Estimating water use during building operations Part 1

Overview
After much anticipation, I would like to treat the topic of quantifying annual water usage during the operation phase of a building. Please note that this procedure is rather similar to the one proposed by LEED and other rating systems, but the assumptions, their variability and validity of such are examined here in more detail. The procedure outlined is thus a predictive tool rather than a relative comparison tool and is thus better fitted for actual-performance oriented rating systems such as the Living Building Challenge or for people who want to understand the variability of their building or home's annual water use.

Surprisingly, I don't hear too much talk about carrying out these types of calculations. It appears most people are focusing on a more practical relative approach such as this post on office water. That post points out how the Building Research Establishment (BRE) in the UK has set a best practice annual water consumption target of 4m³ water per employee (knowing 1 m3 is equivalent to 1,000 liters, this means 4,000 liters).

Please note that this post does start out conceptually, as I believe it is best to understand the whole realistic complexity of a problem if we are to attempt to understand and model it properly. Then we can talk numbers and math which in this case is extremely simple and would make an awesome "green" teaching module in applied math or word problems (you just need to know how to multiply and add).

Procedure
"Real" water use during operation can be somewhat arbitrarily divided into four main sections: water used by occupants inside the building (hygiene purposes like showering, cleaning), water used by occupants to drink, water used in processes (cooling towers, heaters) and water used for landscaping an other uses outside the building. Sadly, an additional use of water is through leaky pipes, something that can happen through the distribution system or in any of the pipes or fixtures inside and around a building.

Also, note that there is a correlation between water use and energy as people for some strange reason refuse to take cold showers and don't like using cold water for washing their hands and cleaning their dishes. This is correlation with energy use is probably a topic deserving attention in a future post.

The basic idea in calculating water use inside a building is to know the number and type of fixtures available in a building. The two general types are those that are flush related, such as toilets, and those that depend on the duration of use such as sinks and faucets. User behavior totally dominates the annual water use. In fact, as can be seen in the simple spreadsheet (at the bottom of this post) that I have put together to illustrate the procedure, the specification of a particular fixture only provides us with one item of information and that is its water demand per use/duration. The rest is all human behavior, proper maintenance and habits. Today, there are more water saving showers available, such as those that allow you to easily reduce or turn off the flow while you are lathering with soap, but you still have to pay a premium for these.

Anyways, I am uploading a simple spreadsheet for flow fixtures so you can investigate yourself. As an example I have entered two showers, the first one based on real data from actual measurements and the second one is how rating systems like LEED would calculate it using more traditional assumptions. You will see that there is a difference per day that is further enhanced when you look at an entire year. The interesting thing is that flow is also variable in these fixtures depending on how much you open or close the valve. Thus behavior is very important for flow fixtures, which is great, because then we can open the valve a little less when we use it and we will be saving a lot of water. The flow issue is different for flush fixtures such as toilets, which will be included in part 2 of this post.

Just in case you were wondering, you can also find out the water demand of your flow fixtures in your home. All you need is a large container, preferably a measuring bowl, so just open the fixture to the level you would normally use it, begin measuring time say 5 seconds and pull out the container. Read the volume gather and divide by the minutes to get your LPM (remember there are 60 seconds in one minute). So, if I applied this procedure to my shower fixture, perhaps I would find that I collected 800 mL in 5 seconds, so 0.8L/(5/60)min= 9.5LPM.