Resistance of Legionella to disinfection in hot water distribution systems.

The efficiency of various disinfection treatments against Legionella was tested on a hot water distribution system (HWDS) pilot unit. The results demonstrated clearly that most Legionella in the networks were fixed in the biofilm at the surface of the pipe (more than 98% for each loop). Chemical treatments (continuous chlorination, hyperchlorination, hydrogen peroxide and peracetic acid mixing) commonly used for the eradication of Legionella in hot water distribution networks appeared to be inadequate for eradicating the bacteria in the biofilm. Unfortunately, the biofilm contained most of the pathogens in an HWDS whereas legislation is only restricted to the Legionella concentration in the water phase. Thermal treatment appeared to be efficient to disinfect most of the biofilm but seemed to promote the biofilm re-growth as well. It was then concluded that the best solution to prevent Legionella contamination in hot water distribution systems would be to have perfect control of the temperature in the networks (temperature > 55 degrees C at all points). Nevertheless, in many cases it is difficult to have such control, so during the time necessary to modify networks, the best solution to control Legionella proliferation appears to be to apply a treatment shock (thermal or chlorination as a function of pipe characteristics). These treatments must be followed by a continuous chlorination that is totally controlled and equipped with alarm systems. This study demonstrates that biofilm sampling devices must be installed in hot water distribution systems to anticipate Legionella contamination and correctly determine the efficiency of the treatments.

Coliform culturability in over- versus undersaturated drinking waters.

The culturability of Escherichia coli in undersaturated drinking water with respect to CaCO3 (corrosive water) or in oversaturated water (non-corrosive water) was tested in different reactors: glass flasks (batch, "non-reactive" wall); glass reactors (chemostat, "non-reactive" wall) versus a corroded cast iron Propella reactor (chemostat, "reactive" wall) and a 15-year-old distribution system pilot (chemostat, "reactive" wall with 1% corroded cast iron and 99% cement-lined cast iron). The E. coli in E. coli-spiked drinking water was not able to maintain its culturability and colonize the experimental systems. It appears from our results that the optimal pH for maintaining E. coli culturability was around 8.2 or higher. However, in reactors with a reactive wall (corroded cast iron), the decline in E. coli culturability was slower when the pH was adjusted to 7.9 or 7.7 (i.e. a reactor fed with corrosive water; pHpHs). We tentatively deduce that corrosion products coming from chemical reactions driven by corrosive waters on the pipe wall improve E. coli culturability.

Escherichia coli behavior in the presence of organic matter released by algae exposed to water treatment chemicals.

When exposed to oxidation, algae release dissolved organic matter with significant carbohydrate (52%) and biodegradable (55 to 74%) fractions. This study examined whether algal organic matter (AOM) added in drinking water can compromise water biological stability by supporting bacterial survival. Escherichia coli (1.3 x 10(5) cells ml(-1)) was inoculated in sterile dechlorinated tap water supplemented with various qualities of organic substrate, such as the organic matter coming from chlorinated algae, ozonated algae, and acetate (model molecule) to add 0.2 +/- 0.1 mg of biodegradable dissolved organic carbon (BDOC) liter(-1). Despite equivalent levels of BDOC, E. coli behavior depended on the source of the added organic matter. The addition of AOM from chlorinated algae led to an E. coli growth equivalent to that in nonsupplemented tap water; the addition of AOM from ozonated algae allowed a 4- to 12-fold increase in E. coli proliferation compared to nonsupplemented tap water. Under our experimental conditions, 0.1 mg of algal BDOC was sufficient to support E. coli growth, whereas the 0.7 mg of BDOC liter(-1) initially present in drinking water and an additional 0.2 mg of BDOC acetate liter(-1) were not sufficient. Better maintenance of E. coli cultivability was also observed when AOM was added; cultivability was even increased after addition of AOM from ozonated algae. AOM, likely to be present in treatment plants during algal blooms, and thus potentially in the treated water may compromise water biological stability.

New approaches to minimize excess sludge in activated sludge systems.

This paper presents three new approaches to reduce excess sludge production in activated sludge systems: 1) modification of conventional activated sludge process with insertion of a sludge holding tank in the sludge return line; 2) chlorination of excess sludge so as to minimize excess sludge production; and 3) utilization of a metabolic uncoupler, 3, 3', 4', 5-Tetrachlorosalicylanilide (TCS) to maximize futile activity of sludge microorganisms thereby leading to a reduction of sludge growth. Pilot study was carried out to evaluate this modified activated sludge process (OSA). It has been confirmed that the OSA process is effective in reducing excess sludge; particularly when the ORP level in the sludge holding tank was kept at -250 mV, more than 50% of the excess sludge was reduced. This process can maintain the effluent quality and even perform with a better sludge settleability than a conventional system. Experimental work on the second approach showed that chlorination treatment of excess sludge at a chlorine dose of 0.066 g Cl2/g MLSS reduced the excess sludge by 60%, while concentration of THMS was found below 200 ppb in the treated sludge. However, such sludge chlorination treatment sacrificed sludge settleability. Thus, it is not feasible to introduce the chlorination step to a conventional system. The third approach confirmed that addition of TCS could reduce sludge growth effectively if the TCS concentration is greater than 0.4 ppm. A 0.8-ppm concentration of TCS actually reduced excess sludge by 45%. It was also experimentally demonstrated that presence of TCS increases the portion of active sludge microorganisms over the entire microbial population.

Escherichia coli resistance to chlorine and glutathione synthesis in response to oxygenation and starvation.

Reduced glutathione (GSH) levels and resistance to chlorine were measured for two isogenic Escherichia coli strains stressed by oxygenation and/or starvation. The E. coli mutant deficient in GSH was not more sensitive to the oxidant than its parent strain when the bacteria were cultured with a low oxygenation rate. Starvation or oxygenation increased the resistance of the parent strain to chlorine, while the resistance of the deficient strain remained unchanged.

Influence of water chlorination on the counting of bacteria with DAPI (4',6-diamidino-2-phenylindole).

Counting bacteria in drinking water samples by the epifluorescence technique after 4',6-diamidino-2-phenylindole (DAPI) staining is complicated by the fact that bacterial fluorescence varies with exposure of the cells to sodium hypochlorite. An Escherichia coli laboratory-grown suspension treated with sodium hypochlorite (5 to 15 mg of chlorine liter-1) for 90 min was highly fluorescent after DAPI staining probably due to cell membrane permeation and better and DAPI diffusion. At chlorine concentrations greater than 25 mg liter-1, DAPI-stained bacteria had only a low fluorescence. Stronger chlorine doses altered the DNA structure, preventing the DAPI from complexing with the DNA. When calf thymus DNA was exposed to sodium hypochlorite (from 15 to 50 mg of chlorine liter-1 for 90 min), the DNA lost the ability to complex with DAPI. Exposure to monochloramine did not have a similar effect. Treatment of drinking water with sodium hypochlorite (about 0.5 mg of chlorine liter-1) caused a significant increase in the percentage of poorly fluorescent bacteria, from 5% in unchlorinated waters (40 samples), to 35 to 39% in chlorinated waters (40 samples). The presence of the poorly fluorescent bacteria could explain the underestimation of the real number of bacteria after DAPI staining. Microscopic counting of both poorly and highly fluorescent bacteria is essential under these conditions to obtain the total number of bacteria. A similar effect of chlorination on acridine orange-stained bacteria was observed in treated drinking waters. The presence of the poorly fluorescent bacteria after DAPI staining could be interpreted as a sign of dead cells.