Gravity flow drainfieldsAKA: absorption field, leach field, tile field, disposal field,

nitrification field, weeping lateral trench

Most commonly used – simple, inexpensive, no energy

Drainfields

Soil absorption system needs to be adequately sized for1. Infiltration and recycle of effluent2. Pathogen (bacteria and viruses) removal by the soil3. Organics and nutrients transformed in soil

Treatment mechanisms

SWIS = subsurface wastewater infiltration system STE = septic tank effluentFrom USEPA Onsite Wastewater Treatment Systems Manual

Water moves through soil in response to a potential energy gradient

4 driving forces for water movement in soil?

Infiltration and recycle of effluent

Water moves through soil in response to a potential energy gradient

Four componentsI. Gravitational Potential – due to weight of water, F=mgII. Hydrostatic Potential – pressure below zone of saturation, P=depth*ρIII. Matrix Potential – negative pressure created through capillary, affinity

of water for soil particlesIV. Osmosis – flow from more dilute to more concentrated

How will soil texture and structure affect these four components??

Infiltration and recycle of effluent

granular platy

blockyprismatic columnar

wedge

%sand%silt %clay

Bacteria and virus removal by the soil

Pathogens need to be removed before contact with ground water

Factors affecting removal:

Pathogens need to be removed before contact with ground water

Factors affecting removal:Soil type – lower porosity better at removing, physical strainingTemperature – thrive at body temperatureAdsorption – adsorbed to soil particles, smaller particles greater surface areaSoil clogging – biomat increases removal efficiencySoil moisture – need moisture to survive, die off in dry soilsPredatation – eaten Antagonism – antibioticsNutrients – die off if there is a lack of nutrients

Pathogen removal by the soil

Treatment via natural soil food web108-109 bacteria in 1 teaspoon or 1 dry gram of soilFactors that influence rates of decomposition?

Anaerobic decomposition

C OH NP S

cellularmaterial

+CH4NH3H2SPO4H2O

+ energyOrganic matter

C OH NP S

cellularmaterial

+ + energy

CO2NO3SO4PO4H2O

Aerobic decomposition

Organic matter

Organics and nutrients transformed by biological and chemical transformations

Image courtesy of the USDA-NRCS

Why do we care about nitrogen and phosphorus in soils?

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NO3- anions are repulsed by soil particles and move freely through the soil

Wastewater from house

Septic tankDrainfield

Effluent

Solids settle out in tank

Water

NO3- in groundwater

NO3- moving through drainfield Groundwater

P compounds complex with Ca, Fe and Al in soil

If nitrogen removal is required it is NOT achieved by the drainfield!!(will be covered in later lectures)NH3-N is dominant form of N in septic tank effluentN cycle in drainfield nitrification – NH3 NO3

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What is a system failure?

Failure

Complex organic mat develops at the gravel soil interfaceReduces infiltrative capacityResults in standing waterAnaerobic conditions increase mat formation (ponding)

Fe2+ + SO42- -> FeS (ferrous sulfide)

Black color, foul odor

Eventually EVERY drainfield will fail (?)Typically need to have reserve area for permit or alternating bedsIf you “rest” field biomat will decompose (H2O2)Effluent screens

Trench

Clogged Zone

Biomat

Design of gravel drainfield – see local regulations for exact numbers

Side view of trench

4” perforated pipe

Gravel (clean – no fines)

Infiltrative area

Geofabric (fines)

Why do you use gravel?1. support pipe and provide infiltrative area2. storage during peak flow3. media dissipates energy4. supports sidewall

Trench Width 1-3’ (width of backhoe blade)Typically spaced 6’ apart, sidewall to sidewallTrench width x length = infiltrative surface

it is assumed for design that this is the infiltrative area

Not to scale…2” gravel above6” below

At least 2-4’ depth to high water table or restrictive layerPlace as shallow as possible (0.5-2’)Length limited by lot sizeLaterals must be <100 feet for non-pressure systemLaterals must be parallel to contours, with a flat bottom (2-6” per 100 feet)

Design of gravel drainfield – see local regulations for exact numbers

How to get water evenly distributed to laterals with gravity system????1. Parallel distribution

Distribution boxesAssume even distribution for each outletBut they can settle and/or intermittent flow results in uneven distributionLaboratory studies have shown their inadequacies

2. Serial distribution/drop box3. Continual

Taken from NY State Building & Environmental Inspection website

Taken from Polylok website

Inlet higher

Outlet lower

Plastic D-box

Concrete D-box

Sep

tic e

fflue

nt

Dis

tribu

tion

box Water tight pipes = manifold

Per

fora

ted

pipe

s =

late

rals

Design of gravel drainfield – parallel distribution, best for level or sloping sitesManifold level for 2’ following distribution box, then sloped (e.g. 1/8-1/4” per foot)

http://www.cet.nau.edu/Projects/WDP/resources/Drainfields.html

Could include looped trench

http://www.ag.ndsu.edu/

http://www.cet.nau.edu/

Design of gravel drainfield – serial distribution – no slope maximum, more flexible

Design of gravel drainfield – continual distribution – no slope requirement

http://www.cet.nau.edu/Projects/WDP/resources/Drainfields.html

trenches are connected effluent passes through trench on

way to next trench

lateral

No biofilmWater could all drain here?

lateral

Biofilm develops Water drains here?

Side view

lateral

D-box settlingAll water drains in one lateral?

gravel trench

gravel trench

gravel trench

lateral

Biofilm developsWater backs up and drains to different lateral or system failure?

Gravel-less chamberInfiltrator

Gravel-less options:•Cannot get heavy machinery to site•Gravel not available or expensive•Eliminates problems with fines in gravel

100% recycled polystyrene EZ Flow

GeotextileEljen Corporation

http://www.ag.ndsu.edu/