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.
Aqua Engineering & Equipment