The measurement of BDOC : an index of the bacterial regrowth potential in water
Some attempts have been made to obtain an assessment of either the easily Assimilable Organic Carbon (AOC) or the total amount of Biodegradabte Dissolved Organic Carbon (BDOC) in drinking water. The first approach was developed in several methods. One of these methods consists of seeding pasteurized samples of water with pure cultures of bacteria (Pseudomonas fluorescens P17, P17 + Spirillum NOX). The growth of bacteria is monitored by a spread plate technique. AOC is expressed as equivalent amount of carbon (μg C eq. acetate/l) by using known yield coefficients of these strains in acetate or in oxalate. In the second approach, the BDOC content of water is evaluated by the reduction of Dissolved Organic Carton (DOC) in a water sample incubated up to 30 days with suspended indigenous bacteria or 5-7 days with bacteria fixed on sand-particles. This work was undertaken to determine the existing relationship between values of BDOC recorded by using fixed bacteria on sand and : - the maximum growth of a pure strain of Pseudomonas fluorescens P17. Water samples (60 ml) were pasteurized and inoculated with a subculture of Pseudomonas fluorescens (initial concentration = 500 CFU/ml). The maximum growth M 20 °C ± 1 °C was recorded by a pour plate method in PCA agar alter 3-4 days of incubation. - the maximum growth of strains P17 and NOX inoculated simultaneously in pasteurized water samples (40 ml) (500-1000 CFU/ml for each species). Each maximum growth at 20 °C ± 1 °C after seven, eight and vine days of incubation was recorded (spread plate method on R2A agar) and converted in AOC (growth constants : 4.1 106 CFU per μg C for P17, 1.2 107 CFU per μg C for NOX). - the maximum growth of a mixed indigenous population of bacteria. Water samples (500 ml) were filtered (0,2 μm filter) and reinoculated with one percent of river water. The maximum growth was recorded alter 3-4 days of incubation at 20 °C ± 1 °C by microscopic epifluorescent counts (acridin orange coloration). Comparisons were done with different types of water including river water (Seine, Oise and Marne), partially treated water (conventional treatments), fully treated water, ground water, mineral water and distilled water. The study bas demonstrated : - a pour correlation (r = 0.716; n = 28) between BDOC values and the maximum growth of Pseudomonas fluorescens. This observation can be explained by the limited spectrum of activity and the low affinity of Pseudomonas fluorescens for organic molecules. Under these conditions, the growth yield was 5.9 108 CFU per mg C (BDOC). - a better correlation (r = 0,850; n = 31) between BDOC and the maximum growth of P17 for water samples seeded simultaneously with P17 and NOX. Under these conditions, no correlation (r = 0.264 n = 31) was found between values of BDOC and maximum growth of NOX, demonstrating varying affinities of the NOX strain for ozonation by-products. In spite of this limit, the relation between BDOC and AOC values remained relatively good (r = 0.769; n = 31) with a factor of conversion of 1 mg BDOC for 90 μg C eq. acetate. - a significative correlation (r = 0.943; n = 30) between values of BDOC (fixed bacteria on sand) and the maximum growth of indigenous bacteria by direct microscopic examination epifluorescent counts. The growth yield was then 1,7 109 cells per mg of BDOC. In conclusion, the rapid estimation of BDOC by the technique using indigenous bacteria fixed on sand particles can be considered as a good predictor of the potential bacteria regrowth for different types of water.