There are many options for the average consumer today to address their drinking water needs. You can purchase from the various bottled water options, drink straight from your tap, or install a home filtration system to improve the quality of water you drink, and for some people, cook with.
From an efficiency standpoint it might be surprising for you to know that the most popular in home filtration system on the market, although very effective at removing impurities in water, is not so effective when considered from a water efficiency standpoint. To understand why this is let's first take a very quick look at the basic process of purification as done in a Reverse Osmosis (RO) system.
The primary principle of Reverse Osmosis (RO) systems is to push water through a special membrane that will not allow impurities in the water to pass through. This is accomplished via pressure so no direct electrical force is required to produce purified water in an RO system. Many RO systems will contain pre and post filtering as well. The purpose of the pre-filter is to help remove larger contaminants from the water and to prolong the life of the membrane which is the key "cleansing" step in generating pure water in an RO system. By using pre-filtration the life of the primary membrane can be extended. Sometimes pure water is considered by some to taste flat. This is due to the elimination of all the contaminants (minerals etc.) that ultimately help give water its taste. By using a charcoal filter in the post-filtration stage the water is "polished" to take away the "flatness" and give it a more appealing taste.
The key to efficiency in an RO system is the ability to push water through the purification membrane. This is achieved via pressure. Most RO systems require a minimum water pressure of 40 PSI to work effectively. The higher the PSI the more water can be pushed through the membrane. Since it is not possible to push all the water through the membrane any water than cannot pass through is re-routed to the drain as waste-water. This is where the inefficiency of RO systems comes into effect. You may be surprised to know that the efficiency of typical home RO systems range from 8-30% efficiency. What does that mean? It means that for every 1 liter of water you use, approximately 2.5 to 11 liters of water is wasted and sent back down the drain. According to information published by ANSI/NSF1 and manufacturers of RO systems the typical efficiency of point of use RO systems is between 8 and 15%. Only more expensive high-end RO systems achieve higher efficiencies. This means that the average home RO system actually generates approximately 6-11 liters of waste-water for every liter of purified water produced. This water is typically routed back down the drain.
Now, some people might think that in comparison to the amount of water used in a home for bathing, washing, and flushing toilets, drinking/cooking water represents a much smaller portion of our daily domestic use. And, you are right, it does. However, let's consider the impacts of an RO system over the course of a year. First let's assume that we are using a typical point-of-use RO system with efficiency between 8 and 15%. Second we will assume that on average each person in the home uses 1 liter of purified water per day (1 L/d). We use this for simple calculations and to take into account those who use purified water for cooking, coffee, tea etc.
If your family consists of 4 people, using the above assumptions, you would use 4 liters (4 people * 1 L/d) of purified water per day. However, don't forget that with an 8-15% efficiency you would have also wasted 23 to 46 liters of water in generating those 4 liters of purified water. Over the course of one year you would use approximately 1460 liters of purified water (4 L/d * 365 days) and wasted approximately 8300 to 16800 liters in generating the 1460 liters of water you actually used. So, how much water is 16800 liters anyway? Well, it's enough water to fill a 15' round by 48" deep swiming pool, or enough water to take approximately 350 5 minute showers, or supply all of the water required for toilet flushing for a single person for one whole year. That's a lot of water, isn't it?
There are additional technologies available that can help improve the efficiency of a point-of-use RO home systems and newer systems that are considered "Zero Waste" as they have no connection to a drain. If you are considering purchasing an RO system you may want to inquire into the efficiency of the system (ask for published specifications) and also inquire about any additional add-ons that can be used to improve the efficiency of the system before you make your decision. Consider also that commercial systems are typically much more efficient than a basic home system (achieving efficiencies of 35-75% and possibly better), and commercial distilled water systems (Vapour Compression Systems) are even still more efficient achieving efficiencies of approximately 80-90%. That means that for every 1 liter of water produced you generate between 0.1 and 0.25 liters of waste water.
This doesn't mean you should purchase bottled water from around the world as that has other obvious negative impacts to the environment, but if you prefer a higher quality of water consider re-fillable 5 gallon bottles from a localbottled water company, look into a zero waste or high efficiency water purification systems, or consider drinking water from your municipal supply. Most importantly I hope this article begins to generate more conscious and informed approach to water conservation. It's everyone's reponsibilty to think about conservation and efficiency.
1NSF International, The Public Health and Safety Company™, is a not-for-profit, non-governmental organization, and is considered a world leader in standards development, product certification, education, and risk-management for public health and safety. These types of specifications can also be found on many RO system manufacturers websites and sites with information in regard to the operation of RO systems (http://www.nsf.org/business/newsroom/regworld01-2/rw_page5.html).