Septic System Design Flow Rate Specifications
Sewage flow rates & model regulations for septic designers

  • SEWAGE FLOW & DESIGN FLOW ESTIMATES - CONTENTS: What wastewater design sewage flow rate should be used by septic system designers. Septic flow rates for homes using either traditional (pre 1979) and more recent (water saving) plumbing fixtures
  • POST a QUESTION or READ FAQs about the septic blackwater or wastewater input flow rate assumptions that should be used by septic system and septic component designers - model septic regulations

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Sewage or wastewater flow rate assumptions & specifications for septic designers: these model septic design regulations discuss sewage flow rates to be used in designing onsite wastewater treatment facilities for individual households.

This document uses the New York State wastewater treatment standard for individual household septic systems (Appendix 75-A) to provide an example of state regulated design and installation of both conventional tank and leach field septic systems and alternative septic system designs, including raised septic systems, septic mound systems, intermittent sand filter septic systems, and evaportion-transpiration septic systems.

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Sewage Design Flow Rates & Regulations

Model source: New York State Septic Regulation Appendix 75-A.3 - Sewage flows - Effective Date: 12/01/90 or later and cited in detail at REFERENCES. The page top photo of campers at Camp Virginia (1950's) appears in our detailed discussion of OUTHOUSES & LATRINES.

Definition of daily design flow of wastewater

The daily design flow is the volume of wastewater that should be assumed when determining the volume of wastewater that the sewage system (septic system or soakbed or drainfield or other treatment system) must be capable of handling. "Handling" means both adequate treatment and disposal without discharge of wastewater to the surface, to nearby waterways or to other unacceptable locations.

The following table gives septic system design flow rates that shall be assumed by septic system designers, giving flows in gallons of wastewater per day per bedroom in the home served by a private onsite wastewater disposal system, that is, a private septic system. Different septic design flow rates are assumed depending on the modernity and efficiency or water usage rate of the plumbing fixtures in the home. --DF

(a) Roof, footing, garage, cellar and surface water drainage must be excluded from the system. Water softener, water recharge and backwash wastes normally are not to be discharged to the system unless a separate subsurface discharge to an area 250 feet from wells or water courses is unavailable.

(b) Minimum design flows for various configurations of plumbing fixtures shall be as shown in Table 1.

NYS Appenndix A-75 Table 1: Septic System Design Flow Rate Data

Plumbing Fixture Age & Water Flow Rates

Septic System Design Flows
Gallons per Day (GPD) per Bedroom


Post-1994 Water-saving plumbing fixtures: water-saving or low flow-rate fixtuires designed / produced after 1994

1.6+ gallons per flush (GPF) toilet &
2.5+ gallons per minute (GPM) flow-rate faucets & shower heads

Pre-1994 Standard plumbing fixtures: conventional fixtures produced before 1994

3.5 gallons per flush (GPF) toilet &
3.0 gallons per minute (GPM) flow-rate faucets & shower heads
Pre-1980 Standard plumbing fixtures: conventional plumbing fixtures produced before 1980 that use the larger water volumes shown below

3.5 + (or more) GPF toilet &
3.0 + (or more) GPM maximum flow rate faucets & shower heads
Water-saving toilet,

1GPF or less with new standard fixtures (3.0 GPM max)
Waterless toilet (e.g. composting toilet) with new standard fixtures (3.0 GPM max)
75 Greywater Only

Notes to the table:

This table is adapted from the New York State Department of Health regulations Appendix 75-A part 3 with additional text and comments to clarify the different categories of plumbing fixture water usage rates.

GPF = gallons per flush at the toilet. Note that for dual-flush toilets that use a smaller flush volume when only liquid waste is present may use still less water but the flow rate assumptions in the NYS table address the alternative, solid-waste larger flush volumes of these fixtures.

GPM = gallons per minute of water flow at the faucet or shower head

Other estimates of daily water usage can be found at WATER QUANTITY USAGE GUIDE

Actual Septic or wastewater drainfield size (leach field size, soakbed size, soakaway bed size, absorption trench size) is given as drainfield trench lengths and dimensions in Table 4A, Septic Wastewater Effluent Input Flow Rate (Gallons per Day) found at SEPTIC DRAINFIELD SIZE



Continue reading at SEPTIC DRAINFIELD SIZE where we provide wastewater application rates and a detailed table of septic soakbed or leachfield or drainfield size as a function of soil percolation rate and wastewater volume flow in gallons per day, or select a topic from the More Reading links shown below.

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SEWAGE FLOW & DESIGN FLOW ESTIMATES at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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