Urban stormwater runoff can add significantly to phosphorus, heavy metals and other pollutant loadings

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catch basin
On average, there is approximately one catch basin for every 15 people who live in an urban area in Canada. Retrofit devices can help address stormwater runoff challenges.

By Stephen Braun

Non-point source pollutants are a big problem, with urban stormwater runoff playing a significant role in adding phosphorus, heavy metals and other pollutant loadings to our receiving waters. For example, in the Lake Simcoe watershed in Ontario, over 30% of phosphorus loadings to the lake are now identified as coming from urban stormwater sources. Less than 3% of total loadings are coming from water pollution control plants.

A re-adjusted focus towards stormwater treatment will go beyond the Lake Simcoe watershed if the costs of marginal performance increases at water pollution control plants exceed the costs of treatment of stormwater on a total pollutant loadings basis.

In any case, it is certain that low impact development (LID)-type stormwater approaches and green infrastructure designs need to be implemented on a broad scale. They will go a long way towards addressing stormwater runoff pollutants in new development areas and in retrofit situations.

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As well, retrofits of older stormwater management facilities to bring them up to state of the art, will ensure their better success at removing sediment and the attached/adhered pollutants which are the real target. But, in large urban areas built prior to any stormwater management controls, retrofits for stormwater quality can often remain problematic and expensive.

Fortunately, another “almost non-point” treatment option is available for urban areas. This involves retrofitting existing catch basins to provide reliable stormwater treatment.

Transforming catch basins into effective stormwater treatment devices has been an idea with some staying power, and interest in retrofits is once again seeing a big resurgence. However, catch basin retrofit ideas can also irk municipal maintenance departments, which are tasked with taking care of the various next “great” ideas. This means that retrofits must perform to their stated objectives and must be worth any extra effort required.

One of the newer types of catch basin retrofit inserts available in the past few years has already seen over a thousand installations in over 20 Ontario municipalities. It will soon be used in Alberta and elsewhere.

The CB Shield® is a patented Canadian product, manufactured in two locations in Ontario. The company supports stormwater plans with engineered designs, and also arranges for catch basins to be installed with custom field-fitted devices. The shielding device uses the existing sump of a catch basin to capture and store pollutant-laden sediment. It has a simple fibre-reinforced plastic construction, along with stainless steel riveting and hinges, making it extremely strong and durable. Its lightweight construction (<8 kg per part) allows for quick removal and replacement during maintenance periods.

The CB Shield acts like a seatbelt for catch basin sediments. The “crashes” that the shield protects from are high intensity rainfall events that would otherwise scour out the contents of an unprotected catch basin sump.

Laboratory testing of shielded and unshielded catch basins indicates that both can capture sediment, albeit the shielded one a little more efficiently. But without a shield, a catch basin is terrible at keeping its sediment under higher flow conditions, which scour out everything it might have caught. Field testing indicates similar results. Accordingly, the catch basin shielding device is seen also as a hedge against future climate change, where we will most likely see increases in high intensity rainfall events.

The value proposition of retrofitting catch basins for reliable stormwater quality treatment is fairly clear, especially for older urban areas where no other stormwater management is in place:

  • 50% – 60% reliable long-term sediment capture (per Environmental Technology Verification test sediment particle size distribution).
  • No digging up streets, no construction impacts, just quick and customized installation.
  • Additional easement and/or purchase of land not required for stormwater management.
  • A stormwater quality control system that spreads out risk, as a failure at one or a few locations is not significant.
  • Utilizing a maintenance system that is typically already in place, and extending the time between maintenance periods (thereby reducing costs).
  • Keeping sediment from downstream areas, where removal is more expensive.
  • Cleaner watercourses and lakes receiving stormwater runoff.

Many cost comparisons have been completed for existing urban areas. All indicate that the implementation cost for a catch basin retrofit program is typically an order of magnitude less expensive than other stormwater management approaches. Catch basin retrofits are simpler and less intrusive. They can quickly transform older existing urban areas to ensure downstream sediment and pollutant loadings are reliably reduced.

The CB Shield device uses the existing sump of a catch basin to capture and store pollution-laden sediment.

Interestingly, many CB Shield devices have also been installed at “greenfield” or new development areas. Catch basin sumps in Ontario (Ontario Provincial Standard Drawing – OPSD 705.010) store approximately 0.2 m3 or 300 kg of sediment. This weight of sediment takes approximately three years to accumulate in the sump under average conditions prior to maintenance. This is often an extra year beyond a typical two-year catch basin maintenance cycle.

The large accumulated mass of sediment also indicates why the approach of storing sediment in the sump for typical vacuum truck cleaning is preferable to lifting top-mounted “hanging bag” devices. In any case, keeping sediment from being transported to downstream areas results in significant operational cost savings. Sediment removal from pipes and ponds is usually many times more expensive than the cost of cleaning it at a catch basin.

Similarly, protecting catch basins with shields can provide a very effective pre-treatment function prior to runoff being discharged to an LID-type or green infrastructure system. The City of Kitchener, Ontario has performance specifications for enhanced catch basin devices being in place to pre-treat runoff prior to discharge to its infiltration systems. The CB Shield insert satisfies its current requirements.

Maintenance of a CB Shield protected catch basin isn’t much different than a regular catch basin, and will, on average, add one more minute of labour per catch basin cleaning. Device removal takes approximately 10 to 30 seconds, with the catch basin then cleaned by vacuum truck in the same fashion it would have been cleaned. This is followed by replacement of the device (another 10 to 30 seconds).

A significant number of these catch basin inserts have now finished their fifth winter season and no extra maintenance has been reported. This is due to the inserts being specifically designed to ensure flow blockage potential would not be increased, and to laboratory testing under very high flow conditions (> 70 L/s).

Suitable testing of any stormwater device, including a catch basin retrofit device, is critical to knowing its true performance capabilities. As well, verification of performance under controlled laboratory conditions must be completed by an accredited third party in order to have value. In the case of the CB Shield device, a protocol was developed by a third-party lab which closely followed Environmental Technology Verification (ETV) Canada’s Procedure for Laboratory Testing of Oil Grit Separators (TRCA, 2014).

Results from the testing allowed verification to be completed according to ETV/ISO 14034:2016 criteria, and posting of testing results has been made available on the ETV Canada website.

Approval agencies and municipalities, including their maintenance departments who are responsible for the devices, will also want to see some field testing results from field pilot studies or larger installations. As well as confirming general performance characteristics, field testing also indicates a catch basin device’s ability to withstand harsh winter conditions, and demonstrates ease of maintenance.

On average, there is approximately one catch basin for every 15 people who live in an urban area in Canada. Catch basin retrofit devices are part of the answer for addressing our urban stormwater runoff challenges, either as pre-treatment to LID and green infrastructure, or as establishing a minimum basic water quality level in otherwise hard to treat areas.

The preservation and enhancement of aquatic life, which includes swimming at the beaches or paddling in our canoes, depends much on the quality of our stormwater runoff. We may be able to treat our own drinking water and even fill a pool, but you can’t really treat a river, never mind a Great Lake.

Stephen Braun is with CB Shield Inc. Email: stephen.braun@cbshield.com

Read the full article in ES&E Magazine’s June/July issue below.

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