Regrowth In Distribution Systems Research Articles

The Minus Approach Can Redefine the Standard of Practice of Drinking Water Treatment

Chlorine-based disinfection for drinking water treatment (DWT) was one of the 20th century's great public health achievements, as it substantially reduced the risk of acute microbial waterborne disease. However, today's chlorinated drinking water is not unambiguously safe; trace levels of regulated and unregulated disinfection byproducts (DBPs), and other known, unknown, and emerging contaminants (KUECs), present chronic risks that make them essential removal targets. Because conventional chemical-based DWT processes do little to remove DBPs or KUECs, alternative approaches are needed to minimize risks by removing DBP precursors and KUECs that are ubiquitous in water supplies. We present the "Minus Approach" as a toolbox of practices and technologies to mitigate KUECs and DBPs without compromising microbiological safety. The Minus Approach reduces problem-causing chemical addition treatment (i.e., the conventional "Plus Approach") by producing biologically stable water containing pathogens at levels having negligible human health risk and substantially lower concentrations of KUECs and DBPs. Aside from ozonation, the Minus Approach avoids primary chemical-based coagulants, disinfectants, and advanced oxidation processes. The Minus Approach focuses on bank filtration, biofiltration, adsorption, and membranes to biologically and physically remove DBP precursors, KUECs, and pathogens; consequently, water purveyors can use ultraviolet light at key locations in conjunction with smaller dosages of secondary chemical disinfectants to minimize microbial regrowth in distribution systems. We describe how the Minus Approach contrasts with the conventional Plus Approach, integrates with artificial intelligence, and can ultimately improve the sustainability performance of water treatment. Finally, we consider barriers to adoption of the Minus Approach.

The role of metal oxides on oxidant decay and disinfection byproduct formation in drinking waters: Relevance to distribution systems

Maintaining a residual disinfectant/oxidant (e.g., chlorine and chlorine dioxide), is a generally used strategy to control microbial contaminants and bacterial regrowth in distribution systems. Secondarily oxidant, such as hypobromous acid (HOBr), can be formed during chlorination of bromide-containing waters. The decay of oxidants and formation of disinfection byproducts (DBPs) due to the interaction between oxidants and selected metal oxides were studied. Selected metal oxides generally enhanced the decay of these halogen-containing oxidants via three pathways: (1) catalytic disproportionation to yield an oxidized form of halogen (i.e., halate) and reduced form (halide for chlorine and bromine or chlorite for chlorine dioxide), (2) oxygen formation, and (3) oxidation of a metal in a reduced form (e.g., cuprous oxide) to a higher oxidation state. Cupric oxide (CuO) and nickel oxide (NiO) showed significantly strong abilities for the first pathway, and oxygen formation was a side reaction. Cuprous oxide can react with oxidants via the third pathway, while goethite was not involved in these reactions. The ability of CuO on catalytic disproportionation of HOBr remained stable up to four cycles. In chlorination process, bromate formation tends to be important (exceeding 10 µg/L) when initial bromide concentration is above 400 µg/L in the presence of dissolved organic matter. Increasing initial bromide concentrations increased the formation of DBPs and calculated cytotoxicity, and the maximum was observed at pH 8.6 during chlorination process. Therefore, the possible disinfectant loss and DBP formation should be carefully considered in drinking water distribution systems.

Aeromonas species from non-chlorinated distribution systems and their competitive planktonic growth in drinking water.

Aeromonas is included in the Dutch Drinking Water Decree as an indicator for elevated microbial regrowth in non-chlorinated drinking water distribution systems (DWDS). The temporal and spatial diversity of Aeromonas species in ten DWDS and their planktonic growth characteristics for different carbon sources was investigated. Genotyping of the gyrB gene of isolates showed a non-systematic temporal and spatial variable prevalence of seven different Aeromonas species in these DWDS and no correlation with AOC-P17/NOX and Aeromonas concentrations. Pure cultures of these seven species showed a high affinity to low concentrations (μg/L) of individual amino acids and fatty acids, compounds associated with biomass. Growth occurred at 0.5 μg-C/L of an amino acid mixture. Growth of a mixed community of A. rivuli, A. salmonicida, A. sobria and A. veronii in drinking water occurred in pasteurized samples, however, no growth and decay occurred in competition with the autochthonous bacteria (non-pasteurized samples). This community also failed to grow in non-pasteurized distribution samples from a location with clear increase in planktonic Aeromonas concentrations in the transported drinking water. For competitive planktonic growth of Aeromonas an amino acid concentration of ≥5 μg-C/L is required. AOC-P17/NOX concentrations showed that such concentrations are not expected in Dutch drinking water. Therefore, we suspect that competitive planktonic growth is not the major cause of the observed non-compliance with the Aeromonas standard in non-chlorinated DWSD.Importance The occurrence of the bacterial genus Aeromonas in non-chlorinated drinking water in the Netherlands is regarded as an indication for elevated microbial regrowth in the distribution system. Identification of the prevalent species in ten distribution systems by genotyping yielded seven different species, with A. rivuli, A. veronii and A. sobria as the most dominant ones. Planktonic growth experiments of pure cultures confirmed former published affinity of Aeromonas for certain biomass compounds (amino and fatty acids). In competition with the autochthonous microflora, however, planktonic growth was not observed, only after addition of a threshold amino acid concentration of 5 μg-C/L. Based on our results and further observations we deduced that planktonic growth of Aeromonas in the DWDS is not very likely. Benthic growth in loose deposits and planktonic release is a more plausible explanation for the observed planktonic increase of Aeromonas.

Microbial Ecology and Water Chemistry Impact Regrowth of Opportunistic Pathogens in Full-Scale Reclaimed Water Distribution Systems

Need for global water security has spurred growing interest in wastewater reuse to offset demand for municipal water. While reclaimed (i.e., nonpotable) microbial water quality regulations target fecal indicator bacteria, opportunistic pathogens (OPs), which are subject to regrowth in distribution systems and spread via aerosol inhalation and other noningestion routes, may be more relevant. This study compares the occurrences of five OP gene markers ( Acanthamoeba spp., Legionella spp., Mycobacterium spp., Naegleria fowleri, Pseudomonas aeruginosa) in reclaimed versus potable water distribution systems and characterizes factors potentially contributing to their regrowth. Samples were collected over four sampling events at the point of compliance for water exiting treatment plants and at five points of use at four U.S. utilities bearing both reclaimed and potable water distribution systems. Reclaimed water systems harbored unique water chemistry (e.g., elevated nutrients), microbial community composition, and OP occurrence patterns compared to potable systems examined here and reported in the literature. Legionella spp. genes, Mycobacterium spp. genes, and total bacteria, represented by 16S rRNA genes, were more abundant in reclaimed than potable water distribution system samples ( p ≤ 0.0001). This work suggests that further consideration should be given to managing reclaimed water distribution systems with respect to nonpotable exposures to OPs.

Slowly biodegradable organic compounds impact the biostability of non-chlorinated drinking water produced from surface water

It is possible to distribute drinking water without a disinfectant residual when the treated water is biologically stable. The objective of this study was to determine the impact of easily and slowly biodegradable compounds on the biostability of the drinking water at three full-scale production plants which use the same surface water, and on the regrowth conditions in the related distribution systems. Easily biodegradable compounds in the drinking water were determined with AOC-P17/Nox during 2012–2015. Slowly biodegradable organic compounds measured as particulate and/or high-molecular organic carbon (PHMOC), were monitored at the inlet and after the different treatment stages of the three treatments during the same period. The results show that PHMOC (300–470 μg C L−1) was approximately 10% of the TOC in the surface water and was removed to 50–100 μg C L−1. The PHMOC in the water consisted of 40–60% of carbohydrates and 10% of proteins. A significant and strong positive correlation was observed for PHMOC concentrations and two recently introduced bioassay methods for slowly biodegradable compounds (AOC-A3 and biomass production potential, BPC14). Moreover, these three parameters in the biological active carbon effluent (BACF) of the three plants showed a positive correlation with regrowth in the drinking water distribution system, which was assessed with Aeromonas, heterotrophic plate counts, coliforms and large invertebrates. In contrast, the AOC-P17/Nox concentrations did not correlate with these regrowth parameters. We therefore conclude that slowly biodegradable compounds in the treated water from these treatment plants seem to have a greater impact on regrowth in the distribution system than easily biodegradable compounds.

Biodegradability of DBP precursors after drinking water ozonation

Ozonation is known to generate biodegradable organic matter, which is typically reduced by biological filtration to avoid bacterial regrowth in distribution systems. Post-chlorination generates halogenated disinfection byproducts (DBPs) but little is known about the biodegradability of their precursors. This study determined the effect of ozonation and biofiltration conditions, specifically ozone exposure and empty bed contact time (EBCT), on the control of DBP formation potentials in drinking water. Ozone exposure was varied through addition of H2O2 during ozonation at 1 mgO3/mgDOC followed by biological filtration using either activated carbon (BAC) or anthracite. Ozonation led to a 10% decrease in dissolved organic carbon (DOC), without further improvement from H2O2 addition. Raising H2O2 concentrations from 0 to 2 mmol/mmolO3 resulted in increased DBP formation potentials during post-chlorination of the ozonated water (target Cl2 residual after 24 h = 1–2 mg/L) as follows: 4 trihalomethanes (THM4, 37%), 8 haloacetic acids (HAA8, 44%), chloral hydrate (CH, 107%), 2 haloketones (HK2, 97%), 4 haloacetonitriles (HAN4, 33%), trichloroacetamide (TCAM, 43%), and adsorbable organic halogen (AOX, 27%), but a decrease in the concentrations of 2 trihalonitromethanes (THNM2, 43%). Coupling ozonation with biofiltration prior to chlorination effectively lowered the formation potentials of all DBPs including CH, HK2, and THNM2, all of which increased after ozonation. The dynamics of DBP formation potentials during BAC filtration at different EBCTs followed first-order reaction kinetics. Minimum steady-state concentrations were attained at an EBCT of about 10–20 min, depending on the DBP species. The rate of reduction in DBP formation potentials varied among individual species before reaching their minimum concentrations. CH, HK2, and THNM2 had the highest rate constants of between 0.5 and 0.6 min−1 followed by HAN4 (0.4 min−1), THM4 (0.3 min−1), HAA8 (0.2 min−1), and AOX (0.1 min−1). At an EBCT of 15 min, the reduction in formation potential for most DBPs was less than 50% but was higher than 70% for CH, HK2, and THNM2. The formation of bromine-containing DBPs increased with increasing EBCT, most likely due to an increase in Br−/DOC ratio. Overall, this study demonstrated that the combination of ozonation and biofiltration is an effective approach to mitigate DBP formation during drinking water treatment.

A survey of indicator parameters to monitor regrowth in unchlorinated drinking water

The objective of our study was to explore microbiological parameters that are suitable as indicators for regrowth in distribution systems that receive unchlorinated drinking water in the Netherlands.

Efficiency of removal of biogenic substances from water in the process of biofiltration

The studies of changes in the level of biological stability of water throughout the process of biofiltration were conducted in real conditions for surface and infiltration water. In both cases, the most effective removal of organic substances was ascertained for the biodegradable fraction, whereby the majority of water samples after this process were characterized by the concentration of biodegradable dissolved organic carbon lower than the concentration limit in terms of the growth of micro-organisms. In the majority of water samples from both filtrates, successful removal of phosphate ions also ensured limiting the threat of micro-organism regrowth in distribution systems. Unfortunately, the concentration of inorganic nitrogen was not lower than the concentration limit in terms of biological stability in any of the filtrate samples. The most important factor affecting the efficiency of the removal of nutrient substrates from water was their content in the water prior to undergoing filtration.

Fluorescence spectroscopy as a tool for determination of organic matter removal efficiency at water treatment works

Abstract. Organic matter (OM) in drinking water treatment is a common impediment responsible for increased coagulant and disinfectant dosages, formation of carcinogenic disinfection-by products, and microbial re-growth in distribution system. The inherent heterogeneity of OM implies the utilization of advanced analytical techniques for its characterization and assessment of removal efficiency. Here, the application of simple fluorescence excitation-emission technique to OM characterization in drinking water treatment is presented. The fluorescence data of raw and clarified water was obtained from 16 drinking water treatment works. The reduction in fulvic-like fluorescence was found to significantly correlate with OM removal measured with total organic carbon (TOC). Fluorescence properties, fulvic- and tryptophan-like regions, were found to discriminate OM fractions of different removal efficiencies. The results obtained in the study show that fluorescence spectroscopy provides a rapid and accurate characterization and quantification of OM fractions and indication of their treatability in conventional water treatment.

Biostability analysis for drinking water distribution systems

The ability to limit regrowth in drinking water is referred to as biological stability and depends on the concentration of disinfectant residual and on the concentration of substrate required for the growth of microorganisms. The biostability curve, based on this fundamental concept of biological stability, is a graphical approach to study the two competing effects that determine bacterial regrowth in a distribution system: inactivation due to the presence of a disinfectant, and growth due to the presence of a substrate. Biostability curves are a practical, system specific approach for addressing the problem of bacterial regrowth in distribution systems. This paper presents a standardized algorithm for generating biostability curves and this will enable water utilities to incorporate this approach for their site-specific needs. Using data from pilot scale studies, it was found that this algorithm was applicable to control regrowth of HPC in chlorinated systems where AOC is the growth limiting substrate, and growth of AOB in chloraminated systems, where ammonia is the growth limiting substrate.

Organic matter transport in New York City drinking-water-supply watersheds

Organic matter (OM) in streams that provide drinking water is a potential energy source for bacterial regrowth in distribution systems and a precursor for disinfection byproducts. Baseflow concentrations of OM were measured over a 3-y period in 60 streams divided evenly between water-supply regions east and west of the Hudson River (EOH or WOH) in New York State. A baseline of OM concentrations in the 2 regions was generated, and land use/cover variables were related to those baseline concentrations. Dissolved organic C (DOC), biodegradable DOC (BDOC), and particulate OM (POM) reflected regional differences in land use and point-source discharges. Three-year mean concentrations for these variables and for total organic C (TOC) were significantly lower in the WOH than in the EOH by factors of 1.5 to 2.3. Size fractionation of POM showed similarities between regions with >70% of particles in the 0.5- to 10-μm size class. DOC made up most of the TOC in both regions, and DOC:POC ratios ranged from 1.7 to 54.4. Stepwise multiple linear regressions revealed that agriculture and forest land uses explained most of the variation in OM concentrations in the WOH, whereas wetlands and point-source discharges, primarily associated with wastewater treatment plants, explained most of the variation in OM concentrations in the EOH. Despite the potential problems from OM for drinking water quality, OM is a natural and important component of stream ecosystems, so its total elimination from watersheds is neither advisable nor possible. Our data from watersheds in the WOH region with high percentages (>97%) of forested land use and from small to mid-sized watersheds in the EOH with no point-source discharges provide lower limits for OM concentrations and targets for best management practices.

Evaluating the impacts of treatment modifications on regrowth potential in distribution systems: a new screening procedure using water quality modeling

The treated water Biodegradable Organic Matter (BOM) content is one of the main factors controlling bacterial regrowth in Distribution Systems (DS). This paper presents an approach to predicting regrowth potential with the objective of helping utilities in the preliminary selection of alternative treatment scenarios. The proposed approach comprises four phases: (i) applying, at bench scale, different types of treatment to a raw water to obtain treated waters to be tested for their regrowth potential; (ii) applying an experimental procedure to the various treated waters which consists of the study of bacterial growth during a Biodegradable Dissolved Organic Carbon (BDOC) test; (iii) partitioning BDOC into three classes of various biodegradability compounds by fitting the bacterial growth curve with a biodegradation model; (iv) using the BDOC and partitioning data to simulate bacterial growth in a DS with a model describing bacterial dynamics in the DS. The various phases of the approach are detailed using a case study for which different types of treatments were tested.

Uncertainty Analysis in a Mechanistic Model of Bacterial Regrowth in Distribution Systems

The first generation of mechanistic models of bacterial regrowth in distribution systems (DS) provides insight into cause-and-effect relationships. However, the state of knowledge about the processes included in these models is insufficient to warrant deterministic predictions. Even if the process descriptions are reasonable, the uncertainty in values of key system constants limits predictions of bacterial growth. A new mechanistic model was developed to incorporate the accepted knowledge of physical, chemical, and biological processes with the hydraulic features in order to capture the unsteady state behavior of the DS. Sensitivitytesting showed that the extent of bacterial regrowth was affected mainly by the rate constants for chlorine decay reactions in bulk water and on the pipe wall and by the maximum growth rate constant of attached bacteria. A simple hypothetical network was used to evaluate the effects of uncertainty in these three system constants by running 100 Monte Carlo simulations. Cumulative probability plots showed a wide range of predictions for concentrations of bacteria and chlorine in bulk water at various nodes in the DS. The magnitude of these concentrations and the range of values were greatly affected by water residence time to each node. Once the chlorine residual is depleted, bacterial growth is mainly influenced by the amount of substrate available. However, high values of the coefficients for the maximum growth rate of attached bacteria, the chlorine decay in bulk water, and the chlorine decay by wall reaction did not necessarily lead to the maximum bacterial growth at a given location.

Bacterial Regrowth Model for Water Distribution Systems Incorporating Alternating Split-Operator Solution Technique

This paper presents a new mathematical model of bacterial regrowth in distribution systems, which combines hydraulic calculations, inclusive of dispersion, with a description of the following microbial processes: free and attached growth, detachment, endogenous respiration, and inactivation by chlorine. The microbial process description has been simplified from previous models based on a sensitivity analysis. The alternating split-operator algorithm is used to solve this model. This method differs from previous approaches by decoupling the transport and reaction processes, allowing a choice among numerical algorithms that is best suited for each part of the model. The proposed solution resolves sharp fronts of a concentration profile more accurately than traditional finite difference methods. The results of the model are also compared against EPANET to show the importance of accounting for dispersion, as would occur during low water demand conditions when velocity is low. Use of the model to understand the interaction among key parameters affecting bacterial regrowth is illustrated for a simple hypothetical network.

Comparison of bacterial regrowth in distribution systems using free chlorine and chloramine: a statistical study of causative factors

Bacterial regrowth was investigated over a 15-month period in distribution systems (DSs) of Durham and Raleigh in North Carolina. These two water utilities were chosen because they are adjacent to one another, have similar service area characteristics, and treat surface waters of similar characteristics with conventional processes (coagulation–sedimentation and dual-media filtration). The finished waters have similar chemical quality and regrowth potential as measured by assimilable organic carbon (AOC). The major difference in treatment is the choice of final disinfectants (chlorine in Durham and chloramine in Raleigh). Ten sampling sites (monthly sampling) were chosen in each system to give wide geographic coverage and correspondingly, a wide range of water residence times. Significant losses were observed in both chlorine and chloramine residual in the DSs that produced bacterial regrowth as measured by heterotrophic plate count (HPC). The frequency distributions for log HPC (133 observations from Durham and 135 observations from Raleigh) were statistically the same in the chlorinated and chloraminated DSs. A correlation analysis indicated that disinfectant residual is the most important factor determining HPC level. However, the resulting R2 value for a non-linear regression model that also included AOC, temperature, and pH as independent variables was less than 0.7. Bacterial regrowth as measured by HPC, is dependent upon a complex interaction of chemical, physical, and operational parameters that may not be captured by such a simple statistical relationship.

Removal of natural organic matter - a fresh approach

Natural organic matter (NOM) has been shown to be one of the major parameters that affects water quality and treatment processes. NOM reduces the effectiveness of water treatment by interfering with the flocculation process, makes treatment with activated carbon and membrane filtration less efficient and is a precursor to the formation of disinfectant by-products (DBP). Furthermore, NOM acts as a food source for micro-organisms resulting in bacterial regrowth in distribution systems. These concerns have resulted in the removal of NOM from raw water being of prime concern for water authorities. The elevated levels of NOM in Australian water supplies have resulted in priority being given to research into methods of removing NOM by the Australian Water Quality Centre (AWQC). Early work showed that some types of anion exchange resins were very effective for NOM removal and that while resin column systems were rapidly fouled by waters with high concentration of suspended matter, a stirred system had no such limitation. This lead to the development of a resin with a high adsorptive capacity for NOM by the Commonwealth Scientific & Industrial Research Organisation (CSIRO) in collaboration with the AWQC which will be manufactured under licence by Orica Australia Pty Ltd. This resin then formed the basis for a novel process for NOM removal developed by the AWQC in collaboration with Orica Australia Pty Ltd. Both the MIEX® resin and process have been patented internationally. This paper outlines the process, gives examples of some of the benefits and provides recent results from an operating pilot plant with a capacity 160 kL/day.

The Pseudomonas group as an indicator of potential regrowth in water distribution systems.

The regrowth of micro-organisms in the water distribution system is assisted by the presence of biodegradable organic matter (BDOC). The low concentration of available organic carbon in this type of water favours the growth of bacteria belonging to the Pseudomonadaceae family, as this group can grow better at low concentrations of substrate than other bacteria also present in the distribution system. Although the genus Aeromonas has already been adopted as an indicator of this potential regrowth, members of the Pseudomonadaceae have not yet been proposed as indicators of potential bacterial regrowth in the water distribution system. The results are presented of a year-long study of the Barcelona distribution system in which the presence of Pseudomonas and Aeromonas was analysed and the validity of these micro-organisms as indicators of potential regrowth in the distribution system was assessed. It seems that, at least in the drinking waters of the Barcelona area, Pseudomonas is a better indicator of potential bacterial regrowth than Aeromonas.

Suspended bacterial biomass and activity in full-scale drinking water distribution systems: Impact of water treatment

Bacterial regrowth in drinking water distribution systems is a source of concern since it could result in non-compliance with water quality regulations, taste and odor problems, and may be associated with an increased risk of gastrointestinal illnesses. Bacterial regrowth is dependent on several factors, including the amount of substrate available for growth, such as biodegradable dissolved organic carbon (BDOC), oxidant residuals and the presence of corrosion. To investigate the impact of nutrients levels and oxidant residual maintenance, a full-scale investigation of two distribution systems was completed. This study presents data obtained from two distribution systems (DS) fed by different treatment trains: one DS has low BDOC concentrations and no free oxidant residual (St. Rose plant, Quebec, Canada), and the other has a high BDOC concentration and moderate concentrations of free oxidant residual (Pont Viau plant, Quebec, Canada). Monitoring included heterotrophic plate counts (HPC), total direct count by epifluorescence after acridine orange staining (AODC), direct viable counts (DVC-CTC method) and the measurement of bacterial production by 3H-thymidine incorporation. Results show that bacterial biomass (AODC and DVC) and bacterial production are lower in the DS fed by the treatment plant with a low BDOC concentration in the plant effluent. This difference is observed in warm water but not in cold water. The results suggest that HPCs are not a good indicator of bacterial regrowth in DSs. Finally, statistical analysis demonstrated that the treatment type and cumulative surface to volume ratio are the significant factors affecting regrowth in the distribution systems studied in warm waters.

TOC removal in biological filters

Optimizing the coagulant dosage offers the most effective means of maximizing TOC removal; biofiltration may be used to augment removals.Laboratory‐scale biologically active sand filters were used to evaluate the effect of natural organic matter (NOM) source and empty bed contact time (EBCT) on the removal of total organic carbon (TOC) from coagulated and ozonated NOM solutions. Ozonation at dosages in the range of 2.0 to 4.0 mg ozone/mg TOC were used to enhance biodegradation of the organic matter. TOC removal in the biofilters ranged from 16 to 33 percent of the biofilter influent concentration. TOC removal was significantly affected by the source of the organic carbon but was independent of EBCT in the range of 4 to 20 min. In the context of an entire water treatment plan, including coagulation and biofiltration, the contribution of biofiltration to TOC removal was relatively small compared with total removals across the entire treatment train. In one biofiltration experiment, however, removal of the biodegradable fraction of TOC was 100 percent, suggesting that biofilters may be effective in reducing subsequent regrowth in distribution systems.

Biological Processes in Drinking Water Treatment

Biological processes have the potential to remove pollutants that may be ineffectively removed by conventional treatment, such as biodegradable organics, synthetic organic compounds, ammonia, nitrate, iron, and manganese. Biooxidation of organic matter and ammonia decreases available substrates for microbial regrowth in distribution systems, reduces tastes and odors, and decreases the amount of precursor available to form disinfection by‐products. Biological removal of certain contaminants could extend the service time or capacity of subsequent physicochemical processes. In US water treatment, the practice is often to impair or preclude development of biological activity by prechlorination, rigorous scouring of filter media, and frequent backwashing of granular activated carbon. Practical experience in Europe with media containing microbial activity has been favorable.