Boosting supply for Bangalore
A £230M OECF funded scheme will deliver an additional 270Ml/d of treated water to Bangalore in southern India. Robert Hyde of Mott MacDonald reviews the process design of the new WTW required to meet this demand.
As a further part of the process design programme, the operation and performance of the existing WTW at TK Halli was reviewed. Each of the existing stages (I to III) comprises:
- pre-chlorination (1mg/l);
- coagulant (alum) addition;
- flash mixing;
- flocculation and clarification using clariflocculators;
- rapid sand filtration; and
- disinfection (1mg/l chlorine).
Locally produced block alum contains 20 to 25% insoluble filter sand-size material. Alum is made up as a 5% stock solution and is fed directly by gravity from the stock tanks via a v-notch constant-head doser to the point of application. The poor quality of the alum makes it difficult to control the flowrate and prevent blocking of the holes of the pipe distributing the alum across the width of the raw water inlet channel.
Proposed new works
Stage IV of the Cauvery River water supply scheme is to deliver 270Ml/d of treated water to Bangalore. The philosophy behind the process design was that:
- the new works would be capable of producing the appropriate quantity of water of a quality to meet the standards for drinking water set by the Indian Central Public Health and Environmental Engineering Organisation;
- the process design and control of the works should be compatible with local conditions. Bangalore is one of the world's major software centres and India has high-tech capabilities (having recently put a satellite into space). However, difficulties are experienced with consistent dosing of chemicals at TK Halli.
The process stages proposed for the new works, based on the on-site investigations and assessment of raw water quality, are similar to those adopted for the existing treatment plant at TK Halli; i.e. aeration, pre-chlorination, coagulation with alum, clarification, rapid gravity filtration and final chlorination. A design and build contract was awarded in April 1999 with construction due for completion in April 2001.
The aerator will be a three-tier waterfall type of similar design to the aerators serving Stage III (see above). Flat bottom floc blanket clarifiers or clariflocculators were proposed for the clarification stage. However, after tender evaluation and negotiations, Pulsator clarifiers were accepted as an alternative. The clarifiers will be desludged automatically using a timer-based system with facilities for manual override.
The rapid gravity filters will be declining rate sand filters with combined air scour and water wash for more effective backwashing and to minimise the volume of backwash water. Backwash control will be provided through a PLC allowing fully automated backwashing in addition to manual operation of each stage or manual initiation of the process.
Vacuum chlorinators will be used for pre- and post-chlorination in preference to the chlorine absorption towers installed for Stage III. Pre-chlorination has been proposed to assist with algae removal as required. The chlorine dose at the inlet will be controlled in proportion to the inlet flow. The disinfection dose will be controlled in proportion to the outlet flow and trimmed manually in order to maintain a set point final chlorine residual value in the treated water leaving the works.
After considerable discussion it was apparent that alum would continue to be supplied from the same source with little prospect that the quality of supply would be improved. With this in mind it was decided that the alum solution would be prepared in stock tanks and the solution would be transferred to a dosing tank, leaving the insoluble material in a sump in the stock tank for disposal prior to make up of the next batch. Alum make up will be carried out as a manual operation.
A channel will be provided to bypass the clarifiers at times of low raw water turbidity, up to 80% of the time. This will enable the works to be operated at a lower coagulant dose and with a smaller discharge to waste. Sludge from the clarifiers will be treated by polyelectrolyte addition with thickening in picket-fence type continuous thickeners followed by polyelectrolyte addition and dewatering using centrifuges.