Post on 28-Jan-2016
description
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/