AWWA WQTC60763

The Water Authority-Cayman (WAC) supplies drinking water to the island of GrandCayman. The WAC water system consists of two reverse osmosis (RO) plants andpumping facilities at Red Gate Road capable of producing a total of 2.1 million USgallons/day. The plant is linked to three storage reservoirs with a combined storagecapacity of 3 million US gallons. A third RO plant is located at Lower Valley,approximately 10 miles east of George Town. This plant and pumping facility arecapable of producing 0.8 million US gallons/day and are linked to two storagereservoirs with a combined storage capacity of 3 million US gallons. Water is producedfrom plants via RO, by pumping saline groundwater from ~250 deep wells through aseries of prefilters to remove silt before circulation through a series of RO membranes.The feed water to both plants contains approximately 1-2 mg/L H₂S. The product wateris passed through a counter flow degasifier, is treated with acid thereby reducing thepH to 6.0 or less. Water is disinfected with 65% Ca hypochlorite, Zn orthophosphate isadded to inhibit corrosion and NaOH is added to control the pH. The first phase of thisfour-year study is designed to characterize the bacterial ecology of the WAC waterdistribution system from inlet to outlet in order to understand the relationship betweenbiofilms and planktonic organisms. A comparative media study found that R2A at 26°Cis the more appropriate media for growth of HPC although there was not a significantdifference in counts obtained from R2A and mHPC; feedwater organisms grow best onmodified mHPC media at 35°C. The HPC counts after treatment are low (<6/mL), butepifluorescence direct counts show several log greater levels of bacteria present.Several dozen colonies have been subcultured and work is underway to identify majorgroups, particularly those found throughout the system. Water temperature is animportant factor in the bacterial ecology of the WAC distribution system, with maximumtemperatures varying from 24-27°C in winter (November-April) to 28-33°C in summer(May to October). Unique to this setting is the use of plastic or PVC piping materialthroughout the system. Samples from pipe breaks are being analyzed to compare thebiofilms in the piping to bacteria in the water phase. DNA characterization of selectedisolates is underway. This study represents the first comprehensive microbiologicalcharacterization of a desal water system. Includes 14 references, tables, figures.

Product Details

Edition:

Vol. - No.

Published:

11/15/2004

Number of Pages:

13

File Size:

1 file , 13 MB