Suspended solids carried in stormwater cause a range of harmful effects in rivers, lakes and oceans. TSS capture and removal is a critical component of effective environmental protection.

What is TSS? 

TSS stands for Total Suspended Solids, and is the term used to refer to the solid particles suspended in water. It is defined as the total amount of solid material, suspended in water, that is retained by a filter.

TSS comprises both organic and inorganic matter, and can consist of a range of materials, including sand, grit, metal particles and decaying plant or animal matter. TSS is considered a waterborne pollutant. 

How is TSS harmful to the environment? 

The most visible environmental impact that TSS has on the environment is siltation and sedimentation. Suspended solids transported into rivers, lakes and oceans eventually settle on the bottom. Over time this accumulated sediment can clog and alter the topography of bodies of water. 

A less obvious but more damaging impact of TSS, however, is in increasing the turbidity (opacity, or cloudiness) of water. In natural environments high turbidity prevents light from reaching aquatic plants, thereby adversely affecting photosynthesis. This reduces the rate at which plants generate oxygen, cutting the amount of dissolved oxygen available to fish and other animals.

Turbidity also increases water temperature, due to light absorption, and may affect the ability of animals to locate food or breed effectively.

TSS affects the equilibrium of the natural environment, and in aquatic ecosystems it can lead to population imbalances that might include fish kills and algal blooms.

What causes TSS in stormwater?

Following rainfall, surface water either infiltrates into the soil or travels across impermeable surfaces until it evaporates, infiltrates or flows into a drainage network.

The built environment typically comprises fewer permeable surfaces than the natural environment, and as rainfall travels across surfaces such as concrete, tarmac and paving, it picks up some of the solids that are on those surfaces.

Any material that is present on roads, paths or other built environments may be carried by surface water to a drain. These suspended solids are what comprise stormwater TSS. 

How do you remove TSS from stormwater? 

Reducing the amount of surface water runoff - through infiltration of rainfall to the underlying soil or other related means - is a primary objective of stormwater management, but in a built environment engineers must assume that there will always be a level of runoff, and that that runoff will contain TSS.

As such, various techniques are available to engineers that need to capture and retain solids carried in stormwater. 

Settling

One way of preventing stormwater TSS pollution is to allow it to settle out before it reaches a natural water source.

Detention basins - dry ponds, wet ponds and constructed wetlands - are temporary or permanent artificial environments that provide residence time to allow TSS to settle out of stormwater. They may be landscaped to provide a local amenity value, and also provide opportunities for other treatment processes such as metal and nutrient removal.

In order to remain effective detention basins require regular maintenance, however, and their construction may require significant space. 

Separation

Hydrodynamic separators use a vortex to remove suspended solids from water. They are connected to a drainage network and separate out solids from water that has already entered the drainage system.

Some separators may also capture litter and other gross solids, as well as floatables including fats, oils and grease (FOG).

They are typically compact, installed in a manhole or vault, and have no moving parts, so require minimal maintenance in order to remain effective. 

Filtration

With reference to stormwater treatment, filtration is the use of a medium through which stormwater passes in order to remove various pollutants. These filtration media may rely on physical, chemical or biological processes to achieve pollutant removal.

Examples of filter media include sand, soil and chemicals such as zeolites, and some stormwater filter systems combine filtration with other processes such as separation and sedimentation.

Like separators, stormwater filter systems are compact and may be connected to the drainage network in a manhole or vault. Maintenance involves emptying sediment traps and changing filter media cartridges.