—orig. developed through MOICANNA
– originally developed through MOICANNA
—orig. developed through MOICANNA
– originally developed through MOICANNA
HERBARIUM
A grower installs reverse osmosis because water chemistry matters.
But reverse osmosis does not make unwanted material disappear.
It divides the incoming water into two streams.
One stream passes through the membrane:
the permeate, or product water.
The other carries a larger share of the material rejected by the membrane:
the concentrate, commonly called reject water.
The product water receives attention.
The concentrate is often sent directly to a drain, as though it were merely the shadow left behind by purification.
But the membrane did not remove responsibility.
It created a second water stream
with a different chemistry and a different possible use.
RO concentrate is not automatically worthless.
It is not automatically safe either.
Its composition depends on the source water, membrane rejection, recovery rate, pretreatment, operating conditions and the contaminants the RO system was installed to remove. Reverse-osmosis concentrate may contain elevated salts, metals, nutrients or other rejected constituents compared with the incoming water.
Water reuse begins by refusing two lazy conclusions:
“It went to the drain, so it must be waste.”
And:
“It came from tap water, so it must be safe.”
The better question is:
What is in this stream,
and which second use can accept it safely?
Reverse osmosis uses pressure to move part of the incoming water through a semipermeable membrane.
The system therefore has three relevant flows:
Two separate performance questions follow:
They are not the same measurement.
A membrane may reject salts effectively while the complete unit still produces a large concentrate stream.
Recovery can be expressed as:
permeate flow ÷ feed flow × 100
The concentrate-to-permeate ratio compares concentrate volume with permeate volume.
These ratios describe volume—not water quality.
EPA reports that a typical point-of-use drinking-water RO unit may discharge five or more units of concentrate for every unit of treated water, with some inefficient systems approaching 10:1. Under the current WaterSense specification, qualifying point-of-use systems discharge no more than approximately 2.3 units of concentrate per unit of permeate. These figures describe that equipment category, not every cultivation-scale RO installation.
The first lesson:
Do not describe an RO system
only by the quality of its product water.
Describe it by:
Brochure efficiency is not enough.
The grower needs to know how the unit performs under the pressure, temperature, source-water chemistry and operating pattern of the actual site.
Measure:
A simple container test can reveal the approximate concentrate-to-product ratio:
Pressure-tank systems, automatic flushing and start–stop operation can complicate a short test. Permanent flow meters provide a better long-term ledger where the system volume justifies them.
A poor ratio may result from:
Maintenance matters because membrane fouling, scaling and pressure losses can reduce performance. But maximum recovery should not become another grower obsession: forcing recovery beyond the system’s design can increase concentration at the membrane surface, scaling risk and product-quality problems.
Follow the equipment specification rather than improvising pressure or restrictor modifications.
Better lesson:
A grower who measures only the permeate
knows only half the machine.
RO concentrate is not a standard product.
It is a concentrated expression
of the water that entered the membrane.
If the RO system was installed to reduce nitrate, arsenic, PFAS, boron, metals or another undesirable constituent, that constituent may become enriched in the concentrate stream.
That history determines whether reuse is sensible.
EC is a useful first measurement, but EC alone cannot identify sodium, chloride, bicarbonate, boron, nitrate or other individual constituents.
For possible soil or landscape irrigation, sodium risk should be considered alongside salinity. Sodium adsorption ratio, or SAR, compares sodium with calcium and magnesium and helps assess the risk of soil dispersion and reduced infiltration. Its meaning also depends on the EC of the water and the receiving soil.
The useful analysis depends on the proposed second use, but may include:
Do not test the water only after choosing its new job.
Choose the job
after understanding the water.
The question is not:
“Can RO concentrate be reused?”
Some water streams can be reused after suitable assessment, treatment and control.
Others may be impractical or inappropriate to reuse within the cultivation site.
The useful question is:
“Which use, if any, can safely accept this chemistry—and what controls would that require?”
Depending on its composition, treatment and the design of the non-potable system, concentrate may be suitable for uses such as:
Scale, staining, corrosion, aerosol formation and worker exposure still need consideration.
Reclaimed-water storage and distribution must be designed to prevent cross-connections with potable plumbing and unintended backflow.
Applicable plumbing and non-potable-water requirements should be addressed by a qualified professional.
Concentrate may sometimes be used for suitable non-crop landscapes or salt-tolerant plants.
But “salt tolerant” is not permission to irrigate blindly.
Repeated application can accumulate salts in the root zone, and sodium can damage soil structure even where the plants initially appear healthy. Drainage, rainfall, soil texture, irrigation frequency and the individual ions present all matter.
Blending concentrate with rainwater, tap water or another low-salinity source can reduce the final EC.
It does not make the original constituents disappear.
A responsible blend is calculated from the volumes and chemistry of both streams, then tested after mixing.
Do not dilute until one EC number looks acceptable while ignoring sodium, chloride, bicarbonate or the constituent the membrane was installed to reject.
RO concentrate should not be returned automatically to a cannabis nutrient tank.
It may contain precisely the background salts the RO system was intended to remove, while the nutrient programme adds another ionic load on top of them.
Direct crop reuse requires a water analysis, a defined target for the completed solution, knowledge of the medium and a plan for cumulative salts.
In many small systems, a non-crop second use is safer and more practical than trying to force concentrate back through the root zone.
The rule:
Concentrate does not need the most prestigious second use.
It needs the safest useful one.
Some RO concentrate should leave the reuse conversation early.
Do not assign it to irrigation or uncontrolled cleaning when:
At utility scale, RO concentrate may contain elevated regulated contaminants and can require specialised treatment or disposal. The exact risk in a cultivation facility depends on the original water and process, but the principle is the same: concentration can turn a small source-water problem into a more significant second stream.
Water scarcity does not make unsuitable water suitable.
Ecological intention
is not a treatment process.
Indoor cultivation moves large quantities of water through plants and air.
That makes condensate a potentially valuable water stream.
It does not make it sterile or automatically feed-ready.
Water may contact:
Moisture, sediment, biofilms, favourable temperatures and stagnation can support microbial persistence or growth in water systems and on wet HVAC surfaces. Low EC therefore does not establish microbiological safety.
Condensate reuse begins with system design:
Depending on the intended use, treatment may include:
Treatment should respond to measured risk, not be assembled as a collection of fashionable devices.
For reuse in nutrient preparation, analyse both chemical and microbiological quality and continue monitoring the collection system. Returning condensate through an established water-treatment train is generally more defensible than assuming that water collected from the air is equivalent to laboratory-distilled water.
Better lesson:
The room may return part of the water
the crop transpired.
The grower must still decide
whether the collection system kept it usable.
Fertigation runoff is not simply nutrient solution that missed the roots.
Its composition has already been altered by:
Depending on the system, it may also carry:
This makes runoff potentially recoverable—but not automatically reusable.
Reuse may require:
Research in container-grown horticulture shows that recirculation can reduce water and fertiliser loss, but success depends on water treatment, pathogen management and control of accumulating ions. A recycled solution can retain an acceptable EC while its individual ionic ratios drift away from the intended formulation.
The objective is not permanent recirculation at any cost.
The objective is safe resource efficiency
without allowing yesterday’s imbalance
to become tomorrow’s feed.
Better lesson:
Runoff is not free nutrient solution.
It is used chemistry
carrying the history of the root zone.
Rainwater can provide a low-mineral blending or irrigation source, but the collection system contributes its own chemistry and biology.
Roof material, atmospheric deposition, bird and animal waste, gutters, first-flush conditions, storage tanks, light and stagnation all affect quality.
Rainwater is not purity.
It is water with a catchment history.
Household greywater may contain sodium, surfactants, fragrances, fats, disinfectants and microorganisms.
It should not be sent casually into cannabis containers or mixed into a nutrient reservoir.
Greywater reuse requires a system designed for its source and intended use—not a hose redirected by ecological enthusiasm. Reviews of greywater reuse consistently identify large variation in chemical and microbial quality between sources and the need for treatment appropriate to the intended application.
Not every recovered litre needs to become crop water.
Replacing potable water in an appropriate cleaning, flushing or landscape task is still reuse.
The goal is not to force every stream
back into the reservoir.
The goal is to match water quality
to the least demanding safe use.
Avoid creating unnecessary water demand.
Keep different water streams separate.
Once unlike streams are mixed, the whole volume inherits the uncertainty of the most contaminated one.
Record:
Measure the chemical variables relevant to the proposed use.
Assign an operational category:
These are internal management categories,
not regulatory certifications.
Filtration, disinfection, blending, membrane treatment and storage control should bridge a defined gap between present quality and intended use.
Treatment without a target
is equipment collecting around uncertainty.
Test the treated stream.
A complete ledger records:
What is not separated cannot be classified.
What is not measured
cannot be responsibly reused.
Water reuse becomes dangerous when enthusiasm outruns classification.
Do not:
EPA describes water reuse as reclaiming water from different sources, treating it and reusing it for beneficial purposes.
The word treating matters:
reuse is not simply the act
of collecting a discarded stream.
The goal is not to reuse every drop.
The goal is to waste fewer litres
without distributing their problems elsewhere.
Reverse osmosis is not a machine
that creates pure water and nothing else.
It is a separator
that creates two responsibilities.
A mature grower does not ask only:
“Can this litre be reused?”
They ask:
Wastewater is not always waste.
But water does not become reusable
merely because the grower dislikes throwing it away.
A second use is responsible
only when it does not create a second problem.
Factual Note
Reverse osmosis separates feed water into permeate and concentrate. Permeate is the fraction that crosses the membrane; concentrate carries a larger share of the dissolved constituents rejected by the membrane. Concentrate quality depends on feed-water chemistry, membrane rejection, recovery, pretreatment and operating conditions. It should not be assumed to have one standard composition.
Membrane rejection and system recovery describe different properties. Rejection describes how effectively a constituent is retained by the membrane. Recovery describes the fraction of feed water converted to permeate. Increasing recovery reduces concentrate volume but raises the concentration burden within the membrane system and may increase fouling or scaling risk if the unit is not designed for that operating point.
EPA reports that many conventional point-of-use RO systems discharge five or more units of concentrate for every unit of permeate, while some inefficient units may approach 10:1. To earn the WaterSense label, a point-of-use RO system must demonstrate that it sends no more than 2.3 units of concentrate to the drain for each unit of permeate produced. Cultivation and commercial RO systems may operate at different recoveries, so actual permeate and concentrate flows should be measured.
EC indicates the combined conductive effect of dissolved ions but does not identify them. Assessing RO concentrate for irrigation may require individual measurements of sodium, chloride, bicarbonate, boron or other source-specific constituents. For soil irrigation, sodium adsorption ratio helps assess sodium relative to calcium and magnesium, while the water’s EC and the receiving soil also affect infiltration risk.
RO concentrate should not be reused casually when the membrane is being used to remove a constituent of health or environmental concern. Reverse-osmosis concentrate can contain elevated salts, nutrients, metals and micropollutants compared with the feed water, and advanced municipal systems may require specialised concentrate treatment and disposal.
HVAC and dehumidifier condensate may contain little dissolved mineral material at the point of condensation, but collection surfaces, dust, biofilms, cleaning residues, metals, storage and distribution can alter its quality. Low EC does not establish microbiological safety. Warm or stagnant water and wet HVAC components can support microbial growth, so crop reuse requires clean collection, suitable storage, testing and treatment appropriate to the intended use.
Fertigation runoff can contain water and recoverable nutrients, but its nutrient ratios may differ from the original solution and it may transport suspended material, poorly absorbed ions or plant pathogens. Recirculating irrigation can reduce water and fertiliser discharge, but it requires filtration, sanitation, nutrient rebalancing and monitoring of accumulating constituents. A stable EC does not prove that the ionic composition remains suitable.
Rainwater, greywater, condensate, RO concentrate and fertigation runoff are not interchangeable sources. Each requires a fit-for-purpose assessment based on origin, chemistry, biological risk, storage and intended use. Water reuse involves reclaiming a water stream, treating it as necessary for a defined beneficial use and verifying that it is suitable for that use. Collection alone does not establish safety or suitability.
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Free member access. Join early. Keep the archive open.
The VADEMECUM is becoming a living archive of practical plant knowledge.
Free member access.