Stormwater can contain a lot of pollution from the debris and dirt that has collected on the built-up surfaces on which rain falls before it finds its way to any stormwater management system. It is necessary to remove these suspended solids before they pollute the natural watercourses to which the stormwater is ultimately led.
The two simplest processes used for suspended solids removal are sedimentation and filtration. The process of sedimentation is very efficient in the removal of suspended solids and this process can take place in any volume of stormwater that is detained for some time in ponds, or other devices like chambers. The efficiency of removal of suspended matter will depend on the characteristics of the water, the depth to which it is retained, the period for which it is detained and other factors like particle size, design of outlets and inlets, temperature, overflow rate and others.
Suspended solids removal through filtration can be achieved by running the stormwater runoff through gravel beds that work as effective filters that hold back the solids allowing only the water to pass through. The same filtering can be achieved by the installation of screens of small size that act to prevent the suspended solids from moving further. The simple installation of grills on the inlets that take the water into drainage systems can work effectively to remove suspended solids. A major problem with this method is the requirement of constant maintenance to see that the grills remain relatively free and are cleaned of any solids that have accumulated.
A well-designed stormwater management system will combine all of these methods so that the least solids go through to the water that is finally discharged from it. Ideally, this system must start with initial filtering at the inlets, detention of water to create the sediments of suspended solids, and then a filtration through gravel or other media before the water is finally allowed to go into the natural watercourses.
This system should be fairly effective in removing suspended solids and help reduce pollution of groundwater. This system will not help to remove any dissolved solids for which more sophisticated systems are necessary. For any installed system to remain effective, there needs to be a proper schedule of maintenance that will ensure that every element of the stormwater management system will continue to function as it is meant to.
Detention ponds, retention ponds and stormwater chambers will require to be periodically desilted to remove all the accumulated sediments. Filter media may need to be washed or replaced whenever it becomes less effective. The maintenance aspect must be given a lot of importance in the design to ensure that it is relatively easy to perform.
The majority of suspended solids in stormwater come from runoff and erosion, caused by the speed of the flowing water. Attention to this, and controlling this so that the water does not attain a scouring velocity can also help in reducing the number of suspended solids that find their way into a stormwater management system.