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Greywater Reuse Flow Requirements Estimator

This estimator converts a greywater reuse volume and operating schedule into average and peak treatment-flow requirements. It is intended for preliminary sizing of treatment equipment, transfer pumps, or distribution systems when the daily reuse quantity is known but the required instantaneous capacity is not. The calculator first spreads the daily volume across the selected operating hours, then applies a user-entered peak factor to account for nonuniform production or demand. Results are shown in liters per minute and cubic meters per hour. This is a screening calculation rather than a full hydraulic or process design: equalization, fixture simultaneity, source-flow surges, pump head, pipe losses, treatment contact time, and equipment turndown can all affect the final required capacity.

Inputs

L/day
h/day
×
Result
—
Peak design flow
Average operating flow—
Peak flow—
Peak flow—

1. Enter daily reuse volume
Use the daily volume the greywater system is expected to treat or deliver.

2. Enter operating hours
Specify the number of hours per day the treatment or pumping equipment will operate.

3. Choose a peak factor
Enter a project-specific multiplier of at least 1.0 to represent short-term flow peaks.

4. Review average operating flow
This indicates the continuous throughput needed during the selected operating window.

5. Check the peak design flow
Compare it with equipment rated capacity while separately checking hydraulic head and process constraints.

Average flow (L/min) = Daily volume ÷ (Operating hours × 60) Peak design flow (L/min) = Average flow × Peak factor Peak flow (m³/h) = Peak flow (L/min) × 0.06

The peak factor is not a universal constant. It should reflect the source and demand pattern being evaluated.

What the result means

Peak design flow is a preliminary capacity target for the greywater treatment or delivery system under the entered operating assumptions.

Final design may require flow equalization, redundancy, minimum contact time, or other process-specific criteria not represented by this simple conversion.

Given
Daily volume = 3,000 L; operating time = 12 h/day; peak factor = 1.8.

Calculation
Average flow = 3,000 ÷ (12 × 60) = 4.167 L/min.
Peak flow = 4.167 × 1.8 = 7.50 L/min.
Equivalent = 7.50 × 0.06 = 0.45 m³/h.

Result
7.50 L/min

A preliminary treatment or transfer unit should be checked for at least this flow, subject to pressure and process design requirements.

Should operating hours match building occupancy?

Not necessarily. Use the hours the treatment or pumping equipment is actually expected to process the daily volume, which may differ from occupancy hours if equalization storage is used.

How do I choose a peak factor?

Base it on measured or design source-flow patterns, reuse demand patterns, equalization capacity, or project criteria. The calculator deliberately does not impose one standard factor.

Can equalization storage reduce peak equipment flow?

Often it can smooth short-term surges, but the effect depends on tank volume, controls, and the timing of source and demand flows. A time-based hydraulic analysis is needed for final sizing.

Is the result a pipe-sizing flow?

It can be an input to pipe sizing, but pipe diameter also depends on velocity limits, pressure, elevation, and friction losses.

Why compare liters per minute and cubic meters per hour?

Different pumps and treatment units are rated in different flow units. Showing both makes equipment comparisons easier without changing the underlying calculation.