Three flow rates a quotation should distinguish
Feed flow is the new water entering the system. Gross filtrate flow is membrane output while filtration is running. Net output is the volume available for the agreed use after internal water consumption and non-producing periods. Each figure needs a time basis and defined feedwater and temperature conditions.
WAVE documentation distinguishes production, wash duration and recovery. A software default is not evidence of achievable project recovery. In crossflow UF, the circulation flow through membrane tubes must also be distinguished from fresh feed and permeate flow.
Example: 12 m³/h becomes 10.4 m³ per cycle
This original calculation is illustrative rather than a commercial membrane operating point. A cycle lasts 60 minutes, including 55 minutes of filtration at 12 m³/h. Gross filtrate volume is 12 × 55 / 60 = 11 m³. Within the cycle, backwashing consumes 0.6 m³ of this filtrate and forward flushing adds 0.4 m³ of feedwater sent to waste.
With unchanged tank inventories and no other losses, useful output is 10.4 m³. Total incoming water is 11.4 m³ and waste volume is 1 m³. Cycle recovery is therefore 10.4 / 11.4 = 91.2 %. Dividing filtration minutes by cycle minutes gives a different measure.
| Per 60-minute cycle | Volume |
|---|---|
| Gross filtrate produced | 11.0 m³ |
| Filtrate used for backwashing | 0.6 m³ |
| New feedwater used for flushing | 0.4 m³ |
| Useful water / waste outlet | 10.4 m³ / 1.0 m³ |
Close the daily balance as well
Now assume 22 cycles per day, with two additional hours reserved for cleaning and other downtime. If those operations consume another 2 m³ of filtrate, daily delivery is 22 × 10.4 − 2 = 226.8 m³/day. Averaged over 24 hours, this is 9.45 m³/h.
Daily incoming water in this example is 22 × 11.4 = 250.8 m³, while waste totals 24 m³. Daily recovery is 90.4 %. The plant cannot meet a demand of 240 m³/day despite its 12 m³/h filtration rate. Actual CIP frequency belongs in the assessment period; the two hours here are an illustrative allowance, not a recommendation for daily CIP.
Connect UF output to RO demand and storage
Downstream RO needs its feed flow during the planned operating hours. Producing 10 m³/h of RO permeate at 75 % recovery requires 13.33 m³/h of RO feed before flushing and other losses. Upstream UF must provide that net supply, or a suitable operating schedule and storage arrangement must support the demand.
A tank can bridge individual backwashes and short demand peaks. It cannot resolve an ongoing daily production deficit. Check usable tank volume, start and stop levels, and the consequences of several trains washing at the same time.
What to request when comparing quotations
During a pilot, totalise fresh feed, delivered product and waste over a representative period. Record changes in tank inventories and sample useful-water quality. More water outside specification does not increase usable production.
- Net delivery in m³/day, together with the feed, temperature and quality conditions supporting it.
- The filtration, backwash and CEB sequence, plus the CIP frequency assumed in the calculation.
- Water source for every wash, residual volume and the destination of discharged streams.
- Available production during maintenance and whether standby capacity is included.
Common questions
Is UF recovery the same as availability?
No. Recovery compares useful and incoming water volumes. Availability describes service time or capacity. The same wash can affect these measures differently.
Can membrane area simply be multiplied by flux?
That gives gross production during filtration if the selected flux is sustainable. Integrate operating time, water consumption and maintenance conditions to establish net delivery.