Reverse Osmosis Filter Discharge Water Introduction

Reverse Osmosis (RO) filters use membrane technology to filter impurities (minerals, calcium, chloride, sodium, chlorine, etc) from the water.  The systems require periodic backwash to clean the filter.  Depending on the model, most RO filter systems* will discharge 4 to 20 gallons (15.1 L to 75.7 L) of backwash water for every gallon of filtered water it produces.  This discharge water is commonly directed to the sewer drain line.  The discharge water from an RO system is only slightly less pure than the source water entering the filter.  The concentration of minerals in the discharge water is often less than 25% greater than the source water; well within safe drinking water standards and certainly suitable for irrigation or other alternate reuse applications.

* In recent years, some manufacturers have developed innovative RO filter systems referred to as “zero-discharge”; discharging the backwash water into the water supply pipes.   The “zero-discharge” refers to the fact that the backwash water is not discharged into the sewer system. 

Estimating Volume of the Resource

The amount of discharge water produced depends on the efficiency of the RO filter and the amount of RO water used.   Somewhat paradoxically, the less efficient the filter system, the more discharge water is available for reuse.    A typical family of four will use 2 to 5 gallons (7.57 L to 18.92 L) of RO water per day, depending on their water use habits. Some homes use RO water just for drinking, other use it for all cooking, rinsing produce, brewing coffee and tea, ice trays and ice makers, watering indoor plants, etc.   Assuming the RO filter is 16% efficient (discharging 5 gallons (18.92 L) for every 1 gallon (3.78 L) of filtered water), a typical home will produce 3,600 to 9,000 gallons (13.6 m3 to 34.1 m3) of discharge water per year; all needlessly wasted when drained into the sewer system.  An older, less efficient RO filter might produce more than 30,000 gallons (113.5 m3) of discharge water per year.

RO water filters are also used in commercial, industrial and institutional settings; each filter producing thousands of gallons of discharge water every day.  The filtered water is used for everything from: biology and chemistry laboratories, car washing (rinse cycle), photography processing, rinsing computer chips and hard discs, etc.  In most all applications, there are potential reuse applications on-site for the discharge water.   

Potential Uses of RO Discharge Water

RO discharge water can be collected, and used similar to other alternate water sources.   The discharge water has the same sanitary qualities as the potable source water for the RO filter; the only difference is the discharge water will have slightly elevated concentrations of minerals and water treatment chemicals.   As long as the sanitary conditions are maintained during storage and transfer, the water can be used the same as potable water (though we never recommend it for direct human consumption).   RO discharge water is one of the few alternate water sources that can be safely used for above surface irrigation – when properly handled.  If the discharge water is stored for more than a day, it is no longer suitable for spay head type irrigation unless additionally treated.

The sanitary quality of RO discharge water suggests there are better uses for it than just irrigation.  This water is perfectly suited to use for laundry and flushing toilets in the home, when the santitation of the water is maintained.   Unlike rainwater or condensate collection, the RO filter will continually discharge a relatively constant quantity of water; 8 to 50 gallons per day (30.3L to 189.2 L) when the home is occupied.   In commercial settings, the potential uses include: pre-rinsing, laundry, toilet and urinal flushing, irrigation, washing hardscapes, make-up water for cooling towers and water cooled condensers, evaporative cooling, decorative fountains, swimming pools, water cooled machinery, vehicle wash, etc.