Stormwater mitigation in new residential properties is now compulsory in many New Zealand cities. Stormwater detention is a vital element in making new builds compliant with council regulations. Almost all of those tanks are plumbed to do one job. Promax believes the same tank can do two.
What stormwater detention means
Stormwater detention refers to rainwater being captured and stored in a holding device (invariably a tank) either above or underground. The tank's capacity is designed to hold stormwater from a specific rain event, which differs between council jurisdictions. However, in most cases, capacity is based on 10-year Average Recurrence Interval (ARI) events and can be up to 50-year ARI events. Public infrastructure is typically designed to handle up to 100-year ARI rain events. The tank is not there to hold your water. It is there to hold the network's water back for a while, so the system is not asked to take everything at once. If the distinction is new to you, start with detention versus retention.
How the orifice controls the outflow
When new builds are required to mitigate their own stormwater on-site, the water slowly drains at a constant rate to the stormwater main. To achieve this, each house has a small tank with an outlet, or orifice. The tank size is determined by the stormwater engineer OR local council-approved solution. The orifice restricts the outflow from the tank to the stormwater main and is designed to release the detained volume inside the tank over a 24-hour period. By the time the next front arrives, the tank has emptied itself and is ready to do the job again. Our pre and post development detention calculator is a useful place to check the numbers for a site.
Caught, then let go
The consequence of that design is that none of the stormwater is used for beneficial purposes like irrigation. Instead, somewhat wastefully, it's caught and let go. A household can watch a full tank of rain drain into the main over a day, then turn around and draw on the mains supply to flush its toilets. Nothing is wrong with the engineering here. The tank does exactly what the council asked it to do. The gap is that the brief only ever asked it for one thing, when the tank standing there is perfectly capable of doing two.
Detention and retention in the same tank
Promax believes that the most sustainable method of stormwater mitigation is detention AND retention inside the same tank. The dual-purpose tank is designed so that not all stormwater is released, with some retained for greywater reuse throughout the dwelling and property. For example, a 4,000 litre tank might have 1,850 litres of detention volume in the top section and 2,150 litres of retention volume in the bottom section. The top of the tank stays free for the next rain event, which is what the council requires. The bottom holds a working supply, which is what the household actually wants.
Three ways it eases infrastructure pressure
A tank of this nature eases infrastructure pressure in several ways:
- During the rainy season, the household can use tank water for the laundry, toilet flushing, outside taps and other non-potable use. As a result, there's less draw on the dams during the rainy season, allowing them to fill faster.
- It reduces pressure on the stormwater network, with households reusing much of the water instead of releasing it to the council's stormwater infrastructure.
- There is no additional pressure on the wastewater network, as the same amount of water is used through the household. It's just no longer coming from the main supply.
The tank does not need to be bigger
Usually, dual-purpose tanks are the same size as those designed and plumbed solely for detention. As a tank manufacturer, there is no additional benefit to upsizing the tank; instead, innovative tank design and smarter plumbing in a new house will significantly enhance and protect potable and stormwater infrastructure. This matters on a new residential site, where space is the constraint that decides most other things. If a detention tank fitted the footprint, the dual-purpose version still fits. What changes is the design of the tank and the plumbing around it, not the hole it goes in. The same logic applies when you size up your stormwater tank.
Why homeowners resent detention tanks
Homeowners often view detention tanks as a grudge purchase. They see no tangible benefit from installing it. The biggest advantage is that flood events are mitigated; however, they cannot fully appreciate the positive effect because the tank is unobtrusively doing its job. This is a hard thing to sell, and the difficulty is structural rather than presentational. The tank succeeding looks identical to the tank doing nothing at all. The return on the money spent is an event that did not happen, somewhere further down the pipe, to somebody who will never know it was avoided.
Reuse is the part people can see
On the other hand, if homeowners can capture and reuse the water, the result is visible and tangible. It's much easier to sell the concept of using the water instead of slowly releasing it to the stormwater network. That is not really a marketing point, it is an adoption point. Mitigation that people value is mitigation that gets plumbed in properly, maintained and used, rather than resented. Promax is committed to sustainability, and our dual-purpose tanks tick a lot of boxes in that respect. They allow our customers to reuse stormwater while significantly easing the pressure on New Zealand's infrastructure. Contact the Promax sales team to learn more about detention and retention in one tank.