How to apply stormwater treatment to optimize environmental benefits and achieve regulatory compliance

14th July 2019

What you need to know in order to design effective stormwater treatment while remaining compliant with local regulations.

Growing climate unpredictability and increasing urbanization are generating significant amounts of stormwater, and thus an increasingly urgent problem. Not only does stormwater threaten waterfront properties, creating problems with insurance and reducing their value, it also affects public health, environmental quality, and the economics of industries that depend on water for their existence.

Specifying treatment options

In specifying stormwater treatment, engineers have a wide range of choices that can help in managing runoff quality. The options start with reducing the load of pollution being generated before it occurs, an approach sometimes known as non-structural best management practices (BMPs) or Low Impact Development (LID) - also known as sustainable drainage systems (SuDS) in the UK, and water-sensitive urban design (WSUD) in Australia and the Middle East.

This approach can take the form of design practices or behaviors.

For example, a design practice might involve reducing the volume of stormwater runoff being generated by reducing the amount of impervious surfaces such as roofs, parking lots and pavements on a development site.

Another option could be to choose building materials carefully – parking lots with dirt surfaces, for example, are known to generate far higher sediment pollution loads than paved or gravel parking.

A behavioral solution might consist of, for example, ensuring trash cans are readily available in areas where people generate litter to encourage use of these receptacles rather than discarding potentially polluting waste on the ground. Another option might be regular street sweeping to ensure sediments are removed from roads before storms transport them into the stormwater drainage system.

Best management practices

A further option is to design a ‘structural best management practice’ (BMP). These solutions capture pollution from stormwater at a point along the drainage path before it is discharged to a receiving water body.

Structural BMPs can be surface based, such as swales, retention basins and rain gardens, or subsurface structures. Such solutions allow stormwater to infiltrate gradually into the ground or retain it until the storm peak has passed.

Often, the type of solution chosen is constrained by the amount of space available: in an extremely urban setting, even subsurface solutions may be restricted by the presence of many below-ground utilities.

Structural BMPs can be designed to provide a wide and varied range of treatment options, depending on the budget, treatment target and runoff volumes being considered. If the aim is only to capture large (>4mm diameter) visual pollutants, trash traps can be installed.

Oil/water interceptors can be used in areas where vehicle operation and maintenance mean hydrocarbons are an issue, and gravity separators can be installed to remove settleable solids. Filters can be employed to remove the finer solids and dissolved pollutants.

During installation, it is important to be aware that many stormwater BMPs only operate at the treatment level anticipated if they are correctly installed and protected from construction runoff after installation and while construction is still ongoing.

The importance of maintenance

Maintenance is the most important factor in ensuring that treatment systems continue to operate optimally. Structural BMPs, whether proprietary or non-proprietary, are designed to capture and hold pollutants from stormwater runoff that accumulate in the BMP over time.

Most BMPs are designed with storage capacities that will cope with one to five years’ worth of the predicted annual pollutant load. However, non-routine events can create an urgent need for maintenance, for example an oil spill in the catchment or an extreme rain event, which could cause major land erosion and fill the drainage system with sediments and gross debris.


Maintenance is the most important factor in ensuring that treatment systems continue to operate optimally.

Ease of cleaning is therefore an important consideration, and any viable solution must be capable of being cleaned quickly and without expensive specialist tools. If personnel access is required, the costs rise considerably because health and safety often requires a team rather than individual to be present, and may potentially call for expensive monitoring and other equipment.

Stormwater regulations

Stormwater treatment must be seen as an ongoing project: engineers need to help the end user remain compliant with regulations at all times. In the UK, US and Australasia, regulations are among the most stringent in the world and all recognize that the quantity and quality of stormwater runoff from certain sites must be managed.

However, continued problems show that each area still needs to improve regulation and enforcement.


Since the early 2000s, government and industry have been working to address surface water management issues, with changes to the Building Regulations in 2004 and the introduction of PPS25, leading ultimately to the Flood & Water Management Act 2010.

Defra has also issued guidance on rainfall runoff management, and there is a code of practice, BS8582, to help local authorities and developers in planning and assessing drainage schemes. CIRIA’s SuDS manual is also seen as the primary guidance for SuDS systems, and the Susdrain website is a useful resource.


Australia’s Department of the Environment and Energy has produced the Introduction to Urban Stormwater Management, which outlines both challenges and approaches. In Australia, unlike the UK, the stormwater system is always separate from the wastewater system. Industry body Stormwater Australia has submitted recommendations that have been accepted by the Senate Environment and Communications Reference Committee, including recognition of the need for complementary planning and building regulatory approaches.

New Zealand’s On-Site Stormwater Management Guideline, issued in 2004, is intended to facilitate design of stormwater systems, providing detailed advice on selection and design of devices.


A 2008 review by the National Research council produced a report that sets out requirements for managing stormwater pollutant discharges and the responsibilities of permittees. The Water Environment Federation’s Stormwater Institute also provides information on best-in-class urban runoff solutions, and WEF also has a Stormwater Testing and Evaluation for Products and Practices workgroup (STEPP), which has issued recommendations for a national testing and evaluation program.


The consequences of incorrect application, installation and maintenance of stormwater treatment systems can be seen in the news virtually every day. The negative effects include algal blooms, beach closures due to high bacterial concentrations following a rain event, and declines in coldwater fish populations due to hot stormwater running off sun-warmed surfaces in the summer.

Stormwater can also reduce recreational opportunities, with devastating effects for communities that rely on income generated by tourism.

By keeping in mind environmental benefits and regulatory requirements when applying stormwater treatment, engineers can help their end users at every stage, keep their businesses running and maintaining their compliance.


  • There are stringent regulations in place in the UK, USA and Australasia (among other regions) that must always be kept front of mind when applying stormwater treatment.
  • Specifying stormwater treatment should allow room for engineers to consider the types of environments, how runoff water can be managed more effectively and if there are any natural ways to redirect stormwater.
  • Maintenance is the most important factor when ensuring that treatment systems are performing optimally and within regulatory boundaries. Contractors and engineers must keep in mind and communicate the way stormwater is changing, and the increase in rainfall (for example) that is expected to add pressure to systems in the coming years.