5.1 INDIVIDUAL RESIDENTIAL SPRAY IRRIGATION SYSTEM (IRSIS)
This system must serve individual residences and are permitted by the municipal
SEO. An individual spray irrigation system means "an individual sewage
system permitted under Section 7 of Act 537 (recent amendments Act 149) that
services a single dwelling, and which treats and disposes of sewage using a
system of piping, treatment tanks, and soil renovation through spray
irrigation". The IRSIS systems must be monitored annually
regardless of the pretreatment. The IRSIS (individual spray irrigation
system) may be permitted through the local agency's sewage enforcement officer.
If the spray irrigation system services for commercial, industrial, or multiple
family dwelling will require PADEP approval. The general requirements are listed below:
1). Soil Requirements: 10” mottling/high water table and 16” bedrock, loose shale, loose gravel. The depth to the limiting zone is measured from the top of the mineral soil.
2) Slope: The ground slope over the length of the irrigation field is as follows:
0-25% mature forest cover
0-12% grassed fields
0-4% cultivated fields, agriculture
3) Spray Field Area: For a 3 bedroom homes ranges from 10,000 to
80,000 ft2.
4) System Components:
Pretreatment Tank:
2 compartment septic tank or 2 septic tanks
or Aerobic Unit. The aerobic tank must contain an NSF seal , electrical
and mechanical components shall comply with the standards for the National
Electric Code and visual/ audible alarms are required. Where intermittent
use or there is a lack or dependable service and maintenance, an aerobic tank is
not recommended.
Pump tank: The lift tank shall be constructed of materials consistent
with specification in Chapter 73, Section 73.45. The dose volume shall be
a minimum of 100 gallons and the lift tank shall have a minimum liquid capacity
of 200 gallons.
Dosed Intermittent Sand Filter (Free Access or Buried)
A. Free Access Intermittent Sand Filter- A free access filter is designed to allow
periodic maintenance and filter accessibility. A small sand surface area is
required for the free access filter as the sand can be replaced as
needed.
If Aerobic Pretreatment is Used: The minimum surface area is 40 square
feet for an aerobic unit for a 3 bedroom home and for each additional bedroom
the area will be increased by 10 sq feet.
If Anaerobic Pretreatment is Used: The filter requires two filter units or a two compartment or chamber tank with each tank or chamber providing a surface area of 40 square feet for a septic unit for a 3 bedroom home and for each additional bedroom the area will be increased by 10 ft2 per tank or chamber. Alternating dosing is required.
The material within the chamber will meet the following requirements:
1)The fine aggregate shall have an effective size of between 0.3 to 0.6 mm, a uniformity coefficient of less than 3.5 and less than 4% of the coarse aggregate passing the # 100 sieve. The sieve analysis shall be conducted in accordance with PADOT PTM # 616 and the uniformity coefficient shall be determined by using PADOT PTM # 149.
2) The sand shall not contain more than 10% by weight deleterious material as determined by PADOT PTM# 510.
3) The percentage of weight loss after 5 cycles of immersion and drying cannot be greater than 12% as determined using a sodium sulfate solution in accordance with PADOT PTM # 510.
B. Buried Sand Filter The Buried Sand Filter is designed to be entirely subsurface, except where seasonal water table or bedrock dictate. The filter must be located in stable fill. The larger buried filter surface area is designed to provide enough sand material to treat sewage effluent indefinitely.
1) The size of the sand filter depends on the estimated daily flow in accordance with Chapter 73, Section 73.16(a). The minimum surface area will be 600 ft2 and for a single family home the minimum size will be based on a sizing factor of 1.5 sq ft per gallon per day.
2)General Construction Specifications
a) At least 2 inches of coarse aggregate shall surround underdrains and distribution pipes. Coarse aggregate used in underdrains and distribution system shall meet the requirements in PADOT specifications publication # 408, Section 703. The size and grading of the aggregate shall meet AASHTO 57 requirement from a PADOT certified stockile. The transition layer will be a minimum of 4 inches of coarse aggregate and shall be placed above the aggregate underdrain material.
b)At least 24 inches of sand must be placed over the transition layer. The size and grading shall meet bituminous concrete sand Type B#1 or #3 requirement from a PADOT certified stockpile. The sieve analysis shall be conducted in accordance with PTM # 616 and # 100. A soundness test in accordance with PTM # 510. The soundness loss shall be a maximum of 12 % and test annually.
c)Minimum depth of earth cover is 12 inches and soil cover must extend beyond the filter area by at least 3 feet.
Chlorine contact/ disinfection facility
Disinfection is required prior to spray irrigation and will produce an effluent of less than 200 fecal coliform colonies per 100 ml. The chlorinator must maintain a chlorine residual of 0.2 to 2.0 ppm and provide a thirty minute contact time. Storage tanks provide further disinfection action and server to retain the effluent until it is distributed to the spray field. The minimum liquid capacity of an individual residential spray irrigation system storage tank shall be:
| Number of Bedrooms | Minimum
Capacity (gallons) |
| 3 or less | 2,000 |
| 4 | 2,500 |
| 5 | 3,000 |
Note: Additional storage may be required where more than 5 days
of storage is needed due to climatic conditions or when spray fields are located
in flood prone areas.
Spray field
1) The effluent shall be discharged to the spray field once per day.
Irrigation should take place early in the morning ( 4 to 5 am).
2)Spray Field Sizing
| Soil Conditions |
Slope | 3 bedroom | 4 bedroom | each additional bedroom |
| Rock > 20 inches and |
< 12 % | 10,000 ft2 | 12,500 ft2 | 2,500 ft2 |
| Water Table > 20 inches | > 12% | 20,000 | 25,000 | 5,000 |
| Rock 16 to 20 inches and | < 12% | 15,000 | 18,750 | 3,750 |
| Water Table > 40 inches | > 12 % | 30,000 | 37,500 | 7,500 |
| Depth to Rock > 20 inches | < 12% | 20,000 | 25,000 | 5,000 |
| Water Table 10 to 20 inches | > 12 % | 40,000 | 50,000 | 10,000 |
| Rock 16 to 20 inches and | < 12% | 40,000 | 50,000 | 10,000 |
| Water Table > 40 inches | > 12 % | 80,000 | 100,000 | 20,000 |
Important Note: Conventional on-site disposal uses loading depths of 1.5 to over 3 sq ft/gpd, but the loading rate for the spray irrigation system is 25 to 200 sq feet/gpd. It is my professional opinion that the sizing of the spray irrigation option is extremely oversized, especially when compared to less effective systems such as conventional elevated sand mounds and seepage beds. It is my opinion, a number of factors can be used to reduce the size of the spray field. These include: an allocation for interceptive loss in forested areas, irrigating multiple times per day, and a more realistic loading depth (probably approximately 0.125 inches per week and sized at an average daily flow peak flow of 400 gpd with multiple spray periods). This would reduce the disposal bed area to a minimum size of 5000 square feet.
For more detailed information on this alternative, visit the following webpage. For large volume spray irrigations systems, flows greater than 2000 gpd.
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