Control of Trihalomethanes in Wastewater Treatment Guanghui Hua and Steven Yeats ontrolling trihalomethanes (THMs) cause health and regulatory concerns. DBPs discharged with wastewater treatment have been linked to an increased risk of cer - Guanghui Hua, Ph.D., and Steven Yeats, plant final effluent has become an im - tain cancers and adverse reproductive effects. P.E., are with the water and wastewater Cportant issue across the United States. Chlori - THMs) and haloacetic acids (HAAs) are the process engineering group in the nation is by far the most widely used method two most abundant groups of known DBPs. Gainesville office of the engineering firm for disinfecting treated wastewater. The reac - The U.S. Environmental Protection Jones Edmunds & Associates. This article tion of free chlorine with certain organic com - Agency (EPA) Stage 2 D/DBP rule sets the max - was presented at the Florida Water Re - pounds in the wastewater leads to the imum contaminant level for four THMs and sources Conference in April 2009. formation of disinfection byproducts, includ - five HAAs at 80 g/L and 60 g/L, respectively, ing THMs. Limited knowledge is available on a basis of a locational running annual aver - about the formation and control of THMs age for drinking water. The four regulated formation during chlorine disinfection is crit - during wastewater disinfection. THMs are chloroform, bromodichloromethane ical for wastewater utilities to meet the FDEP’s Many utilities in Florida discharging to sur - (BDCM), dibromochloromethane (DBCM), THM surface water discharge standards. face waters are having difficulty complying with and bromoform. Figure 1 shows the chemical limits for bromodichloromethane and dibro - structures of the THMs. Trihalomethane Formation mochloromethane, the two regulated THMs. To protect surface water quality, the During Chlorination This article presents a comprehensive evaluation Florida Department of Environmental Protec - of factors affecting THM formation during tion (FDEP) sets regulatory limits for THMs When chlorine is introduced into water, chlorination and details available technologies for treated wastewater effluents discharged to it reacts with DBP precursors to form byprod - for controlling THMs in wastewater. The article surface waters. Unlike the Stage 2 D/DBP rule ucts. Natural organic matter (NOM) present also recommends ways for wastewater utilities for drinking water, the FDEP sets a regulatory in source water is the primary precursor for to meet the effluent THM limitations. limit for each THM species. The annual aver - the formation of DBPs in drinking water Disinfection is a vital process to inactivate age limits for chloroform, BDCM, DBCM, and (Reckhow et al, 1990). NOM is a complex mix - pathogenic microorganisms in drinking water bromoform are 470.8, 22, 34, and 360 g/L, re - ture of organic compounds derived from the and wastewater, but since the 1970s it has been spectively, for a Class II or Class IIIµsurface decay of vegetation and animal material. recognized that disinfection can produce harm - water discharge (Table 1). The organic matter found in municipal ful byproducts and cause health concerns. Because the limits for chloroform and wastewater is significantly different from drink - Chorine is by far the most widely used chemical bromoform are relatively high, it is not diffi - ing water supplies. The organic matter in waste - disinfectant in water and wastewater treatment. cult for wastewater utilities to meet the chlo - water typically consists of proteins, carbohydrates, Chlorine readily reacts with certain or - roform and bromoform standards under and oils and fats (Metcalf & Eddy, 2002). ganic compounds in water and wastewater to typical wastewater chlorination conditions. In addition to organic matter, bromide form disinfection byproducts (DBPs) that Consequently, controlling BDCM and DBCM ions are important inorganic DBP precursors. Bromide ions are ubiquitous in water and wastewater and are quickly oxidized by chlo - Cl Br Br Br rine to bromine during chlorination. Subse - quent reactions between bromine and organic H C Cl H C Cl H C Br H C Br matter result in the formation of brominated THMs and HAAs. These reactions are sum - marized in Equations 1-3. Cl Cl Cl Br Chloroform Bromodichloromethane Dibromochloromethane Bromoform HOCl + Organics DBPs (Equation 1) HOCl + Br - HOBr + Cl - (Equation 2) Figure 1: The THM Species HOBr + Organics DBPs (Equation 3) When ammonia is present in treated efflu - Class III : Rec rea ti on , Prop agati on and ents, it reacts with chlorine to form chloramines. Class I: Class II : Maintenance of a Hea lt hy , We ll- The formation of chloramines can be summa - Potable Shell fish Balance d Popu lati on of Fish and THMs rized in its simplest form by Equations 4-6. Wa ter Prop agati on or Wil dli fe Supp ly Harvesti ng Predo minantl y Predo mi nantl y NH 3 + HOCl NH 2Cl + H 2O (Equation 4) Fresh Wa ter Marine Wa ter NH 2Cl + HOCl NHCl 2 + H 2O(Equation 5) CHCl3 5.67 μg/L 470 .8 μg/L 470 .8 μg/L 470 .8 μg/L NHCl 2 + HOCl NCl 3 + H 2O (Equation 6) CHBrCl2 0.27 μg/L 22 μg/L 22 μg/L 22 μg/L CHBr2Cl 0.41 μg/L 34 μg/L 34 μg/L 34 μg/L The distribution of mono-, di-, and CHBr3 4.3 μg/L 360 μg/L 360 μg/L 360 μg/L trichloramines primarily depends on the chlo - rine-to-ammonia ratio and the pH. Mono - Table 1: Criteria for Surface Water Quality Classifications (FDEP, FAC 62-302.530) Continued on page 8 6 • APRIL 2010 • FLORIDA WATER RESOURCES JOURNAL Continued from page 6 pH, chlorine dose, and temperature. As shown in Figure 5, Chloroform yield chloramine is predominant at low Cl 2 –N ra - Several mechanistic and empirical mod - gradually decreased while bromoform yield tios (e.g. < 5 mg Cl 2/mg N). Free chlorine els have been developed to describe the forma - gradually increased as bromide was increased. residual appears when the chlorine-to- tion of THMs. For example, Equation 7 shows CHCl 2Br and CHClBr 2 yields passed through ammonia ratio exceeds a certain point (theo - an empirical model to predict the formation of the maximum at 2-10 mol/L (160-800 g/L) retical value at 7.6). This is often referred to as the four regulated THMs (Westerhoff, 2006). bromide concentratioµn and decreased µwhen break-point chlorination (Figure 2). bromide ions were increased. Bromoform was 1.098 0.152 - 0.068 In general, chloramines form fewer THMs = 0.0412[TOC] [Cl 2] [Br ] dominant when the bromide concentration THMs and HAAs than free chlorine. Dihalo - [Temp] 1.069 [pH] 0.263 (Equation 7) was increased to 30 mol/L (2400 g/L). genated HAAs are the predominant known Figure 6 showsµthat THM conµcentration DBPs identified in chloraminated waters (Hua This model predicts that the concentra - increased as reaction time and pH value in - and Reckhow, 2008). tion of THM increases with increasing total creased. The formation of THMs exhibited an Figure 3 shows a conceptual model of the organic carbon (TOC), chlorine dose, bro - initial rapid phase within a few hours, followed formation pathways of THMs and HAAs dur - mide, temperature, pH, and reaction time. by a more steadily increasing phase. ing chlorination and chloramination. Based Bromide ions affect the yield of THMs and Varying pH also strongly influenced the on this model, formation of THMs, greatly influence the speciation of THMs. formation of THMs. THM concentration in - trichloroacetic acid (TCAA), and Consequently, the presence of bromide is crit - creased by a factor of 2.8 and 1.6, respectively, dichloroacetic acid (DCAA) proceeds through ical for controlling individual THM species. when increasing pH from 5 to 7 and 10 for a common dihalogenated intermediates. The distribution of THM species depends 24-h contact time. Further halogenation of dihalogenated primarily on the bromide-to-chlorine ratio. The chlorine dose and reaction tempera - intermediates by chlorine leads to the forma - Figure 4 shows a qualitative profile of THM ture also have a significant impact on THM tion of THMs and TCAA. THMs form speciation as a function of the Br/Cl 2 ratio. formation. The concentrations of THM in - through base-catalyzed hydrolysis, and the for - Chloroform predominates at low Br/Cl 2 ratios. creased nearly linearly with an increasing chlo - mation of THMs is favored at high pH values. More brominated THM species are formed rine dose (Figure 7). A more than 100-percent Certain chemical factors prohibit chloramines with increasing bromide concentrations. The increase in THMs was observed when increas - from adding a third halogen to dihalogentaed maximum levels of BDCM and DBCM appear ing the reaction temperature from 5 oC to 30 oC intermediates; therefore, chloramines are not at a medium range of bromide to chlorine ra - (Figure 8); therefore, wastewater utilities may active in the formation of THMs. tios. Thus, the formation of BDCM and DBCM experience increased THMs in the chlorine Numerous water quality and treatment can be reduced or minimized either by reduc - contact basin during warm-weather operation factors affect THM formation. The formation ing the bromide concentration to low levels or compared to cold-weather operation. and speciation of THMs primarily depends on by increasing bromide concentration to high Figure 9 shows the influence of ammonia source water characteristics and specific disin - levels to shift the speciation toward the forma - on the formation of DBPs during chlorination. fection conditions. The type and abundance of tion of chloroform or bromoform. The ratio of chlorine to ammonia had a great organic precursors and bromide and iodide Figures 5-8 present examples of the effect impact on the THM yield. THM concentra - concentrations have been shown to signifi - of bromide concentration, pH, reaction time, tions decreased rapidly as the ammonia-to- cantly influence the concentration and specia - dose, and temperature on THM formation.