Other Septic Tank Effluent Dispersal Options

Transcription

1 Other Septic Tank Effluent Dispersal Options CEE484 Decentralized and Onsite Waste water Management and Reuse April 16, 2007 Department of Civil and Environmental Engineering University of Washington

2 Septic Effluent Dispersal Options Mound System Pressure Distribution Evapotranspiration Drip Irrigation

3 Mound Systems What is a mound system? Developed in North Dakota in late 1940s A soil absorption system elevated above the natural soil surface e in a suitable fill material What conditions favor the use a mound System?

4 What conditions favor the use a mound System? Overcomes site restrictions that prohibit use of drainfields Slowly permeable soils High water table Shallow permeable soils over crevice or porous bedrock

5 What are the design components of a mound System? Suitable fill material An absorption area Distribution network Cap Top soil

6 Design issues for a mound System Parameter Criteria Slope Depth of unsaturated fill Cap and vegetation 0-6% (percolation rates<1.0 in/hr) 0-12% (percolation rates>1.0 in/hr) in. to seasonally saturated horizons 1.0 to 1.5 ft Depth to impermeable layer Mound base area Mound absorption bed area 3-55 ft Infilration rate of native soil Infiltration rate of fill material

7 Infiltration rates to determine base area of Mound Native on site soil Sand, sandy loam Loam, silt loams Silt loams, silty clay loams Clay loams, clay Infiltration rate gal/ft 2 -day

8 Mound fill and infiltration rates Material Characteristics Infiltration Rate gal/ft 2 -day Medium Sand 0.05 mm to 0.25 mm 1.2 Sandy loam 5-15% clay 0.6 Sand/sandy loam 88-93% sand 1.2

9 Mound Design Example Design Assumptions Design Flow Fill depth below excavation Fill infiltration rate Natural soil infiltration rate 450 gal/d 2.0 ft 1.2 gpd/ft gpd/ft 2

10 450 gpd Absorption area within mound = 1.2 gpd/ft 375ft Assume 15 ft wide bed :Bed length = 15ft total height of bed = 2 ft below excavation ft = 25 ft 9 in of bed depth + 1ft cover = 3 ft 9 in. At 3 :1 slope the sides extend out 3.75 ft (3.0) = ft Overall width = 15 ft + 2(11.25 ft) = 37.5 ft Overall lenght = 25 ft + 2(11.25ft) = 47.5 ft 2 2 = 2 Minimum base area = 450gpd 0.25gpd/ft = 1800ft which compares for 37.5 by 47.5 = 1780 ft. do you extend it out more? 2 2 to area

11 Advantages and disadvantages of a mound system Advantages Disadvantages Used in land not suitable for conventional drainfield Design and construction costs higher Can be used in most climates More skilled installer needed Can be used with creative landscaping Placement may limit land use options Difficult and expensive to repair Requires pumping instead of gravity flow

12 Pressure Dosing System Consist of a wet well or dosing chamber with pump and controls When is it use necessary? Septic tank can not drain by gravity Drain field pipe is very long (>500 linear ft) Drain field is designed to use a pressurized system When Mound system is used Advantages for septic tank drainfields? Better and more equal distribution to drainage area Provides preferable saturation and unsaturated operating conditions with better aeration in off time

13 Dosing Chamber with automatic pump Can be a separate chamber or at end of septic tank

14 Dosing siphon Drainfield about 2 ft lower elevation

15 How many doses per day and flow rate for pump? Rule of thumb ~60-75% of volume in manifold and distribution piping per dose Example : drainfield soil = Drainfield area 3Bedroom Home, silt loam, porous silt 360 gpd = 0.45 gpd/ft 800 ft trench at 2 ft width and 8 ft apart, trench length = = 400 ft 2ft assume manifold pipe is 48 ft long, total 4 inch pipe lenght = = 448 ft πd π(4 /12) Pipe area = = % of 292 gal = 219 gal maximum dosing frequency = daily flow = 360 gal. = 800 ft = ft and long term acceptance rate = 0.45 gal/ft gal/day 219 gal/dose 2 - d,total volume in pipes = (448 ft)( ft )(7.48 gal/ft = 1.6 doses/day and dose volume for pump = 219 gallons ) = 292 gal what is dose chamber volume? reserve capacity + alarm capacity + dose volume + permanent lower reservoir reserve capacity equal at least 1day holding assume chamber about 3 ft diameter assume pump depth needed is 1ft volume

16 Type of Information for Pump Selection Pump Head vs flow Pump Efficiency (x10) 80.0 Head, ft System Head vs Flow Flow, gpm

17 Pump Selection Need pump flow rate Take dose volume and divide by percolation rate Or design pipe orifices -get orifice flow rate for given size orifice and flow equal that times # of orifices For an orrifice, Q = CA A = orifice opening area, C 2gH, where H = pressure or head on orifice = orifice coefficient, 0.70 to 0.90 Need pump operating head for the design flowrate

18 Pump Operating Head Function of flow rate Head = pressure head desired in pipe + elevation lift head + headloss due to friction loss in pipe + misc losses due to bends etc From hazen williams equation h Q 0.43CHD L = L then hl 1.85 = KQ, where K = L CHD These are small pumps 1/8 to 1/2 Hp range

19 Evapotranspiration Arid climates Effluent to sand bed (24 to 30 in) above a liner Surface of sand covered with shallow layer of topsoil and water tolerant vegetation may be added Water moves up via capillary flow in sand May be combined with lower level percolation

20 What determines bed area Septic tank effluent application rate Precipitation rate Evapotranspirate rate Percolation rate if combined system A = ET Q Pr+ P

21 Subsurface Drip Dispersal The system distributes wastewater to the soil, through a network of drip tubing installed below the ground surface. The system is composed of: Wastewater source Pretreatment device(s) Pump tank Controller Filtering device Drip dispersal field(s)

22 Pretreatment Device Selection Process Sites capability to accept and treat wastewater Type of drip tubing being used and manufacturers recommendations. Available treatment devices: Anaerobic Treatment Aerobic Treatment Aerobic Treatment Unit Sand Filter Media Filter Constructed Wetland Others?

23 Constructed Wetland Aerobic Treatment Unit Media Filter Septic Tank/Pump Tank

24 Drip Field Layout

25 Drip Field Layout

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

27 Pump Tank / Subsurface Drip System High head pump Water level sensors

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

29 Automatic Back Flushing Disk Filters

30 Media Filter System

31 Drip Field Components Supply line Water distribution devices (flow splitting) Zones Supply manifold Drip laterals Return manifold Air relief/vacuum breaker

32 Drip Dispersal Drain Field

33 Drip Field Components

34 Drip Field with Looped Runs

35 Drip Line Drip line approved by the manufacturer for use with wastewater. Pressure Compensating or Non-Pressure Compensating emitters. The tubing is generally ½ inch diameter with an emitter in the tubing. The operating pressure is typically psi for non-pressure compensating and psi for pressure compensating emitter systems, with water exiting the pressure compensating emitter under pressure. Bioslime and root intrusion control is achieved by bactericides, herbicides and flushing.

36 Drip Line

37 Supply Manifold, Return Manifold, and Return Line

38 Landscape Irrigation Incorporated into the landscape Water Reuse

39 What are advantages and disadvantages of drip irrigation?

40 Advantages and Disadvantages Advantages Can be used in wide range of site conditions Water reuse Disadvantages More complex Greater maintenance Higher energy Higher capital cost