As urban populations continue to rise around the world, water resources are falling under ever increasing pressure. Many cities are now moving towards recycling and reusing a widening range of urban water sources as a solution to meet rising demand. Among others, these include harnessing roof catchment areas and harvesting urban stormwater. While these strategies have multiple benefits for the community, both stormwater and roof water need to be effectively treated prior to reuse. While stormwater harvesting is becoming more common around Australia, little is known about levels of contaminants, and their removal or treatment through available natural and passive filters. This project aimed to address these knowledge gaps, and to assess the safe use of treated stormwater for the irrigation of vegetables and crops.
At the CERES Environmental Park in Brunswick, Victoria, an inner suburb of Melbourne, two catchment areas were selected as urban water sources. Harvested water was then treated via matched systems to allow pair wise comparison of results.
The Roof Runoff Catchment supplied harvested rainwater to two separate above ground storage tanks. The supply line to one tank included the use of an enviss™ REUSE treatment system. The other stored untreated rainwater.
The Road and Roof Runoff Catchment supplied harvested storm water combined with rain water to two recycling systems; an enviss™ REUSE system, and a RootZone system, which is a reed-bed filter. Both systems treated runoff from this highly urbanised catchment model, which incorporated a zinc plated steel sheet roof and a multiple use car park. The catchment runoff was split evenly and supplied to each filter system. Effluent from each treatment system was monitored, fed into a holding pond, and then supplied to an irrigation system for a vegetable garden. Vegetables grown were assessed for suitability for human consumption.
For both these catchments, monitoring points were set up to measure standard water quality parameters, heavy metals and microorganism concentrations and to allow comparison of the consistency of treatments achieved.
The Roof Runoff Catchment harvested rainwater of significantly variable quality. This was observed, even between two sections of the same roof and may have been due to different orientations of each roof section, and to specific features in the local environment. Overhanging branches are thought to have caused a major increase in pollutant levels on one side of the roof, by allowing birds and animals access to contaminate part of the roof surface.
The effluent water quality from both storage tanks was similar, despite a noticeably higher contaminant load for the tank fed from the roof section overhung with branches. Water in this tank was effectively treated by the enviss™ REUSE system which reduced contaminants to the levels found in the tank fed directly with untreated rainwater from the roof section without overhanging branches.
Regular quarterly maintenance of roofs, gutters and tanks is recommended, as per the Australian Stormwater Harvesting Guidelines to help roof catchment systems supply water within usable limits for food irrigation purposes.
The Road and Roof Runoff Catchment produced raw stormwater runoff within the typical ranges found in the international literature. This runoff required treatment, as E. coli, phosphorus and iron levels were above guidelines set for irrigation.
While the two treatment systems performed well, the RootZone filter provided better treatment for most of the studied pollutants. However, nitrogen levels were more effectively reduced by the enviss™ REUSE system, possibly due to the unwanted presence of nitrogen fixing weeds in the RootZone filter. Both systems reduced E. coli concentrations by around 99 per cent but, this E. coli removal rate varied between wet weather events.
Regular maintenance and inspection of urban stormwater treatment systems is essential to ensure they operate as originally planned. Changes in catchment condition can also reduce treatment efficiency and annual monitoring of runoff quality is recommended to ensure effective treatment can occur with the installed treatment plant.
Road runoff stormwater treated with passive filtration did not result in elevated levels of contamination in vegetables when compared with those irrigated with mains water. This is an important finding and suggests that treated urban stormwater can successfully be used for urban food production and agriculture.