Simulated Distribution System Tests Research Articles

Simultaneous prediction of trihalomethanes, haloacetic acids, haloacetonitriles and haloacetamides using simulated distribution system tests

This study analysed the spatial and temporal occurrence of 29 disinfection by-products (DBPs) formed by chlorination and chloramination.

Pilot Plant Performance Comparing Carbon-Based and Membrane-Based Potable Reuse Schemes

Two pilot-scale advanced water treatment plants using tertiary-treated biological nutrient removal wastewater effluent were operated simultaneously to demonstrate and compare treatment performances and process operations. Final effluent from both the plants was compared for its suitability to recharge a potable water supply aquifer. The two treatment approaches included a carbon-based treatment process (flocculation/sedimentation, ozone-biofiltration, granular activated carbon (GAC) adsorption, ultraviolet (UV) disinfection; 20,000 L/day) and a membrane-based treatment process (ultrafiltration, reverse osmosis, UV-hydrogen peroxide advanced oxidation; 160,000 L/day). Both processes were found to be reliable in producing water that met all United States Safe Drinking Water Act's primary maximum contaminant levels (MCLs). The carbon-based process often exceeded the secondary MCL for total dissolved solids due to background salinity in the source water. Results showed that both systems performed comparably with respect to removal of pathogens, fecal indicators, and contaminants of emerging concern. Total organic carbon (TOC) was removed to below 0.5 mg/L by the membrane-based process. For the carbon-based process, GAC media exhaustion over the course of the pilot operation resulted in an increase in finished water TOC from <0.5 to ∼4 mg/L. Disinfection byproducts, including trihalomethanes and haloacetic acids were not measured above their respective MCL after 8 h of free chlorine contact in a simulated distribution system test. N-nitrosodimethylamine was removed below 10 ng/L by the GAC with 20 min of empty bed contact time after 20,000 bed volumes.

Chlorine addition prior to granular activated carbon contactors improves trihalomethane control

Optimizing granular activated carbon (GAC) operation can reduce costs for removing disinfection byproducts or their precursors. This article investigates prechlorination to enhance the removal of total trihalomethane (TTHM) precursors and reduce TTHM concentrations in finished water. Rapid small‐scale column tests (RSSCTs) and pilot tests using virgin and reactivated GAC were conducted on water with and without prechlorination. Prechlorination (1 or 2 mg Cl2/L) increased (by 15 and 30%, respectively) the number of bed volumes treated by GAC. On the basis of size exclusion chromatography, prechlorination followed by GAC preferentially removed low‐molecular‐weight dissolved organic carbon by GAC. Simulated distribution system (SDS) chlorination tests on GAC effluent showed lower TTHM concentrations with prechlorination. Prechlorination lowered the bromine incorporation factor values in GAC effluents after SDS tests, which suggests a reduction in toxicity potential from trihalomethanes. RSSCT results were validated by pilot testing, showing that RSSCTs can be used to optimize the prechlorination process.

Predicting the Formation of Haloacetonitriles and Haloacetamides by Simulated Distribution System Tests

Unintended chemical reactions between disinfectants and natural organic matter (NOM) or anthropogenic compounds in natural waters result in the formation of disinfection by-products (DBPs) during drinking water treatment. To date, numerous groups of disinfection by-products have been identified in drinking water, some of which are suspected to be of public health importance and thus are regulated in the water industry. Recent studies have suggested that some unregulated nitrogen-containing DBPs, such as haloacetonitriles (HANs) and haloacetamides (HAcAms), may have greater toxicity than the currently regulated groups (trihalomethanes, THMs, and haloacetic acids, HAAs). There is only sparse information on the behaviour of the HANs and HAcAms in distribution systems. It is however known that HANs can be hydrolysed to the HAcAms, which in turn can hydrolyse to form dihaloacetic acids (DHAAs).Simulated distribution systems tests (SDS) have been successfully applied to predict the formation of THMs and HAAs using a simple and inexpensive lab-based technique, and have been recommended by the US Environmental Protection Agency (EPA) to American water utilities for collecting information about the levels of DBPs occurring in their distribution systems. SDS tests aim to simulate the water quality, disinfectant residuals, and water ages of a real distribution system, allowing easy sampling at prescribed time intervals for analysis of DBP formation. These tests are also a useful tool for considering the impact of potential changes to distribution practices, such as switching from chlorination to chloramination, for example.Therefore, a sampling survey was conducted in four surface water treatment plants in the UK to examine the formation of HANs and HAcAms in both real distribution systems and SDS tests. The samples were extracted using liquid-liquid extraction and analysed by gas chromatography with electron capture detection (GC-ECD). The research sought to determine whether SDS can be a useful predictive tool for HANs and HAcAms in distribution systems and what levels of prediction error are to be expected.

Disinfection byproduct formation kinetics of a water treatment plant on Kinmen Island

Chloramine has often been used as a chlorine alternative for trihalomethane (THM) and haloacetic acid (HAA) control. However, nitrogenous disinfection byproduct (N-DBP) formation and nitrification in distribution have always been major concerns in chloramination practices. On Kinmen Island, the high organic nitrogen content in raw water may increase the nitrogenous DBP formation. Simulated distribution system tests were conducted to explore the DBP formation kinetics in the distribution system. Lower haloacetonitrile (HAN4) formation (0.26 μg L−1) with chloramination than with chlorination (10.48 μg L−1) was observed from the 24 hours of reaction time. The nitrogen sources contributing to the dichloroacetonitrile (DCAN) formation kinetics were explored with 15N-chloramination. The results showed that nitrogen sourced from organic nitrogen was more dominant in DCAN formation with low chloramine dosage. This suggests that chloramine contributes to less DCAN formation in practice, especially for short distribution systems. In summary, the results provide evidence that simultaneous post-chloramination and pre-chlorination would be a feasible disinfection strategy applied to control regulated THM and HAA formation on Kinmen Island.

Disparity in disinfection byproducts concentration between hot and cold tap water

The quality of water entering a distribution system may differ substantially from the quality at the point of exposure to the consumer. This study investigated temporal variations in the levels of regulated and non-regulated disinfection byproducts (DBPs) in cold and hot tap water in a home on a medium-sized municipal water system. In addition, samples were collected directly from the water plant with some being held in accordance with a simulated distribution system (SDS) test protocol. The location for this work was a system in western Massachusetts, USA that uses free chlorine as a final disinfectant. Very little short term variability of DBPs at the point of entry (POE) was observed. The concentration of DBPs in the time-variable SDS test was similar to concentrations in the cold water tap. For most DBPs, the concentrations continued to increase as the cold water tap sample was held for the time-variable SDS incubation period. However, the impact of heating on DBP levels was compound specific. For example, the concentrations of trihalomethanes (THMs), dichloroacetic acid (DCAA) and chloropicrin (CP) were substantially higher in the hot water tap than in the cold water time-variable SDS samples. In contrast, the concentration of trichloroacetic acid (TCAA) was lower in the heated hot tap water, but about equal to that observed in the cold tap water. The situation was more pronounced for dichloroacetonitrile (DCAN), bromodichloroacetic acid (BDCAA), bromochloroacetic acid (BCAA) and 1,1,1-trichloropropanone (TCP), which all showed lower concentrations in the hot water then in either of the cold water samples (instantaneous or time-variable SDS). The latter was viewed as a clear indication of thermally-induced decomposition. The ratio of unknown total organic halide (UTOX) to TOX was substantially lower in the hot tap water as the THM to TOX ratio became correspondingly larger. The results of this study show that DBP exposure in the home is not well represented by concentrations measured in cold water taps where most compliance monitoring is done.

GAC removal of organic nitrogen and other DBP precursors

The increased mutagenicity of disinfection by‐products (DBPs) containing bromide (Br−) and nitrogen requires a renewed evaluation of best available treatment technologies for DBP control. The aim of this article is to document the removal of organic nitrogen during granular activated carbon (GAC) treatment and to illustrate how GAC treatment alters DBP speciation. Rapid small‐scale column tests (RSSCTs) with GAC were conducted on pretreated surface water sources to evaluate the simultaneous removal of carbonaceous and nitrogenous DBP precursors: dissolved organic nitrogen (DON), dissolved organic carbon (DOC), organics absorbed by ultraviolet light at 254 nm (UV254), and Br−. Simulated distribution system tests were conducted with RSSCT effluent samples throughout natural organic matter breakthrough, and free chlorine was used to evaluate the formation of halogenated carbonaceous and nitrogenous DBPs. GAC preferentially removed UV254‐absorbing material over DOC, which was removed more effectively than DON. Br− was not removed. Consequently, effluent ratios of Br− to DOC and Br− to DON changed during GAC treatment, and the ratio of brominated DBPs to chlorinated DBPs shifted during the GAC breakthrough cycle; brominated DBPs dominated earlier in the breakthrough of DOC. Neither DON nor nitrogenous DBP precursors were removed efficiently during GAC treatment.

Modelling of haloacetic acid concentrations in a United Kingdom drinking water system

Haloacetic acids (HAAs) are formed during the chlorination of water containing organic matter but predictive models for their formation are not well established at present and have not been applied to UK water systems. In this paper two different modelling approaches have been applied and evaluated to a selected UK drinking water system. The first approach involved the development of statistical predictive models for the HAA formation by using multiple linear regression with appropriate water quality and operational parameters obtained from sampling, and the second approach employed the simulated distribution system (SDS) test. Statistically significant predictors were trihalomethane (THM) levels, pH, temperature, total chlorine total organic carbon, UV254, bromide concentration and residence time, but the importance of each varied with HAA species. The models that generally explained most of the variance of individual and total HAAs included THMs as a predictor variable. The use of the SDS-HAA test included quantification of individual HAA species. The concentration of total HAA and individual compounds in the SDS test and field samples were comparable immediately after chlorination, but with increasing residence time the concentration of HAAs in the selected water distribution system were greater than those found in the SDS test.

A framework for the implementation and design of pilot-scale distribution systems

In recent years, model distribution systems have become valuable tools for understanding physical, chemical and microbiological processes that occur in full-scale distribution systems. These systems enable numerous treatment strategies to be examined in controlled conditions and without adversely affecting the water quality at a customer9s tap. Many model reactors have been developed for obtaining water quality data for evaluating very specific processes that occur in the distribution system (e.g. trihalomethane formation). The simulated distribution system (SDS) test is used to predict the formation of trihalomethanes in the distribution system, annular reactors (AR) are commonly used for biofilm studies and pipe rig systems can be used for a variety of studies including corrosion and biofilm. Of the three model reactors, pipe rig systems provide the most flexibility, however relatively few standard or de facto testing procedures have been developed in comparison to the other laboratory-based model systems (e.g. SDS test protocol). There are several hydraulic configurations that can be modelled along with various lengths and diameters of different pipe materials. However, this flexibility and the lack of standard pipe rig designs cause difficulties in making broad comparisons among research projects. This paper outlines the important design variables of a pipe rig system and provides examples of different options for each variable. Based on a thorough review of previous pipe rig studies, a framework was developed to assist in the design and selection of pipe rig configurations. It is recommended that the framework provides a method for selecting the physical and hydraulic configuration for future pipe rig investigations. Ultimately, the framework would provide the water industry with potential designs that would allow for improvement in data evaluation and strategies for minimizing water quality degradation in distribution systems.

Field and material-specific simulated distribution system testing as aids to understanding trihalomethane formation in distribution systems

The evolution of disinfection by-products (DBP) in drinking water is triggered at the initial point of disinfectant application and, in most cases, continues to some extent throughout the distribution system. A comprehensive study using an extensive field monitoring program, together with an improved simulated distribution system (SDS) test, is presented, in which the effects of increased chlorine demand exerted by internal pipe walls are included in the evaluation of potential DBP exposure to the public. In the field study, comparing both pre-formed trihalomethane (THM) and THM formation potential among several distribution system locations eliminated inherent uncertainties associated with the complexities of network hydraulics, leaving exposure to the internal pipe environment as the primary factor of interest. Findings in this paper confirm the reliability of using the material-specific SDS (MS-SDS) test as a better representation of DBP evolution in a real distribution system. In the case study, the increased chlorine demand of unlined cast-iron results in lower levels of trihalomethanes than those dictated by testing the finished water alone. The MS-SDS test is readily adaptable for pilot-plant studies where real distribution system conditions are inaccessible.Key words: chloroform, disinfection by-products, distribution systems, field monitoring, simulated distribution system.

Evaluating surrogates for disinfection by‐products

Data analysis suggests that UV‐254 absorbance is a better indicator than TOC concentration for chlorination by‐product formation in SDS tests—and is substantially easier, faster, and less costly to measure.Each of four low‐bromide waters was coagulated at three pH values (5.5, 7.0, and 8.0), and then simulated distribution system (SDS) chlorination tests were performed. The object of the study was to determine whether total organic carbon (TOC) or UV‐254 absorbance can be employed as an indicator of the trihalomethane (THM) and haloacetic acid (HAA) concentrations formed upon chlorination. Results showed that removal of TOC and UV‐254 always increased with decreasing coagulation pH, but the extent of removal varied with the different waters. Maximum reduction of SDSTHM and SDSHAA5 formation was achieved after coagulation at pH 5.5. UV‐254 proved to be a better surrogate for chlorinated by‐product formation than TOC. The applicability of the correlations to waters with higher bromide levels and different chlorination pH values should be evaluated.