Each year approximately 650GL of water, nearly double Melbourne’s annual consumption, falls in the urban areas of the city. As such, stormwater has been identified as a valuable resource that offers benefits for water conservation and water recycling.
There has been a dramatic increase in stormwater recycling projects in Victoria that use general urban run-off as an urban water supply source for non potable purposes.
However, there is still a need to investigate any human health and environmental risks of reusing urban stormwater. Monash University and the University of New South Wales were awarded a grant to identify any such risks, as well as the potential benefits of substituting stormwater for potable water in urban environments.
The Institute for Sustainable Water Resources at Monash University and the Centre for Water & Waste Technology at the University of New South Wales provided crucial knowledge about the actual water harvesting performance of urban stormwater recycling systems using runoff from paved and unpaved urban areas.
Three demonstration sites with storm water recycling systems in operation were monitored from late 2006 through to early 2008. In addition, treatment barriers were ‘challenge tested’ with microbial surrogates to evaluate pathogen removal performance. These sites were located at Royal Park, Altona Green and Monash University Clayton Campus.
The flow of water through the systems was monitored so that the water supply performance could be evaluated. Investigation into the level of faecal contamination and other water contaminants moving through the system (from source to point-of-use) was also conducted to evaluate the health risk associated with stormwater water reuse.
The three sites provided varying results that have given the project team an insight into the contaminants that can be anticipated in different stormwater recycling environments.
The assessment of the water supply performance found that even though storage capacity constraints are routinely put forward as the limiting factor in a stormwater harvesting system, it is likely that capacity is not actually the key system performance constraint. For example, in the case of the Monash University and Altona Green sites, stormwater inflows to the store limited the harvesting potential of these systems.
It was also evident that employing stormwater treatment devices which intercepted or lead to infiltration of a significant proportion of the catchment runoff can dramatically reduce stormwater inflows and therefore the benefits and disadvantages of their use should be compared during the design process.
When evaluating health risks from faecal pathogens the source of faecal contamination is critical. Faecal contamination in stormwater can occur from leaking sewerage infrastructure and from wildlife such as ducks in open storage ponds.
Overall results from the Quantitative Microbial Risk Assessment undertaken at the three sites indicated that when human faecal contamination was present, human enteric virus risks dominated. Contamination from wildlife may pose a risk for transmission of bacterial pathogens to humans; however the likelihood of this event was unknown and expected to be low. A framework for incorporating these type of rare, but potentially important risk events into the overall fit-for-purpose evaluation is needed.
Overall, the key stormwater treatment barriers performed well in terms of physical and chemical pollutant removal to provide fit-for-purpose open space irrigation water. The Royal Park wetland and pond treatment train also performed well in terms of removing nitrogen, phosphorus and suspended solids for waterway health protection.
This research will help to ensure stormwater recycling systems are achieving fit- for-purpose quality, protecting public health and protecting the environment.
The findings will help increase the certainty surrounding the design of stormwater recycling systems and therefore reduce the resources required by developers and local government to design and assess systems through the development of guidelines for system design and monitoring.
By optimising the design of stormwater recycling systems, it will be possible to ensure they do not waste money through unnecessary over design or require expensive modifications due to under design.