Is it better to store rainwater in offices or just use mains?
Rainwater and water of any kind is a precious commodity. We are being encouraged to store rainwater in office buildings by various industry bodies and government. But the question is – from an ecological perspective is it better to store rainwater in tanks and reuse within an office for toilet flushing or is it better to let the rainwater flow to the local catchment and be recovered as part of the overall water cycle? It is a question we get asked a lot so we did a lifecycle analysis of basement rainwater storage in an office building in Brisbane. Here are the results.
First up the most important thing with lifecycle analysis, in my opinion, is to define the boundary. Are you looking at the impact of an isolated option in an isolated building or the impact of an unit flow in the entire system? This important first step is highlighted by the lifecycle analysis of rainwater storage in office buildings.
A background of the options.
Option 1 is easy. Use mains potable water that has been treated to drinking water standards to flush toilets and urinals. Calculate the amount of pollutants from the rainwater running into the local water course.
Option 2 install rainwater tanks in the basement of your building. Run separate pipes and risers from the tank to your toilets. Install filters and pumps. Calculate the reduction in pollutants running into the water course by reusing the rainwater on site.
The lifecycle result?
Well, to my surprise rainwater storage turns out to be a better option by about 30-40%. Surprised?
The largest ecological impact of two options was amount of energy that was needed to pump the mains water to the office building. I would have thought it was the excavation, concrete, additional pipework etc but no over 50 years its actually the water pump energy.
But there is a proviso here.
In option 1 for our office building the design didn’t use mains water pressure to circulate water through the building. It had a separate booster set to control the water pressure. Which meant that when we looked at option 2 we were already allowing for water pump energy so the rainwater option had additional pumps but no additional pumping energy.
However, here is the proviso. If your option 1 uses the mains water pressure to circulate water through the building when you look at option 2 you will need additional water pumping energy. The result of this is that rainwater storage is still marginally better but only by about 2-5% and will vary by +/- 5% depending upon which state you are in.
So the lifecycle result appears to show that rainwater storage is an ecologically better option than using mains water for toilet flushing. But what about if we considered the unit flow in the overall system rather than our isolated building?
Let’s consider the unit flow as the ‘provision of water for toilet flushing’ and then work out the most ecologically effective way of getting there.
So, we have already analysed two options for the provision of water for toilet flushing – option 1 mains water and option 2 rainwater on site and found that the largest ecological impact was pumping energy. So if we looked at the system rather than the building our response would be not how do we capture rainwater, it would be how do we reduce the ecological impact of the water pumping energy.
The easiest option from an analysis perspective is to use renewable energy for water pumping. Now there are many ways you could do this and I am sure there is some smart nerdy stuff with water turbines etc but for the sake of an easy comparison let’s just say we use utility green energy as that allows for all the ecological impact of the renewable energy infrastructure etc.
Option 3 stick with mains water for flushing the toilets but the water company uses green power.
The result of option 3 is a 60-70% lower ecological impact than storing rainwater on site.
So yes, rainwater storage on site for toilet flushing is the best option if we compare it to a high carbon energy source but that doesn’t its a better ecological option for Australia as a whole. The better ecological option for Australia as a whole is to convert the water utility to green power.
But that will cost a fortune I hear you say. Well, the cost of installing, operating and maintaining the rainwater system in your office building would pay for 100 years of green power for the equivalent amount of water consumption.
Now, yes this is just looking at the ecological impact. It doesn’t consider the water impact, the climate resilience in the system, the age of the municipal infrastructure etc. But what it does highlight is that bang for buck, or ecological bang for buck, making our municipal systems better is a much better option that local storage for Australia.