Short answer
Four recoverable water streams exist on almost every industrial site: RO reject, where recovery typically runs 50 to 75 percent leaving a quarter to a half of feed as reject; cooling tower blowdown, recoverable at 75 to 90 percent with appropriate treatment; boiler condensate; and treated STP output. Most go to drain because they are not metered, and a reuse figure without a meter behind it fails at the first assurance question.
Reuse is claimed far more often than it is measured
Almost every industrial sustainability report contains a reuse percentage. Almost none of them can point at the meter it came from. The number is usually derived from a design assumption about a treatment plant's rated output, multiplied by an assumed operating time, which is an estimate dressed as a measurement.
That was tolerable when reuse figures were narrative. It is not tolerable now. Where disclosures carry independent assurance, an assurance provider asks how the number was derived, and a design rating is not a derivation.
The four streams, and what each is actually worth
RO reject
Reverse osmosis recovery rates normally range from 50 to 75 percent, meaning 25 to 50 percent of the feed water is discharged as reject. On a large site with RO on boiler feed, process water and polishing duty, that is a substantial continuous stream.
It is also frequently reusable as it stands. Where a cation exchange softener sits ahead of the RO unit, the concentrate can be of high enough quality to use as make up for an evaporative cooling tower or other process water systems, which requires no additional treatment at all.
Cooling tower blowdown
Blowdown is necessary to control cycles of concentration, and it is usually sent straight to drain. It is recoverable: cooling tower blowdown can be treated in a single stage of RO and achieve recoveries of 75 to 90 percent, with pretreatment such as ultrafiltration and activated carbon adsorption improving filtration characteristics.
Boiler condensate
Hot, already treated, already purchased. Reclaiming condensate offsets the cost of producing highly purified water and reduces boiler make up from external supply. Condensate is the highest value water on the site per litre, and the return ratio is a metric most plants cannot state.
Treated STP output
The largest volume stream and typically the lowest quality, suitable for landscape irrigation, flushing, dust suppression, and with further treatment, cooling make up. The barrier is rarely treatment capability. It is that nobody can prove where the treated volume went, so the reuse claim cannot be substantiated.
| Stream | Points to meter | The claim it supports |
|---|---|---|
| RO systems | Feed, permeate, reject flow, plus reject destination | Recovery percentage as a measured value, and reject volume actually reused |
| Cooling towers | Make up, blowdown, recovered volume, conductivity | Cycles of concentration achieved and blowdown recovered, rather than assumed |
| Boiler house | Condensate return flow and temperature, feedwater make up | Condensate return ratio, and thermal energy recovered with it |
| STP and ETP | Inflow, treated output, output by end use, discharge | Reuse volume by application, and reuse as a share of non potable demand |
| Site balance | All intakes and all discharges | Total withdrawal, consumption, discharge and reuse in the structure GRI 303 and BRSR expect |
ZLD: where it applies and what it demands
Zero liquid discharge is mandated under CPCB and state pollution control board orders for certain high load sectors, such as distilleries and breweries above defined production volumes, where no liquid discharge to water bodies or land is permitted. Beyond the sectors where it is mandated, ZLD style conditions increasingly appear in individual consent conditions, particularly in water scarce regions.
Operating a ZLD or near ZLD site raises the measurement stakes considerably, because the balance has to close. Every litre in must be accounted for as product, evaporation, reuse or solid residue. A site that cannot close its balance cannot demonstrate ZLD, whatever its treatment train is capable of on paper.
The order of work
- Meter the streams before designing the recovery. Most reuse projects are sized on assumed volumes and then under perform because the assumption was wrong. A quarter of measurement is cheaper than a mis sized plant.
- Take the free recoveries first. RO reject to cooling make up where quality allows, and condensate return improvement, both of which require little or no new treatment.
- Then size the treatment. With measured volumes, quality data and a real demand profile at the point of reuse.
- Instrument the reuse permanently. So the claim is continuously evidenced rather than validated once at commissioning.
What is RO reject and can it be reused?
RO reject, or concentrate, is the portion of feed water rejected by a reverse osmosis membrane, typically 25 to 50 percent of feed given recovery rates of 50 to 75 percent.
It is frequently reusable. Where a cation exchange softener precedes the RO unit, the concentrate can be of sufficient quality to use directly as evaporative cooling tower make up or in other process water systems, with no additional treatment.
Can cooling tower blowdown be recovered?
Yes. Cooling tower blowdown can be treated in a single stage of RO with recoveries of 75 to 90 percent. Pretreatment combinations such as ultrafiltration with activated carbon adsorption have been found effective at improving RO filtration characteristics on this stream.
Is zero liquid discharge mandatory in India?
ZLD is mandated under CPCB and state pollution control board orders for certain high load sectors, such as distilleries and breweries above defined production volumes, where no liquid discharge to water bodies or land is permitted.
It is not a blanket national requirement across all manufacturing, but ZLD style conditions appear in individual consents, particularly in water scarce regions, and reuse expectations continue to rise through customer and investor requirements independently of statutory triggers.
How do you prove a water reuse percentage to an auditor?
With metered volumes at the point of production and at the point of use, timestamped and continuous, plus the site level balance that shows total withdrawal, consumption, discharge and reuse consistently.
A design rating multiplied by assumed operating hours is an estimate, and where a disclosure carries independent reasonable assurance, that derivation does not hold up.
References
- Boiler feedwater and reverse osmosis for modern steam systems, Kurita America
- Recovery of cooling tower blowdown water for reuse, ScienceDirect
- Cooling tower blowdown treatment and reuse options, New Logic Research
- CPCB effluent discharge standards, compliance guide
- Zero liquid discharge and water reuse in recirculating cooling towers
