Sewage Characteristics, Flows, and Design Basis
What effluent is and how much is produced
Design begins with a defensible flow estimate. Residential flows are typically derived from bedroom count or fixture units, while non-residential flows depend on occupancy, fixture use, and operating hours. Candidates must know that design flow is not the same as measured water use; codes apply minimum allowances and demand categories. Sewage strength matters too: blackwater carries the highest organic and pathogen load, greywater less, and segregated flows can change treatment selection.
Building the design basis
Assemble occupancy, fixture counts, and any commercial processes. Apply the governing flow allowances, then add any required safety factors or allowances for garbage grinders, dishwashers, or high-use fixtures. The result sets tank volume, treatment capacity, and field area. Hydraulic and organic loading must both be considered; an undersized tank passes solids forward and shortens field life.
Traps in questions
- Using actual water meter readings instead of code allowances.
- Forgetting to add flow for an added bedroom or fixture during renovation.
- Treating all wastewater as equivalent when a commercial process changes strength.
- Confusing peak instantaneous flow with daily design flow.
Rule of thumb
Size the tank for retention time at design daily flow, and size the field for the same daily flow; never size one component on a different basis from the other.
Worked example
A four-bedroom dwelling is proposed. Using the applicable per-bedroom allowance, the daily design flow is established, then a tank volume is selected to provide the required retention, and the leaching bed area is computed from that same daily flow. If a basement kitchenette adds a fixture, the flow is recalculated and every downstream component is revisited, not just the tank.