The Controversy Over Waste Water
Ratios in Residential Reverse Osmosis.
The
following is a little information to help you decide if you want to change your
Reverse Osmosis (RO) waste ratio. Membranes work by splitting the
incoming water stream into a permeate and a concentrate stream; commonly known
as the good water and the wastewater. The Concentrate stream is just what it
sounds like. It carries the great majority of TDS to drain. This
stream is also “rinsing” the membrane at the same time. The flow of the
concentrate stream is designed to be higher than the permeate. It
is this split we are here to discuss.
The
average residential RO system is manufactured with a standard flow restrictor,
producing a 4 to 5 parts waste stream to a 1 part product stream.
Many people find this objectionable; due to conservation concerns and/or water
shortages. Consumers often ask if they can cut down on this waste
stream; the answer is YES……BUT there is a cost. The cost is
membrane life. The “waste” stream is an integral part of the
process, carrying dissolved solids out and off the membrane; this is part of
the design.
The
most common dissolved solid in tap water is calcium. As Calcium drops out
of solution, it will scale the membrane a little at a time. The high
volume “waste stream” takes a good deal of scale to drain. When we cut
back on the volume and flow rate of the waste stream, membrane deposits build
much quicker. The average membrane life is 3 to 5 years; membrane
life can drop to as low as several months when the waste stream is reduced.
Membrane life can still take a backseat when the availability of water is of a
major concern.
Using
Softened water extends membrane life. This includes naturally soft or
salt softened water. If your feed water is softened, meaning the calcium
has been replaced with a non-scaling salt, you can send less water to
waste. When softening with sodium you end up with deposits that dissolve
rather than scales. So a small amount of flushing can dislodge some
of the buildup, extending membrane life, while still reducing waste
water.
Higher
pressures will result in a more efficient RO system, efficiency being measured
in terms of less water waste and higher quality product water. Reverse
Osmosis systems assume common US household pressures, ranging from 35 to 60
PSI. Although a higher pressure is desirable for best efficiencies,
most RO systems components cannot handle these pressures. People in
areas with higher pressures NEED to add pressure reducing valves to their
incoming water lines; or purchase RO systems built specifically for high
pressure applications if you plan on using your system above 70
PSI.
Water
temperature also affects efficiency. Warmer water produces faster and
higher quality water with less waste water. Most membranes are
rated for temperatures around 80 degrees F. Beware; HOT WATER
(above 80 F) WILL MELT/destroy YOUR MEMBRANE.
In
my opinion, the most important thing about re-using waste water is
Fluoride. Fluoride in small concentrations is hazardous to humans
and other mammals. You will find conflicting information but no matter
what, you will be concentrating the fluoride. SO NEVER consume the waste
stream! Safer uses for waste/concentrate stream include
irrigation, washing, washing machine and pool top off.
Reverse
Osmosis is a process that works extremely well to remove/reduce Total Dissolved
Solids from a water stream. Weighing the costs of waste/concentrate
or membrane life is solely the consumer’s decision but there is no
alternative. Do not be fooled when manufactures or sellers claim
you can have both less waste and long membrane life, you cannot. But you
can make an informed decision. Please feel free to let us know of
any other uses you have come up with for your concentrate stream.
Marianne Brizio
Aqua Engineering & Equipment
AquaFX
