I spent a very rainy cold Yesterday peering into holes across urban Queenstown with 30 other urban stormwater devotees with Stormwater360 leading the tour. We were kicking off the NZ Stormwater Conference looking at a range of stormwater interception devices – three vortex systems and one cartridge system. (I have no affiliations with Stormwater360 or Conteches – consultant and suppliers for devices below).
Councils typically have controls on water quality leaving a site. Water containing fine-sediment, oils, phosphate, heavy metals etc is toxic to aquatic organisms and accumulates in nature e.g. mercury and fish. However council rules are normally based on (the very outdated) so-called Total-Suspended-Sediment (TSS) method – this misses fractions below 40µm which is the most toxic to aquatic organisms. Current thinking involves analysing representative stormwater streams to see what is actually in the water – quite logical really one would think, but one which the state-sector often resists.
The overall approach is to detain water for long enough that pollutants can be floated off and collected, and for sediments to fall out of solution for later removal.
One technology for this is the vortex separator. These detain water in a shaped chamber and employs water flowing through to impart a spin to the water. This then extends the holding time so sediments can fall out (and in some systems be spun out through a screen). They all need cleaning tho’ as they are designed to trap anything that is not water. There have been some system failures in NZ as people are reluctant to clean or maintain things here. Overseas, systems are often leased so the supplier can protect the system and ensure the approach keeps its reputation.
Vortex systems date from the late 1960’s and are now highly refined and capable of removing sediment down to 20µm (which is about 80% of sediment-bound heavy metals for instance). Before this technology mechanical systems required ten to twenty times the area – some of these installs are about a square metre and easily retrofitted.
Usually several different devices or approaches are installed together as a ‘treatment-train’ starting with a trash separator, then an oil-separator, followed by various levels of sediment removal to clean the water to the point where it will only cause an ‘acceptable’ level of damage to organisms and our home planet. Where there is room a system will often start with a trash-separator and coarse-sediment control, followed by a rain-garden (growplan blog old post) to attenuate the flow and take some of the load off the vortexes so they can do their work.
Finally, we all need to advocate and agitate when our local town plan comes up for review so we can get these old codes and rules updated with infrastructure that really protects the planet, using science that we have free access to and underpinned by theory that is peer-reviewed. It’s our Earth to protect.
Andoh, etal 2002. Improving Water Quality Using Hydrodynamic Vortex Separators And Screening Systems (link goes to a Researchgate pdf size 650Kb). This a very useful introduction to stormwater controls and the use of vortex technology in a number of fluid-based domains.