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Long-Term Comparison of Disinfection By-Product Formation Potential in a Full Scale Treatment Plant Utilizing a Multi-Coagulant Drinking Water Treatment Scheme

Andrew Skeriotis, Nancy Sanchez, Marla Kennedy, David Johnstone, Christopher Miller
2016 Water  
A comparative study of two coagulants, aluminum sulfate (Alum) and aluminum chlorohydrate (ACH), used in parallel in a full scale water treatment plant (WTP) in Ohio from October 2009 to December 2012, was conducted to determine disinfection by-product (DBP) formation potential removal based on both dissolved organic matter (DOM) and fluorescence-derived metrics. Water quality parameters and fluorescence intensity of water samples collected before and after coagulation were measured three times
more » ... easured three times per week and fluorescence matrices were analyzed using parallel factor (PARAFAC) analysis, while DBP formation potential was measured in a weekly basis in pre-and post-coagulation water samples. This study revealed that Alum consistently removed more trihalomethane (THM) formation potential per mg/L of dissolved organic carbon (DOC) than ACH. ACH treated waters averaged approximately 33% more THM formation potential when normalized to DOC. Similarly, haloacetic acid (HAA) formation potential averaged 10% higher in ACH treated waters. From the fluorescence analysis, PARAFAC components C1 and C2 (humic-like fluorophores groups) removal were 23% and 16% higher, respectively, with Alum when compared to ACH. Monte Carlo simulations, based on neural network models developed from the field data, were performed to compare DBP formation across a wide range of conditions. At similar pH, the model results showed that ACH coagulated water had 13% and 20% higher THM and HAA formation potential, respectively, when compared with Alum. The observations from this study reveal that a coagulant's preferential removal of DBP precursors has an impact on DBP formation despite similar DOC removal. 2 of 18 portion, quantified as dissolved organic carbon (DOC) [2], reacts with chlorine during disinfection to form THMs [3] and HAAs. A robust approach to limit the formation of DBPs is to maximize DOC removal via coagulation. Optimization of the coagulation process includes the selection of appropriate coagulant and dose in order to achieve DOC removal levels that will keep the treatment facility in compliance while controlling associated chemical costs. Optimal coagulant doses including preoxidation can be difficult to determine because of the dynamic nature of DOC. Jar tests are typically used to optimize treatment processes; however, there may be some limitations. An issue with jar tests is that only one sample of source water is typically used, therefore, the spatial and temporal variability of the nature of DOC is not properly captured [4] . Since the chemical character of DOC is dynamic, treatment processes must respond to these dynamics and include adaptive strategies. WTPs must be able to adjust their treatment processes as the DOC in the surface water changes, which is a limitation associated with the use of jar tests. The purpose of this study is to compare two coagulants, aluminum sulfate (Alum) and aluminum chlorohydrate (ACH), effectiveness in removal of DOC and DBP formation potential at a full-scale plant that has the ability to run side by side treatment trains utilizing two different coagulants. Samples collected three times a week from October 2009 to December 2012 including raw water, Alum treated settled water, and ACH treated settled water were analyzed for DOC, specific ultraviolet absorbance (SUVA), fluorescence excitation-emission matrices (EEM), and DBP formation potential (performed weekly). Further analysis of EEMs of raw and settled water samples was performed utilizing parallel factor (PARAFAC) analysis. NOM is a complex heterogeneous mixture of organic compounds consisting of aliphatic and aromatic compounds [5] . This mixture of organic compounds defines the nature of the DOC and its reactivity with chlorine, as well as the treatability of the source water. Aromatic compounds, which are more easily removed through coagulation [6, 7] are generally associated with humic acids which consist of large molecular weight compounds. These compounds account for over half of the mass of the DOC in water [5] and are precursors for THM formation. Aliphatic or low molecular weight compounds are usually removed at a lesser extent by coagulation (compared with aromatic structures) and therefore might have a significant contribution to DBP formation. In addition to potential DBP formation, other water quality water issues caused by NOM include: (i) taste and odor along with color issues; (ii) increases in coagulation and disinfection doses which in turn lead to larger sludge volumes and increased DBP formation; (iii) promotion of biological growth in the distribution system; (iv) increased levels of complex metals and adsorbed organic pollutants; (v) provides sources and sinks for carbon; and (vi) can intercede photochemical microbial processes [4, 8] . Furthermore, DOC is also responsible for affecting WTP treatment train processes, such as shortened run times with membrane filtration, granulated activated carbon fouling and decreased the performance of oxidants and disinfectants [9] . There are two main sources of NOM that comprise the nature of DOC. The allochthonous NOM is derived from the degradation of terrestrial plant matter which is dissolved and transported through rivers, lakes and estuaries within a watershed and include: agricultural runoff, urban runoff, forested land, soil and aquatic environments [10] . Autochthonous NOM is produced by microbial processes in the water. NOM concentration, composition and chemistry can exhibit significant variations in nature and are a function of temperature, pH and ionic strength, as well as photolytic and microbial processes [4] . There are literally thousands of different chemical compounds in NOM consisting of both hydrophilic and hydrophobic components. The hydrophobic compounds within NOM are aromatic compounds with phenolic structures and conjugated double bonds while the hydrophilic part consists of aliphatic compounds are low molecular weight structures, including carbohydrates, proteins, sugars and amino acids [11] . Hydrophobic and high molecular weight compounds in NOM have been shown to be a precursor to the formation of DBPs [12] . In waters with low DOC however, hydrophilic compounds may play a contributing role in the formation of DBP and furthermore, can lead to the
doi:10.3390/w8080318 fatcat:nt3ca7hktze3xj4azc2hinczcu