How is a well pump sized for a residential home?

Match the pump curve to the total dynamic head — the vertical lift from pumping water level to the pressure tank, plus friction losses in the drop pipe and distribution piping, plus the target discharge pressure converted to feet of head — while keeping the pump within the well's tested sustainable yield and above the manufacturer's minimum flow for motor cooling.

What determines the size of a leach field?

The percolation rate measured onsite, converted through the state's design table to a required absorption area per gallon of daily design flow; slower-percolating soils require substantially more trench area than fast-percolating soils for the same flow.

Why does a septic system need a reserve area?

Most health departments require a second parcel, identical in size to the primary leach field and kept free of structures or heavy traffic, reserved for a replacement field if the primary system eventually fails — protecting the property's long-term ability to treat wastewater onsite.

Engineering·13 min read·July 12, 2026

🪣 Private Water Wells & Septic System Design

Engineering fundamentals for private water supply and onsite wastewater disposal: well types and yield, pump sizing, septic tank sizing, leach field design, and separation distance requirements.

📖Plumbing Systems — Illustrated Guide (Full Access)

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When a Project Needs Private Systems

Most plumbing engineering work assumes a municipal water main and public sanitary sewer at the property line. Rural sites, large-lot suburban parcels, and many agricultural or recreational facilities do not have that luxury — the engineer of record must instead design a private water supply (a well) and an onsite wastewater treatment system (a septic system) that perform the same functions the municipal utility would otherwise provide. The decision to go private is not always driven purely by absence of utilities: even where a municipal main exists nearby, extension costs, easement acquisition, tap fees, and utility capacity limits sometimes make a private system the more economical choice for a single building on a large parcel. The Authority Having Jurisdiction (AHJ) for private water and septic systems is almost always the county or state health department rather than the local building or plumbing code office, and the applicable regulations are typically found in a state's onsite wastewater treatment system (OWTS) code and a separate private well construction code — neither of which is the IPC or UPC, although both codes cross-reference private systems for the building-side connection.

Topics covered

private wellseptic systemleach fielddrain fieldwell yieldonsite wastewaterperc testseptic tank sizingwell pump sizingOWTS