How (and why) to harvest stormwaterJenny Griffin | October 14, 2021
Urban stormwater is generally considered a problem that requires proactive management to prevent soil erosion and flooding and associated impacts such as damage to infrastructure and property, and a reduction in water quality as a result of silt and other surface pollutants entering freshwater systems with runoff. City managers typically allocate a significant portion of their budget toward building infrastructure to collect and channel stormwater away from built-up areas. But couldn’t this water be captured and used to augment freshwater supplies that are rapidly dwindling in many regions of the world?
As populations in cities across the world grow, so too does their demand for water. Since climate change is making many regions drier, droughts are becoming more common and more prolonged. Consequently, many heavily populated cities are facing water scarcity as the demand increases and water supplies dwindle.
Stormwater, which is currently viewed as a problem that needs to be managed, could serve as a valuable resource if we manage it more creatively and sustainably. Capturing and storing stormwater could provide a sustainable backup supply that could be used for non-potable water needs, such as irrigating parks and sports grounds, for example. This alone would reduce the pressure on potable water used to provide drinking water to the city’s inhabitants. However, since stormwater is essentially rainwater (that may or may not be tainted with surface debris and other pollutants), it can also be used as a source of drinking water if treated appropriately. The volume of freshwater lost as stormwater runoff in most urban areas is significant. If this was captured instead of being diverted to drains that empty into rivers and eventually the ocean, it could provide a sustainable source of water that would alleviate pressure on freshwater sources needed to grow crops and for maintaining environmental integrity of freshwater ecosystems.
Fundamentals of stormwater collection
There are two primary sources of rainwater runoff that we can capture for reuse: 1) rainwater that runs off rooftops, and 2) surface runoff from pavements and roads (stormwater). Unlike rainwater harvesting, which tends to be decentralized, with homeowners taking the initiative to collect, store and treat the rainwater harvested off their rooftops, stormwater harvesting is more centralized, and generally tends to be channeled to large reservoirs (either on the surface or underground) where it is stored and may either be treated onsite or sent to a water treatment facility before being distributed to the water distribution network.
The primary limitation of centralized stormwater harvesting is that storage facilities that can store large volumes of collected stormwater are in many cases unavailable, particularly in built up urban areas where vacant land is scarce.
One solution to the stormwater storage dilemma is to use the harvested water to replenish underground aquifers. So instead of building large reservoirs to store the collected stormwater, it is treated and used to recharge groundwater supplies, where it is once again available as a source of water for urban use -- an approach known as 'managed aquifer recharge' (MAR).
Considering that around 20% of all groundwater aquifers globally are over-exploited, the MAR approach offers several benefits:
• In coastal aquifers, it can prevent saltwater intrusion from seawater, thereby protecting the groundwater from becoming brackish.
• It can help replenish groundwater in aquifers where there is a high demand, and help meet this demand.
• It can improve the natural environmental flow of water in watersheds.
However, in order for the MAR approach to be viable certain geological conditions are required. When these conditions are absent, alternative, more costly storage options will be necessary.
Many cities in Spain, Australia and other water-stressed countries around the world have started to collect, store and use stormwater. The city of Melbourne has implemented several harvesting projects to collect stormwater, clean it and reuse it for irrigation and other purposes. The cleaning process typically involves filtering water through a gross pollutant trap followed by settlement in a sediment chamber, before biofiltration through a garden bed and followed by UV disinfection to kill any pathogens.
One of the primary constraints with the MAR method is that because stormwater harvesting is still a relatively new concept, scientifically-sound guidelines on best practices to ensure environmental health and safety tend to be lacking. Stormwater may not require the rigorous treatment that recycled wastewater has to undergo, but since a wide range of contaminants can be washed off roads, pavements and other above-ground surfaces, stormwater will need to undergo some form of treatment to ensure it does not pose a public health threat. The World Health Organization (WHO) does have guidelines for best practices on reusing treated wastewater, but this currently does not include treating harvested stormwater for reuse. Hopefully this will change soon so that more and more cities can safely tap into this underutilized resource to boost their water supply.
Building sustainable cities
Collecting, storing and reusing stormwater not only improves water security, it offers several environmental benefits, too. Stormwater harvesting reduces the frequency, volume and peak discharge rates of runoff, which in turn reduces the volume of stormwater that flows into urban rivers, streams and canals, ultimately reducing the likelihood of flooding in cities and towns, improving water quality, ecological integrity and biodiversity in urban waterways.
As the world gets hotter and rainfall patterns become increasingly less predictable, stormwater harvesting is a climate change mitigation strategy that allows urban areas to prepare for flooding events, while at the same time also building drought resilience. Stormwater harvesting prevents environmental degradation, improves water security, and builds climate resilience, thereby offering great potential as a sustainable development tool, particularly for densely populated water-stressed cities around the world.