Sewage Treatment Plant
National Environmental Engineering Research Institute, Nagpur, has designed the Sewage Treatment Plant at Kandla. In view of limited land area at Kandla, better effluent quality and process control, the design of the plant is based on treatment by lagoon type aeration tank working on Extended Aeration principle.
The plant is divided into several units, where the sewage is proposed to be treated in preliminary treatment units like Screen Chamber, Grit Chamber and Grease Trap for the removal of floatable particles, gritty matters and Oil/Grease before it passed on to the biological treatment units.
(A) Dry/Wet Well (Screen Chamber)
This is the unit where raw sewage is collected. The unit is provided with screen bars inclined at 45o to horizontal. All floatable particles are removed by means of the screen. Further, the unit is provided with three numbers of 15 HP pumps for alternate use. The raw sewage is then pumped to distribution chamber from where gravity flow condition is achieved throughout the process.
(B) Distribution Chamber
The unit is provided to facilitate proper grit removing in the grit chamber by channelizing the flow into two equal halves.
(C) Grit Chamber
The grit withdrawal mechanism of the unit consists of two grit pumps carried on an electrically operated traveling bridge. Each grit pump serves two grit channels by means of twin suction legs of 50mm dia. The travel velocity of the bridge shall not exceed 0.04 mtr/sec. In order to remove the grit properly.
(D) Grease Chamber
As Oil/Grease tends to form a film on liquid surfaces, the outlet of the grease chamber is provided at the bottom of the tank. While, Oil/Grease is removed through the outlet provided at surface of the tank to the near by sump as and when required. The bottom outlet feeds the biological unit i.e. Aeration Tank.
(E) Aeration Tank
Sludge settled out of sewage which has been previously
aerated is known as “Activated Sludge”. In this process certain amount of activated sludge is mixed with the incoming settled sewage in the aeration tank to form what is called “Mixed Liquor” and then aerated. Aeration serves two-fold purpose. It supplies Oxygen by means of mechanical aerators for Bio-Oxidation and keeps the active flock containing bacterial mass moving so that they come in contact with organic substrate which are adsorbed and used as food for bacterial growth and multiplication. Thus, the dead organic matter present in the sewage is converted into living organic matter in the bacterial cell. In order to maintain mixed liquor in optimal condition, the re-circulation flow to the aeration tank should be adjusted properly to maintain the desired level of 4000 mg/L MLSS (Mixed Liquor Suspended Solids) in the aeration tank. This condition is recommended by NEERI in order to maintain food-to- microorganism ratio, envisaging maximum BOD removal. In the process owing to the bacterial action and continuous agitation, the non-settleable particulates having repulsive forces in between are forced to come closer to form a flock. The density of the floc formed is relatively very high to that of individual particle. This tends to quick settling of the sludge in the settling tank.
(E) Hopper Bottom Settling Tank
The inlet pipe is provided at 2/3rd of the depth of settling tank for proper distribution of the flow. The supernatant from the tank is collected in rectangular channel discharging into the effluent tank. And, the settled sludge is discharged hydrostatically to the re-circulation sump.
