Pressure Distribution Systems
Pressure Distribution Systems
This chapter establishes requirements for the design and installation of pressure distribution systems, particularly for conventional private sewage disposal.
701 — General
701.1 Scope. The provisions of this chapter shall govern the design and installation of pressure distribution systems.
702 — Design Loading Rate
702.1 General. A Pressure distribution system shall be permitted for use on any site meeting the conventional private sewage disposal system criteria. There shall be not less than 6 inches (152 mm) to the top of the distribution piping from original grade for any pressure distribution system. The minimum required suitable soil depths from original grade for pressure distribution systems shall be in accordance with Table 702.1 .
A table appears here in the adopted code. Refer to the official source for the full table.
702.2 Absorption area. The total absorption area required shall be computed from the estimated daily wastewater flow and the design loading rate based on the percolation rate for the site. The required absorption area equals wastewater flow divided by the design loading rate from Table 702.2. Two systems of equal size shall be required for systems receiving effluents exceeding 5,000 gallons (18 925 L). Each system shall have a minimum capacity of 75 percent of the area required for a single system and shall be provided with a suitable means of alternating waste applications. A dual system shall be considered as one system.
A table appears here in the adopted code. Refer to the official source for the full table.
702.3 Estimated wastewater flow. The estimated wastewater flow from a residence shall be 150 gallons (568 L) per bedroom per day. Wastewater flow rates for other occupancies in a 24-hour period shall be based on the values in Table 802.7.2.
Worked example
Scenario: A new residential septic system requires a pressure distribution field. The engineer needs to specify the minimum cover over the distribution pipes.
Explanation: According to Section 702.1, the top of the distribution piping must be at least 6 inches (152 mm) below the original grade.
703 — System Design
703.1 General. Pressure distribution systems shall discharge effluent into trenches or beds. Each pipe connected to an outlet of a manifold shall be counted as a separate distribution pipe. The horizontal spacing of distribution pipes shall be 30 inches to 72 inches (762 mm to 1829 mm). The system shall be sized in accordance with the formulas listed in this section. Systems using Schedule 40 plastic pipe shall be sized in accordance with the formulas listed in this section or in accordance with the tables listed in Appendix B. Distribution piping shall be installed at the same elevation, unless an approved system provides for a design ensuring equal flow through each of the perforations and the effluent is uniformly applied to the soil infiltrative surface (see Appendix A, Figure A-5).
703.2 Symbols. The following symbols and notations shall apply to the provisions of this chapter:
Ch = Hazen-Williams friction factor.
D = Distribution pipe diameter, inches (mm).
d = Perforation diameter, inches (mm) .
Dd = Delivery pipe diameter, inches (mm).
Dm = Manifold pipe diameter, inches (mm).
f = Fraction of total head loss in the manifold segment.
FD = Friction loss in the delivery pipe, feet of head (mm of head).
Fi = Friction factor for ith manifold segment.
FN = Friction loss in the network pipe, feet of head (mm of head).
h = Pressure in distribution pipe, feet ofhead (mm ofhead).
hd = In-line pressure at distal end oflateral, feet of head (mm of head).
LD = Length of delivery pipe, feet (mm).
Lj = Length of ith manifold segment, feet (mm).
N = Number of perforations in the lateral.
q = Perforation discharge rate, gpm (L/min).
Qj = Flow rate ith manifold segment, gpm (L/min).
Qm = Flow rate at manifold inlet, gpm (L/min).
703.3 Distribution pipe. Distribution pipe size, hole diameter and hole spacing shall be selected. The hole diameter and spacing shall be equal for each manifold segment. Distribution pipe size shall not be required to be the same for each segment. Changes in pressure in the distribution pipe shall be less than or equal to 10 percent by conforming to the following formula:
For SI: 1 foot = 304.8 mm.
where:
The Hazen-Williams friction factor, Ch, for each pipe material shall be determined in accordance with Table 703.3.
A table appears here in the adopted code. Refer to the official source for the full table.
703.4 Manifolds. The diameter of the manifold pipe shall be determined by the following equation:
For SI: 1 inch = 25.4 mm.
where:
Fi = 9.8 × 10-4 Qj
q = 11.79d2
Qj = Nq
The fraction of the total head loss at the manifold segment, f, shall be less than or equal to 0.1. The in-line pressure at the distal end of the lateral, hd, shall be a minimum of 2.5 feet (762 mm) of head. Distribution pipes shall be connected to the manifold with tees or gO-degree (1.57 rad) ells. Distribution pipes shall have the ends capped.
703.5 Friction loss. The delivery pipe shall include all pipe between the pump and the supply end of the distribution pipe. The friction loss in the delivery pipe, FD, shall be determined by the following equation:
For SI: 1 inch of head = 25.4 mm of head.
The Hazen-Williams friction factor, Ch, for each pipe material shall be determined in accordance with Table 703.3.
The friction loss in the network pipe shall be determined by the following equation:
FN = 1.31 hd (Equation 7-3)
For SI: 1 inch of head = 25.4 mm of head.
Pipe in the system shall be increased in size if the friction loss is excessive.
703.6 Force main. Size of the force main between the pump and manifold shall be based on the friction loss and velocity of effluent through the pipe. The velocity of effluent in a force main shall be not more than 5 feet per second (1524 mm/sec).
Worked example
Scenario: A designer is laying out a new pressure distribution field for a commercial building. They need to determine the appropriate spacing for the lateral distribution pipes.
Explanation: Section 703.1 dictates that the horizontal spacing of distribution pipes must be between 30 inches and 72 inches (762 mm to 1829 mm).
704 — Bed and Trench Construction
704.1 General. The excavation and construction for pressure distribution system trenches and beds shall be in accordance with Chapter 6. Aggregate shall be not less than 6 inches (152 mm) beneath the distribution pipe with 2 inches (51 mm) spread evenly above the pipe. The aggregate shall be clean, nondeteriorating 0.5-inch to 2.5-inch (12.7 mm to 64 mm) stone.
Worked example
Scenario: During the construction of a pressure distribution trench, the contractor needs to confirm the minimum aggregate depth required beneath the distribution pipe.
Explanation: Section 704.1 states that aggregate shall be not less than 6 inches (152 mm) beneath the distribution pipe, with an additional 2 inches (51 mm) spread evenly above the pipe.
705 — Pumps
705.1 General. Pump selection shall be based on the discharge rate and total dynamic head of the pump performance curve. The total dynamic head shall be equal to the difference in feet of elevation between the pump and distribution pipe invert plus the friction loss and a minimum of 2.5 feet (762 mm) where using low pressure distribution in the delivery pipe and network pipe.
705.2 Pump and alarm controls. The control system for the pumping chamber shall consist of a control for operating the pump and an alarm system to detect a pump. Pump start and stop depth controls shall be adjustable. Pump and alarm controls shall be of an approved type. Switches shall be resistant to sewage corrosion.
705.3 Alarm system. Alarm systems shall consist of a bell or light, mounted in the structure, and shall be located to be easily seen or heard. The high-water sensing device shall be installed approximately 2 inches (51 mm) above the depth set for the "on" pump control but below the bottom of the inlet to the pumping chamber. Alarm systems shall be installed on a separate circuit from the electrical service.
705.4 Electrical connections. Electrical connections shall be located outside the pumping chamber.
Worked example
Scenario: An engineer is selecting a pump for a pressure distribution system. The distribution pipe invert is 10 feet above the pump, friction loss is calculated at 5 feet, and low pressure operation is used.
Explanation: Section 705.1 requires the total dynamic head to be calculated as the elevation difference (10 feet) plus friction loss (5 feet) plus a minimum of 2.5 feet for low-pressure systems. Total dynamic head = 10 + 5 + 2.5 = 17.5 feet.
706 — Dosing
706.1 General. The dosing frequency shall be a maximum of four times daily. A volume per dose shall be established by dividing the daily wastewater flow by the dosing frequency. The dosing volume shall be a minimum of 10 times the capacity of the distribution pipe volume. Table 706.1 provides the estimated volume for various pipe diameters.
A table appears here in the adopted code. Refer to the official source for the full table.
Worked example
Scenario: A pressure distribution system handles 600 gallons per day (GPD) of wastewater. The system is designed for the maximum dosing frequency.
Explanation: Per Section 706.1, the maximum dosing frequency is four times daily. The volume per dose would be 600 GPD / 4 doses/day = 150 gallons per dose. This volume must also be a minimum of 10 times the distribution pipe volume.
Frequently asked questions
What is the minimum depth required for pressure distribution piping?
The top of the distribution piping must be not less than 6 inches (152 mm) from the original grade.
How often can a pressure distribution system dose effluent?
The dosing frequency is a maximum of four times daily.
What are the allowed horizontal spacing requirements for distribution pipes?
Distribution pipes must be spaced horizontally between 30 inches and 72 inches (762 mm to 1829 mm).