Date of visit: 29/07/2024
Members visited: Deeksha, Ayushi, Apeksha, Nikita, Neelima, Avinash, Krishna, Taabish, Trisha, Amulya, Ruchitha and Ruthu.
As Bachelors of Planning students interning at Biome, we recently had the opportunity to visit one of the Sewage Treatment Plant (STP) in Bengaluru on July 29, 2024, which has a treatment capacity of 40 million liters per day (MLD). The visit provided us with valuable insights into the complex and essential process of treating sewage. Upon our arrival at the plant, we were welcomed by a knowledgeable staff member who guided us through the treatment process, focusing on the activated sludge process (ASP). This process is crucial for removing contaminants from the sewage and making the water safe for discharge or reuse.
What is an STP?
As per CPHEEO manual, 80% of water supply may be expected to reach the sewers. Sewage treatment plant (STP) treats sewage or wastewater generated from domestic, industrial, or commercial sources to remove contaminants and produce treated effluent that is safe for discharge or reuse.
Three stages of sewage treatment are:
Sewage treatment plant technologies:
Activated Sludge Process (ASP)
Sequential Batch Reactor (SBR)
Membrane Bio Reactor (MBR)
Moving Bed Bio Reactor (MBBR) / Fluidized Aerobic Bed rector (FAB)
(Source: KSPCB)
Treatment Process at STP
Pumping and Screening: The journey of sewage treatment begins with the raw sewage being pumped from the canal to the inlet chamber. The inlet chamber, equipped with three screen channels with the size of 25mm each, removes large debris such as wood, rocks, plastics, papers, and even dead animals. This step is vital to prevent damage to downstream equipment and ensure smooth operation.
Grit Removal and Organic Matter Separation: After screening, the sewage passes through a grit classifier, where sand and other fine particles are removed. In the grit classifier, organic matter is effectively removed using a centrifugal pump. This pump helps separate heavier particles and organic matter from the wastewater.
Conveyor plate: The next stage involves the removal of oil and grease-like substances using a conveyor plate. This marks the end of the primary treatment phase, where solid particles are effectively removed.
Anaerobic Zone: In the anaerobic zone, sewage sludge undergoes methane fermentation, facilitating the decomposition of macromolecular organic matter into simpler compounds. This biological process also helps in the removal of phosphorus content without the need for chemical additives.
Phosphorus is accumulated in the sludge and subsequently removed through sedimentation. The organisms responsible for this process, known as polyphosphate-accumulating organisms (PAOs), store phosphorus by building up reserves of polyphosphate.
PAOs break down internally stored polyphosphate to generate energy for the uptake of volatile fatty acids (VFAs) like acetate. This process results in the release of orthophosphate (PO₄³⁻) into the surrounding water.
Anoxic Zone: The anoxic zone plays a crucial role in the biological removal of nitrogen contents. The anoxic tank facilitates the process of denitrification, where nitrates (NO3) are reduced to nitrites (NO2) and potentially further reduced to nitrogen gas (N2), reducing the nitrogen content in the wastewater.
Denitrification is a process where nitrates (NO₃⁻) are reduced to gaseous nitrogen (N₂) by facultative anaerobes, such as certain fungi. These organisms thrive in anoxic conditions by breaking down oxygen-containing compounds like nitrates to obtain oxygen. In aquatic environments, nitrogen is present in multiple forms, including dissolved nitrogen gas (N₂), ammonia (NH₄⁺ and NH₃), nitrite (NO₂⁻), nitrate (NO₃⁻), and organic nitrogen, which can exist as proteinaceous matter or in dissolved or particulate phases.
6 NO3- + 5 CH3OH → 5 CO2 + 3 N2 + 7 H2O + 6 OH-
Aeration Basin: The sewage then moves to the aeration basin, where oxygen is added to support the growth of aerobic bacteria. These bacteria further break down organic matter. A mixed liquor recycle pump recirculates a portion of the sewage back to the anoxic zone to enhance the denitrification process. After aeration, both Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) are significantly reduced. This process enhances the degradation of organic pollutants, leading to clearer and cleaner effluent.
The BOD value is the amount of oxygen, dissolved in water, that is consumed by aerobic bacteria that degrade a certain amount of organic material in the wastewater.
The COD value indicates the amount of oxygen required to chemically oxidize the totality of organic matter in the wastewater.
An extended aeration process is used, where wastewater remains in the aeration tank for an extended period, typically 24 hours. This allows for more complete degradation of organic material, further reducing both BOD and COD levels.
As in the activated sludge treatment process, microorganisms, including bacteria, are used to degrade and consume the organic matter present in wastewater. Here the wastewater is mixed with a combination of these microorganisms, referred to as "mixed liquor."
As the microorganisms feed on the organic pollutants in the wastewater, they metabolize this organic matter, breaking it down into simpler substances like carbon dioxide (CO₂) and water (H₂O). During this process, the microorganisms require and consume dissolved oxygen, which is supplied by aerating the tank. The aeration ensures that there is enough oxygen available for the microorganisms to thrive and efficiently degrade the organic waste
Standards to maintained as per CPCB
Secondary Classifier: The remaining mixture proceeds to the secondary classifier, where sludge and water are separated. The secondary classifier is crucial as it helps in the removal of fine particles and residual solids from the wastewater after primary and secondary treatments. This step ensures the quality of treated water before further processes or discharge.
Chlorination: The final step in the treatment process involves disinfection. The treated water, now referred to as effluent, is passed through a chlorine contact tank, where chlorine is added to kill any remaining bacteria. The chlorine is neutralized, if necessary, before the water is discharged to the minor irrigation department for further use.
The water, after undergoing secondary treatment, is tested for various parameters such as pH, COD, BOD, and E. coli etc. as per CPCB standards and are tested before being sent to Minor irrigation department. The data represents the monthly average of the recorded values.
LEARNINGS FROM THE VISIT
We gained a deeper understanding of the Activated Sludge Process, which is a core treatment method used in the plant and various stages involved in the process i.e., screening, removal, anaerobic, anoxic and aerobic conditions, secondary classifier and chlorination.
Difference between the anaerobic and anoxic zones with their respective functions. Even though both the processes take place in absence of oxygen, the nutrient removed in each process is different. Anaerobic zone plays a vital role in removing phosphorus content from wastewater while anoxic zone removes all the nitrogen contents. Contributing to pollution reduction.
Knowledge on extended aeration process, as the process in aeration zone takes place for 24 hours and significance of reducing BOD and COD in wastewater treatment while maintaining standards as per CPCB.
Continuous monitoring, regular testing, and thorough data analysis are crucial for optimizing the treatment process and maintaining high standards of quality control. These practices help to identify and address any issues promptly, ensuring that the treatment plant operates efficiently and does not lead to any negative impacts on the environment or public health.
CONCLUSION
The field visit to the Sewage Treatment Plant was an enlightening experience. It underscored the complexity and importance of modern sewage treatment processes. We gained a deeper understanding of the necessity of investing in and maintaining robust water infrastructure. Furthermore, the visit highlighted the need for continuous research and innovation to enhance the efficiency and effectiveness of water treatment systems. This experience has deepened our appreciation for the critical role that such facilities play in sewage treatment infrastructure, safeguarding public health and the environment.
APPENDIX A: Detailed reactions
In the first step of nitrification, ammonia is oxidized to nitrite by ammonia-oxidizing bacteria according to the following equation:
NH3 + O2 → NO2– + 3H++ 2e–
In the second step, nitrite is further oxidized to nitrate by nitrite-oxidizing bacteria according to the following equation:
O2– + H2O → NO3– + 2H+ +2e–
The energy reactions involved in this process can be illustrated as follows:
6 NO3- + 2 CH3OH → 6 NO2- + 2 CO2 + 4 H2O (Step 1)
6 NO2- + 3 CH3OH → 3 N2 + 3 CO2 + 3 H2O +6 OH- (Step 2)
Overall,
6 NO3- + 5 CH3OH → 5 CO2 + 3 N2 + 7 H2O + 6 OH-
APPENDIX B: Consolidated analysis for month june-2024 at STP
RAW SEWAGE ANALYSIS
TREATED WATER ANALYSIS
POWER AND CHEMICAL CONSUMPTION
Written by:
Ruchitha Singh B
Amulya S
Ruthu M
