Gravity flow drainfields AKA: absorption field, leach field, tile field, disposal field, nitrification field, weeping lateral trench

Transcription

1 Gravity flow drainfields AKA: absorption field, leach field, tile field, disposal field, nitrification field, weeping lateral trench Most commonly used simple, inexpensive, no energy

2 Drainfields

3 Treatment mechanisms Soil absorption system needs to be adequately sized for 1. Infiltration and recycle of effluent 2. Pathogen (bacteria and viruses) removal by the soil 3. Organics and nutrients transformed in soil

4 SWIS = subsurface wastewater infiltration system STE = septic tank effluent From USEPA Onsite Wastewater Treatment Systems Manual

5 Infiltration and recycle of effluent Water moves through soil in response to a potential energy gradient 4 driving forces for water movement in soil?

6 Infiltration and recycle of effluent Water moves through soil in response to a potential energy gradient Four components I. Gravitational Potential due to weight of water, F=mg II. Hydrostatic Potential pressure below zone of saturation, P=depth*ρ III. Matrix Potential negative pressure created through capillary, affinity of water for soil particles IV. Osmosis flow from more dilute to more concentrated How will soil texture and structure affect these four components?? %sand %silt %clay granular platy blocky prismatic columnar wedge

7 Bacteria and virus removal by the soil Pathogens need to be removed before contact with ground water Factors affecting removal:

8 Pathogen removal by the soil Pathogens need to be removed before contact with ground water Factors affecting removal: Soil type lower porosity better at removing, physical straining Temperature thrive at body temperature Adsorption adsorbed to soil particles, smaller particles greater surface area Soil clogging biomat increases removal efficiency Soil moisture need moisture to survive, die off in dry soils Predatation eaten Antagonism antibiotics Nutrients die off if there is a lack of nutrients

9 Organics and nutrients transformed by biological and chemical transformations Treatment via natural soil food web bacteria in 1 teaspoon or 1 dry gram of soil Factors that influence rates of decomposition? Anaerobic decomposition Organic matter C O H N P S cellular material + CH 4 NH 3 H 2 S PO 4 H 2 O + energy Aerobic decomposition Organic matter C O H N P S cellular material CO 2 NO 3 + SO 4 + energy PO 4 H 2 O Image courtesy of the USDA-NRCS

10 Why do we care about nitrogen and phosphorus in soils?

11 If nitrogen removal is required it is NOT achieved by the drainfield!! (will be covered in later lectures) NH 3 -N is dominant form of N in septic tank effluent N cycle in drainfield nitrification NH 3 NO NO 3- anions are repulsed by soil particles and move freely through the soil Wastewater from house Water NO 3- moving through drainfield Drainfield Effluent Septic tank Solids settle out in tank Groundwater NO 3- in groundwater P compounds complex with Ca, Fe and Al in soil

12 What is a system failure?

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14 Failure Trench Biomat Complex organic mat develops at the gravel soil interface Reduces infiltrative capacity Results in standing water Anaerobic conditions increase mat formation (ponding) Fe 2+ + SO > FeS (ferrous sulfide) Black color, foul odor Clogged Zone Eventually EVERY drainfield will fail (?) Typically need to have reserve area for permit or alternating beds If you rest field biomat will decompose (H 2 O 2 ) Effluent screens

15 Design of gravel drainfield see local regulations for exact numbers Geofabric (fines) Side view of trench Gravel (clean no fines) 4 perforated pipe Not to scale 2 gravel above 6 below Infiltrative area Why do you use gravel? 1. support pipe and provide infiltrative area 2. storage during peak flow 3. media dissipates energy 4. supports sidewall Trench Width 1-3 (width of backhoe blade) Typically spaced 6 apart, sidewall to sidewall Trench width x length = infiltrative surface it is assumed for design that this is the infiltrative area

16 Design of gravel drainfield see local regulations for exact numbers At least 2-4 depth to high water table or restrictive layer Place as shallow as possible (0.5-2 ) Length limited by lot size Laterals must be <100 feet for non-pressure system Laterals must be parallel to contours, with a flat bottom (2-6 per 100 feet)

17 How to get water evenly distributed to laterals with gravity system???? 1. Parallel distribution Distribution boxes Assume even distribution for each outlet But they can settle and/or intermittent flow results in uneven distribution Laboratory studies have shown their inadequacies 2. Serial distribution/drop box 3. Continual Plastic D-box Inlet higher Concrete D-box Outlet lower Taken from Polylok website Taken from NY State Building & Environmental Inspection website

18 Design of gravel drainfield parallel distribution, best for level or sloping sites Manifold level for 2 following distribution box, then sloped (e.g. 1/8-1/4 per foot) Septic effluent Distribution box Water tight pipes = manifold Perforated pipes = laterals Could include looped trench

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

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21 Design of gravel drainfield continual distribution no slope requirement trenches are connected effluent passes through trench on way to next trench

22 Side view No biofilm Water could all drain here? lateral lateral gravel trench gravel trench Biofilm develops Water drains here? lateral gravel trench D-box settling All water drains in one lateral? lateral Biofilm develops Water backs up and drains to different lateral or system failure?

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

24 100% recycled polystyrene EZ Flow

25 Geotextile Eljen Corporation