Simple life cycle analyses of rainwater harvesting systems may underestimate their energy consumption and carbon dioxide emissions by as much as 60%, according to research, which has used an improved method to estimate systems’ energy use more accurately. However, their energy use is still considered very small when compared to the overall energy use of a modern building.
Rainwater harvesting systems (RWH) can reduce dependence on the mains water supply, relieving pressure on often overused water resources. They are becoming more popular around world, in response to the desire for buildings to become more adaptable and resilient to climate change and as populations grow larger. As a result, some national governments, such as the UK’s, have set targets for rainwater harvesting in new sustainable buildings.
However, non-gravity rainwater harvesting systems require electricity to drive pumps and, in some cases, disinfect water using UV light. Some commentators have therefore questioned whether the energy use and associated carbon emissions outweigh the other environmental benefits of such systems.
Previous studies have used life cycle analyses to establish the energy cost of rainwater harvesting systems. However, these may not have captured the full energy use of the systems. In contrast, the latest study included a more accurate representation of the energy used by the pumps. In particular, the researchers included an estimate of the energy used on start-up as well as that used during operation. The new method also distinguishes between pumping phases and has been developed for RWH systems where no pump energy monitoring equipment exists. The improved method also considered the efficiency of the pump. As rainwater harvesting systems do not always incorporate energy use monitors, the researchers had to estimate energy use based on the volume of rainwater. The new method also estimated the carbon dioxide emissions from the process.
The researchers then tested their improved method on a rainwater harvesting system installed in an office building in the UK where rainwater is used to flush toilets.
Comparing their improved model with the simple method used previously, they found that the energy use and carbon dioxide emissions predicted by the improved method were 60% higher than those predicted by the simple method. For instance, the simple method predicted energy use of 0.32kWh per m3 of rainwater, versus 0.54kWh per m3 from the improved method. Similarly, carbon dioxide emissions increased from 0.34kgCO2e per m3 of rainwater to 0.56 kgCO2e per m3. These results emphasised the importance of the amount of pump start-up energy consumption and efficiency and underlined the necessity to estimate the total energy consumption and CO2 emissions.
The results from the improved method closely matched the observed energy use identified by another study from Australia. The researchers also found that the overall total energy consumption of the rainwater harvesting system accounted for just 0.07% of the total energy use in the office. Thus findings suggest that the energy use of such systems is negligible when compared to the overall energy use of a modern office building.
The results also reveal that the examined rainwater harvesting system used marginally less energy than mains water distribution, and cost less per m3 of water. The research highlighted that emerging gravity systems, which do not require energy for pumping, present opportunities to reduce energy consumption substantially. The model provides a tool for benchmarking rainwater harvesting systems’ energy use, but needs to be applied to further case studies to verify the results presented in the study. The research also stressed that the water-saving benefits of rainwater harvesting systems are significant, and should not be overlooked.
Source: Ward, S., Butler, D., Memon, F.A. (2011) Benchmarking energy consumption and CO2 emissions from rainwater-harvesting systems: an improved method by proxy. Water and Environment Journal. Doi:10.1111/j.1747-6593.2011.00279.x.