Bulletin Board

WHITE PAPER

Re-Use of Reverse-Osmosis Retentate Water
When Inlet Feed Water is Potable

CUNO Incorporated, a 3M Company
Peter M. Meier, Ph.D.
Senior Marketing Manager
3M Water Filtration

December 5, 2007

Preface:

For this paper, a reverse-osmosis system processes potable inlet water and divides it into two exiting streams: a product stream of water that passes through the reverse-osmosis membrane and is depleted in the concentration of all substances retained partially or totally by the membrane; and a retentate stream of water that correspondingly does not pass through the reverse-osmosis membrane and is proportionately enriched in the concentration of all substances partially or totally retained by the membrane. The proportions of depletion and enrichment both depend upon the fraction of inlet water passing through the membrane and the retention of a given dissolved or suspended substance by the reverse osmosis membrane.

Definitions:

Product water: Inlet water that passes through the reverse-osmosis membrane is potable, and ultimately used for consumption in a beverage or food.
Retentate: Inlet water that does not pass through the reverse-osmosis membrane.
Recovery: The fraction of inlet water that passes through the reverse-osmosis membrane: the volumetric flow rate of the product-water stream divided by the volumetric flow rate of the inlet-water flow stream.
Rejection: The fractional concentration (mg/L or ppm – parts-per-million) of a dissolved or suspended substance in the inlet-water that remains in the retentate water.

Assumptions:

The inlet (feed) water to the reverse-osmosis system is potable; that is, meets the EPA‘s Primary Drinking Water Standards (health related), and subscribes to the EPA's Secondary Drinking Water Standards (aesthetic qualities).
For the following discussion and particularly for the calculations in the Table below, it is assumed that all substances are 100% rejected/retained by the reverse-osmosis membrane and the recovery is X. Thus at a concentration of 200 ppm, a substance is enriched in the retentate stream by the inlet concentration divided by 1 minus the recovery. If X is 1/3, for example, a 200 ppm inlet becomes zero in the product water and 200/(1-1/3)=300 ppm in the retentate stream.

CUNO Position for RO Retentate Re-Use:

When the reverse-osmosis retentate-stream water (with all retained/rejected substances enriched in concentration by their respective inlet concentrations divided by (1-X)) meets the EPA's Primary Drinking Water Standards and subscribes to the EPA's Secondary Drinking Water Standards and meets all the requirements of state and local codes pertaining to drinking water, it is, therefore, suitable for all potable applications.
When the reverse-osmosis retentate stream water does not meet the three requirements set forth in the preceding paragraph, it becomes non-potable waste water , and it may only be used for non-portable applications such as irrigation, toilets, non-potable rinse water, etc. Consult local, state, and federal agencies regarding waste-water handling and disposal.
The inlet water stream is generally pre-filtered through a carbon and sediment filter to help protect the life of the reverse-osmosis membrane. The carbon filter will reduce chlorine and chloramines to very low levels. Thus, the bactericidal properties of the chlorine and chloramines are similarly reduced for the product water stream and for the retentate stream. The product-water stream is generally used immediately for the preparation of a beverage. The retentate stream should also be used without standing for more than a few minutes.
Potable retentate water should not be recycled back into the inlet-water stream. This may result in a continuous rise in the inlet concentration of one or more substances, resulting ultimately in the EPA's mandated MCL levels being exceeded, and thus rendering the inlet water no longer potable. It also is likely to result in RO membrane fouling and/or scaling. Rather, it is recommended that potable retentate be sent to and combined into a water stream that will ultimately result in the directing of the water stream to drain. Examples include wash sinks and hot-water tanks.
An inlet water analysis must be procured before retentate re-use decisions can be made. With the inlet concentrations known, concentration changes imparted by the RO system can be estimated according to the method outlined above. A set of example calculations are shown in the table, below. It is recommended that additional water analyses be procured for the RO system retentate water over a period of six months, following installation and start-up.

Example: Chromium is a substance subject to the EPA's MCL* Primary Standard (Enforced)

Inlet Chromium (ppm)**	RO System Recovery X	Retentate Chromium (ppm)	EPA Primary MCL (ppm)	Passes/Not Passes EPA Primary MCL
0.06	1/4	0.080	0.1	Passes
0.06	1/3	0.090	0.1	Passes
0.06	1/2	0.120	0.1	Not Passes
0.07	1/4	0.093	0.1	Passes
0.07	1/3	0.105	0.1	Not Passes
0.08	1/4	0.107	0.1	Not Passes

* MCL = Maximum Concentration Limit
**ppm = parts-per-million or mg/L as reported in an analysis of the inlet (potable) water

Notes:

Plumbing Codes
When installing a reverse-osmosis system, always follow state and local plumbing codes.
Primary Drinking Water Standards
The EPA standards for drinking water fall into two categories - Primary Standards and Secondary Standards. Primary Standards are based on health considerations and are enforced by the EPA. They address three classes of toxic pollutants: pathogens, radioactive elements and toxic chemicals. Primary Standards set a limit, called the Maximum Contaminant Level (MCL), on the highest allowable concentration of a contaminant in drinking water supplied by municipal water systems. The MCL is usually expressed in milligrams per liter (mg/L) which is the same as parts-per-million.
Secondary Drinking Water Standards
Secondary Standards regulate contaminants that cause offensive taste, odor, color, corrosivity, foaming and staining. The concentration limit is called the Secondary Maximum Contaminant Level (SMCL). Secondary Standards are not enforced. They are guidelines for water treatment plant operators and state governments attempting to provide communities with the best quality water possible.
Current Drinking Water Standards
The EPA has set MCLs for total trihalomethanes, pesticides, volatile organic chemicals, inorganic contaminants, microbial contaminants and radionuclides. The EPA periodically issues standards for additional organic and inorganic chemicals, microbes and viruses. Many more organic chemicals known to be present in drinking water are not currently regulated by either state or Federal standards.

Working through state governments, the EPA monitors community drinking water. When a standard is exceeded, the EPA requires that the contaminant levels be reduced to the MCL. The corrective treatment is left to the individual water system, usually a private utility.

Follow the two links below for tables of the EPA's primary and secondary standards, respectively.

http://www.epa.gov/safewater/contaminants/index.html#listmcl – PRIMARY

http://www.epa.gov/safewater/contaminants/index.html#sec – SECONDARY
State Responsibilities:
Ultimately, state regulatory officials set and enforce drinking water standards for EPA-regulated contaminants and for other contaminants. However, states are not permitted to set standards that are less stringent than the MCLs set by the EPA. Some states may set MCL's that are more stringent than those set by the EPA.
White, Grey, and Black Water:
Assuming that the reverse-osmosis inlet water is always potable, "white", "grey" and "black" water considerations are irrelevant unless the retentate stream becomes otherwise contaminated with waste that would then result in a reclassification. In these cases, the retentate water (as "White", "Grey", or "Black" Water) may be used for applications so allowed by state and local governments for these or similar water classifications. "White" water is generally the result of contamination by paper-making-process waste. "Grey" water is generally the result of contamination by laundry waste. "Black" water is generally the result of contamination by human or animal waste.