August 2019

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Pressure Distribution Systems

Roxanne GrooverFlorida Onsite Wastewater Association

Web Version

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How does dosing effectdrainfield longevity?

• Designed to use entire drainfield each dose• Alternately wet and dry conditions• Biomat is partially consumed as oxygen is

drawn down behind wetting front• Can accept many times more pounds of

BOD per sq ft of surface (Hargett, 1984)

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Why use a pump?• Required by 64E-6, Florida Administrative Code for “large”

drainfields (over 1000 sq. ft total required area).

• To overcome a low plumbing stubout or elevation/distancechallenges on a lot.

Code calls this “lift dosing”.

• To control peak loading stress (e.g., Church, Flea Market).

• Establishments with high strength wastes to spread thebiological loading to enhance exposure to bacteria for quickremoval.

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Warning!!!

• Tank watertightness is a must !– Exfiltration pollutes the groundwater.– Infiltration burns up the pump and overloads

the drainfield.

• In some states, all mounded drainfields arepressure dosed.

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Conventional Gravity Distribution• Creeping failure principle….• Entire flow out the nearest and lowest holes.• Locally clog the bottom beneath that hole.• Begin to spread out laterally along the bottom• Once entire bottom surface clogged, begins to

rely on using the sidewall.

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Once we rely on sidewall, whichtype of a gravity system is

superior, a bed or a trench?

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Example Bed System

• Four bedroom home (400 gpd)• Subsurface aggregate bed 12” deep 30 ft

long• Loamy Fine Sand (0.35 gpd/ft2)• GIVEN: 40% pore space, ignore pipe, 1

cubic ft equals 7.5 gallons• FIND: TOTAL STORAGE VOLUME IN THE

DRAINFIELD

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Example Bed System solution• Area = Q (gpd) ÷LTAR (gpd/ft2)• Area = 400 gpd � 0.35 gpd/ft2 = 1143 ft2• Volume = Area (ft2) x Depth (ft)• Volume = 1143 ft2 x 1 ft thick = 1143 ft3• Void volume = volume (ft3) x % pore space• Void volume = 1143 ft3 x 0.40 = 457 ft3• Total Storage Volume = VV(ft3) x 7.5 G/ft3• Total Storage Volume (in gallons)

457ft3 x 7.5 gallons/ft3 = 3429 gallons

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Bed System Safety factor againstsurge overload

• If the above drainfield was dry, but all bottom

and sidewall surfaces were clogged, howmany days could it accept this home’seffluent?

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Bed System Safety factor againstsurge overload - solution

• Total Storage volume = 3429 gallons• Daily Estimated Sewage flow = 400 gpd• Safety Factor = 3429 gal/400 gpd = 8.6

days

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Sidewall Safety factor againstClogged bottom surface

• If the the bottom surface of the example bed

drainfield was completely clogged, howmany gallons of effluent would have to passthrough the sidewall per day per linear footof sidewall to keep this home’s effluent fromsurfacing?

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Bed System Safety factor againstclogged bottom surface - solution• Let bed width = 30 ft• Bed length= 1143ft2/30 ft = 38 ft• Bed perimeter = 30ft + 38ft + 30ft + 38ft

= 136 linear ft• Sidewall infiltration rate = 400 gpd/136 ft =2.94

gal/ linear ft/day• At 12” deep drainfield=LTAR of 2.94 gallons

per sq ft per day

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Example Trench System

• Four bedroom home (400 gpd).• Subsurface aggregate 12”deep 36” wide.• Loamy Fine Sand (0.65 gpd/ft2).• GIVEN: 40% pore space, ignore pipe, 1

cubic ft equals 7.5 gallons.• FIND: TOTAL STORAGE VOLUME IN THE

DRAINFIELD.

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Example Trench System solution• Area = Q (gpd) ÷LTAR (gpd/ft2)• Area = 400 gpd � 0.65 gpd/ft2 = 615 ft2• Volume = Area (ft2) x Depth (ft)• Volume = 615 ft2 x 1 ft thick = 615 ft3• Void volume = volume (ft3) x % pore space• Void volume = 615 ft3 x 0.40 = 246 ft3• Total Storage Volume = VV(ft3) x 7.5 G/ft3• Total Storage Volume (in gallons) 246

ft3 x 7.5 gallons/ft3 = 1846 gallons

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Trench System Safety factoragainst surge overload

• If the above drainfield was dry, but all bottom

and sidewall surfaces were clogged, howmany days could it accept this home’seffluent?

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Trench System Safety factor againstsurge overload - solution

• Total Storage volume = 1846 gallons• Daily Estimated Sewage flow = 400 gpd• Safety Factor = 1846 g/400 gpd = 4.6 days

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Sidewall Safety factor againstClogged bottom surface

• If the the bottom surface of the example trench

drainfield was completely clogged, how manygallons of effluent would have to pass through thesidewall per day per linear foot of sidewall to keepthis home’s effluent from surfacing? Assume threetrenches. Include the ends of the trenches in thecalculation

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Trench System Safety factoragainst clogged bottom surface -

solution

• Total drainfield area= 615 ft2• Total drainfield length = 615 ft2/3ft = 205 ft• Sidewalls = 205 ft x 2 = 410 linear ft• Endwalls = 6 ends x 3 ft = 18 linear ft• Total perimeter = 410 ft + 18 ft = 428 linear ft• Sidewall infiltration rate = 400 gpd/428 linear ft =0.93 gal/

linear ft/day

• At 12” depth=LTAR of 0.93 gallons per sq ft per day

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Comparison Data• BED SYSTEM • 2.94 gal/linear ft• 8.57 days storage

• TRENCH SYSTEM• 0.93 gal/linear ft• 4.62 days storage

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What pumpers need to knowabout dosing systems

• Do not leave dosing tanks bone dry (float)• Will suck in sediment first time used (pump mounted

on blocks helps prevent this)• Turn off, pump chamber dry and refill to top of pump

casing at least• If pump is exposed corrosive gases can attack the

metal• Make sure low level cutoff is above casing of pump• Effluent surrounding pump keeps the pump cool

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“By placing 100% of the pumphousing under effluent 100% of

the time - you will double theeffective pump life”

Chuck Schwabe Zoeller Pump Company

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What Installers need toknow about dosing systems

• Not as simple as picking up a sump pump at thelocal discount hardware store

• Horsepower is not the key• Determine GPM and Total Dynamic Head, then

compare to published pump curves• Small design improvements have big payoffs:

--put check valve in vertical leg of discharge pipe --drill a drain hole in bottom of each line of drainfield

pipe furthest from the pump

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What Installers need to knowabout dosing systems (cont.)

• If tank is not watertight, during high water conditions you arepumping and treating the effluent plus any rainwater that findsits way into the system

• Check pipe penetrations, risers & lids for complete watertightseal

• Including gate or ball valves at key points gives flexibility indealing with problems

• More small design improvements have big payoffs: --set pump beneath four blocks , not just one--use true union connectors so can pull pump easily--use float tree, don’t strap floats to pump discharge pipe

Let’s Talk Design

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Considerations • Many design ‘preferences’ involved• Two competent designers can come up with

very different solutions• Sometimes external forces push solution,

e.g. Site constraints Availability or cost of materials

• Owners suggestions

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Steps in Sizing Low PressureDistribution Networks

• Determine estimated sewage flow (64E-6, Table I)

• Determine soil textural classification and ESHWT (field, site/soil evaluation)

– Use to determine maximum sewageloading rate(s) in trench and bedconfigurations (64E-6, Table III & mound)

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Selecting Pipe Networks:• Header line sizing• Lateral line sizing• Hole diameters and hole spacing-very

important• Transmission line sizing• Total dynamic head=static head + frictional

head + working head• Pump selection

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Supplemental Handouts

1. Scouring gal/min for various diameters2. Discharge rates for various hole diameters3. Friction loss in various pipe diameters4. Friction loss for fittings worksheet example5. Volume per foot for various pipe diameters6. Dosing system decision tree7. Tank sizes and LTAR excerpts from rule8. Pump curve

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Scouring Velocity

• If flow inside pipe gets too slowsuspended materials in pipe get left behind

• Called ‘stranding solids’• Will eventually clog network• If flow at 2 ft / sec or higher will eject

suspended solids with the fluids

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Minimum gpm to achieve scouring velocity (2 ft/sec)for common pipe diametersequation: 4.896 (d2) = GPM

Pipe Diameter (in.)Nominal (actual)

Minimum GPMActual (use)

0.50 (.622) 1.89 (2)

0.75 (.824) 3.32 (4)

1.00 (1.049) 5.39 (6)

1.25 (1.380) 9.32 (10)

1.5 (1.610) 12.69 (13)

2.0 (2.067) 20.92 (21)

2.5 (2.469) 29.84 (30)

3.0 (3.068) 46.08 (46)

4.0 (4.026) 79.35 (80)

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Discharge rates for various sizedholes at various pressures (in gpm)

Operatinghead

3/32 1/8 5/32 3/16 7/32 1/4 5/16 3/8 7/16

1 ft(0.43 psi)

0.10 0.18 0.29 0.42 0.56 0.74 1.15 1.66 2.26

2 ft (0.87psi)

0.15 0.26 0.41 0.59 0.80 1.05 1.63 2.34 3.19

3 ft (1.30psi)

0.18 0.32 0.50 0.72 0.98 1.28 1.99 2.87 3.91

4 ft (1.73psi)

0.21 0.37 0.58 0.83 1.13 1.48 2.30 3.31 4.51

5 ft (2.17psi)

0.23 0.41 0.64 0.94 1.26 1.65 2.57 3.71 5.04

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Volume of effluent per foot of pipe length (in gallons)equation: .0408 (d2) = gallons

Pipe Diameter (in.)Nominal (actual)

Volume per foot(gallons)

0.50 (.622) .016

0.75 (.824) .028

1.00 (1.049) .045

1.25 (1.380) .078

1.5 (1.610) .106

2.0 (2.067) .174

2.5 (2.469) .249

3.0 (3.068) .384

4.0 (4.026) .661

Dosing Tree

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Table II - Septic/pump tank capacityAverageSewage Flow(gpd)

Septic Tank effectivecapacity (gallons)

Residential Pump Tankeffective capacity(gallons)

Commercial PumpTank effective capacity(gallons)

0-200 900 150 225201-300 900 225 375301-400 1050 300 450401-500 1200 375 600501-600 1350 450 600601-700 1500 525 750701-800 1650 600 900801-1000 1900 750 10501001-1250 2200 900 12001251-1750 2700 1350 19001751-2500 3200 1650 27002501-3000 3700 1900 30003001-3500 4300 2200 30003501-4000 4800 2700 30004001-4500 5300 2700 30004501-5000 5800 3000 3000

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PUMP must reach capacityagainst Total Dynamic Head

• Total Dynamic Head =– Static Head, plus– Friction Head, plus– Operating Head

• TDH = SH +FH+OH

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Static Head

• Also known as elevation head.• The vertical distance from off point of

pump (or lowest water level in pumpchamber) to the point of discharge,usually the header pipe.

• NOTE: if lines are not level, choose the

highest point (why?)

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Friction Head

• Resistance to flow of fluid against side wallsof pipes and fittings.

• Friction Head a function of: Pipe diameter (smaller, more friction) Capacity (more flow, more friction) Configuration (+fittings, more friction) Pipe materials (PVC, Steel, Cast Iron) Age (older more friction) why?

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Friction Head

• Represented as equivalent length of pipe, As if you could remove the fitting and

replace it with a length of straight pipe withthe same friction loss

• Table A in SSPMA handout

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Frictional Loss Worksheet

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Operating Head

• Pressure desired at the holes (orifices)• How high the water rises in a standpipe at

the distal lateral hole location.• 1-3 ft is reasonable to keep holes clear• Placing the furthest hole pointing down

allows flow to drain to end of lines whenpump kicks off, also drains off solids

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With TDH & Pump Capacity• Locate point on pump curve• Pumps vary + 10% from published values• Use a pump curve slightly above and to the

right of point plotted

Pump Curve

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Select Central or End manifold

• Central manifold - discharge lateralsarranged off both sides (letter ‘H’)

• End manifold - looks like a typical trenchsystem (letter ‘E’)

• With the same system size, centralmanifolds have half the length ofdischarge laterals, but twice the numberof laterals

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Steps in sizing low pressuredistribution networks (cont.)

• Determine the number of doses per rule. – Moderately limited soil = maximum of 2.– Slightly limited soil = maximum of 6.

• Calculate volume per dose by 64E-6.

• Calculate volume required to fill the laterals one time

• Calculate number of pipe fills per dose. Rule requires

minimum of 4.

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Finally, review your design

• 64E-6 requires 4 fill/cycle– EPA tables assume 10 fills/cycle- discussion• 64E-6 – slightly limited soils maximum of 6

doses per 24 hours, moderately limitedsoils maximum of 4 doses per 24 hour

– PE can specify more frequent doses, but notless than one fill per cycle

Steps to Design

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Bed Example• A ___ gallon per day bed Onsite Sewage Treatment and

Disposal System serves a residential waste establishment.The soil is a _____. The system site has an estimatedseasonal high water table of ___ inches at/below grade atthe drainfield location. A bed system with a length of __feet is preferred (use 36 inch separation between lines).

• GIVEN:• System size (gpd) would be given in class• Bed configuration• Soil type would be given in class• SHWT would be given in class• Length of bed would be given in class

Bed Example

• What is the proper size of the septic tank?– From 64E-6 Table II• What is the proper size of the pump tank?– From 64E-6 Table II• Residential or commercial

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Table II - Septic/pump tank capacityAverageSewage Flow(gpd)

Septic Tank effectivecapacity (gallons)

Residential Pump Tankeffective capacity(gallons)

Commercial PumpTank effective capacity(gallons)

0-200 900 150 225201-300 900 225 375301-400 1050 300 450401-500 1200 375 600501-600 1350 450 600601-700 1500 525 750701-800 1650 600 900801-1000 1900 750 10501001-1250 2200 900 12001251-1750 2700 1350 19001751-2500 3200 1650 27002501-3000 3700 1900 30003001-3500 4300 2200 30003501-4000 4800 2700 30004001-4500 5300 2700 30004501-5000 5800 3000 3000

Bed Example

• What is the long term acceptance rate(LTAR) of the effluent passing through theinfiltrative bottom surface of drainfield?

– GIVEN:• From soil type given in class w/SHWT determination

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Bed Example

• What is the total drainfield area?– Estimated Sewage Flow/LTAR– From 64E-6

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Bed Example• Choose a hole diameter and spacing– ?? ” holes – to be given in class– ?? foot spacing – to be given in class• Choose a distal operating head– ?? Feet – to be given in class• How many holes are there in each lateral?– Trench Length/hole spacing

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Bed Example

• What is the discharge rate through each size hole?

– Use chart in next slide• Depends on operating head & hole size

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Discharge rates for various sizedholes at various pressures (in gpm)

Operatinghead

3/32 1/8 5/32 3/16 7/32 1/4 5/16 3/8 7/16

1 ft(0.43 psi)

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0.18 0.29 0.42 0.56 0.74 1.15 1.66 2.26

2 ft(0.87 psi)

0.15

0.26 0.41 0.59 0.80 1.05 1.63 2.34 3.19

3 ft(1.30 psi)

0.18

0.32 0.50 0.72 0.98 1.28 1.99 2.87 3.91

4 ft(1.73 psi)

0.21

0.37 0.58 0.83 1.13 1.48 2.30 3.31 4.51

5 ft(2.17 psi)

0.23

0.41 0.64 0.94 1.26 1.65 2.57 3.71 5.04

Bed Example

• What is the discharge through each lateral?– # of holes x discharge rate per hole

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Bed Example

• What is the lateral diameter:– From chart in next slide• Based off flow from all holes in lateral

• What is the flow per system?– Flow per lateral x total number of trenches• Based off flow from all laterals

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Bed Example

• End Connection or Center Connection?• What is the manifold diameter?– From chart• Discussion on excessive head loss• Discussion on flow velocity

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Frictional Loss Calculation

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Total Dynamic Head (TDH)=Static Head + Frictional Head + Operating Head

Bed Example

• Pump Selection– Static Head– The elevation from the discharge port at the

pump to the drainfield (from site plan notes)– For class we typically use 7.5 feet

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Bed Example

• Pump Selection– Frictional Head• Use chart to determine frictional losses– Will depend on design in class– Calculated separately at each part of system.

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Bed Example

• Pump Selection– Operational Head– GIVEN:• from initial design• Will vary but most often example will use 3 feet

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Bed Example

• Pump Selection– Total Dynamic Head (TDH)• Static Head + Frictional Head + Operating Head

• Pump Selection– ?? GPM @ ?? ft of head - #16

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Bed Example

• Check number of fills– What is the volume per linear foot of lateral line

(from table)?

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Volume of effluent per foot of pipe length (in gallons)equation: .0408 (d2) = gallons

Pipe Diameter (in.)Nominal (actual)

Volume per foot(gallons)

0.50 (.622) .016

0.75 (.824) .028

1.00 (1.049) .045

1.25 (1.380) .078

1.5 (1.610) .106

2.0 (2.067) .174

2.5 (2.469) .249

3.0 (3.068) .384

4.0 (4.026) .661

Bed Example• Total volume– volume per linear foot * # of laterals * length of

lateral• There will be ? dose cycles per day– Use 64E-6 to help w/# of dose cycles• This will be determined in class

• What is the volume per dose?– Flow/# of doses

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Bed Example

• How many line fills will there be?– Dose Volume/Total Lateral Volume

• Does this meet code requirements?– Must be at least 4 fills according to 64E-6

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Trench Example

• We will follow the same steps for a trenchdesign

• Certain design parameters will change.– 64E-6 requirements for trench separation.

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Questions/Comments

Master Exam1 Hour Time Limit

No notes

Design specifications will be given by instructor along withsupplemental handouts necessary for calculations

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