AWWA WQTC64006

The presence of hydrogen sulfide in drinking water is undesirable for many reasons, notthe least of which is the numerous customer complaints about taste and odor. Forceddraft aerators have been shown to effectively remove this compound from groundwater inFlorida. However, the environment produced in FDAs (large surface area, highlyoxygenated water) is conducive to extensive microbial growth, particularly in the warmFlorida climate. This results in a complex biofilm developing rapidly, and regular andthorough cleaning is required to minimize build up of biofilm which sloughs off and canresult in water with high turbidity. Prior to installation of the FDAs the mean turbiditywas 0.4 NTU with a maximum of 1.0 NTU but following installation the mean turbidityrose to 2.1 NTU with a maximum value of 9.3 NTU. Microscopic examination of thebiofilm showed a complex community of organisms including protozoa, cyanobacteria,amoeba, bacteria and rotifers.Use of denaturing gradient gel electrophoresis (DGGE) showed that bacteria of the genusNitrosomonas formed a substantial component of the bacterial flora. Microbiologicalexamination showed that coliform organisms were also prevalent within the biofilm andin the water leaving the FDAs. These were often Enterobacter cloacae and this specieswas frequently found in the distribution system fed from the treatment plant. Moredetailed examination of a number of strains of these E.cloacae strains showed that thosefound in the FDA effluent were clonal in nature, indicating that they were indeedgrowing in the FDAs. Comparison of the phenotypic characteristics of the E.cloacaestrains found in the FDA effluent and in the distribution system indicated that theyappeared to have the same origin. Examination of the microbiological data for thetreatment plant effluent showed that coliforms were seldom detected by routine samplingwhich was carried out daily. A study was therefore undertaken to determine if coliformorganisms were surviving disinfection. Samples were collected after chlorination (targetresidual disinfectant was 0.8 mg/L and contact time was a minimum of six hours) and100mL and 2L samples were examined. Of the 104 samples of 100mL, none were foundto contain coliforms while 15 of 104 of the 2L samples contained coliforms. Presumablythese organisms were protected from the free chlorine by pieces of biofilm matrixcontained in the FDA effluent. Assuming that a single organism was responsible for thepositive result in the 2L samples, and that approximately 15% of the water containedcoliforms then calculations indicated that as many as 2.8 x 106 coliforms could beentering the distribution system through the treatment plant. A strain of E.cloacaepresent in the water of the distribution system (which other work suggested was biofilm-derived)had identical phenotypic characteristics to some of the organisms found in theFDA effluent. Thus it was concluded that coliform organisms were survivingdisinfection, passing into the distribution system and establishing themselves in biofilms.An aggressive FDA cleaning program was implemented which resulted in a significantimprovement in the turbidities recorded for the FDA effluent with a mean value ofapproximately 0.6 NTU. Particle analysis of the water suggested that biofilm continuedto be sloughed off into the FDA effluent. Further microbiological work was undertakento determine the levels of coliform organisms present in the effluent from the FDAs.Samples of FDA effluent were collected and analyzed over a two month period and werepositive for total coliforms on every occasion. Coliform densities were generally in the10-100 cfu/mL range but on one occasion the concentration was greater than 2400cfu/mL. The continuing presence of coliforms in the effluent of the FDAs was cause forconcern although the frequency of detection of coliforms in the disinfected water wasmarkedly lower than before the cleaning regimen had be

Product Details

Edition:

Vol. - No.

Published:

11/01/2006

Number of Pages:

2

File Size:

1 file , 310 KB