Thursday, 4 August, 2011



Regular readers of the adda articles will no doubt recall an earlier piece in which the importance of Proper Design of a treatment plant was highlighted as a crucial aspect for proper functioning of the plant.  Within this aspect was an innocuous sounding reference to “Environmental factors”.  How critical this can be in making or breaking a treatment plant is best illustrated by the following story from Asia’s Largest WTP at Bhandup, Bombay.

In the early 1980’s, the Bombay Municipal Corporation ( BMC)  was again the happy recipient of aid from the World Bank ( those were the days when India was a certified underdeveloped nation) for implementing a safe and secure water supply system to the burgeoning megaloplolis. 

Binnie & Partners, a respectable engineering consulting house based out of the UK were the  advisors for the project. Tata Consulting Engineers, an equally well respected outfit was their local sidekick.

The Table Top of a mini hillock in Bhandup, a suburb of Bombay was the chosen location for the Huge Water Treatment Complex :  by all counts, the site was ideal for the many advantages it offered.  It was close to Vihar Lake, the source of water for the WTP.  Water could flow by gravity from the lake to the WTP.  And, from the WTP, situated at a fairly good elevation ( I estimate around 100 metres above MSL)  supply of water to the teeming masses of Bombay could also be effected without expending much energy.  Dorr- Oliver (India) was chosen to supply the huge clarifiers required for the plant.  Hindustan Construction Company ( HCC- of Lavasa fame) were the civil contractors for the tankages.  I was biding time as a sub- junior engineer in Dorr-Oliver.

Now friends, don’t even think you can imagine the size and extent of the clarifiers, if your only exposure has been to the clarifiers in your basement/ backyard STP !  There were Twenty clarifiers, each 40 m x 40 m square, in all occupying an area little over 4 Hectares !.  A one time perambulation around these units could well qualify as a good Morning’s walk for us sedentary city folks.

Binnie & Partners, clever fellows, clearly foresaw the pitfalls in clarifying water with very little suspended solids and turbidity coming out of a passive source such as Vihar lake ( Contrast this with a source like a flowing river in spate ).  Therefore, the clarifiers were of a very special design, deliberately incorporating a sludge recirculation system within the clarifier, just to provide sufficient seed sludge mass for the scant amount of incoming solids to cling on to and settle down. Excellent design, by all means.  And the Dorr- Oliver “Pretreator”©, a solids contact clarifier  was the right piece of equipment for this duty.  The clarification stage in the Treatment Plant was followed by the “Aquazur” gravity sand filters of Degremont, France : another celebrity piece of technology in its own right.

Alas, the venerable Binnie & Partners had overlooked the all important “ Environmental conditions” , including Altitude and the surroundings of the site in their design.

The problem reared its ugly head right from the initial commissioning trials.  Cloudballs of light solids would regularly burst out of the clarifier at regular intervals during early morning and noon hours, to slowly subside later on in the day.  For months Binnie, TCE and the Technology team from Dorr-Oliver racked their combined brains to fathom this baffling phenomenon.  Without a  doubt, it was an amazing visual treat to behold these spectacular cloud formations in those huge clarifiers, which nevertheless left all and sundry red faced, and nowhere to hide on the open hill top of Bhandup.

Therein, lies the clue.

The sub-junior marketing engineer from Dorr-Oliver, with a fresh PhD Degree was summoned to the site, to see if he could put his newly learned knowledge to some practical use.  I was a regular visitor to the plant on subsequent days. Watching the clouds in the clarifier tank, fascinated, totally clueless, and having nothing better to do, trying to compare the patterns in the clouds below to those in the sky above.

When all at once I was struck by a bolt of lightning ! Eureka ! !

Now, if you have not already seen the hilltop site of the WTP, I ask you to visit the map again.

The Treatment plant is at an elevation, on a hillock, open to sky, totally isolated from any structures around, or back radiating bodies.  So, at nighttimes, the top layers of water in the clarifier would rapidly suffer drop in Temperature by radiating out energy to the ink black night sky. ( You have heard the story – one could freeze water kept in a shallow pan under a night sky in the desert).  So, in the morning hours, the colder, heavier water layers from the top of the clarifier tend to move down, and the incoming water, at slightly higher temperatures would move up to take the space recently vacated by the heavier masses.  The solid flocs, being light enough, were being carried up along with the warmer flowstreams.  That in a nutshell was what was happening.

For once, I eschewed my avowed reluctance to fall back upon hardcore scientific theory and complex equations : In a scholarly article of nearly 14 pages, using theory of Continuity, energy transport, convective transport, radiation theory, theory of turbulence, buoyancy theory etc., I could prove with sound theoretical, empirical and heuristic arguments that  buoyancy forces pushing up the floc clouds by these Temperature inversions,  are comparable to the inertial forces pulling the flocs down.

Once the cause was understood and acknowledged, the solution was simple enough : We simply weighted down the light flocs with a heavier seed material to aid the inertial forces in their fight against buoyancy.  Inertial forces won the war.  The seed material used was Bentonite clay.

Dr. Ananth S Kodavasal                                                                          June 10, 2011

P.S. : Binnie & Partners and TCE still owe me portion of their consultancy fee, for having saved their bacon in such a high profile, prestigious project of mammoth proportions.


  1. Sir,
    what a fantastic read! As an aspiring civil engineer, it's stories like these that make me want to work harder to achieve my goals.

    I plan on visiting the treatment plant this week for a seminar report of mine titled "Water distribution systems" with the mumbai WDS as a case study. Please don't mind if i highlight this anecdote of yours in my presentation :).

    Once again, i'm grateful for this post. Cheers!

  2. it is indeed a treat to read this kind of issues arising in infrastructure projects. I have heard many stories before but few which are documented like the one you have documented here...