Stormwater and its Treatment
1.1 Stormwater and its benefits
Water originating from precipitation and snow melts is called stormwater. Stormwater can either runoff and join the streams, rivers or water bodies or evaporate from the surface or it can recharge the groundwater via infiltration. Easy to capture and store, it can be a very effective alternative to the mains drinking water and can be utilised for various purposes.
1.2 Why do we need to treat Stormwater?
The best management practices (BMPs) for stormwater, include treatment of this stormwater before it can either be consumed od discharged into the waterways. Stormwater treatment is one of the key BMPs for stormwater.
Therefore, based on the topography, area and the purpose, the stormwater must go through a range of treatment processes in the treatment train before it is declared fit for consumption or discharge.
While Agricultural areas may have a high nutrient content, the industrial areas can usually have high levels of heavy metal concentration.
Both these cases require a tertiary filter in combination with a primary filter, which can remove the unwanted litter, debris and gross pollutants, before they can enter the tertiary stormwater treatment system. Similarly, in urban setups with impermeable surface, the stormwater might have high amounts of litter and gross pollutants which can end up polluting our water bodies, if we do not install primary treatment devices like gross pollutant traps.
1.3 Stormwater Treatment Train
Stormwater treatment requires the stormwater to go through various phases of treatment or various step. A treatment train essentially acts a train or a combination of various treatment processes to achieve maximum removal of pollutants and hence this combination is also known as treatment train.
For effective treatment of stormwater, it must go through the following 3 stages of stormwater treatment train:
- Primary Treatment
- Secondary Treatment
- Tertiary Treatment
1.3.1 Primary Treatment
Primary Treatment aims at the removal of coarse sediments and gross pollutants. Litter like cigarette buts, plastic bags, straws, small pieces of plastic etc usually end up in our oceans. There have been claims by EPA in USA that about 70% of the land pollutants ultimately entering the water bodies and thereby polluting them.
This litter and gross pollutants are usually non-biodegradable and can be fatal for the marine life. An increase in the number of wildlife deaths due to these pollutants has been reported. Installing Gross pollutants, that are simple yet highly effective (removal of Total Suspended Solids) are a must to ensure the water bodies are clean and habitable for the aquatic wildlife, thereby maintaining the marine ecosystem.
1.3.2 Secondary Treatment
Secondary Treatment is required for the removal of oils and grease from stormwater. Stormwater sewers close to carparks can have a high amount of oils from these cars. According to EPA California, even a single quart of oil can contaminate up to a quarter million gallons of drinking water. Further it can cause an oil slick which can span almost 2 acres in size.
The oil does not dissolve in water, forming a thick sludge. This can have a major impact on the marine life and can be fatal for a variety of aquatic species. Therefore, secondary treatment devices are required to ensure that the oils can be prevented from entering our water bodies. With effective designs a 95% removal of oils from stormwater can be achieved by utilising these Secondary Treatment Devices.
1.3.3 Tertiary Treatment
Tertiary Treatment of stormwater is vital for the removal of nutrients like phosphorus and nitrogen as well as heavy metals like copper, lead etc. A high amount of nutrients like nitrogen and phosphorus can cause eutrophication in the water bodies, which can directly impact the aquatic wildlife. Whereas an increase in the heavy metals from a desired level can make the water toxic and can indirectly cause various life-threatening diseases as well. Minamata disease was the result of an increase in the levels of mercury in the water. While the mercury rich water was not consumed directly, however the locals consumed fishes from the contaminated water body, which turned out to be fatal.
Tertiary treatment devices utilise chemical methods of adsorption, absorption, mineralisation, oxidation or biological transformations. Chemical adsorption is the most commonly used method for tertiary treatment of stormwater, and it utilises zeolites, coal filters or filters that can adsorb the nutrients of heavy metals on to the surfaces. These devices must have an effective removal of these nutrients as well as the heavy metals as set by the local councils of the governing environmental body.
Are all stages of Treatment Train required?
For effective treatment of stormwater and to ensure it is fit for consumption by humans, the stormwater must go through all the three stages of stormwater treatment train. However, there are certain scenarios where a single stage or a combination of two stages of the treatment train are sufficient enough.
For instance, in an urban setup, with impermeable surfaces and negligible levels of heavy metal concentration, nutrient concentration and oils, the pollutants would mainly be litter and coarse sediments.
If the stormwater in this scenario needs to be discharges into water bodies, a simple gross pollutant trap is sufficient.
However, in agricultural lands the stormwater that needs to be discharged into the water bodies would also require tertiary treatment along with primary treatment, since it will be rich in nutrients and if discharged merely after primary treatment, it could lead to Eutrophication and ultimately impact the delicate balance of the aquatic ecosystem.
Therefore, as we know “Horses for courses”, different scenarios can require, either a single treatment process or a combination of these processes in the treatment train.
Therefore, stormwater can be a valuable resource, provided we use the best management practices, treat the stormwater effectively and remove the pollutants to the desired limits. To achieve this effective removal of pollutants from stormwater, it is essential to direct the stormwater through all the necessary treatment phases.