Pressure Distribution Design

Transcription

1 Pressure Distribution Design Presented at the 2011 CEHA Annual Educational Conference By Roger J. Shafer, P.E.

2 Distribution Systems Non-pressurized Distribution

3 Distribution Systems Non-pressurized Distribution (cont.)

4 Distribution Systems Non-pressurized Distribution (cont.)

5 Distribution Systems Non-pressurized Distribution (cont.)

6 Distribution Systems Pressure Dosed mounds and beds

7 Distribution Systems Pressure Dosed - trenches

8 Distribution Systems Pressure Dosed sand filters

9 Why Pressure Distribution? Dosed-flow distribution systems are a significant improvement over gravity-flow distribution systems dosed-flow systems reduce the rate of soil clogging, more effectively maintain unsaturated conditions and provide a means to manage wastewater effluent applications to the infiltration system. US EPA Onsite Wastewater Treatment Systems Manual, 2002

10 Pressure Dosing Provides Uniform loading to the soil hydraulic and organic Unsaturated flow through the soil Improved oxygenation Longer retention for better treatment Graphics courtesy of Orenco Systems, Inc.

11 Demand vs. Timed Dosing Demand Dosing Dose occurs when sufficient volume of effluent is collected. Dosing frequency depends on how much water is generated, and when. There is no control over how often dosing occurs. Timed Dosing Timer controls doses per day, and volume of dose. Minimizes the potential for over-loading or saturated conditions during peaks. Useful for controlling surges and high-flow days.

12 Design Goal To Design a Distribution System that Provides the Optimal Effluent Distribution to the Dispersal Field. Uniform Loading to the Soil Unsaturated Flow through the Soil Applies to Trenches; Beds; Mounds; Sand Filters; and Drip Dispersal Systems Size a Pump to meet the Goal Obtain a design Gallon per Minute (GPM) and a Total Dynamic Head (TDH), which is needed to choose a pump.

13 Total Dynamic Head

14 Example Configurations End Feed System Center Feed System Mulit-zone Feed System

15 Design Example 3 Bedroom House = 450 GPD average = 675 GPD Design Soils are a Sandy Loam = 0.72 GPD/SF Required Absorption Area = square feet Shallow Ground Water Use a shallow bed Long and Narrow especially on sloping sites Use 15 X long laterals 4 laterals total Lateral pipe diameter??? Use 3 lateral spacing 9 long manifold manifold diameter???? Orifice size and spacing???

16 Design References US EPA Onsite Wastewater Treatment System Manuals, 1980 and Otis, R.J Design of Pressure Distribution Networks for Septic Tank Soil Absorption Systems. Small Scale Waste Management Project. Converse, James C Pressure Distribution Network Design. Basically extracted and modified version of the Otis paper. WA State Dept. of Health. July Pressure Distribution Systems. This is a design guidance document. Several Health Departments have similar documents. Utah On-Site Wastewater Treatment Training Program

17 Design Software PumpSelect by Orenco Systems, Inc.

18 Rules of Thumb Discharge Assembly What is locally available (in stock)? Consider Head Losses smaller pipe = higher head loss Higher flow rates = higher head losses Consider matching the pump discharge diameter to 2 is typical Transport Line Size Consider Head Losses smaller pipe = higher head loss Higher flow rates = higher head losses 1 to 2 is typical

19 Rules of Thumb (cont) When to Use an Automatic Distributing Valve For large systems, a valve keeps zones small, which allows smaller, more reasonably-sized pumps. For multiple zones at different elevations. For multiple zones in different areas. To alternately dose different zones. Manifold Line Size Consider Head Losses smaller pipe = higher head loss Higher flow rates = higher head losses 1.5 to 2 is typical

20 Rules of Thumb (cont) Lateral Line Size Consider Head Losses smaller pipe = higher head loss Higher flow rates = higher head losses Maintain less than 10% flow differential from 1 st to last orifice Laterals are typically smaller than Manifolds (each lateral transports a smaller flow rate than the manifold) 1 to 1.5 is typical

21 Rules of Thumb (cont) Orifice Size 1/8 orifice Can be used with screened or pretreated effluent, and high-head pumps 5/32 orifice Commonly used with screened effluent and high-head pumps 3/16 orifice Can be used with most applications Minimum size recommended with low-head pumps 1/4 orifice Often not used due to high flow rates from each orifice Requires larger pumps Larger dose volume may not maintain unsaturated flow

22 Rules of Thumb (cont) Orifice Spacing Try to maintain a grid (i.e. 2 by 2 or 3 by 3 etc.) More orifices provides better distribution More orifices causes high flow rates and high head loss = larger pumps Residual Head at Last Orifice (squirt height) 1/8 orifice = minimum 3 5 squirt 5/32 orifice = minimum 2 5 squirt 3/32 orifice = minimum 2 3 squirt 1/4 orifice = minimum 2 squirt

23 Rules of Thumb (cont) Dose Volume Distribution is poor during filling and draining the network (prior to, and after, pressurizing). Dose at least 5 times the volume of the distribution network. Minimize the diameter of long transport lines that drain back.

24 Other Recommendations Screen septic tank effluent. Use higher-head pumps to scour pipes and clear orifices. Use flushing valves to clear debris from slime build-up. Use orifice shields to prevent gravel from masking the orifices. Face orifices downward in cold climates.

25 Questions? Contact Information: