Case study - Yarra Valley Water - The Life Cycle Assessment of a domestic rainwater tank

Yarra Valley Water applied Life Cycle Assessment (LCA) to improve the understanding of the broader environmental implications of water tank production, installation, use and eventual disposal.

Two water tank scenarios were tested in the study:

• A 600 litre plastic tank used for garden watering only using gravity feed (i.e., no pump installed); and
• A 2,250-litre plastic tank used for garden watering and toilet flushing, with a pump installed with an automatic pressure switch. Mains water (via a backflow preventor) is connected to the tank via a float switch to maintain a minimum level of water in the tank for toilet flushing in times of low rainfall.

Completing the LCA was a valuable exercise for YVW, as it added science to the decision-making processes. YVW learnt two key aspects from this work.

Firstly, the LCA identified greenhouse gas emissions as a major environmental impact, however these were relatively small when compared to the annual per capita generation. The overall additional annual greenhouse impacts of having a water tank are roughly equivalent to 20km and 60km per year of car travel for the 600-litre and 2,250-litre tanks respectively.

By conducting the LCA YVW was prompted to ask the question, "can this environmental impact be reduced even more, to further enhance the environmental benefits of using a rainwater tank?"

An analysis of different pump options identified further opportunities for environmental improvements. The smallest commercial water pumps currently being used for water tank applications can deliver around 50 litres per minute. While this may be needed for garden usage, a much slower rate of 5 litres per minute could be tolerated for toilet water. This suggests that pumps used currently may be oversized and alternative solutions to supplying water into the toilet would reduce the greenhouse impact substantially.

Secondly, the LCA demonstrated another significant environmental benefit from the installation of rainwater tanks in Melbourne. The nitrogen captured by the rain, which is diverted from stormwater through water tank collection and use, results in reduced nutrient load entering waterways and Port Phillip Bay.