks provide primary treatment

– What are pros and cons of this?

Conveyance methods for septic tank effluent

• Septic tank effluent gravity (STEG) sewers• Septic tank effluent pressure (STEP)

sewers• Combination of gravity and pressure

sewers

what are advantages of STEG versus conventional gravity sewers?

• Liquid characteristics greatly different• Smaller pipe size - 2 in acceptable• More shallow – not as steep a gradient needed• Can use siphon design for variable terrain

– Manholes not provided– Clean out access ports provided

• Longer distance between clean out ports vs manholes

• Check valve used at lateral/main intersection vs manhole

Other considerations for STEG

• Clean out or “pigging” ports• Air release valves at high points• Odor control • Check valves from service lateral to main

line

what are advantages of STEP versus conventional gravity sewers?

• Shallow depth possible– Good for high groundwater table locations– Less costly construction for installation

• Smaller pipe size - 1.25 to 2 in in acceptable• Can accommodate variable or undulating terrain

– Manholes not provided– Clean out access ports provided

• Longer distance between clean out ports vs manholes

Other considerations for STEP

• Clean out or “pigging” ports• Air release valves at high points• Odor control • Use separate pump pit after existing septic

tank• Consider collection septic tank effluent

from cluster of homes to common pump tank

• Have effluent screening on septic tank

Summary• Conventional gravity collection and conveyance

systems not best choice for many cluster and decentralized wastewater treatment situations

• For untreated wastewater grinder pumps can handle widest range of head requirements

• Vacuum systems applicable for limited head ranges – better for flat terrain

• Grinder pump and vacuum systems result in smaller pipe diameter and less construction costs

Summary

• Conveyance of septic tank effluents can be done by gravity system or pressure, depending on topography

• Smaller pipes can be used for septic tank effluents to reduce construction costs

• Less cleaning issues and lower velocity possible with septic tank effluent conveyance