Stabilité biologique des réseaux de distribution d'eau potable

The maintenance of the quality of water from the outlet of the treatment plant to the consumer tap is a major concern of water distributors. From a biological point of view, this maintenance must be characterized by a stability of biological features, namely bacterial growth from biodegradable organic matter, and protozoan bacterivory which must be not detectable. However, drinking water distribution systems are continuously exposed to a flow of biodegradable organic matter, which can represent around 20–30 % of the total dissolved organic carbon, and a flow of allochthonous microorganisms (bacteria, fungi, protozoa…), coming from the water treatment plant but also from incidents (breaks/repairs) on the distribution network itself. Apart from these microorganisms (heterotrophic bacteria in particular) can grow in this ultra-oligotrophic environment and colonize the all drinking water distribution system. The highest density of microorganisms occurs on the surface of pipewalls where they are organized in microcolonies (biofilm) that are mixed with corrosion products and inorganic precipitates. Five groups of organisms have been identified in distribution networks, in both the water phase and the biofilm: bacterial cells, protozoa, yeast, fungi and algae. The majority of these organisms are not pathogens, nevertheless potentially pathogen bacteria (Legionella…), fecal bacteria (coliforms, E. coli…), and pathogen protozoan cysts (Giardia intestinalis, Cryptosporidium parvum…) can transitorily find favorable conditions for their proliferation in the networks. Bacteria grow from the biodegradable fraction of dissolved organic matter while protozoa grow from dissolved organic matter, other protozoa but especially from bacterial prey items. The protozoan bacterivory was extensively studied in marine aquatic environments and in rivers, lakes,… but very rarely in drinking water distribution networks. Actually, proofs of the protozoan grazing on fixed and free-living bacterial cells were given by photography or film of biofilms accumulation on coupons that were previously immersed in potable water or by direct microscopic observation of bacteria in food vacuole of protozoa from potable water. A single and recent study has estimated protozoan bacterivory rate from laboratory experiences using fluorescent markers. It appears that in an experimental distribution system fed with biologically treated water (ozone/filtration through granular activated carbon), only ciliates present in the biofilm have a measurable grazing activity, estimated at 2 bacteria·ciliate⁻¹·h⁻¹ on average.Bacterial dynamics in drinking water distribution systems is complex and related to different parameters, like the biodegradable fraction of dissolved organic carbon, the presence of a residual of disinfectant, the nature and the state of pipewalls, the relative biomass of free and fixed bacterial, and grazing impact.The preservation of the biological stability of potable water during its storage in reservoir or its transport through the distribution systems can be achieved by (a) the use of chemical disinfectants (in particular by addition of chlorine) which is the widely used technique, or (b) the use of new techniques such as nanofiltration that can eliminate bacteria and significantly decrease the concentrations of organic matter at the inlet of the distribution network and in the potable water.

• (a)The use of oxidant, usually chlorine, induces a number of problems, in particular the development of oxidation by-products like trihalomethans (THM), among which some are recognized as carcinogenic products for animals. In addition, chlorine added at the outlet of treatment plant is consumed in the network and the maintenance of a residual of chlorine along an entire distribution network would need high concentrations of chlorine at the outlet of the treatment plant. This may be incompatible with standards for both residual chlorine and its by-products. Nevertheless, chlorine has a disinfectant effect on planctonic bacteria, if considering that only around 10 % of free bacterial cells are living cells, i.e. are able of respiratory oxidation. However, some studies show that bacteria fixed on granular activated carbon particles can be resistant to chlorine, as well as bacteria in aggregates. Thus, the addition of chlorine in potable water does not inhibit the formation of a biofilm at the surface of pipewalls. In the same way, protozoa transported by potable water can resist to chlorine.
• (b)The above disadvantages permitted the development of membrane filtration techniques like the nanofiltration, which is at the junction between reverse osmosis and ultrafiltration, and which seems to be an interesting alternative to conventional treatments because it presents the advantage to (i) decrease very strongly the concentrations of dissolved organic carbon (on average 90 % for DOC (Dissolved Organic Carbon) and 99 % for BDOC (Biodegradable Dissolved Organic Carbon)), (ii) to remove a very high proportion of almost the entire microorganisms (99 %), precursors of chlorination by-products, and micropollutans, (iii) to decrease the musty flavor of water (2-fold) and (iv) to produce a water that needs low concentration of chlorine.

     

A pilot study of bacteriological population changes through potable water treatment and distribution

This pilot study compares the compositions of bacterial biofilms in pipe networks supplied with water containing either high levels of biodegradable organic matter (BOM) or low levels of BOM (conventionally or biologically treated, respectively). The Microbial Identification System for fatty acid analysis was utilized in this study to identify a large number of organisms (>1,400) to determine population changes in both conventionally and biologically treated water and biofilms. Data generated during this study indicated that suspended bacteria have little impact on biofilms, and despite treatment (conventional or biological), suspended microbial populations were similar following disinfection. Prechlorination with free chlorine resulted not only in reduced plate count values but also in a dramatic shift in the composition of the bacterial population to predominately gram-positive bacteria. Chlorination of biologically treated water produced the same shifts toward gram-positive bacteria. Removal of assimilable organic carbon by the biologically active filters slowed the rate of biofilm accumulation, but biofilm levels were similar to those found in conventionally treated water within several weeks. Iron pipes stimulated the rate of biofilm development, and bacterial levels on disinfected iron pipes exceeded those for chlorinated polyvinyl chloride pipes. The study showed that the iron pipe surface dramatically influenced the composition, activity, and disinfection resistance of biofilm bacteria.     

A Pilot Study of Bacteriological Population Changes through Potable Water Treatment and Distribution

This pilot study compares the compositions of bacterial biofilms in pipe networks supplied with water containing either high levels of biodegradable organic matter (BOM) or low levels of BOM (conventionally or biologically treated, respectively). The Microbial Identification System for fatty acid analysis was utilized in this study to identify a large number of organisms (>1,400) to determine population changes in both conventionally and biologically treated water and biofilms. Data generated during this study indicated that suspended bacteria have little impact on biofilms, and despite treatment (conventional or biological), suspended microbial populations were similar following disinfection. Prechlorination with free chlorine resulted not only in reduced plate count values but also in a dramatic shift in the composition of the bacterial population to predominately gram-positive bacteria. Chlorination of biologically treated water produced the same shifts toward gram-positive bacteria. Removal of assimilable organic carbon by the biologically active filters slowed the rate of biofilm accumulation, but biofilm levels were similar to those found in conventionally treated water within several weeks. Iron pipes stimulated the rate of biofilm development, and bacterial levels on disinfected iron pipes exceeded those for chlorinated polyvinyl chloride pipes. The study showed that the iron pipe surface dramatically influenced the composition, activity, and disinfection resistance of biofilm bacteria.     

Formation of biofilms in drinking water distribution networks,a case study in two cities in Finland and Latvia

The formation of biofilms in drinking water distribution networks is a significant technical, aesthetic and hygienic problem. In this study, the effects of assimilable organic carbon, microbially available phosphorus (MAP), residual chlorine, temperature and corrosion products on the formation of biofilms were studied in two full-scale water supply systems in Finland and Latvia. Biofilm collectors consisting of polyvinyl chloride pipes were installed in several waterworks and distribution networks, which were supplied with chemically precipitated surface waters and groundwater from different sources. During a 1-year study, the biofilm density was measured by heterotrophic plate counts on R2A-agar, acridine orange direct counting and ATP-analyses. A moderate level of residual chorine decreased biofilm density, whereas an increase of MAP in water and accumulated cast iron corrosion products significantly increased biofilm density. This work confirms, in a full-scale distribution system in Finland and Latvia, our earlier in vitro finding that biofilm formation is affected by the availability of phosphorus in drinking water.     

Simple Method for Determination of Biodegradable Dissolved Organic Carbon in Water

Determination of biodegradable dissolved organic carbon in waters is of particular importance for the water treatment industry. A simple method for determining biodegradable dissolved organic carbon which is applicable to surface and drinking water is proposed. It consists of sterilizing the water sample, inoculating it with autochthonous bacteria, and measuring the decrease in dissolved organic carbon concentration due to the carbon oxidization by bacteria. The detailed experimental procedure is discussed, and validation of the method is presented. The method has been used for studying river waters and for drinking water treatment plant design.     

Biofilm formation in drinking water affected by low concentrations of phosphorus

Abstract: There are geographical regions where microbial growth in drinking waters is limited by phosphorus instead of organic carbon. In these drinking waters even a low amount of phosphorus can strongly enhance microbial growth. The formation of biofilm can be limited by low availability of phosphorus in drinking waters with low content of phosphorus. The formation of biofilms on polyvinyl chloride plates was studied in laboratory experiments with water containing 48 microg/L assimilable organic carbon and 0.19 microg/L microbially available phosphorus. We found that low additions of phosphate (1-5 microg/L PO4(3-)-P) to water increased microbial growth in the water and in the biofilm. The effect of phosphorus on microbial growth could be detected by determining either the microbial cell production or the content of ATP in biofilms. Also, in steady-state biofilms, microbial concentrations were higher with phosphorus addition as enumerated by heterotrophic plate counts on R2A-agar and acridine orange direct counting. This work confirms the earlier findings of the importance of phosphorus for microbial growth in humic-rich drinking waters.     

Impacts of the reduction of nutrient levels on bacterial water quality in distribution systems.

This study evaluated the impacts of reducing nutrient levels on bacterial water quality in drinking water. Two American Water System facilities (sites NJ102a and IN610) with histories of coliform problems were involved, and each water utility received two pilot distribution systems (annular reactors). One reactor simulated the conventional treatment conditions (control), while the other reactor was used to assess the effect of biological filtration and subsequent reduced biodegradable organic matter levels on suspended (water column) and biofilm bacterial concentrations in the distribution systems. Biodegradable organic matter levels were reduced approximately by half after biological treatment. For site NJ102a, the geometric mean of the assimilable organic carbon concentrations was 217 microg/liter in the plant effluent and 91 microg/liter after biological filtration. For both sites, plant effluent biodegradable dissolved organic carbon levels averaged 0.45 mg/liter, versus 0.19 to 0.22 mg/liter following biological treatment. Biological treatment improved the stability of free chlorine residuals, while it had little effect on chloramine consumption patterns. High bacterial levels from the biological filters resulted in higher bacterial concentrations entering the test reactors than entering the control reactors. On average, biofilms in the model systems were reduced by 1 log unit (from 1.4 x 10(5) to 1.4 x 10(4) CFU/cm(2)) and 0.5-log unit (from 2.7 x 10(5) to 7.8 x 10(4) CFU/cm(2)) by biological treatment at sites NJ102a and IN610, respectively. Interestingly, it required several months of biological treatment before there was an observable impact on bacterial water quality in the system, suggesting that the effect of the treatment change was influenced by other factors (i.e., pipe conditions or disinfection, etc.).     

利用BDPs同时作为反硝化微生物的碳源和附着载体的研究

饮用水源水中硝酸盐污染已引起世界各国的普遍关注,异养反硝化是去除水中硝酸盐的主要技术之一。利用可生物降解聚合物（BDPs）同时作为反硝化微生物的碳源和附着生长的载体,近年来得到了人们的关注。该系统对进水水质的波动具有良好的适应能力;BDPs对人体无毒无害,不会污染出水水质。随着各种新型BDPs材料的不断涌现以及BDPs材料生产成本的降低,BDPs材料在饮用水源水生物脱氮中会得到越来越广泛的应用。本文对利用可生物降解聚合物进行反硝化的研究进展进行了综合评述。     

Colonization and Disinfection of Biofilms Hosting Coliform-Colonized Carbon Fines

The documented release of carbon fines from granular activated carbon filters is a concern for drinking water utilities, since these particles may carry coliform and even pathogenic bacteria through the disinfection barrier. Such a breakthrough could have an impact on distribution system biofilms. Using total cell counts, specific monoclonal antibody staining, and computerized image analysis, we monitored the colonization of introduced Klebsiella pneumoniae associated with carbon fines in mixed-population biofilms. The particles transported the coliforms to the biofilms and allowed successful colonization. Chlorine (0.5 mg/liter) was then applied as a disinfectant. Most K. pneumoniae along with the carbon fines left the biofilm under these conditions. The impact of chlorine was greater on the coliform bacteria and carbon fines than on the general fixed bacterial population. However, 10% of the introduced coliforms and 20% of the fines remained in the biofilm. The possibility that this represents a mechanism for bacteria of public health concern to be involved in regrowth events is discussed.     

Formation of natural biofilms during chlorine dioxide and u.v. disinfection in a public drinking water distribution system

AIMS: The influence of two disinfection techniques on natural biofilm development during drinking water treatment and subsequent distribution is compared with regard to the supply of a high-quality drinking water. METHODS AND RESULTS: The growth of biofilms was studied using the biofilm device technique in a real public technical drinking water asset. Different pipe materials which are commonly used in drinking water facilities (hardened polyethylene, polyvinyl chloride, steel and copper) were used as substrates for biofilm formation. Apart from young biofilms, several months old biofilms were compared in terms of material dependence, biomass and physiological state. Vital staining of biofilms with 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) and the DNA-specific 4',6-diamidino-2-phenylindole (DAPI) staining resulted in a significant difference in physiological behaviour of biofilm populations depending on the disinfection technique. Compared with chlorine dioxide disinfection (0.12-0.16 mg l-1), the respiratory activities of the micro-organisms were increased on all materials during u.v. disinfection (u.v.254; 400 J m-2). The biofilm biocoenosis was analysed by in situ hybridization with labelled oligonucleotides specific for some subclasses of Proteobacteria. Using PCR and additional hybridization techniques, the biofilms were also tested for the presence of Legionella spp., atypical mycobacteria and enterococci. The results of the molecular-biological experiments in combination with cultivation tests showed that enterococci were able to pass the u.v. disinfection barrier and persist in biofilms of the distribution system, but not after chlorine dioxide disinfection. CONCLUSIONS: The results indicated that bacteria are able to regenerate and proliferate more effectively after u.v. irradiation at the waterworks, and chlorine dioxide disinfection appears to be more applicative to maintain a biological stable drinking water. SIGNIFICANCE AND IMPACT OF THE STUDY: As far as the application of u.v. disinfection is used for conditioning of critical water sources for drinking water, the efficiency of u.v. irradiation in natural systems should reach a high standard to avoid adverse impacts on human health.     

Effects of Carbon Source, Carbon Concentration, and Chlorination on Growth Related Parameters of Heterotrophic Biofilm Bacteria

Abstract To investigate growth of heterotrophic biofilm bacteria, a model biofilm reactor was developed to simulate a drinking water distribution system. Controlled addition of three different carbon sources (amino acids, carbohydrates, and humics) at three different concentrations (500, 1,000, and 2,000 ppb carbon) in the presence and absence of chlorine were used in separate experiments. An additional experiment was run with a 1:1:2 mixture of the above carbon sources. Biofilm and effluent total and culturable cells in addition to total and dissolved organic carbon were measured in order to estimate specific growth rates (SGRs), observed yields, population densities, and bacterial carbon production rates. Bacterial carbon production rates (μg C/L day) were extremely high in the control biofilm communities (range = 295–1,738). Both growth rate and yield decreased with increasing carbon concentrations. Therefore, biofilm growth rates were zero-order with respect to the carbon concentrations used in these experiments. There was no correlation between growth rate and carbon concentration, but there was a significant negative correlation between growth rate and biofilm cell density (r=−0.637, p= 0.001 control and r=−0.57, p= 0.021 chlorinated biofilms). Growth efficiency was highest at the lowest carbon concentration (range = 12–4.5%, amino acids and humics respectively). Doubling times ranged from 2.3–15.4 days in the control biofilms and 1–12.3 days in the chlorinated biofilms. Growth rates were significantly higher in the presence of chlorine for the carbohydrates, humics, and mixed carbon sources (p= 0.004, < 0.0005, 0.013, respectively). The concept of r/K selection theory was used to explain the results with respect to specific growth rates and yields. Humic removal by the biofilm bacteria (78% and 56% for the control and chlorinated biofilms, respectively) was higher than previously reported literature values for planktonic bacteria. A number of control experiments indicated that filtration of drinking water was as effective as chlorination in controlling bacterial biofilm growth. Received: 26 March 1999; Accepted: 3 August 1999; Online Publication: 15 February 2000     

Bacterial nutrients in drinking water.

Regrowth of coliform bacteria in distribution systems has been a problem for a number of water utilities. Efforts to solve the regrowth problem have not been totally successful. The current project, which was conducted at the New Jersey American Water Co.-Swimming River Treatment Plant, showed that the occurrence of coliform bacteria in the distribution system could be associated with rainfall, water temperatures greater than 15 degrees C, total organic carbon levels greater than 2.4 mg/liter, and assimilable organic carbon levels greater than 50 micrograms of acetate carbon equivalents per liter. A multiple linear regression model based on free chlorine residuals present in dead-end sections of the distribution system and temperature predicted 83.8% of the heterotrophic plate count bacterial variation. To limit the growth of coliform bacteria in drinking water, the study concludes that assimilable organic carbon levels should be reduced to less than 50 micrograms/liter.     

Bacterial nutrients in drinking water

Regrowth of coliform bacteria in distribution systems has been a problem for a number of water utilities. Efforts to solve the regrowth problem have not been totally successful. The current project, which was conducted at the New Jersey American Water Co.-Swimming River Treatment Plant, showed that the occurrence of coliform bacteria in the distribution system could be associated with rainfall, water temperatures greater than 15 degrees C, total organic carbon levels greater than 2.4 mg/liter, and assimilable organic carbon levels greater than 50 micrograms of acetate carbon equivalents per liter. A multiple linear regression model based on free chlorine residuals present in dead-end sections of the distribution system and temperature predicted 83.8% of the heterotrophic plate count bacterial variation. To limit the growth of coliform bacteria in drinking water, the study concludes that assimilable organic carbon levels should be reduced to less than 50 micrograms/liter.     

Bacterial community in the biofilm of granular activated carbon (GAC) PreBiofilter in bench-scale pilot plants for surface water pretreatment

Biofilters of granular activated carbon (GAC) are responsible for the removal of organic matters in drinking water treatments. PreBiofilters, which operate as the first unit in a surface water treatment train, are a cost-effective pretreatment for conventional surface water treatment and provide more consistent downstream water quality. This study investigated bacterial communities from the samples of raw surface water, biofilm on the PreBiofilter, and filtrates for surface water pretreatment. A bench-scale pilot plant of PreBiofilter was constructed to pretreat surface water from the Canoochee River, GA, USA. PreBiofilter exhibited a significant reduction of total organic carbon and dissolved organic carbon. The evenness and Shannon diversity of bacterial operational taxonomic units (OTUs) were significantly higher on the biofilm of PreBiofilter than in raw water and filtrates. Similar bacteria communities were observed in the raw water and filtrates using relative abundance of bacterial OTUs. However, the bacterial communities in the filtrates became relatively similar to those in the biofilm using presence/absence of bacterial OTUs. GAC biofilm or raw water and filtrates greatly contributed to the abundance of bacteria; whereas, bacteria sheared from colonized biofilm and entered filtrates. Evenly distributed, diverse and unique bacteria in the biofilm played an important role to remove organic matters from surface water for conventional surface water pretreatment.     

Impacts of the Reduction of Nutrient Levels on Bacterial Water Quality in Distribution Systems

This study evaluated the impacts of reducing nutrient levels on bacterial water quality in drinking water. Two American Water System facilities (sites NJ102a and IN610) with histories of coliform problems were involved, and each water utility received two pilot distribution systems (annular reactors). One reactor simulated the conventional treatment conditions (control), while the other reactor was used to assess the effect of biological filtration and subsequent reduced biodegradable organic matter levels on suspended (water column) and biofilm bacterial concentrations in the distribution systems. Biodegradable organic matter levels were reduced approximately by half after biological treatment. For site NJ102a, the geometric mean of the assimilable organic carbon concentrations was 217 μg/liter in the plant effluent and 91 μg/liter after biological filtration. For both sites, plant effluent biodegradable dissolved organic carbon levels averaged 0.45 mg/liter, versus 0.19 to 0.22 mg/liter following biological treatment. Biological treatment improved the stability of free chlorine residuals, while it had little effect on chloramine consumption patterns. High bacterial levels from the biological filters resulted in higher bacterial concentrations entering the test reactors than entering the control reactors. On average, biofilms in the model systems were reduced by 1 log unit (from 1.4 × 10⁵ to 1.4 × 10⁴ CFU/cm²) and 0.5-log unit (from 2.7 × 10⁵ to 7.8 × 10⁴ CFU/cm²) by biological treatment at sites NJ102a and IN610, respectively. Interestingly, it required several months of biological treatment before there was an observable impact on bacterial water quality in the system, suggesting that the effect of the treatment change was influenced by other factors (i.e., pipe conditions or disinfection, etc.).     

Assessment of biological activity and fate of organic compounds in a reactor for the measurement of biodegradable organic carbon in water

A new, rapid method for the determination of biodegradable dissolved organic carbon (BDOC), especially suited to water industry needs, was recently proposed by the authors. This dynamic method measured the BDOC of circulating water continuously pumped over a biofilm attached to a special support (sinterized porous glass) that fills a system of two glass columns. The BDOC value corresponds to the difference in dissolved organic carbon (DOC) between inflow and outflow water samples. The analytical results are not significantly different from those of other bioassays that use indigenous bacteria, and the total duration of the analysis is less than 3 h. However, a problem common to all the BDOC methods based on attached bacteria is the extent to which the decrease in DOC during the BDOC analysis is due to true biodegradation or to adsorption of organic matter to the reactor. In the present study, a reasonable support is provided for the hypothesis that this decrease, at least in the dynamic method, is predominantly due to microbiological activity. After comparing the support (sinterized porous glass) with a good physical adsorbent (granular activated carbon), the influence of temperature, residual chlorine and sodium azide on the reactor performance was tested, and a sensitivity only attributable to biological activity was observed. Another set of experiments were performed to assess the fate and specific elimination of different organic substances, explicable assuming that biodegradation processes were involved.     

Warming alters coupled carbon and nutrient cycles in experimental streams

Although much effort has been devoted to quantifying how warming alters carbon cycling across diverse ecosystems, less is known about how these changes are linked to the cycling of bioavailable nitrogen and phosphorus. In freshwater ecosystems, benthic biofilms (i.e. thin films of algae, bacteria, fungi, and detrital matter) act as biogeochemical hotspots by controlling important fluxes of energy and material. Understanding how biofilms respond to warming is thus critical for predicting responses of coupled elemental cycles in freshwater systems. We developed biofilm communities in experimental streamside channels along a gradient of mean water temperatures (7.5–23.6 °C), while closely maintaining natural diel and seasonal temperature variation with a common water and propagule source. Both structural (i.e. biomass, stoichiometry, assemblage structure) and functional (i.e. metabolism, N₂‐fixation, nutrient uptake) attributes of biofilms were measured on multiple dates to link changes in carbon flow explicitly to the dynamics of nitrogen and phosphorus. Temperature had strong positive effects on biofilm biomass (2.8‐ to 24‐fold variation) and net ecosystem productivity (44‐ to 317‐fold variation), despite extremely low concentrations of limiting dissolved nitrogen. Temperature had surprisingly minimal effects on biofilm stoichiometry: carbon:nitrogen (C:N) ratios were temperature‐invariant, while carbon:phosphorus (C:P) ratios declined slightly with increasing temperature. Biofilm communities were dominated by cyanobacteria at all temperatures (>91% of total biovolume) and N₂‐fixation rates increased up to 120‐fold between the coldest and warmest treatments. Although ammonium‐N uptake increased with temperature (2.8‐ to 6.8‐fold variation), the much higher N₂‐fixation rates supplied the majority of N to the ecosystem at higher temperatures. Our results demonstrate that temperature can alter how carbon is cycled and coupled to nitrogen and phosphorus. The uncoupling of C fixation from dissolved inorganic nitrogen supply produced large unexpected changes in biofilm development, elemental cycling, and likely downstream exports of nutrients and organic matter.     

Potential for biofilm development in drinking water distribution systems

Regrowth of micro-organisms in drinking water distribution systems is caused by the utilisation of biodegradable compounds which are either present in treated water or originate from materials in contact with drinking water. In the Netherlands most drinking water is distributed without disinfectant residual and regrowth is limited by achieving biostable drinking water. A combination of methods is used to assess the biostability of drinking water. These methods are: (1) determination of the concentration of easily assimilable organic carbon (AOC); and (2) assessment of the biofilm formation rate (BFR). Assimilated organic carbon concentrations in drinking water in the Netherlands range from a few μg C/l in slow sand filtrates and in ground water supplies to values of ～ 50 μg C/l in supplies using ozonation in water treatment. Biofilm formation rate values were found to range from < 1 pg ATP/cm(2)/d in supplies using anaerobic ground water as the source. Increase of heterotrophic plate counts is limited at AOC values below 10 μg C/l. At BFR values below 10 pg ATP/cm(2)/d the risk of exceeding the guideline value for aeromonads (90 percentile < 200 c.f.u./100 ml) is less than 20%. Calculations based on the decrease of the AOC concentration observed in distributions systems confirm that very low concentrations of AOC can cause considerable biofilm formation on the pipe wall. The methods for assessing the biostability of drinking water combine with the assessment of the Biofilm Formation Potential of materials in contact with drinking water, thus providing a framework, the Unified Biofilm Approach, for evaluating the biostability of drinking water and materials.     

The effects of metabolite molecules produced by drinking water-isolated bacteria on their single and multispecies biofilms

The elucidation of the mechanisms by which diverse species survive and interact in drinking water (DW) biofilm communities may allow the identification of new biofilm control strategies. The purpose of the present study was to investigate the effects of metabolite molecules produced by bacteria isolated from DW on biofilm formation. Six opportunistic bacteria, viz. Acinetobacter calcoaceticus, Burkholderia cepacia, Methylobacterium sp., Mycobacterium mucogenicum, Sphingomonas capsulata and Staphylococcus sp. isolated from a drinking water distribution systems (DWDS) were used to form single and multispecies biofilms in the presence and absence of crude cell-free supernatants produced by the partner bacteria. Biofilms were characterized in terms of mass and metabolic activity. Additionally, several physiological aspects regulating interspecies interactions (sessile growth rates, antimicrobial activity of cell-free supernatants, and production of iron chelators) were studied to identify bacterial species with biocontrol potential in DWDS. Biofilms of Methylobacterium sp. had the highest growth rate and M. mucogenicum biofilms the lowest. Only B. cepacia was able to produce extracellular iron-chelating molecules. A. calcoaceticus, B. cepacia, Methylobacterium sp. and M. mucogenicum biofilms were strongly inhibited by crude cell-free supernatants from the other bacteria. The crude cell-free supernatants of M. mucogenicum and S. capsulata demonstrated a high potential for inhibiting the growth of counterpart biofilms. Multispecies biofilm formation was strongly inhibited in the absence of A. calcoaceticus. Only crude cell-free supernatants produced by B. cepacia and A. calcoaceticus had no inhibitory effects on multispecies biofilm formation, while metabolite molecules of M. mucogenicum showed the most significant biocontrol potential.     

Effects of different nutrients on bacterial growth in a pilot distribution system

The growth of bacterial communities in drinking water distribution systems can lead to the development of problems incompatible with water quality requirements. This study was carried out in order to determine which factors promote bacterial growth in distribution networks. A pilot distribution system was used to perform these experiments. After addition of three different inorganic elements to the network (N, P and S) the results obtained show that they did not contribute to the growth or bacteria either in the circulating water or on the surface of the pipes of the distribution system. However, when organic carbon was added an increase in the number of circulating bacteria was observed though the number of bacteria in the biofilms of the network was constant. These results indicate that in the drinking water of the Barcelona distribution system the factor that controls the growth of bacteria is organic carbon. Moreover, bacteria from the biofilm growing on the surface of the pipes may cause the problems of bacterial growth.