On Thursday, Nov 8th, we visited Lokhandwala Sewage Treatment Plant, greater Mumbai Municipal Corporation; This visit was aimed to make us understand the basic process of domestic sewage treatment in Mumbai. Through my observations, I concluded that the plant's treatment processes are effective, but there is still room for improvement.

"Mr. Purushottaman and his first period fundamentals of environmental science class of 15 students visited the Lokhandwala Sewage Treatment Plant, greater Mumbai municipal corporation to understand the basic sewage treatment processes. Mr. Padmanabhan guided the group on the tour of the Versova sewage pumping station and Lokhandwala sewage treatment plant. The tour lasted 3 hours. We observed the operations at the pumping station and the treatment plant in treating the domestic sewage.

VERSOVA PUMPING STATION:

This is mere a pumping station for channeling the sewage. Except for some physical filtration by bar screens, no other treatment is given here to the sewage. The domestic sewage is collected from the nearby areas in a deep sump. The sump is at least 40 feet deep (below MSL) to insure gravity flow of sewage from the households till the sump at pumping station. The collected sewage needs to be lifted up to the mean sea level (MSL) so that it can be easily discharged into the sea after treatment. So the basic work of the pumping station is to pump up the sewage collected at 40 feet deep sump to the mean sea level. 3500 million litre of sewage is daily collected from Mumbai, which needs to be discharged to the sea after treatment. To pump this sewage there are 52 sewage pumping stations at Mumbai, one of which is at Versova. The Versova pumping station can pump 300 MLD (million liters per day) of sewage. For the purpose, the station has 6 numbers of 250 HP pumps, each with capacity of 60 MLD. Out of these 6 pumps, 5 pumps run continuously 24hrs X 365 days. The 6th pump is kept as spare. The average BOD of domestic sewage collected at Versova pumping station is 200ppm. As the collected domestic sewage have fair amount of large floating particles and large amount of grit or silt, they need to be separated out before pumping, and otherwise they can clog the machinery and damage the pumps. For the purpose first the sewage is allowed to be stagnant for some time in the chamber (so that the silt and grit gets settled down) and then bar screens is used to filter out the large suspended matters. 2 large bar screen filters could be seen at the pumping station removing the larger wastes. After the filtration the sewage is pumped to Lokhandwala sewage treatment plant.

The present cost of electricity alone for running these pumps and bar screens account around Rs 24 lakhs INR per month.

LOKHANDWALA SEWAGE TREATMENT PLANT:

The sewage from pumping stations pumped up to the sea level is brought at the Sewage Treatment Plant for the treatment so that it is good enough to be released into the sea. Physical and biological treatments are applied here to the sewage. The average BOD of the sewage after treatment is ensured to be below 30ppm before discharge into the sea.

The wastewater treatment plant at Lokhandwala can be broadly divided into two main units:

i. Physical treatment (WWTF - wastewater treatment facility)– removal of floating material and grit

ii. Biological treatment (Lagoons)– removal of organic matters

At present the design capacity of WWTF is 180 MLD and that of lagoons is 90 MLD.

Based on the hydraulic calculations, the lagoons can take a peak load of 245 MLD corresponding to average flow of about 125 MLD. Hence it is advised to operate both WWTF and lagoons for a daily average flow of 125 MLD, the rest being by-passed to the creek.

The main features of WWTF are:

Design flow (MLD) No. of Screens No. of grit channels

180 3 4

The various units of WWTF are:

a. Screen Chambers: consists of mechanical bar screens to remove floating materials.b. Aerated Grit Chambers: consists of aeration system, grit removal system, grit chamber

drainage system and telescopic & mud valves. Intense aeration is provided to separate the organic matter from the grit. Vigorous aeration will not only oxidize the organic matter but will also physically separate them from the sand.

c. Partial flume for flow measurement and velocity control.d. Splitter box for splitting the flow downstream of WWTF.e. Domestic water system – for catering the need of domestic water consumption.f. Plant water system – for washing the screens, grit chambers and other equipments.

The main features of lagoons are:

Design flow (MLD) No. of Streams No. of Cells No. of aerators

90 2 3per stream 26 per stream

The lagoons are divided into 2 equal streams each being capable of handling 45 MLD of sewage flow. Both the streams are identical.

Each stream of lagoon is divided into 3 cells viz.

a. AAL: Aerobic Aerated Lagoons with 15 aerators of 37 KW each. 65% to 75% BOD reduction. Hydraulic retention time of 1.4 days.

b. FAL – 1: Facultative Aerated Lagoons with 9 aerators of 15 KW each. Partially aerobic ponds. Hydraulic retention time of 1.8 days.

c. FAL – 2: Facultative Aerated Lagoons with 2 aerators of 15 KW each. Hydraulic retention time of 1.1 days.

After the BOD and suspended particles are reduced sufficiently, the treated sewage is discharged into the adjoining creek (Malar Bay).