Caroni WTP

The contract for the process, mechanical and ICA works associated with the upgrade of the Caroni Water Treatment Plant, Trinidad, was awarded to Naston in December 1998. The ?2.75 million contract was based on a "fast-track" approach to provide a new 77 Ml/d stream to increase output of the plant from 295 to 378 Ml/d, for the new millennium.

Raw Water Intake
Raw water is drawn from the Caroni river and pumped to the existing treatment plant and to the adjacent new stream. Eight centrifugal, vertical turbine pumps were installed, each capable of delivering 730 l/s and driven by 190 kW motors. Installation was carried out in planned phases to allow continued supply to the existing plant.

Mixing, Flocculation, and Sedimentation
Occasional raw water turbidity in excess of 1000 NTU and colour values in excess of 500 Hazen were established parameters. The water is dosed with alum, lime (when required) and polyelectrolyte, mixed by a hydraulic jump mixer before passing to the flocculation stage. Six clarification units were provided, including primary and secondary flocculation using hydraulic baffles. Rectangular horizontal flow sedimentation tanks were constructed with automatic travelling bridge scrapers for sludge removal.

Filtration
Six rapid gravity sand filters were provided, designed for semi-automatic operation from individual control consoles. Washwater pumps and air blowers were installed to provide combined air / water washing. The control system incorporates "slow start" and "run to waste" facilities to prevent cryptosporidium breakthrough.

Treated Water
Filtered water is disinfected using Chlorine gas and passes through twin, one-hour contact tanks before undergoing pH correction with lime. Three centrifugal vertical turbine pumps were installed, each capable of delivering 450 l/s and driven by 450 kW, 4160V motors. A full surge analysis was carried out on the 40 km treated water supply network to ensure that the existing facilities provided adequate protection.

Chemical Dosing
Existing alum, lime and polyelectrolyte systems were refurbished and uprated to serve the existing plant and new stream. A new 40 tonne capacity Chlorine drum store was built to house automatic evaporators and chlorinators using remote vacuum injection. The system was designed in accordance with HS(G)40 and includes Chlorine gas leak detection complete with emergency automatic shutdown capability.