CHENNAI: The rising population of Chennai may have resulted in increased pressure on the sewage network of the city as Metro Water is planning to renovate and improve the existing network.
Catering to a population of 86 lakh, which is still growing, the city’s sewage treatment plants have a capacity to treat 732 MLD of sewage, provided the sewage network undergoes renovation and improvement. “The existing infrastructure is inadequate and the rapid expansion of the city requires immediate rehabilitation of sewer system in Chennai city to avoid entry of untreated sewage into the waterways,” says a Metro Water engineer.
Surprisingly, Metro Water does not have adequate figures of total sewage generated by the city as some parts of the city come under the jurisdiction of urban local bodies and some under the newly added areas where the sewage network is being laid. Sources in Chennai Metro Water say that it could be more than 250 MLD of sewage if the schemes are completed and implemented by giving connections to the households in the newly added areas.
Meanwhile, the existing sewage network is under duress during the rains. The sewage treatment plants have to treat sewage that exceeds their capacity. “We have to treat 735 MLD of sewage which exceeds our capacity. On normal days only 492 MLD of sewage is treated,” says the Metro water official.
The sewage which is generated by the city undergoes several processes before being disposed. “The sewage generated in various zones is sent to the sewage pumping station. This is then sent to upper delivery channel from where it is sent to inlet chamber and from there to screen chamber where
physical separation of floating and suspended organic and inorganic materials takes place,” says a Metro water engineer. From there on the city’s sewage undergoes several process including treating the solid as well as liquid waste before disposing the treated water into the water channels.
The sewage is treated as per Tamil Nadu Pollution Control Board norms and discharged safely. “We monitor all parameters before letting it into the waterways,” says a Metro Water engineer. Not all sewage is treated and disposed safely through the pumping station. The growth in population as well as new residential buildings has resulted in huge number of outfalls that pump the sewage directly into the water bodies. This has posed a huge problem during the DMK regime as the Cooum clean-up project was done without giving a thought to plug outfalls.
Currently, Chennai Metro Water has identified 337 sewage outfalls in Cooum River basin, Buckingham Canal and Adyar Basin. “Though the erstwhile Chennai city is fully covered with sewerage system, a portion of the untreated sewage finds it way to the waterways due to illegal connections,” says the engineer.
The state government has put forth an action plan to prevent flow of untreated sewage into waterways and also to plug all 337 sewage outfalls. The first phase is currently being implemented. Chennai Metro Water has already plugged 51 outfalls and it will be plugging a few more.
The second phase of the project has already been sanctioned at a cost of Rs 163 crore.
How sewage is treated
1. Inlet chamber
Raw sewage is pumped into here
2. Screen chamber
Suspended organic and inorganic materials are separated
3. Grit chamber
Coarse particles like sand, ash and clinkers, egg shells, bone clips and inert materials are removed.
4. Primary clarifier
Sedimentation of settleable solids takes place. Primary clarifier also reduces the organics load on secondary treatment units.
5. Aeration tank
Sewage containing waste organic matter is aerated and micro-organisms metabolise the soluble and suspended organic matter.
6. Secondary settling tank
Separation of biological sludge takes place. This ensures the final effluent quality and also the return of adequate sludge to maintain the MLSS level in the aeration tank.
7. The treated water from secondary clarifier us collected and treated in a water collection tank, and is then pumped for final disposal.
8. The excess sludge from secondary clarifier and sludge from primary clarifier is pumped to sludge thickener for thickening of sludge.
9. The thickened sludge from sludge thickener is pumped to sludge digester for anaerobic sludge digestion.
10. The digester sludge is taken to centrifuge or sludge drying bed for sludge dewatering.
11. The filtrate from dewatered sludge is collected in a sump and pumped back in inlet chamber