Page 226 - WaterSense at Work

October 2012
Consider water purification systems that require the least amount of backwashing or
regeneration. For membrane processes such as reverse osmosis, consider a system
with a high recovery rate for its size. For deionization systems, consider systems that
regenerate based on the volume of water treated or conductivity. For distillation
systems, consider units that use air-cooled coils, rather than water-cooled coils and
recover at least 85 percent of the feed water.
For water softeners, consider demand-
initiated systems instead of systems with manual or auto-initiated regeneration. In
addition, consider installing multiple smaller, more efficient cation exchange water
softeners that can be alternated to minimize the frequency of regeneration and allow
for a constant, uninterrupted supply of soft water.
Savings Potential
The water use of a water purification system is dependent upon the level of purifica-
tion required, incoming water quality, volume of use, and purified water demand.
Water use is also specific to the type of water purification system used.
Carbon filtration and deionization systems are typically regenerated off site. If regen-
erated off site, the water use of these systems will not directly affect the water use of
the facility. However, minimizing the frequency of removal and regeneration will help
to reduce the water use of these systems.
The water use of distillers is dependent upon the method of cooling and the amount
of reject water used to clear the boiler of scale buildup. Water savings can be maxi-
mized if air-cooled coils are used rather than water-cooled coils. Additionally, systems
that produce less reject water will consume less water overall.
For filtration processes, water use is determined by the water quality requirements
and frequency of the backwash phase. Optimizing the frequency of the backwash
phase by initiating backwash only when a pressure drop occurs across the filter me-
dia will ensure less water is used overall.
The water efficiency of a reverse osmosis process can be determined by the recovery
rate, which is defined as the ratio of permeate to feed. Systems with higher recovery
rates are considered more efficient, because they are able to produce more purified
water from the same amount of feed.
Recovery rates can vary widely depending upon the type of membrane and quality
of incoming water. Some less efficient reverse osmosis systems, for example, have a
recovery rate of 33 to 50 percent.
The recovery rate can be maximized by increas-
ing the number of stages of membrane pressure vessels, which allows for higher
pressures to be achieved in order to more effectively overcome natural osmosis. A
one-stage system can achieve a recovery rate of 50 percent, while two- and three-
stage systems can achieve recovery rates of 75 percent and 90 percent, respectively.
For example, the Sandia National Laboratories in Albuquerque, New Mexico, installed
a high-efficiency reverse osmosis system with pretreatment before the membranes.
Water Purification
Pagliaro, Tony. 1995. “Commercial/Industrial Reverse Osmosis Systems: General Design Considerations.”
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