Other Septic Tank Effluent Other Septic Tank Effluent Dispersal Options Dispersal Options

CEE484CEE484Decentralized and Onsite WasteDecentralized and Onsite Wastewater Management and Reusewater Management and Reuse

April 16, 2007April 16, 2007

Department of Civil and Environmental Department of Civil and Environmental EngineeringEngineering

University of WashingtonUniversity of Washington

Septic Effluent Dispersal OptionsSeptic Effluent Dispersal Options

•• Mound SystemMound System•• Pressure DistributionPressure Distribution•• EvapotranspirationEvapotranspiration•• Drip IrrigationDrip Irrigation

Mound SystemsMound Systems•• What is a mound system?What is a mound system?

•• Developed in North Dakota in late 1940sDeveloped in North Dakota in late 1940s•• A soil absorption system elevated above the natural soil surfacA soil absorption system elevated above the natural soil surface in a suitable fill e in a suitable fill

materialmaterial

•• What conditions favor the use a mound System?What conditions favor the use a mound System?

•• What conditions favor the use a mound System?What conditions favor the use a mound System?•• Overcomes site restrictions that prohibit use of drainfieldsOvercomes site restrictions that prohibit use of drainfields

•• Slowly permeable soilsSlowly permeable soils•• High water tableHigh water table•• Shallow permeable soils over crevice or porous bedrockShallow permeable soils over crevice or porous bedrock

What are the design components of What are the design components of a mound System?a mound System?

•• Suitable fill materialSuitable fill material•• An absorption areaAn absorption area•• Distribution networkDistribution network•• CapCap•• Top soilTop soil

Design issues for a mound SystemDesign issues for a mound System

1.0 to 1.5 ft1.0 to 1.5 ftCap and vegetationCap and vegetation

Infiltration rate of fill materialInfiltration rate of fill materialMound absorption bed areaMound absorption bed area

InfilrationInfilration rate of native soilrate of native soilMound base areaMound base area

33--5 ft5 ftDepth to impermeable layerDepth to impermeable layer

2020--24 in. to seasonally saturated 24 in. to seasonally saturated horizonshorizonsDepth of unsaturated fill Depth of unsaturated fill

00--6% (percolation rates<1.0 in/hr)6% (percolation rates<1.0 in/hr)00--12% (percolation rates>1.0 in/hr)12% (percolation rates>1.0 in/hr)

SlopeSlope

CriteriaCriteriaParameterParameter

Infiltration rates to determine base Infiltration rates to determine base area of Moundarea of Mound

0.250.25Clay loams, clayClay loams, clay

0.500.50Silt loams, Silt loams, siltysilty clay clay loamsloams

0.750.75Loam, silt loamsLoam, silt loams

1.21.2Sand, sandy loamSand, sandy loam

Infiltration rateInfiltration rategal/ftgal/ft22--dayday

Native on site soilNative on site soil

Mound fill and infiltration ratesMound fill and infiltration rates

Sand/sandy loamSand/sandy loam

Sandy loamSandy loam

Medium SandMedium Sand

MaterialMaterial

1.21.20.05 mm to 0.25 mm0.05 mm to 0.25 mm

1.21.28888--93% sand93% sand

0.60.655--15% clay15% clay

Infiltration RateInfiltration Rategal/ftgal/ft22--dayday

CharacteristicsCharacteristics

0.25 gpd/ft2Natural soil infiltration rate1.2 gpd/ft2Fill infiltration rate2.0 ftFill depth below excavation

450 gal/dDesign FlowDesign Assumptions

Mound Design Example

more? out it extend youdo ft. 1780 47.5by 37.5 for

area to compares which1800ft0.25gpd/ft

450gpd area base Minimum

ft 47.5 2(11.25ft) ft 25 lenght Overallft 37.5 ft) 2(11.25 ft 15 widthOverall

ft 11.25(3.0) ft 3.75 out extend sides the slope 1:3 At in. 9 ft 3 cover ft 1

depth bed of in 9 excavationbelow ft 2 bed of height total

ft 5215ft

375ft length Bed :bed wideft 15 Assume

ft 375gpd/ft 1.2

gpd 450 mound withinarea Absorption

22

2

22

=

==

=+==+=

==+

+=

==

==

Advantages and disadvantages of a mound systemAdvantages and disadvantages of a mound system

Difficult and expensive to repairDifficult and expensive to repair

Requires pumping instead of Requires pumping instead of gravity flowgravity flow

Placement may limit land use Placement may limit land use optionsoptions

Can be used with creative Can be used with creative landscapinglandscaping

More skilled installer neededMore skilled installer neededCan be used in most climatesCan be used in most climates

Design and construction costs Design and construction costs higherhigher

Used in land not suitable for Used in land not suitable for conventional conventional drainfielddrainfield

DisadvantagesDisadvantagesAdvantagesAdvantages

Pressure Dosing SystemPressure Dosing SystemConsist of a wet well or Consist of a wet well or ““dosing chamberdosing chamber”” with with pump and controlspump and controlsWhen is it use necessary?When is it use necessary?

Septic tank can not drain by gravitySeptic tank can not drain by gravityDrain field pipe is very long (>500 linear ft)Drain field pipe is very long (>500 linear ft)Drain field is designed to use a pressurized systemDrain field is designed to use a pressurized systemWhen Mound system is usedWhen Mound system is used

Advantages for septic tank drainfields?Advantages for septic tank drainfields?Better and more equal distribution to drainage areaBetter and more equal distribution to drainage areaProvides preferable saturation and unsaturated Provides preferable saturation and unsaturated operating conditions with better aeration in off timeoperating conditions with better aeration in off time

Dosing Chamber with automatic pumpCan be a separate chamber or at end of septic tank

Dosing siphonDrainfield about 2 ft lower elevation

How many doses per day and flow rate for pump?How many doses per day and flow rate for pump?

Rule of thumb ~60-75% of volume in manifold and distribution piping per dose

ft 1 is needed depth pump assumediameter ft 3 about chamber assume

holdingday 1 least at equalcapacity reservevolume reservoir lower permanent volume dose capacity alarm capacity reserve

volume? chamber dose is what

gallons 219 pump for volume dose anddoses/day 6.1gal/dose 219gal/day 360 frequency dosing maximum

gal 219 gal 292 of 75%

gal 292)gal/ft )(7.482ftft)(0.0087 (448 pipes in volume total,ft 008724

)12/4(4

area Pipe

ft 44848 400 lenght pipe inch 4 total long, ft 48 is pipe manifold assume

ft 4002ft

ft 800 length trench apart, ft 8 and widthft 2 at trench

ft 008gpd/ft 0.45

gpd 360 area Drainfield

d-gal/ft 0.45 rate acceptance term long and silt porous loam, silt soil drainfield

gal. 360 flow daily Home, 3Bedroom :Example

32222

2

22

2

+++

===

=

=====

=+=

==

==

==

=

ππD

Type of Information for Pump SelectionType of Information for Pump Selection

0.0

20.0

40.0

60.0

80.0

100.0

0 1000 2000 3000 4000 5000 6000 7000 8000

Hea

d, f

t

Flow, gpm

Pump Head vs flow

System Head vs Flow

Pump Efficiency (x10)

Pump SelectionPump Selection

Need pump flow rateNeed pump flow rateTake dose volume and divide by percolation Take dose volume and divide by percolation raterateOr design pipe orifices Or design pipe orifices --get orifice flow rate for get orifice flow rate for given size orifice and flow equal that times # given size orifice and flow equal that times # of orificesof orifices

Need pump operating head for the design Need pump operating head for the design flowrateflowrate

0.90 to 0.70 t,coefficien orifice C area, opening orifice A orifice on head or pressure H where,2gHCAQ orrifice, an For

==

==

Pump Operating HeadPump Operating Head

Function of flow rateFunction of flow rateHead = pressure head desired in pipe +Head = pressure head desired in pipe +

elevation lift head + elevation lift head + headlossheadloss due due to friction loss in pipe + to friction loss in pipe + miscmisclosses due to bends etclosses due to bends etc

From From hazenhazen williamswilliams equationequation

85.1

2.63H

0.541.85

L0.54

L2.63

D0.43CLK where,KQh then

Lh0.43CHDQ

⎥⎥⎦

⎤

⎢⎢⎣

⎡==⎟

⎠⎞

⎜⎝⎛=

These are small pumps – 1/8 to 1/2 Hp range

EvapotranspirationEvapotranspiration

Arid climatesArid climatesEffluent to sand bed (24 to 30 in) above a Effluent to sand bed (24 to 30 in) above a linerlinerSurface of sand covered with shallow layer Surface of sand covered with shallow layer of topsoil and water tolerant vegetation of topsoil and water tolerant vegetation may be addedmay be addedWater moves up via capillary flow in sandWater moves up via capillary flow in sandMay be combined with lower level May be combined with lower level percolationpercolation

What determines bed areaWhat determines bed area

Septic tank effluent application rateSeptic tank effluent application ratePrecipitation ratePrecipitation rateEvapotranspirateEvapotranspirate rateratePercolation rate if combined systemPercolation rate if combined system

PETQA

+−=

Pr

Subsurface Drip DispersalSubsurface Drip DispersalThe system distributes wastewater to The system distributes wastewater to the soil, through a network of drip tubing the soil, through a network of drip tubing installed below the ground surface.installed below the ground surface.The system is composed of:The system is composed of:

Wastewater sourceWastewater sourcePretreatment device(s)Pretreatment device(s)Pump tankPump tankControllerControllerFiltering deviceFiltering deviceDrip dispersal field(s)Drip dispersal field(s)

Pretreatment DevicePretreatment DeviceSelection ProcessSelection Process

Sites capability to accept and treat wastewaterSites capability to accept and treat wastewaterType of drip tubing being used and manufacturers Type of drip tubing being used and manufacturers recommendations.recommendations.

Available treatment devices:Available treatment devices:Anaerobic TreatmentAnaerobic TreatmentAerobic TreatmentAerobic Treatment

•• Aerobic Treatment UnitAerobic Treatment Unit•• Sand FilterSand Filter•• Media FilterMedia Filter•• Constructed WetlandConstructed Wetland

Others?Others?

Septic Tank/Pump Tank

Constructed WetlandAerobic Treatment Unit

Media Filter

Drip Field LayoutDrip Field Layout

Drip Field LayoutDrip Field Layout

Pump Tank/PumpPump Tank/PumpThe pump tank stores treated effluent until The pump tank stores treated effluent until the drip field is ready to be dosed. the drip field is ready to be dosed. A high head pump delivers water from the A high head pump delivers water from the pump tank through the filtering device to pump tank through the filtering device to the drip dispersal system.the drip dispersal system.Pump is sized to supply required water Pump is sized to supply required water volume at the needed pressure.volume at the needed pressure.

Pump Tank / Pump Tank / Subsurface Drip Subsurface Drip

SystemSystemHigh head pumpHigh head pumpWater level sensorsWater level sensors

Filtering DeviceFiltering DeviceThe filtering device can be media (including The filtering device can be media (including sand), disk or screen.sand), disk or screen.Its main purpose is to remove larger particles Its main purpose is to remove larger particles from the wastewater so they do not cause from the wastewater so they do not cause problems with the drip emitters. Generally problems with the drip emitters. Generally remove particles greater than 100 microns.remove particles greater than 100 microns.Depending upon wastewater quality, the filter Depending upon wastewater quality, the filter may need to be an automatic cleaning system.may need to be an automatic cleaning system.

Automatic Back Flushing Disk FiltersAutomatic Back Flushing Disk Filters

Media Filter System Media Filter System

Drip Field ComponentsDrip Field Components

Supply lineSupply lineWater distribution devices (flow splitting)Water distribution devices (flow splitting)ZonesZonesSupply manifoldSupply manifoldDrip lateralsDrip lateralsReturn manifoldReturn manifoldAir relief/Vacuum breakerAir relief/Vacuum breaker

Drip Dispersal Drain FieldDrip Dispersal Drain Field

Drip Field ComponentsDrip Field Components

Drip Field with Looped RunsDrip Field with Looped Runs

Drip LineDrip LineDrip line approved by the manufacturer for use Drip line approved by the manufacturer for use with wastewater.with wastewater.Pressure Compensating or NonPressure Compensating or Non--Pressure Pressure Compensating emitters.Compensating emitters.The tubing is generally The tubing is generally ½½ inch diameter with an inch diameter with an emitter in the tubing.emitter in the tubing.The operating pressure is typically 15The operating pressure is typically 15--25 psi for 25 psi for nonnon--pressure compensating and 15pressure compensating and 15--45 psi for 45 psi for pressure compensating emitter systems, with pressure compensating emitter systems, with water exiting the pressure compensating emitter water exiting the pressure compensating emitter under pressure.under pressure.BioslimeBioslime and root intrusion control is achieved and root intrusion control is achieved by bactericides, herbicides and flushing.by bactericides, herbicides and flushing.

Drip LineDrip Line

Supply Manifold, Return Supply Manifold, Return Manifold, and Return LineManifold, and Return Line

Water ReuseWater ReuseLandscape IrrigationLandscape IrrigationIncorporated into the Incorporated into the landscapelandscape

What are advantages and What are advantages and disadvantages of drip irrigation?disadvantages of drip irrigation?

Advantages and DisadvantagesAdvantages and Disadvantages

Higher capital costHigher capital cost

Higher energy Higher energy

Greater maintenanceGreater maintenanceWater reuseWater reuse

More complexMore complexCan be used in wide Can be used in wide range of site conditionsrange of site conditions

DisadvantagesDisadvantagesAdvantagesAdvantages