Successfully dosing pipe networks Kevin Sherman, Ph.D., P.E., D.WRE Vice President of Engineering Clearstream Wastewater Systems, Inc.
What is a successful dose? Spreads effluent throughout soil disp. area Uses entire soil dispersal area each dose Alternately wet and dry conditions Partially consume biomat as oxygen is drawn down behind a wetting front Can accept many more pounds of BOD 5 per square foot of surface (Hargett, 1984)
Why call it successful? Design preferences play a big role There are many ways to arrive at workable mixes of hole diameter, pipe diameter, hole spacing, etc. There are several rules of thumb (ROT), tricks and traps, etc. Cost and availability of materials Site constraints
Rule of Thumb #1 Flow through supply pipes should achieve scouring velocity
Scouring velocity = 2 ft/sec If flow in pipe gets too slow, suspended materials in pipe get left behind Called stranding solids Can eventually cause blockages in network At 2 ft/ sec or higher, will retain suspended solids with the fluid
Minimum gpm to achieve scouring velocity for common pipe diameters Pipe Diameter (in.) Minimum GPM (gallons per minute) 1.25 10 1.5 12 2 21 3 46
Rule of Thumb #2 Hole spacing must be between 2 ft (min) and 7 ft (max) Ch 7080 Table VI specifies 2 ft (min) to 3 ft (max)
Hole spacing detail If too close a hole s wetting area overlaps with its neighbor s Too far away dry areas between them (not using entire soil disp. area) If use 20 ft long pieces, choose 4 or 5 foot spacing, 1 st hole ½ distance chosen (i.e. 2 ft or 2.5 ft) Easier to construct because each 20 ft stick has identical hole pattern
Central versus End manifold Manifold fancy word for supply pipe with lateral connections Central manifold - discharge laterals arranged off both sides (letter H ) End manifold - looks like a gravity piping system (letter E ) With the same system size, central manifolds have half the length of discharge laterals, but twice the number
3 1 4 2
1 2 3 4 5
Rule of Thumb #3 Hole orientation up or down?
PUMP OFF BALL VALVE (2) LATERAL PIPE (2) MANIFOLD PIPE GATE VALVE (2) SWEEP ASSEMBLY (2) 14
PUMP ON PUMP OFF 15
PUMP ON PUMP OFF
Rule of thumb #4 No hole size smaller than 3/16 with septic tank effluent No hole size smaller than 1/8 with pretreated effluent Any hole larger than 1/4 is science fiction
Discharge rates for various sized holes at various pressures (in gpm) Operating head 1 ft (0.43 psi) 2 ft (0.87 psi) 3 ft (1.30 psi) 4 ft (1.73 psi) 5 ft (2.17 psi) 1/8 5/32 3/16 7/32 1/4 0.18 0.29 0.42 0.56 0.74 0.26 0.41 0.59 0.80 1.05 0.32 0.50 0.72 0.98 1.28 0.37 0.58 0.83 1.13 1.48 0.41 0.64 0.94 1.26 1.65
Sample Problem 4 BR Classification I residence loamy fine sand w/ 5% rock fragments 42 to periodically saturated soil <2% slope hill top landscape position 600 gallons per day flow rate 0.6 gal/ft 2 /day soil
Soil Dispersal Area Size 600 gallons per day 0.6 gallons/ft 2 per day 1000 ft 2
Designer chooses 8 laterals in a bed configuration with an end manifold feeding each pressure lateral What is the width of the soil dispersal area?
Width of Soil Disp. Area 3 ft max spacing between pressure laterals, 8 lines need 7 spaces + 1 ft either side = 3 x 7 = 21 +2 = 23 ft 1000 sq ft / 23 ft = 43.5 2 = 41.5 equal length pressure laterals = 43. 5 ft long by 23.0 ft wide
Plan View: Soil disp. system End manifold 21 ft Edge of rock 23 ft 3 ft 3 ft 3 ft 3 ft 3 ft 3 ft 3 ft Discharge laterals 43.5 ft -2.0 ft between end of pipe and edge of rock = 41.5 ft 43.5 ft
Design Details & Decisions End manifold splits the 8 lines into 8 pressurized laterals Pump volume < 25% of design flow Each lateral accepts 600/4 = 150 gallons/8 lines = 18.75 gallons per lateral each dose Assume a Pump delivery rate of 80 gpm Lateral delivery rate 80 gpm / 8 laterals = 10.0 gpm per lateral Assume 3/16 holes @ 3 ft of head
Discharge rates for various sized holes at various pressures (in gpm) Operating head 1 ft (0.43 psi) 2 ft (0.87 psi) 3 ft (1.30 psi) 4 ft (1.73 psi) 5 ft (2.17 psi) 1/8 5/32 3/16 7/32 1/4" 0.18 0.29 0.42 0.56 0.74 0.26 0.41 0.59 0.80 1.05 0.32 0.50 0.72 0.98 1.28 0.37 0.58 0.83 1.13 1.48 0.41 0.64 0.94 1.26 1.65
Number of Holes per lateral Space holes at 3 maximum per Ch 7080 The lateral is 41.5 ft long, so what is the total number of holes per lateral? 41.5 ft / 3 ft per hole = 13.8 holes, use 14 10 gpm / 0.72 gpm/hole = 13.9 holes (ok) What diameter should the laterals be?
41.5
manifold length 10 gpm per lateral w/ end manifold * 8 laterals w/ end manifold
Matching Pump to Network Commercially available Microsoft Excell spreadsheet Created by Larry Stephens, PE Great for evaluating what if scenarios quickly Available at scscons@yahoo.com
This is the pump design point!
What to do with this information Pump-flo.com pump selection software Basic version free, premium $99 /yr Input your favorite pump manufacturer Select type of pump (sewage effluent, solids handling, or grinder) you want Select pump selection criteria (design point, speeds, viscosity of fluid, etc.) Provides a pump curve with the design point on it
Zoeller 6280 series 2 or 3 vertical NPT discharge (choose 3 ) model 6284 Pump curve slightly above and to right of design point [81.5 gpm @ 14.8 ft TDH}
Compare to Ch 7080 Standards Perforation discharge Q = 19.65(0.60) diameter perf 2 *(head) 1/2 = 0.72 gpm * 112 holes = 80.6 gpm 81.5 gpm Residual head above 1 ft for 3/16 holes Pump TDH 5 above pipe friction and elevation change = 0.2 (dose tank)+ 1.3 (force main) + 10 (elev) + 5 = 16.5 TDH pump operates@ 96 gpm @21 Dose = 4x volume of distribution and supply pipes 83 of 3 schedule 40 + 332 of 1.25 schedule 40 = 57.72 gallons *4 = 231 gallons 39% of design flow <10% variance of flow for all perforations head of 3 used up to 16 holes OK