G 11 - Irrigation Field Construction  

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  • The Administrative Authority may permit subsurface drip irrigation, mini-leachfield or other equivalent irrigation methods which discharge graywater in a manner which ensures that the graywater does not surface. Design standards for subsurface drip irrigation systems and mini-leachfield irrigation systems follow:


    Standards for a subsurface drip irrigation system are:


    Minimum 140 mesh (115 micron) 1-inch (25mm) filter with a capacity of 25 gallons (94.6L) per minute, or equivalent, filtration shall be used. The filter backwash and flush discharge shall be caught, contained and disposed of to the sewer system, septic tank or, with approval of the Administrative Authority, a separate mini-leachfield sized to accept all the backwash and flush discharge water. Filter backwash water and flush water shall not be used for any purpose. Sanitary procedures shall be followed when handling filter backwash and flush discharge or graywater.


    Emitters shall have a minimum flow path of 1,200 microns and shall have a coefficient of manufacturing variation (Cv) of no more than 7 percent. Irrigation system design shall be such that emitter flow variation shall not exceed 10 percent. Emitters shall be recommended by the manufacturer for subsurface use and graywater use, and shall have demonstrated resistance to root intrusion. For emitter ratings, refer to Irrigation Equipment Performance Report, Drip Emitters and Micro-Sprinklers, Center for Irrigation Technology, California State University, 5730 N. Chestnut Avenue, Fresno, California 93740-0018.


    Each irrigation zone shall be designed to include no less than the number of emitters specified in Table G-3, or through a procedure designated by the Administrative Authority. Minimum spacing between emitters is 14 inches (356mm) in any direction.


    The system design shall provide user controls, such as valves, switches, timers and other controllers, as appropriate, to rotate the distribution of graywater between irrigation zones.


    All drip irrigation supply lines shall be PVC Class 200 pipe or better and Schedule 40 fittings. All joints shall be properly glued, inspected and pressure tested at 40 psi (138kPa), and shown to be drip tight for five minutes, before burial. All supply lines will be buried at least 8 inches deep. Drip feeder lines can be poly or flexible PVC tubing and shall be covered to a minimum depth of 9 inches (229mm).


    Where pressure at the discharge side of the pump exceeds 20 psi (138kPa), a pressure-reducing valve able to maintain downstream pressure no greater than 20 psi (138kPa) shall be installed downstream from the pump and before any emission device.


    Each irrigation zone shall include an automatic flush valve/vacuum breaker to prevent back siphonage of water and soil.


    Standards for the mini-leachfield system are (see Figure 5):


    Perforated sections shall be a minimum 3-inch (76mm) diameter and shall be constructed of perforated high-density polyethylene pipe, perforated ABS pipe, perforated PVC pipe, or other approved materials, provided that sufficient openings are available for distribution of the graywater into the trench area. Material, construction and perforation of the piping shall be in compliance with the appropriate absorption field drainage piping standards and shall be approved by the Administrative Authority.


    Clean stone, gravel or similar filter material acceptable to the Administrative Authority, and varying in size between 3/4 inch (19mm) to 2½ inches (64mm) shall be placed in the trench to the depth and grade required by this Section. Perforated sections shall be laid on the filter material in an approved manner. The perforated sections shall then be covered with filter material to the minimum depth required by this Section. The filter material shall then be covered with landscape filter fabric or similar porous material to prevent closure of voids with earth backfill. No earth backfill shall be placed over the filter material cover until after inspections and acceptance.


    Irrigation fields shall be constructed as follows:

    Minimum Maximum
    Number of drain lines per irrigation zone 1
    Length of each perforated line 100 ft. (30480mm)
    Bottom width of trench 6 in. (152mm) 18 in. (457mm)
    Total depth of trench 17 in. (432mm) 18 in. (457mm)
    Spacing of lines, center to center 4 ft. (1219mm)
    Depth of earth cover of lines 9 in. (229mm)
    Depth of filter material cover of lines 2 in. (51mm)
    Depth of filter material beneath lines 3 in. (76mm)
    Grade of perforated lines level 3 in./100 ft. (76mm/30480mm)


(Ord. 95-0069 § 10 (part), 1995.)