Chlorination of model drinking water biofilm: implications for growth and organic carbon removal

The influence of chlorine on biofilm in low organic carbon environments typical of drinking water or industrial process water was examined by comparing biomass and kinetic parameters for biofilm growth in a chlorinated reactor to those in a non-chlorinated control. Mixed-population heterotrophic biofilms were developed in rotating annular reactors under low concentration, carbon-limited conditions (< 2 mg/L as carbon) using three substrate groups (amino acids, carbohydrates and humic substances). Reactors were operated in parallel under identical conditions with the exception that chlorine was added to one reactor at a dose sufficient to maintain a free chlorine residual of 0.09-0.15 mg/L in the effluent. The presence of free chlorine resulted in development of less biofilm biomass compared to the control for all substrates investigated. However, specific growth and organic carbon removal rates were on the average five times greater for chlorinated biofilm compared to the control. Observed yield values were less for chlorinated biofilm. Although chlorinated biofilm's specific organic carbon removal rate was high, the low observed yield indicated organic carbon was being utilized for purposes other than creating new cell biomass. The impacts of free chlorine on mixed-population biofilms in low-nutrient environments were different depending upon the available substrate. Biofilms grown using amino acids exhibited the least difference between control and chlorinated kinetic parameters; biofilm grown using carbohydrates had the greatest differences. These findings are particularly relevant to the fundamental kinetic parameters used in models of biofilm growth in piping systems that distribute chlorinated, low-carbon-concentration water.     

The potential for biofilm growth in water distribution systems.

Biofilms on pipe walls in water distribution systems are composed of bacteria in a polymeric matrix, which can lead to chlorine demand, coliform growth, pipe corrosion and water taste and odour problems. The majority of previous studies have been laboratory or pilot plant based and few results are available for field conditions. In this study, field observations of biofilm were made using biofilm potential monitors. The monitor results were compared with pipe samples taken from the distribution system and with laboratory pipe reactors. An empirical equation quantified the inhibitory effects of free chlorine and decrease of temperature on biofilm growth. With water having total organic carbon concentrations in the range 1.5-3.9mg/1 a free chlorine residual of 0.2 mg/l was needed to reduce biofilm concentration to below 50 pg ATP cm2. Pipe material influenced biofilm activity far less than chlorine with mean biofilm activity being ranked in the order glass (136 pg ATP/cm2) < cement (212 pg ATP/cm2) < MDPE (302 pg ATP/ cm2) < PVC (509 pg ATP/cm2).     

Dynamic simulation of multicomponent reaction transport in water distribution systems

Given the presence of nutrients, regrowth of bacteria within a distribution system is possible. The bacterial growth phenomena, which can be studied by developing a multicomponent (substrate, biomass and disinfectant) reaction transport model, is governed by its relationship with the substrate (organic carbon) and disinfectant (chlorine). The multicomponent reaction transport model developed in the present study utilizes the simplified expressions for the basic processes (in bulk flow and at pipe wall) such as bacterial growth and decay, attachment to and detachment from the surface, substrate utilization and disinfectant action involved in the model. The usefulness of the model is further enhanced by the incorporation of an expression for bulk reaction parameter relating it with the organic carbon. The model is validated and applied to study the sensitive behavior of the components using a hypothetical network. The developed model is able to simulate the biodegradable organic carbon threshold in accordance with the values reported in the literature. The spread of contaminant intruded into the system at any location can also be simulated by the model. The multicomponent model developed is useful for water supply authorities in identifying the locations with high substrate concentrations, bacterial growth and lower chlorine residuals.     

Determination of nutrients limiting biofilm formation and the subsequent impact on disinfectant decay.

Understanding the contribution of both organic and inorganic nutrients to biofilm development and the subsequent impact of developed biofilms on disinfectant decay are important requirements for distribution system management strategies. Nutrient limitation may be one way to control biofilm development without increasing disinfectant dosing. Little is known, however, of the nutrient requirements of biofilms in distribution systems. Indeed, the effects on biofilm development due to the addition of nutrients to distribution systems and what impact biofilm development may have on disinfectant decay is still poorly understood. This study used annular reactors to determine the nutrients limiting for biofilm development in drinking water from two different Sydney sources and the subsequent effects of biofilm development on disinfectant decay. It was found that biofilm development in Sydney water was limited by organic carbon and that biofilm development promoted chloramine decay. Moreover, biofilm development occurred in the presence of chloramine. The ability of biofilms to respond to increases in disinfectant concentrations was dependent on the biomass of the biofilms. In a comparative study using chlorinated drinking water containing very low levels of organic carbon, biofilm development was not detected. Removal of organic carbon resulted in greater persistence of chlorine, which led to greater biofilm control. It was also shown that biofilms could contribute cells to the aqueous phase. The results of the study indicate that treatment and system management strategies should incorporate organic carbon removal to limit biofilm development through a combination of retarding bacterial growth and enhancing disinfectant persistence.     

Determination of biodegradable dissolved organic carbon in waters: comparison of batch methods

An effect of different types of bacterial inocula upon the final biodegradable dissolved organic carbon (BDOC) result was investigated in samples of both low and high BDOC concentrations. Stream water and leaf leachate samples were incubated either with free, suspended bacteria or with bacteria attached to the stream sediment particles or attached to artificial substrata. The time course of dissolved organic carbon (DOC) decomposition was observed using absorbance analysis of DOC. BDOC determination by means of commonly used suspended bacteria as the inoculum made for an underestimation of BDOC between 5% and 25%, compared with attached bacterial community (biofilm). The reason for these findings could be the higher microbial diversity, higher metabolic activity of attached bacteria and abiotic adsorption of organic molecules to inorganic support and biofilm matrix surfaces. Adsorbed DOC is easily hydrolyzed and utilized by biofilm bacteria.     

Effects of temperature and biodegradable organic matter on control of biofilms by free chlorine in a model drinking water distribution system

This study used annular reactors (AR) to investigate, under controlled laboratory conditions, the effects of temperature and biodegradable organic matter (BOM) on the free chlorine residual needed to control biofilm accumulation, as measured by heterotrophic plate count (HPC) bacteria. Biofilm was grown on PVC coupons, initially in the absence of chlorine, at 6, 12, and 18 degrees C, in the presence and absence of a BOM supplement (250 microg C/L) added as acetate. During the early stages of chlorine addition, when no measurable free chlorine residual was present, a reduction in biofilm HPC numbers was observed. Subsequently, once sufficient chlorine was added to establish a residual, the biofilm HPC numbers expressed as log CFU/cm2 fell exponentially with the increase in free chlorine residual. Temperature appeared to have an important effect on both the chlorine demand of the system and the free chlorine residual required to control the biofilm HPC numbers to the detection limit (3.2 Log CFU/cm2). For the water supplemented with BOM, a strong linear correlation was found between the temperature and the free chlorine residual required to control the biofilm. At 6 degrees C, the presence of a BOM supplement appeared to substantially increase the level of free chlorine residual required to control the biofilm. The results of these laboratory experiments provide qualitative indications of effects that could be expected in full-scale systems, rather than to make quantitative predictions.     

Investigation of microbially available phosphorus (MAP) in flemish drinking water

Several researchers have reported phosphorus growth limitations of heterotrophic bacteria instead of main energy source--organic carbon. Usually this phenomenon was noticed in waters with high organic carbon content, where phosphorus concentration was deficient to maintain the growth on level suggested by high organic carbon amount. We analysed the microbially available phosphorus (MAP) and assimilable organic carbon (AOC) in several drinking waters in Flanders, Belgium. Our aim was the investigation whether organic carbon or phosphorus is the restricting nutrient in specific water and determination of the impact of some treatment processes on MAP content. We obtained a wide range of MAP concentrations being from 0.3 to 15.2 microg P-PO(4)/l in finished drinking water. In a treatment unit applying ozone, MAP was found to be the nutrient that limits bacterial growth instead of organic carbon. Moreover, ozone caused slight MAP decrease. Granular activated carbon (GAC) filtration was able to diminish further the MAP content significantly but not below a certain level. The biofilm monitor supplied with the MAP-limited water resulted in significantly lower biofilm formation rate (BFR) value than the same installation fed with AOC-limited water.     

Bacterial dynamics in the drinking water distribution system of Brussels

Water samples and pipe coupons were collected from the Brussel's drinking water distribution system (DS). A treated surface water and various groundwaters feed this DS. Parameters related to bacterial regrowth have been measured on these samples: temperature, concentrations of free residual chlorine, concentration of biodegradable dissolved organic carbon (BDOC), abundance of suspended bacteria, densities of fixed bacteria and levels of bacterial activity. Results showed that groundwaters were less susceptible to favor bacterial regrowth in the DS pipes. Treated surface water and mixed waters had the highest potential of bacterial regrowth in the DS dead ends. Results also showed that the potential regrowth induced by the distribution of a treated surface water could be reduced if: (1) the BDOC levels were below 0.25 mg C/l at the outlet of the surface water treatment plant; (2) a significant free chlorine residual was present within the whole DS. Second-stage biological filtration using granular activated carbon is now under construction at the surface water treatment plant feeding a part of this DS. This treatment implementation should reduce BDOC levels and chlorine demand of the treated surface water and will further reduce the slight regrowth phenomena observed in this DS.     

Minimizing biofilm in the presence of iron oxides and humic substances

Based upon circumstantial evidence linking elevated coliform bacteria counts in drinking water distribution systems with unlined cast iron pipe, it was hypothesized that adsorption of humic substances by iron oxide containing corrosion products (CPs) can stimulate and/or support biofilm development. Using porous media consisting of iron-oxide-coated glass beads (IOCBs) or actual iron CPs, experiments were performed to evaluate the effectiveness of different corrosion control and disinfection treatments in reducing biofilm when humic substances were the carbon source. Free chlorine was the most effective treatment in minimizing biofilm. Addition of phosphate alone did not significantly reduce biofilm using the CPs, but there was weak evidence it did using the IOCBs. The combination of free chlorine and phosphate was more effective at minimizing biofilm than free chlorine alone when CPs were the media. The presence of humic substances was a major factor when considering biofilm minimization based on results of experiments using both types of iron oxide media. The combination of humic substances and CPs led to an increase in biofilm biomass when free chlorine was not present, similar to conditions that could occur at distribution system dead-ends. Treatment to raise the pH to 9 did not reduce biofilm in experiments using both media, and actually increased biofilm in the experiment using CPs under the conditions tested.     

Soluble microbial products (SMP) formation kinetics by biofilms

Soluble microbial products (SMP) formation kinetics were investigated by employing a laboratory-scale biofilm reactor and naturally-grown oligotrophs. The experimental results indicated that the majority of effluent soluble organic carbon(SOC) was SMP, while only a small fraction of the effluent SOC was the residual original substrate. Utilization-associated products (UAP), which were produced directly from substrate metabolism, were more important than biomass-associated products (BAP), which were produced by basic metabolism. The SMP contained mainly high-molecular-weight organic compounds, although the organic carbon source to a biofilm reactor was a low-molecular-weight compound. The steady-state concentrations of the effluent SMP and SOC were directly proportional to the influent substrate concentrations in this study. An extended steady-state biofilm model was developed by incorporating into the steady-state biofilm model an SMP formation model based on two types of SMP (i.e. UAP and BAP). The model described successfully the experimental substrate utilization, SMP formation, and the removal of total soluble organic matter (SOC).     

Disinfectant efficacy of chlorite and chlorine dioxide in drinking water biofilms

The drinking water industry is closely examining options to maintain disinfection in distribution systems. In particular this research compared the relative efficiency of the chlorite ion (ClO2-) to chlorine dioxide (ClO2) for biofilm control. Chlorite levels were selected for monitoring since they are typically observed in the distribution system as a by-product whenever chlorine dioxide is applied for primary or secondary disinfection. Previous research has reported the chlorite ion to be effective in mitigating nitrification in distribution systems. Annular reactors (ARs) containing polycarbonate and cast iron coupons were used to simulate water quality conditions in a distribution system. Following a 4 week acclimation period, individual ARs operated in parallel were dosed with high (0.25mg/l) and low (0.1mg/l) chlorite concentrations and with high (0.5 mg/l) and low (0.25mg/l) chlorine dioxide concentrations, as measured in the effluent of the AR. Another set of ARs that contained cast iron and polycarbonate coupons served as controls and did not receive any disinfection. The data presented herein show that the presence of chlorite at low concentration levels was not effective at reducing heterotrophic bacteria. Log reductions of attached heterotrophic bacteria for low and high chlorite ranged between 0.20 and 0.34. Chlorine dioxide had greater log reductions for attached heterotrophic bacteria ranging from 0.52 to 1.36 at the higher dose. The greatest log reduction in suspended heterotrophic bacteria was for high dose of ClO2 on either cast iron or polycarbonate coupons (1.77 and 1.55). These data indicate that it would be necessary to maintain a chlorine dioxide residual concentration in distribution systems for control of microbiological regrowth.     

Control of Coliform Growth in Drinking Water Distribution Systems

The occurrence of coliforms in drinking water distribution systems may be explained by (i) inadequate water treatment, (ii) post-treatment contamination, or (iii) coliform growth in the network. In order to confirm the third hypothesis, a 24 h-starved dense suspension of Escherichia coli was injected into an experimental water distribution system. Results from this experiment clearly indicate that E. coli may find ecological conditions in drinking water distribution systems which will allow growth, particularly in the biofilm phase. Chlorination is an appropriate tool to limit coliform much more easily than the total heterotrophic bacterial biomass. However, in all cases, biofilm associated bacteria are more difficult to kill than suspended bacteria, because of chlorine consumption by the pipe material, and because of a diffusion-limited reaction between chlorine and the biofilm.     

Evaluation of disinfectant efficacy against biofilm and suspended bacteria in a laboratory swimming pool model

Laboratory reactor systems designed to model specific environments enable researchers to explore environmental dynamics in a more controlled manner. This paper describes the design and operation of a reactor system built to model a swimming pool in the laboratory. The model included relevant engineering parameters such as filter loading and turn-overs per day. The water chemistry in the system's bulk water was balanced according to standard recommendations and the system was challenged with a bacterial load and synthetic bather insult, formulated to represent urine and perspiration. The laboratory model was then used to evaluate the efficacy of six chemical treatments against biofilm and planktonic bacteria. Results showed that the biofilm was able to accumulate on coupons and in the filter systems of reactors treated with either 1-3 mg/L free chlorine or 10 mg/L polyhexamethylene biguanide (PHMB). All the treatments tested resulted in at least a 4 log reduction in biofilm density when compared to the control, but shock treatments were the most effective at controlling biofilm accumulation. A once weekly shock dose of 10 mg/L free chlorine resulted in the greatest log reduction in biofilm density. The research demonstrated the importance of studying a biofilm in addition to the planktonic bacteria to assess the microbial dynamics that exist in a swimming pool model.     

以氯化方法实现生物膜法短程硝化反应

以稳定运行的全程硝化的生物膜反应器的一个反应单元为研究对象,通过外加氯实现对硝化反应中两类细菌的选择性抑制。分别以冲击性和连续性方式投加氯,可以在生物膜法生物脱氮反应中实现短程硝化。当短程硝化反应实现后,通过改变投药方式可节省投药量并维持短程硝化反应。     

A study of substrate removal in a microbial film reactor

A mathematical model is formulated describing the mechanism of substrate removal by a microbial slime over which a film of liquid, containing the substrate as dissolved biodegradable material, is flowing. It is assumed that a lack of either organic carbon, oxygen, or both simultaneously, can limit the overall rate of the process. Basic chemical engineering principles of interfacial mass transfer, diffusion and biochemical reaction are used in the formulation of the model and the resulting set of equations is solved by digital computer using typical kinetic parameters taken from the literature. Predictions of whether organic carbon, oxygen, or both simultaneously, limit the process, the substrate removal rate, and the active depth of the biofilm are made.Data were obtained in support of the model by measuring substrate removals on a vertically mounted experimental biofilm reactor over a range of hydraulic and organic loadings typical of industrial-scale operation. Good agreement between the experimental results and the model predictions was obtained with the exception of the data pertaining to hydraulic loadings approaching the minimum wetting rate. These data deviated from the predicted values at high substrate concentrations indicating that under conditions of low hydraulic load the model is less satisfactory for describing the system.Conversely it may be that at low hydraulic loads and high applied substrate concentrations experimental accuracy is poor.     

供水管网余氯衰减变化规律及模型研究

研究了水温T、总有机碳TOC及初始氯浓度C0对主体水余氯衰减系数kb的影响;通过管网水质模拟中试装置探究初始氯浓度、流速对管网总余氯衰减的影响;推算管壁余氯衰减系数kw,并分析余氯、细菌总数、浊度及UV(254)指标间的相关性。结果表明,一级模型可以较好地描述管网总余氯衰减变化,在主体水中仅水温较高时一级模型相关系数较高,而平行一级模型拟合度不易受水温影响,相关系数>0.96;各因素对kb影响的主次关系为T>TOC>C0;总余氯衰减系数k、kw与管网初始氯浓度及流速均呈正相关,kw与雷诺数之间存在良好的指数关系;细菌总数(HPC)与初始氯浓度呈负相关、与浊度无明显相关性、与UV(254)呈正相关,浊度与UV(254)相关性良好。     

高温、高藻期臭氧/生物活性炭工艺的处理效果研究

针对高温、高藻期原水较难处理的特点,采用臭氧/生物活性炭工艺进行了中试研究。试验结果表明,臭氧/生物活性炭工艺对有机物的去除效果明显,对CODMn的平均去除率为73.76%,对UV254的平均去除率为86.38%。高温条件下,大量生长的细菌随出水流出反应器,在投氯量为1 mg/L时可杀灭生物活性炭工艺出水中的大部分细菌,剩余细菌数〈10 CFU/mL,对细菌的杀灭率为99%,能够保证出水的微生物安全性。同时为避免细菌在活性炭表面大量繁殖而堵塞活性炭微孔,应适当缩短反冲洗周期,以3～4 d为宜。臭氧/生物活性炭工艺对藻类的平均去除率为75%,且在其出水中未检测出藻毒素。     

Measurement of streamwater biodegradable dissolved organic carbon with a plug-flow bioreactor

Plug-flow biofilm reactors were colonized by microorganisms indigenous to streamwater and used to measure concentrations of biodegradable dissolved organic carbon (DOC) in streamwater. We performed experiments to determine the influence of physical, chemical, and biological factors on the operation of the bioreactors. Colonization required several months and the biological removal of DOC within the reactors was influenced by hydraulic residence time, dissolved organic carbon concentration, and water temperature. The large microbial biomass within a bioreactor buffered bioreactor performance from changes in chemical and physical parameters.     

Testing and validation of a multiple nonlinear regression model for predicting trihalomethane formation potential

This paper summarizes a testing and validation analysis of a previously developed model for predicting trihalomethane formation potential (THMFP) in chlorinated waters containing THM precursors. The original model, in the form of a nonlinear multiple regression equation, tended to overpredict THM formation potential at lower chlorine concentrations and underpredict at higher chlorine concentrations. The model proved to be more accurate in simulating THM formation potential in waters with moderate levels of organic carbon/THM precursors. While the model did not provide a high degree of predictive accuracy, the general format of the nonlinear model represents a rational framework for developing source specific models applicable to a given water source.     

Association between haloacetic acid degradation and heterotrophic bacteria in water distribution systems

The occurrences of trihalomethanes (THMs), haloacetic acids (HAAs) and heterotrophic bacteria were monitored in five small water systems over a nine-month period to investigate the association between HAA degradation and heterotrophic bacteria populations. The sampling sites were chosen to cover the entire distribution network for each system. An inverse association between heterotrophic bacteria and HAA concentrations was found at some locations where chlorine residuals were around or less than 0.3 mg L⁻¹. At other sample locations, where chlorine residuals were higher (over 0.7 mg L⁻¹), no HAA reduction was observed. A high heterotrophic bacteria count accompanied with a low chlorine residual could be used as an indicator for HAA degradation in distribution systems.