Abstract

In this study, diffused aeration was applied to remove trihalomethane (THM) compounds from chlorinated, treated water containing high dissolved organic carbon (DOC) of 6.8 ± 1.2 mg/L. Increasing air-to-water volumetric ratio (rA/W) from 16 to 39 enhanced total THM (TTHM) removal from 60 to 70% at 20 °C and from 30 to 50% at 4 °C. Although bromodichloromethane has lower Henry's law constant than chloroform (CF), it was removed by a higher degree than CF in some aeration trials. Albeit obtaining high removals in aeration, TTHM reformed, and their concentration surpassed the Canadian guideline of 100 ppb in about 24 hours at 20 °C and 40 hours at 10 °C in all attempted air-to-water ratios. The water age in the system investigated in this study varied from 48 hours in midpoint chlorine boosting stations to 336 hours in the nearest endpoint. This study showed that THM removal by aeration is not a viable solution to control the concentration of these disinfection by-products in high-DOC treated water and in distribution systems where water age exceeds 24 hours; unless, it is going to be installed at the distribution endpoints.

INTRODUCTION

The primary purpose of drinking water treatment is to eradicate pathogenic and non-pathogenic bacteria and to control their growth in distribution systems (Von Gunten 2003; Özdemr 2014; Bertelli et al. 2018). A worldwide accepted strategy to control bacterial accumulation is through the use of chemical disinfectants such as chlorine (Boorman 1999; Bitton 2014). Accordingly, regulatory agencies require water purveyors to retain a regulated amount of chlorine residual in water throughout the distribution system to hinder the microbial growth and ensure inactivation of pathogens.

The Province of Manitoba (Canada) requires a minimum contact time of 20 minutes in chlorine disinfection. Free chlorine residual must be above 0.5 mg/L immediately after chlorination and not less than 0.1 mg/L throughout the distribution system (Manitoba Water Stewardship 2007). Although preventing water-borne diseases merits applying chlorine to water entering the distribution system, chlorine is very reactive with natural organic matter (NOM) present in water and forms disinfection by-products (DBPs) such as trihalomethanes (THMs). THMs are possible carcinogens and can be associated with early-term miscarriage and developmental problems (Richardson 2003; Barceló 2012; Nieuwenhuijsen et al. 2015). Thus, these chemicals are regulated all over the world. The maximum acceptable concentration of total THM (TTHM) in Guidelines for Canadian drinking water quality is 100 μg/L (Health Canada 2017).

Approximately 87% of drinking water plants in the Province of Manitoba, Canada, use surface water as their drinking source (Statistics Canada 2011). Many surface water sources in the Canadian prairies have a high concentration of dissolved organic carbon (DOC), ranging from 8 to 25 mg/L, therefore, almost 70% of the potable water systems in Manitoba exceed Federal and Provincial water quality standards for THMs (Goss et al. 2017).

There are several strategies to reduce the concentration of THMs including enhanced removal of THM precursors (i.e. DOC), application of alternative disinfectants which prevent the formation of THMs, and post-removal of THMs following their formation (Wang et al. 2015). Aeration of treated water, i.e. post-removal of THMs, has garnered much attention due to the simple mechanism and low capital and operating costs in comparison to precursor removal processes or the introduction of alternative disinfectants (Ghosh et al. 2015).

Aeration to control THMs

The aeration process uses a liquid–gas concentration gradient to transport volatile compounds from liquid to gas phase. Due to the volatile nature of THMs, they can be effectively removed by this process. When the concentration of THMs in the water is higher than the equilibrium concentration at the air–water interface, THMs will diffuse from water to the air. At the interface, the partial pressure of THMs in the air (P) has a direct relationship with their mole fraction in water (X) based on Henry's Law (American WWA & Edzwald 2000): 
formula
(1)
where H is Henry's law constant (atm).

The overall mass transfer of THMs is a function of Henry's law constant. The equations used for the design of different air stripping systems can be found in detail elsewhere (American WWA & Edzwald 2000).

Factors affecting THMs reduction using aeration

Air-to-water volumetric ratio (rA/W)

The air-to-water volumetric ratio is the amount of available air for mass transfer per unit volume of water. Many studies suggested that THMs removal enhances with increasing rA/W, but this increase is very dependent on several factors including the type of aeration, water temperature, and the THM species (Brooke & Collins 2011; Cecchetti et al. 2014). Several studies have analyzed the effect of temperature, THMs species, types of aeration, and air-to-water volumetric ratio on THMs air stripping. Table 1 summarizes these findings and the important operational parameters employed in these studies.

Table 1

Application of different aeration setups to remove THMs

Water source/treatment processWater quality parameters
Design variables of the aeration unit
THMs removala, %
THMs reformationReference
pHDOC mg/LType/aeratorWater temperature (°C)Water volume (L or L/min)Air flow (L/min)rA/WbCFBDCMCDBMBFTTHM
Synthetic water, RO-filtered and distilled water spiked with THM speciesc,d n.a.e Diffused/fine bubble stone diffuser 20 (1) 1.5 22.5 95 (58) 78 (38) 50 (15) 30 (10) 70 (32) n.c.f (Brooke & Collins 2011
1.5 30 100 (88) 88 (55) 60 (32) 40 (22) 75 (55) 
45 100 (95) 98 (70) 85 (38) 58 (22) 85 (62) 
60 100 (98) 100 (78) 90 (48) 65 (30) 90 (68) 
n.a. Spray/ BETE spiral nozzle 20 – – 22.5 95 80 52 32 n.a. 
30 98 85 62 40 n.a. 
45 100 98 85 55 n.a. 
60 100 99 90 65 n.a. 
Synthetic water, tap water spiked with THM species n.a. n.a. Pilot scale diffused (membrane tube fine bubble cylindrical diffuser) 20 750 140 33.6 97 93 81 64 84 n.c. (Sherant 2008
Real water, field study, Blacklick Valley, Pennsylvania n.a. n.a. Diffused, full scale 14–21 (158) 1,800 11.4 ∼42 ∼56 n.a. n.a. ∼60 12 days (aeration maintained TTHM under 80 μg/L even at a distant location) 
Real water, Polk County Babson Park WTP, Florida 7.4 Spray BETE spiral nozzle (TF10) 26.5 (11.4) n.a. 58.3 54.7 45.9 n.a. 54.7 130 hours (compared to 30 hours for non-aerated sample) (Duranceau & Smith 2016
GridBeeMedora Corp, nozzle 48.6 44.8 36.9 n.a. 45.2 110 hours (compared to 30 hours for non-aerated sample) 
Real water, Oviedo Mitchell Hammock WTP, Florida 7.9 1.6 Spray BETE spiral nozzle (TF10) 23.5 (11.4) n.a. 52.9 53.3 44.5 31.6 48.1 10 hours (compared to 5 hours for non-aerated sample) 
GridBeeMedora Corp, nozzle 38.6 42.8 36.3 26.4 37.7 10 hours (compared to 5 hours for non-aerated sample) 
Water source/treatment processWater quality parameters
Design variables of the aeration unit
THMs removala, %
THMs reformationReference
pHDOC mg/LType/aeratorWater temperature (°C)Water volume (L or L/min)Air flow (L/min)rA/WbCFBDCMCDBMBFTTHM
Synthetic water, RO-filtered and distilled water spiked with THM speciesc,d n.a.e Diffused/fine bubble stone diffuser 20 (1) 1.5 22.5 95 (58) 78 (38) 50 (15) 30 (10) 70 (32) n.c.f (Brooke & Collins 2011
1.5 30 100 (88) 88 (55) 60 (32) 40 (22) 75 (55) 
45 100 (95) 98 (70) 85 (38) 58 (22) 85 (62) 
60 100 (98) 100 (78) 90 (48) 65 (30) 90 (68) 
n.a. Spray/ BETE spiral nozzle 20 – – 22.5 95 80 52 32 n.a. 
30 98 85 62 40 n.a. 
45 100 98 85 55 n.a. 
60 100 99 90 65 n.a. 
Synthetic water, tap water spiked with THM species n.a. n.a. Pilot scale diffused (membrane tube fine bubble cylindrical diffuser) 20 750 140 33.6 97 93 81 64 84 n.c. (Sherant 2008
Real water, field study, Blacklick Valley, Pennsylvania n.a. n.a. Diffused, full scale 14–21 (158) 1,800 11.4 ∼42 ∼56 n.a. n.a. ∼60 12 days (aeration maintained TTHM under 80 μg/L even at a distant location) 
Real water, Polk County Babson Park WTP, Florida 7.4 Spray BETE spiral nozzle (TF10) 26.5 (11.4) n.a. 58.3 54.7 45.9 n.a. 54.7 130 hours (compared to 30 hours for non-aerated sample) (Duranceau & Smith 2016
GridBeeMedora Corp, nozzle 48.6 44.8 36.9 n.a. 45.2 110 hours (compared to 30 hours for non-aerated sample) 
Real water, Oviedo Mitchell Hammock WTP, Florida 7.9 1.6 Spray BETE spiral nozzle (TF10) 23.5 (11.4) n.a. 52.9 53.3 44.5 31.6 48.1 10 hours (compared to 5 hours for non-aerated sample) 
GridBeeMedora Corp, nozzle 38.6 42.8 36.3 26.4 37.7 10 hours (compared to 5 hours for non-aerated sample) 

aData in parentheses are for aeration process that was conducted at the water temperature of 1 °C.

brA/W in spray aeration depends on the average droplet travel distance (havg) which equals to height of the headspace in the water tank and the Sauter mean diameter of water droplets (dSMD), rA/W = 1.5 havg/dSMD.

cComposition of TTHM: CF 40% – other species 20%.

dTHMs removal values may not be precise. They are estimated from the provided figures in this study.

eNot available.

fNot conducted.

Temperature

Temperature is a particularly important parameter in removing THMs by aeration since the solubility of gases decreases with temperature and, consequently, Henry's law constant increases (American WWA & Edzwald 2000). The magnitude of the temperature dependence on H is determined by standard enthalpy change (Δ) for the dissolution of a component in water (American WWA & Edzwald 2000) (Equation (2)). Henry's law constants of THM species at 20 and 4°C are provided in Table 2. 
formula
(2)
where R is the universal gas constant (8.206 L·atm·mol–1·K–1), H1 is a known value of Henry's constant at temperature T1 (K), H2 is the calculated Henry's law constant at the desired temperature T2 (K).
Table 2

Henry's law constant for THM species at two temperatures encountered in water treatment

THM speciesH (atm), 20 °CaH (atm), 4 °Cb
Chloroform (CF) 170 58 
Bromodichloromethane (BDCM) 118 30 
Dibromochloromethane (DBCM) 47 17 
Bromoform (BF) 35 
THM speciesH (atm), 20 °CaH (atm), 4 °Cb
Chloroform (CF) 170 58 
Bromodichloromethane (BDCM) 118 30 
Dibromochloromethane (DBCM) 47 17 
Bromoform (BF) 35 

bCalculated based on the empirical equations provided by Nicholson et al. (1984).

Type of aerator

There are four different types of aerators, including packed bed columns, diffused or bubble aeration, spray aeration, and surface aeration. A detailed description of aerator design can be found in other references (American WWA & Edzwald 2000; Ghosh et al. 2015). All aerator types can achieve high levels of THM removal, but the most apt ones are those with minimal operational and maintenance costs, high energy efficiency, and are able to be installed in the current water treatment facilities (Brooke & Collins 2011; Ghosh et al. 2015). The most popular aeration systems employed are diffused and spray aeration types with spray aeration being more energy-efficient and less susceptible to THM speciation (Brooke & Collins 2011; Cecchetti et al. 2014; Ghosh et al. 2015). Additionally, in diffused aeration, air bubbles reach saturation with THMs at a certain depth, where no more THMs will be transferred into the air bubbles; thus, the diffused aeration is not recommended for depths higher than 5 m (American WWA & Edzwald 2000).

Although spray aeration outperforms diffused aeration, it has some disadvantages as well. A sensitivity analysis in spray installations demonstrated that 4.5–6 m of the headspace above the water level in the storage tank is required to optimize THMs removal (Cecchetti et al. 2014). Since many treated water reservoirs in the Canadian Prairies are shallow (<5 m) clearwells and do not have the required headspace for spray installation, diffused aeration was selected as the focus of this research.

Concentration of bromine in the water source

The formation of brominated THMs depends on the concentration of bromine in water. As reflected in Table 2, increasing the amount of bromine in THM compounds decreases Henry's law constant and reduces the removal efficiency via air stripping. Therefore, theoretically, removing bromoform by aeration is more difficult than removing chloroform.

Moreover, the presence of brominated THMs alters the phase that controls mass transfer and, consequently, the efficacy of the aeration process and reformation of THMs. Removal of brominated THMs is controlled by gas-film diffusion and spray aeration (with adequate ventilation) provides a lower concentration of THMs in the gas phase; this means higher driving force available for the diffusion of THMs from water droplets to the headspace air (Brooke & Collins 2011). Therefore, the difference between CF and BF removal was reported to be only 12% in spray aeration compared to 56% in diffused aeration (Brooke & Collins 2011).

Reformation of THMs post aeration

Chlorine and organic matter are continuously reacting in the water distribution system, therefore, THMs continue to form following the aeration process. There is not much information about the THMs reformation after aeration, and the reported reformation times vary significantly from 10 to 130 hours (Table 1). A recent study evaluated the reformation of THMs after spray aeration in two water treatment facilities using groundwater as the source with low concentrations of total organic carbon (TOC) ranging from 1.6 and 2 mg/L, and the results showed very different trends (Duranceau & Smith 2016). In one facility, spray aeration maintained the level of TTHM below 80 μg/l (US guideline) in the distribution system even after 130 hours (∼5.5 days). In contrast, in the other facility (OWTP) where water had even slightly lower TOC levels (i.e. 1.6 vs. 2 mg/L), THM concentrations remained below the guideline for only 10 hours (Duranceau & Smith 2016). The prompt THM reformation was justified to be due to higher bromide concentration in OWTP (i.e. 0.12 vs. <0.05 mg/L) (Duranceau & Smith 2016). This justification was made according to the fact that HOBr reacts with NOM about one order of magnitude faster than HOCl (Hua et al. 2006), therefore the presence of HOBr may be the reason for quick reformation of THMs following aeration.

Research objectives

Many Canadian potable waters have high DOC levels ranging from 4 to 5 mg/L, and are distributed through long distribution systems. The high water DOC and long travel time are the main reasons for high concentration of THMs at the tap. This research investigates the potential application of aeration for removal of THM from these waters. No current studies are investigating air stripping of THMs from waters with DOC higher than 2 mg/L. Therefore, the first objective of this research was to study the applicability of aeration for the control of THMs in high DOC treated water. The reported THMs reformation time in which THMs exceed the guidelines varies significantly from 10 to 130 hours. Therefore, another goal of this research is to evaluate re-formation of THM throughout the typical Canadian distribution system.

MATERIALS AND METHODS

Plant description

The River Regional Water Treatment Plant (WTP)-Letellier (called Letellier hereafter) is a conventional lime/caustic soda softening water treatment supplied by the Red River (Figure 1). The Red River represents a typical surface water quality throughout the Manitoban prairies containing high concentrations of DOC (>10 mg/L) and hardness (>300 mg/L as CaCO3) (Goss et al. 2017; Archibald & Fehr 2018). The water treatment process at Letellier does not sufficiently remove DOC (removal ∼30.4–42.4%) to prevent excessive THMs formation. The TTHM concentration can reach 170 μg/L at the endpoints of the distribution system. Many other conventional lime/soda softening plants treating high DOC waters have a similar problem.

Figure 1

Schematic of Letellier existing water treatment process (Archibald & Fehr 2018).

Figure 1

Schematic of Letellier existing water treatment process (Archibald & Fehr 2018).

Letellier has four underground rectangular reservoirs, i.e. clearwells, to store treated water which are located under the plant's facilities. Clearwells are connected to each other through pipes with a nominal size of 0.25 m, and the maximum water depth is about 3.5 m. The plant uses gas chlorine as a disinfectant and its annual average chlorine dose is 3.7 mg/L resulting in 1.5 mg/L of free chlorine residual at the clearwell.

Pembina valley water distribution system

Letellier, Morris and Stephenfield WTP are operated by Pembina Valley Water Co-op (PVWC) which serves 14 municipalities and approximately 50,000 people. PVWC service area covers 9,000 square kilometres through a complex distribution system. The section of distribution systems which is covered by Letellier WTP is depicted in Figure 2. There are several boosting stations to monitor and adjust water pressure and chlorine residual. Supplementary chlorine is supplied by injecting sodium hypochlorite to the pipe/mid-reservoir for a required amount of time.

Figure 2

Letellier WTP distribution system (Archibald & Fehr 2018).

Figure 2

Letellier WTP distribution system (Archibald & Fehr 2018).

In summer and during a peak flow, water age in Altona could be 2 days; continuing to Winkler, it reaches to 4.6 days. Under average flow, water travelling time in Altona and Winkler reaches 2.9 days (69.6 hours) and 6.8 days (163.2 hours), respectively. The water age in winter could be twice as high as in the summer, estimated at about 6 days (144 hours) in Altona and 14 days (336 hours) in Winkler under the average flow.

General methodology

Water sample collection

Sampling was conducted twice, on September 13th, 2017, and on February 21st, 2018, to evaluate the effects of seasonal DOC and temperature changes. Six litres of water were collected from the Letellier WTP clearwell, treated chlorinated water, in organic-free 1 L amber bottles lacking headspace and were maintained at 4 °C until DOC and THMs formation analysis. The general water quality parameters of both samples are provided in Table 3.

Table 3

General water quality of samples taken from the Letellier treated water clear well

Parameter (unit)Fall sample Sep. 13th, 2017Winter sample Feb. 21st, 2018
pH (–) 7.25 7.60 
Alkalinity (mg/L as CaCO360 106 
Total hardness (mg/L as CaCO3136 270 
Ca hardness (mg/L as CaCO3104 174 
Non-carbonate hardness 76 164 
DOC (mg/L) 6.8 ± 1.2 6.3 ± 1.5 
Parameter (unit)Fall sample Sep. 13th, 2017Winter sample Feb. 21st, 2018
pH (–) 7.25 7.60 
Alkalinity (mg/L as CaCO360 106 
Total hardness (mg/L as CaCO3136 270 
Ca hardness (mg/L as CaCO3104 174 
Non-carbonate hardness 76 164 
DOC (mg/L) 6.8 ± 1.2 6.3 ± 1.5 

Determination of chlorine uptake in the water distribution system

Prior to studying the (re)formation of THMs in the distribution system, the free chlorine uptake of water samples was measured in order to determine the required amount of chorine needed for THMs formation at each water age. This approach allows us not to over-chlorinate water for THMs formation and prevent the plausible overestimation of THMs. Sodium hypochlorite was used to chlorinate water and the residual free chlorine in water samples was quantified by the DPD method (HACH method 8021) using a HACH Dr-2000 spectrophotometer.

The chlorine uptake of water samples was measured for both summer and winter samples with respect to various water travel times of 3, 7, 10, 24, 48, 72, 168 (7 days), and 336 hours (14 days), to mimic the water age in the distribution as explained in the previous section. The required amount of chlorine stock solution was added to different 70 mL amber bottles to provide 20 mg/L of chlorine; bottles were maintained in an incubator for different water times. The incubator temperature was set at 20 °C for summer and at 20 and 10 °C for winter samples (Figure 3). The free chlorine uptake at each travel time was calculated by deducting the free chlorine that remained after that time from the initial chlorine concentration, i.e. 20 mg/L.

Figure 3

Amount of free chlorine uptake at different water travelling times: (a) summer sample was taken on September 13th, 2017; (b) winter sample was taken on February 21st, 2018.

Figure 3

Amount of free chlorine uptake at different water travelling times: (a) summer sample was taken on September 13th, 2017; (b) winter sample was taken on February 21st, 2018.

Measurement of THM compounds

THM species were measured by a liquid–liquid extraction method according to Standard Methods 6232B (APHA 2012) using Agilent 7890A GC System (Agilent Technologies, Santa Clara, CA) equipped with an electron capture detection (ECD) and a Combi PAL CTC Analytics autosampler.

Aeration setup and procedure

The aeration experiments were completed in 900 mL of water samples, summer and winter, in a cylindrical glass reactor with a diameter of 6 cm and height of 44 cm. A spherical fine bubble stone diffuser, Fisherbrand™, with diameter of 2.5 cm and an average pore size of 60 μm was placed in the bottom of the reactor using a PTFE tube. The schematic of the setup is provided in Figure 4.

Figure 4

Bench-scale diffused aeration setup.

Figure 4

Bench-scale diffused aeration setup.

Oil-free air, GC grade, was supplied via a compressed air cylinder, and a VWR 0–5 L/min flow meter was used to control air flow at 1.5 L/min. The required air-to-water ratio was provided by changing the time of the aeration ranging from approximately 9 to 23 minutes. Conducting the experiments at room temperature allowed for stabilization of the aerator's temperature at 20 °C, which resembles the average temperature of the plant's reservoir in summer. In regards to the low-temperature level, the reactor was immersed in an ice bath to control the temperature at 4 °C, which is the temperature of the plant's clearwell during cold seasons (Anderson 2016, personal communication).

For this study, four different air-to-water volumetric ratios were used to evaluate THMs removal, including 16, 24, 31, and 39. These rA/W values were selected based on the intention of installing 2, 3, 4 or 5 air blowers with a capacity of 34 m3/min at Letellier WTP's clearwell (Anderson 2016, personal communication). The annual average flow of the plant is 4.4 m3/min (72.25 litres per second) so the aforementioned air to water volumetric ratio was calculated by dividing varying air flows by a mixed water flow. These rA/W values were in the typical range studied for diffused aeration (Table 1). Aeration experiments were carried out in duplicates, and THMs concentration was measured before and after each test to quantify the removal of THMs. To evaluate the chlorine loss during aeration, the water sample was spiked with the required amount of chlorine stock solution prior to aeration to provide 3 mg/L of free chlorine in 1 L of sample. pH and free chlorine were measured before and after aeration. SDS-THMs analysis was used on the water following aeration to study the reformation of THMs after completion of the aeration process.

Simulated distribution system trihalomethanes (SDS-THMs)

THMs (re)formation, both THMs formation in the distribution system without aeration as well as the reformation of THMs after aeration, was measured in various water ages according to the Method 5710C, Simulated Distribution System Trihalomethanes (SDS-THMs) (APHA 2012). The sample under study was chlorinated to have residual free chlorine of 4 mg/L and analysed for THMs after the simulated, i.e. mimicking the distribution system, water travel time. The 4 mg/L free chlorine residual was selected based on 3–5 mg/L free chlorine recommended for Total THM Formation Potential test (APHA 2012). The equations in Figure 4 were used to determine the required amount of chlorine (as sodium hypochlorite), i.e. free chlorine uptake +4 mg/L of residual free chlorine, at the respective travel time. No pH adjustment was made on the simulated distribution system samples.

For SDS-THM, i.e. THMs (re)formation experiments, two incubation temperatures, 20 °C for summer and 10 °C for winter samples, were used as representatives of water temperature in the Letellier WTP distribution system during warm and cold seasons. These temperatures were selected according to the Manitoba soil temperature records in the target region from the Manitoba Agriculture Ministry (Government of Manitoba n.d.).

RESULTS AND DISCUSSION

Effect of water age and seasonal variation (T and DOC) on THMs

Formation in the distribution system

The SDS-THMs test showed that the formation of THMs correlates well with water travelling time for both summer, September 13th, 2017, and winter samples, February 21st, 2018 (Figures 5 and 6). In summer, the concentration of total THMs was very close to the guideline (100 μg/L), even at the plant reservoir (at the travel time of 0), and plateaued at 400 μg/L after 14 days (336 h) of incubation at 20 °C. This suggests that THMs can exceed the guideline even at the plant's reservoir during warmer seasons. For winter samples that were incubated at 10 °C, THMs exceeded the guideline in approximately 50 hours and reached 150 μg/L after 14 days (Figure 5). During winter, the rate of THMs formation over time is more moderate than it is in summer, and the final TTHM level is much lower (150 vs. 400 μg/L). Lower TTHM concentration of winter samples compared to summer samples is due to the lower incubation temperature of winter samples or seasonal change in DOC characteristics. Figure 7 demonstrates the profound effect of temperature on the THMs formation. The same data presented in Figure 6 reappeared in Figure 7, the blue line, to visualize the effect of temperature more conveniently. It can be seen that doubling the incubation temperature, from 10 to 20 °C, doubles the average THMs concentration in winter samples, 280 vs. 140 μg/L, after 14-days incubation time.

Figure 5

TTHM concentration versus time for summer sample, incubation temperature = 20 °C (error bars for n = 2).

Figure 5

TTHM concentration versus time for summer sample, incubation temperature = 20 °C (error bars for n = 2).

Figure 6

TTHM concentration versus time for winter sample, incubation temperature = 10 °C (error bars for n = 2).

Figure 6

TTHM concentration versus time for winter sample, incubation temperature = 10 °C (error bars for n = 2).

Figure 7

Effect of incubation temperature on TTHM concentration for the winter sample in different water ages. Please refer to the online version of this paper to see this figure in colour: http://dx.doi.org/10.2166/wqrj.2020.016.

Figure 7

Effect of incubation temperature on TTHM concentration for the winter sample in different water ages. Please refer to the online version of this paper to see this figure in colour: http://dx.doi.org/10.2166/wqrj.2020.016.

The effect of seasonal change in DOC characteristics on THMs formation can be discerned by comparing Figures 5 and 7. At the same incubation temperature of 20 °C, summer samples continued to have higher THMs formation potential compared to the winter sample, 400 vs. 280 μg/L, even though the DOC level was similar in both samples (Table 3). This can be attributed to the seasonal variability of DOC characteristics. The Red River water DOC is more hydrophilic in summer than in winter, and in this source water, hydrophilic fraction has been reported to have higher THM formation potential (THMFP) than hydrophobic fraction of DOC (Goss & Gorczyca 2013). Higher THMFP of hydrophilic fraction has been also documented by other researchers (Hwang et al. 2000; Marhaba & Van 2000).

Removal of THMs by diffused aeration

Diffused aeration was conducted on water samples taken in summer and winter from the plant's reservoir and at two temperatures, 20 and 4 °C, in an effort to simulate water temperature in the reservoir during each season (Anderson 2016, personal communication). Each water sample was aerated in duplicate at different rA/W values of 16, 24, 31, and 39. A 20-mL aliquot of sample was taken immediately before aeration for THMs analysis at the time of zero. Then, chlorine was added to a 1-L water sample to have 3 mg/L of free chlorine residual and evaluate chlorine loss during aeration. This amount was adequate to study reformation of THMs after 7 hours. Figure 8 demonstrates the average removal achieved for different THM species and total THM.

Figure 8

Diffused aeration removal of four THM species as a function of air-to-water ratio at (a) summer sample and at 20 °C and (b) winter sample and at 4 °C (error bars for n = 2).

Figure 8

Diffused aeration removal of four THM species as a function of air-to-water ratio at (a) summer sample and at 20 °C and (b) winter sample and at 4 °C (error bars for n = 2).

The data shows the TTHM removal was in the range of 60–70% at 20 °C and 30–50% at 4 °C. As expected, high temperature enhanced THM removal which can be due to a larger Henry's law constant or a decrease in resistance to mass transfer (Albin & Holdren 1985). Although it has been reported that air-to-water ratio has a significant effect on THMs removal by diffused aeration (Albin & Holdren 1985; Brooke & Collins 2011), in this study, increasing the volumetric air-to-water ratio did not result in statistically significant higher TTHM removal, especially at 20 °C. No control was conducted to evaluate the formation of THMs during aeration time. However, according to the data provided in Figure 5, the change in TTHM concentration during a specific amount of time, ΔTTHM vs. Δt, at 20 °C follows this relationship: ΔTTHM = 7.2131 × Δt0.6483, R2 = 0.96. Using this formula, the total THM could have been formed during the maximum air-to-water ratio, i.e. aeration time of 0.39 hours, was calculated to be 3.93 μg/L. Even by considering THMs formation of 3.93 μg/L during aeration, and assuming all are in the form of chloroform, the effect of increasing air-to-water ratio on TTHM removal was still evaluated to be insignificant.

Free chlorine loss during aeration was relatively small in the range of 0.45–0.8 mg/L at 20 °C and 0.4–0.5 at 4 °C, which is comparable to the range provided in the literature (Sherant 2008; Brooke & Collins 2011). Therefore, considering the current chlorine residual of about 1.5 mg/L at the plant's clearwell, all the aerated samples should maintain the minimum chlorine residual of 0.5 mg/L at the plant suggested by the province of Manitoba guidelines (Manitoba Water Stewardship 2007).

In some of the trials, bromodichloromethane had higher removal compared to chloroform despite its lower Henry's law constant. A similar trend was also observed in spray aeration of a treated natural, not synthetic, water (Sherant 2008; Duranceau & Smith 2016). Recently, a pilot study using PAX Surface Aeration Technology was conducted in Plumas, an end point in the Portage la Prairie WTP distribution system, Manitoba. This study also acquired a higher average removal of BDCM, 80.1%, versus CF, 60.8% (Parsons 2018). This phenomenon can be explained by the presence of a complex matrix of organic matter in natural water samples. Organic materials can act as a surfactant and change the solubility of components and, consequently, decrease the value of apparent H (O'Haver et al. 2004; Shah et al. 2016). Organic matter has a nonpolar hydrocarbon chain that can interact with hydrophobic and less polar solutes, such as CF, and increase their solubility (Shah et al. 2016). However, some studies reported that the presence of humic acid did not change Henry's law constant of THM species (Nicholson et al. 1984). Therefore, the reason for the lower removal of CF compared to BDCM in aeration studies that use natural waters containing NOM is not clear, and more research is required to explain this phenomenon.

REFORMATION OF THMS POST AERATION

The SDS-THM test has been performed on all aerated samples, both summer and winter, at different air-to-water ratios. Reformation studies were conducted at two temperature values, 20 and 10 °C, which is identical to conditions of the summer and winter temperatures in the distribution system (Anderson 2016, personal communication). Water samples were collected immediately after the aeration process in 70 mL amber bottles with PTFE screwed caps. These samples were chlorinated and incubated for the required water travelling times of 7, 10, 24, 72, and 168 hours. For each travel time, the required amount of chlorine was added according to the equations in Figure 3, considering that aerated water has about 2 mg/L of free chlorine residual.

The data of THMs reformation following the aeration process is provided in Figures 9 and 10. THMs continued to form with a very similar trend observed for non-aerated samples, regardless of the applied rA/W value and the removal rates. The TTHM concentration for the summer sample, aerated and incubated for the reformation study at 20 °C, exceeded the guideline in less than 24 hours and reached 260 μg/L after 7 days. For the winter sample, TTHM stayed under the Canadian guideline for about 40 hours, most likely due to the lower incubation temperature of 10 °C. This is much lower than the reported 160 hours (Duranceau & Smith 2016), which is due to higher water DOC used in this study (6.8 mg/L).

Figure 9

TTHM concentration after aeration versus water age for the summer sample, incubation temperature = 20 °C (error bars for n = 2 for each rA/W).

Figure 9

TTHM concentration after aeration versus water age for the summer sample, incubation temperature = 20 °C (error bars for n = 2 for each rA/W).

Figure 10

TTHM concentration after aeration versus water age for the winter sample, incubation temperature = 10 °C (error bars for n = 2 for each rA/W).

Figure 10

TTHM concentration after aeration versus water age for the winter sample, incubation temperature = 10 °C (error bars for n = 2 for each rA/W).

CONCLUSIONS

The objective of this study was to evaluate the aeration as a strategy to control THMs formation in high DOC treated water that is transported in a long distribution system, typical in Canada. Water samples were collected from Letellier water treatment plant representing such a system. The following conclusions can be made from this study:

  • THMs removal of 72 and 50% was achieved using diffused aeration at 20 and 4 °C, respectively, at the maximum tested air-to-water ratio of 39.

  • At 20 °C, all air-to-water ratios were able to reduce the concentration of TTHM to the range of 33.4–36.3 μg/L; while at 4 °C, the lowest TTHM concentration of 50 μg/L was obtained at the maximum rA/W.

  • THMs continued to form after the aeration process and stayed under the guideline only for about 20 hours at 20 °C and 40 hours at 10 °C in the water used in this study. Therefore, it may not be possible to control TTHM concentration under the regulatory level by aeration in distribution systems where water age is greater than 40 hours.

To conclude, aeration may be suitable for locations close to the endpoints of the distribution system where water does not remain for a long time following the aeration process, or be installed in more than one location throughout the distribution system to strip off newly formed THMs.

ACKNOWLEDGEMENTS

The authors would like to express their gratitude to Jake Fare from Pembina Valley Water Cooperative Inc. and Ken Anderson from Associated Engineering, for their financial and technical support. We also would like to thank the operators of the Letellier WTP, especially Trevor Hodgins, for facilitating sampling and providing general water quality. Last but not least, we are grateful to Dr. Victor Wei, the manager of the University of Manitoba's Civil/Environmental Engineering laboratories, for his assistance in THMs analysis.

REFERENCES

REFERENCES
American WWA
&
Edzwald
J.
2000
Chapter 6
.
Water Quality and Treatment
.
McGraw-Hill Professional Publishing
,
New York
, pp.
6-1
6-65
.
APHA
2012
Standard Methods for the Examination of Water and Wastewater
, 22nd edn.
American Public Health Association
,
Washington, DC
.
Archibald
G.
Fehr
J.
2018
Pembina Valley Water Cooperative Inc., Annual Report 2017
.
Winnipeg, Manitoba
.
Barceló
D.
2012
Emerging Organic Contaminants and Human Health
.
Springer
,
Berlin, Heidelberg
.
Bertelli
C.
Courtois
S.
Rosikiewicz
M.
Piriou
P.
Aeby
S.
Robert
S.
Greub
G.
2018
Reduced chlorine in drinking water distribution systems impacts bacterial biodiversity in biofilms
.
Front. Microbiol.
9
,
1
11
.
Bitton
G.
2014
Microbiology of Drinking Water: Production and Distribution
.
John Wiley & Sons
,
Hoboken, NJ
.
Boorman
G. A.
1999
Drinking water disinfection byproducts: review and approach to toxicity evaluation
.
Environ. Health Perspect.
107
,
207
217
.
Brooke
E.
Collins
M. R.
2011
Posttreatment aeration to reduce THMs
.
J. Am. Water Works Assoc.
103
(
10
),
84
96
.
Cecchetti
A. R.
Roakes
H.
Collins
M. R.
2014
Influence of selected variables on trihalomethane removals by spray aeration
.
J. Am. Water Works Assoc.
106
(
5
),
91
92
.
Duranceau
S. J.
Smith
C. T.
2016
Trihalomethane formation downstream of spray aerators treating disinfected groundwater
.
J. Am. Water Works Assoc.
108
(
2
),
E99
E108
.
Ghosh
A.
Seidel
C.
Townsend
E.
Pacheco
R.
Corwin
C.
2015
Reducing Volatile Disinfection By-Products in Treated Drinking Water Using Aeration Technologies
. .
Government of Manitoba
n.d.
Soil Temperature-14 Day History
. .
Health Canada
2017.
Guidelines for Canadian Drinking Water Quality – Summary Table
.
Water and Air Quality Bureau, Healthy Environments and Consumer Safety Branch
.
Hwang
C. J.
Sclimenti
M. J.
Krasner
S. W.
Barrett
S. E.
Amy
G. L.
2000
Disinfection By-Product Formation Reactivities of Natural Organic Matter Fractions of A Low-Humic Water
.
American Chemical Society
,
Washington, DC
,
USA
.
Manitoba Water Stewardship
2007
Operational Guidelines for Monitoring and Reporting Public and Semi-Public Water Systems
. .
Nicholson
B.
Maguire
B. P.
Bursill
D. B.
1984
Henry's Law constants for the trihalomethanes: effects of water composition and temperature
.
Environ. Sci. Technol.
18
(
7
),
518
521
.
Nieuwenhuijsen
M. J.
Toledano
M. B.
Eaton
N. E.
Fawell
J.
Elliott
P.
2015
Chlorination disinfection byproducts in water and their association with adverse reproductive outcomes: a review
.
Occup. Environ. Med.
57
(
2
),
73
85
.
O'Haver
J. H.
Walk
R.
Kitiyanan
B.
Harwell
J. H.
Sabatini
A. D.
2004
Packed column and hollow fiber air stripping of a contaminant-surfactant stream
.
J. Environ. Eng.
130
(
1
),
4
11
.
Parsons
T.
2018
Reduction of trihalomethane disinfection by-products in the Yellowhead Regional Water Co-op
. In
Presented by: Western Canada Water Conference & Exhibition
,
Winnipeg, Manitoba
.
Shah
A.
Shahzad
S.
Munir
A.
Nadagouda
M. N.
Khan
G. S.
Shams
D. F.
Rana
U. A.
2016
Micelles as soil and water decontamination agents
.
Chem. Rev.
116
(
10
),
6042
6074
.
Sherant
S. R.
2008
Trihalomethane Control by Aeration
.
The Pennsylvania State University
. .
Statistics Canada
2011
Survey of Drinking Water Plants
.
https://doi.org/16-403-X
.
Wang
X.
Mao
Y.
Tang
S.
Yang
H.
Xie
Y. F.
2015
Disinfection byproducts in drinking water and regulatory compliance: a critical review
.
Front. Environ. Sci. Eng.
9
(
1
),
3
15
.