Sanitary Sewers & Storm Water Drainage: A Difference Citizens Must Understand

You’ve seen one drain, you’ve seen them all. They are all the same, right? Not so! It’s important to understand the difference between sanitary sewers and storm sewers so we can prevent environmental damage.

The sanitary sewer is a system of underground pipes that carries sewage/wastewater (also called “Badd rav” in local language) from bathrooms, sinks, kitchens, and other plumbing components to a wastewater treatment plant where it is filtered, treated and discharged.

Wastewater, after leaving our home’s plumbing system, is carried through a network of underground pipes that lead to a wastewater treatment plant where it goes through a series of treatment processes.

Once the wastewater meets standard effluent regulations in the treatment plant it is then released back into the environment’s waterways or reused to replenish amenity ponds or irrigate landscapes.

The storm sewer is a system designed to carry rainfall runoff also called storm-water discharge (“roode waen” in local language) and other drainage. It is not designed to carry sewage or accept hazardous wastes.

The runoff is carried through underground pipes or open surface drains and discharges into local streams, rivers and other surface water bodies without any treatment. Storm drain inlets are typically found in curbs and low-lying outdoor areas. Disposal of chemicals or hazardous substances to the storm sewer system damages the environment especially water bodies.

Motor oil, cleaners, paints and other common household items that get into storm drains can poison fish, birds, and other aquatic life, and can find their way into drinking water supplies too.

Primarily there are two types of conveyance Systems-Combined Sewer system and separate sewer systems. Combined systems as the name suggests carry some or all of the stormwater and sewage mixed together, while a separate system of sewage conveyance is designed to carry stormwater and sewage while always keeping both apart. Even if the collection principles are different, the operational standards remain the same. Meeting the wastewater and storm water challenge is a transformative act, designed to improve people’s living conditions and foster cities’ resilience against pollution and waterlogging.

In our city although the sewerage system exists in most of the areas but these sewers are designed to carry peak wastewater discharge safely and not storm water discharge. Though huge emphasis is given to construct sewerage systems, we seem to be lagging way behind on the drainage front. This is the main reason behind the instantaneous water-logging and local flood like situations that often occur in many parts of the city during high intensity rainfall.

Over the years increase in urbanization and construction activities in our cities and towns of valley have resulted in more impermeable surfaces and less open soil cover and thus during precipitation less infiltration occurs resulting in a considerable increase of the surface run-off.  The lack of effective storm water drainage system thus causes severe local inundation during rainfall as the runoff does not find any path to travel except the existing sewer system which by design is incapacitated to carry such peak flow.

The peak stormwater discharge is always much greater in quantity than sewage and in such a typical situation during rainfall what happens is that these sewers though designed for wastewater are coerced to run full and carry huge quantity of flow which makes our existing conveyance system neither a combined system nor a separate one but something betwixt. In such instances where exceptionally high surface runoff occurs (such as large rainstorms), the load on individual tributary branches of the sewer system also cause a back-up to a point where raw sewage flows out of input sources such a toilet, causing inhabited buildings to be flooded with a toxic sewage-runoff mixture, incurring massive financial burdens for cleanup and repair which is often seen in downtown area of the city.

When sewer systems experience these higher than normal throughputs, relief systems cause discharges containing human and industrial waste to flow into rivers, streams, or other water bodies.

During such situations another frightful thing that happens is that the combined flow of sewage and stormwater exceeds the capacity of The sewage treatment plants (STPs) or effluent treatment plants(ETPs) leaving them overloaded beyond their highest capacity as they are designed to handle such sewage flow only.

Consequently, what happens is that an Operator shuts down the plant and directly discharges the incoming combined flow to the outfall channel or stream thereby bypassing all the treatment/cleaning processes. Combined flow thus floods waterways with contaminants including microbial pathogens, suspended solids, chemicals, trash, and nutrients that deplete dissolved oxygen which can therefore lead to contamination of drinking water supplies too. 

On the contrary also, in areas where there are some dedicated storm water drains, the people inadvertently discharge household sewage directly in storm water drains which is detrimental to the water body in which sewage is disposed.

This abominable practice does happen in many areas throughout Kashmir valley especially in places close to river banks and water bodies as most of the households lack proper sewage disposal systems or Septic tanks. These are perhaps the grey areas where huge work is needed to be done as there must be some kind of proper sewage disposal system. Therefore, it should always be kept in mind that both the above mentioned situations don’t arise at all.

Although we can design combined sewers too but mixing sewage and storm water will also tantamount to constructing STPs/ETPs designed to handle very high discharge (both wastewater and stormwater) which shall be unwise and uneconomical to build and will also involve huge maintenance and operating cost for pumping as well as treating huge quantity of mixed sewage and storm water which otherwise is not required.

Also it would require us to provide a large diameter of sewers and it would become practically arduous to achieve minimum self-cleansing velocities during normal dry weather flow thus causing silt deposition. Ideally thus, storm water and wastewater should travel very different paths and not blend together despite both may end up in the same place.

Water quality in urban streams is highly influenced by emissions from Wastewater Treatment Plants and from sewer systems particularly by overflows from combined systems. During storm events, this causes random fluctuations in discharge and pollutant concentrations over a wide range.   

For the reasons above, countries across the world are shifting from a combined sewer system to the separate one. We should too start a paradigm shift in our strategy and plan a separate stormwater drainage system in our cities and towns. One such endeavor, although there could be more, is being done by the Urban Environmental Engineering department at Charar-e-shareef town where a parallel system of sewers is being laid together thereby separating Sewage from storm water at source which should definitely set a precedent for all our future sewerage and drainage projects. It is better to opt for such a construction practice wherever a new sewer network is proposed to be constructed as it results in a considerable cost savings compared to constructing separate drainage network at a future stage.

Also in places where sewerage systems exist we should prioritize the construction of a proper drainage system for such catchments after calculating the exact discharge from precipitation data available or using any other acceptable methods for the whole catchment. Perhaps to deal with our flood and waterlogging problem it would be desirable to chalk out a comprehensive master plan suitable for drainage of the entire catchment.

Furthermore, towards the end, on a different albeit a germane point, it is appropriate to mention that with an increasing percentage of paved surfaces, the hydrological response of urban catchments is becoming more rapid, peak flows are increasing, and more pollutants are transported with the runoff.

This peak flow frequently results in swelling of water bodies in a very short span of time causing a potential flood threat which often arises in our case. Different techniques like retention basins, infiltration basins rain water harvesting etc. can be supplemented with drainage systems for better storm water management as to store part of storm water in order to release it at a later stage to decrease peak flow during rains which is a challenging issue that must be deliberated upon by the hydrological experts of the pristine valley. 

In a context of accrued sustainability, transparency, environmental protection and related regulations, sewer and drainage networks must be considered an integral part of the full water cycle. Till we keep behaving oblivious to above facts we shall never achieve our goal of abating pollution in water bodies and mitigating waterlogging regardless how many Treatment plants we may construct.

In conclusion, what we glean from above discussion is:

· that we must understand the need to separate storm water from Sewage

· that we must shift towards separate sewer system

· that we must give equal importance to stormwater disposal when compared to sewage disposal.

·That we should never allow the occurrence of the situation that would overload the Effluent treatment plants

·that in order to preserve our water bodies there is an immense need to educate the masses about proper disposal of sewage and storm water so that people would understand the difference and exercise extreme caution before disposing off the same to prevent both from blending together. Perhaps a slogan like “badd rav alag roode waen alag” or anything similar as the authorities may deem fit should be made viral through social media and other mass media platforms so that people get an idea of the grave ramifications of this infelicitous practice.

Author is a Civil Engineer.