Can RO Reject Water Be Reused?

RO (Reverse Osmosis) is a technology which is used extensively for producing purified water in a household, commercial buildings or industries. In the process, some of the feed water is filtered out as purified water, and the rest of the water exits as RO reject water or concentrate.

RO reject water is regarded as wastewater by many. It is important to understand the quality, but as often it can be reused for appropriate applications. The use of RO reject water for re-use can minimise the wastage of water, reduce freshwater consumption and enhance the water efficiency.

What is RO Reject Water?

RO reject water has a greater amount of dissolved salts and other impurities than the original feed water. Quality will vary based on the feed water and type of RO system.

RO reject water has a higher TDS and mineral content due to its non-removal of these during the RO process, which may make it unsuitable for drinking or applications that demand high-quality water. It can be useful for a few non-potable purposes, however.

Where Can RO Reject Water Be Reused?

Chemical and physical quality are the most important parameters for assessing the suitability of RO reject water reuse.

. Floor Cleaning

RO reject water may be used to clean floors, outdoor areas, other surfaces, and for other purposes where drinking quality water is not needed.

If the TDS of the reject water is very high, however, surfaces should be checked for salt deposits.

. Toilet Flushing

If the systems are suitable, the reject water from RO can be stored and also used for flushing toilets.

This can greatly cut down the amount of fresh water used for everyday sanitation.

. Gardening and Landscaping

RO reject water can be used in certain landscape applications, however the salinity level needs to be taken into consideration.

High TDS water or presence of ions like sodium and chloride could kill salt sensitive plants and impact soil quality. Thus, prior to use of irrigation water, it is important to test it.

. Vehicle and Outdoor Cleaning

Water which has been rejected can be used for washing vehicles, outdoor equipment, pathways and other uses where purified water is not needed.

This is an easy method to save the use of freshwater.

. Construction Activities

Depending on quality and application needs, reject water from a RO system can be used for certain construction activities, e.g., curing or cleaning.

Water quality should be checked prior to use when the water may impact the material or process.

Can RO Reject Water Be Reused in Industrial Processes?

Yes, in some cases.

RO reject water can be reused by industries for applications which do not require a low TDS water. Any of the following applications are possible: cooling tower makeup water, washing water, utility water or other appropriate applications.

Industrial re-use, however, requires appropriate water analysis. If there is a lot of salt, silica, hardness or other contaminants, they can lead to scaling, corrosion or other operational issues.

Can RO Reject Water Be Treated Again?

Yes. RO reject water can be further treated to be recovered to higher levels of water recovery for industries that require more.

The treatment will be:

• Additional RO stages

• Ultrafiltration

• Nanofiltration

• Evaporation

• Crystallization

• Ion exchange

Advanced systems can recover more water and minimize concentrate to be disposed of.

RO Reject Water and Zero Liquid Discharge

In industries that have clean water disposal as a critical constraint, RO reject water can be integrated into a Zero Liquid Discharge (ZLD) system.

The concentrate in a ZLD system is subsequently processed to achieve water re-use and reduce or prevent liquid discharge. If necessary, evaporators and crystallizers can be employed.

This method can be very beneficial for industries with heavy waste water generation and in regions where effluent discharge is governed by strict standards.

What Should Be Checked Before Using RO Reject Water?

Test the quality of RO reject water before reusing. Important parameters include:

• TDS

• pH

• Hardness

• Chloride

• Sulphate

• Silica

• Sodium

• Specific contaminants related to the source water

An intended use should also be taken into account. The water that is acceptable for use on floors may not be acceptable for irrigation or industrial cooling.

Benefits of Reusing RO Reject Water

RO reject water can be reused with the following benefits.

. Reduces Water Wastage

Water which is rejected can be reused for appropriate non-potable uses rather than disposal to the drain.

. Saves Freshwater

Reuse of reject water will avoid the need for freshwater for cleaning, flushing, landscaping and utility uses.

. Lowers Water Costs

The decreased amount of water used will help to lower water procurement and utility expenses.

. Supports Sustainable Water Management

The use of water for reuse benefits businesses and institutions by enhancing water efficiency and minimizing their environmental footprint.

What Are the Limitations?

RO reject water can't be used for all applications.

It has a more concentrated dissolved content, leading to:

• Scaling

• Corrosion

• Soil salinity

• Damage to sensitive plants

• Reduced equipment performance

It is not to be used for drinking nor for cooking without proper treatment to ensure the drinking-water quality specifications are met.

How Can Businesses Improve RO Water Recovery?

The overall design of the RO system can be optimized to minimize the reject water production in the system.

Useful measures include:

• Proper pretreatment

• Regular membrane cleaning

• Optimized recovery rates

• Efficient membrane selection

• Multi-stage RO systems

• Concentrate recovery systems

• Water quality monitoring

With proper designing of the RO plant, it is possible to recover more water and still retain the performance of the membrane.

Conclusion

The reject water from the RO system can be used again provided the quality of the water is adequate and its application is appropriate. Often used to flush toilets, clean floors, wash outdoors, landscape and in some industrial uses. Industries may opt for another treatment or combine it with a Zero Liquid Discharge system for increased recovery.

Quality of RO reject water must always be checked and its quality must be matched before its reuse. By carefully planning, reject water can be used as a good resource rather than just a waste of water.

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