The objective of this research was to study the occurrence and seasonal variations of disinfection by-products (DBPs), including traditional carbonaceous and emerging nitrogenous DBPs, in a full-scale drinking water treatment plant (DWTP) for nearly 2 years. The removal efficiencies of each DBP through the treatment processes were also investigated. This DWTP takes raw water from the Yangtze River in East China. The quality of the raw water used in this DWTP varied with different seasons. The results suggested that DBP concentrations of the finished water were higher in spring (82.33 ± 15.12 μg/L) and summer (117.29 ± 9.94 μg/L) with higher dissolved organic carbon (DOC) levels, but lower in autumn (41.10 ± 5.82 μg/L) and winter (78.47 ± 2.74 μg/L) with lower DOC levels. Due to the increase of bromide concentration in spring and winter, more toxic brominated DBPs increased obviously and took up a greater proportion. In this DWTP, DBP concentrations increased dramatically after pre-chlorination, especially in summer. It is noteworthy that the removal of DBPs during the subsequent treatment was more obvious in spring than in the other three seasons because the pH value is more beneficial to coagulation in spring.
Disinfectants, usually chlorine, can react with natural organic matter (NOM) in aqueous environments to form disinfection by-products (DBPs) in drinking water treatment processes (Richardson et al. 2007). More than 600 kinds of DBPs have been identified in drinking water, and trihalomethanes (THMs) and haloacetic acids (HAAs) are the most prevalent ones (Richardson et al. 2002; Richardson et al. 2007). Other DBPs such as haloacetonitriles (HANs), chloral hydrate (CH), haloketones (HKs) and trichloronitromethane (TCNM) have also been observed in chlorinated or chloraminated waters (Richardson et al. 2007; Wei et al. 2010).
Knowing the formation of DBPs from a specific water source is crucial for water quality management and treatment process design. DBP surveys have been conducted in many countries since the mid-1970s, starting in America, and then in Europe, Australia and Asia.
Because China possesses vast lands with various climatic and geographical conditions, the source water quality varies greatly in different areas.
Yangtze River is one of the most important drinking water sources in East China. But in recent years, raw water quality has deteriorated due to pollution (Jiang et al. 2011). Although advanced treatment processes such as ozonation and activated carbon adsorption have been applied in some large DWTPs in East China, the occurrence of THMs and other DBPs is still of great concern for the public.
In this study, the occurrence of DBPs including THMs, HANs, HKs, and TCNM in a full-scale DWTP with advanced treatment processes in East China was investigated continuously for almost 2 years with special interest in their speciation and seasonal variation. Reduction efficiencies of DBPs through the water treatment processes were also evaluated. The results of this research can contribute to the understanding of DBP formation characteristics and so to the optimization of water quality management and the operation of DWTPs in East China.
MATERIALS AND METHODS
Chemicals and reagents
Commercial sodium hypochlorite (NaOCl) solution 4–4.99%, NaOH (≥98%), KH2PO4 (≥99.0%), Na2CO3 (≥99.0%) and NaHCO3 (≥99.0%) were purchased from Sigma-Aldrich (St Louis, MO, USA) and used without further purification. DBP standard solutions of EPA 551.1 and 552.2 halogenated volatile mix including four THMs (chloroform (CF), bromodichloromethane (BDCM), dibromochloromethane (DBCM) and bromoform (BF)), four haloacetonitriles (HANs) (dichloroacetonitrile (DCAN), bromochloroacetonitrile (BCAN), trichloroacetonitrile (TCAN) and dibromoacetonitrile (DBAN)), two haloketones (HKs) (1,1-dichloropropanone (DCP) and 1,1,1-trichloropropanone (TCP)), trichloronitromethane (TCNM) and other DBP species (tetrachloromethane (PCM), tetrachloroethylene (PCE) and trichloroethylene (TCE)) were purchased from Sigma-Aldrich (USA). The extraction solvents including methyl tert-butyl ether (MtBE) and acetonitrile of high performance liquid chromatography (HPLC) grade were purchased from J.T. Baker (USA). Analytical grade reagents including Na2S2O3 and H2SO4 were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China) and used without further purification. All solutions were prepared using ultra-pure water produced from a Milli-Q water purification system (Millipore, USA).
Description of the DWTP and sample collection
The target DWTP is located in the Yangtze River delta of East China, taking water from Yangtze River. The treatment process with a capacity of 1,480,000 m3/d includes pre-chlorination (in order to control algae and improve the efficiency of coagulation, the dosage is 0.5 mg/L), coagulation/flocculation (using aluminum sulfate as the coagulant), sedimentation, filtration, ozonation, biological activated carbon (BAC) filtration and post-chlorination (the dosage changing in order to keep the concentration of residual chlorine in the range from 1.0 mg/L to 1.2 mg/L in the finished water). The flow chart of the treatment processes is shown in Figure 1.
The samples of raw water and effluent from different treatment units were collected on July 18, September 18, December 26 in 2012, April 17, July 10, September 26, November 26 in 2013 and April 6 in 2014. After collection, the samples were all immediately filtered through 0.45 μm membrane filters (Millipore Corp., USA) and stored in the dark at 4 °C until used. The raw water (collected before pre-chlorination in the plant) in this study was pre-chlorinated (the dosage is 0.2 mg/L) at the water source pump station for algal growth inhibition during long-distance water delivery, which is different from the pre-chlorination in this plant, so the concentration of DBPs in raw water was also investigated in this work. The characteristics of the raw water samples are summarized in Table S1 and shown in Figure S1 (available with the online version of this paper).
The concentration of NaOCl was calibrated using the N,N-diethyl-p-phenylenediamine (DPD) colorimetric method (APHA 1998). Turbidity was measured using a HACH 2100N turbidimeter, and pH was measured with a regularly calibrated pH meter (FE20-FiveEasy, Mettler Toledo, Switzerland). A DR/890 portable colorimeter (Hach, USA) was used to detect the concentration of NH3-N. Dissolved organic carbon (DOC) and total nitrogen (TN) were measured using a Shimadzu TOC-VCSH analyzer (Shimadzu, Japan) and the detection limit of total organic carbon (TOC) and TN was 0.1 mg-C/L. UV254 was measured using a spectrophotometer (SQ-4802, UNICO, Shanghai) with a 1 cm quartz cell at the 254 wavelength. Specific ultraviolet absorbance (SUVA) was calculated as the ratio of UV254 and DOC. The concentrations of chloride (Cl−), bromide (Br−), nitrate and nitrite were measured using an ion chromatograph (Dionex ICS-2000, USA) equipped with a conductivity detector, a Dionex AS11-HC analytical column (250 mm × 4.0 mm i.d.) and a Dionex AG11-HC guard column (50 mm × 4.0 mm i.d.).
Methods for analyzing THMs, HANs, HKs, TCNM, PCM, PCE and TCE were based on US EPA Method 551.1 (Munch & Hautman 1995) and modification reported in our previous research (Lin et al. 2014). Ten mL of samples were extracted with 2 mL MtBE immediately after the designed reaction time and analyzed using a gas chromatograph (GC-2010, Shimadzu, Japan) equipped with an electron capture detector and a HP-5 capillary column (30 m × 0.25 mm i.d., 0.25 μm film thickness, J&W, USA). The detection limits of the DBPs are shown in Table S2 (available online).
RESULTS AND DISCUSSION
Concentration and speciation of typical DBPs
As shown in Table S1 and Figure S1 the raw water quality changed seasonally, which may result from climate change and different frequencies of biological activities. However, the water source is a closed reservoir located in the center of the river that can hardly have been influenced by human activities. Due to the intrusion of salt water in spring and autumn (Zhang et al. 2011) the concentration of bromide would increase. DBP concentrations in the raw and finished water of the DWTP are summarized in Tables S3 and S4 (available with the online version of this paper). In the finished water, the total DBP concentrations reach their maximum value in summer (117.29 ± 9.94 μg/L) and minimum in autumn (41.10 ± 5.82 μg/L).
THM4 (0.46–24.64 μg/L) dominated in the DBPs in the raw water from Yangtze River, and other measured DBPs including HAN4, TCNM and HK2 ranged from ND to below 1 μg/L, which indicated that DBPs detected in finished water were mostly formed in the treatment processes. As in the finished water, THM4 (26.27–91.86 μg/L) were also the highest among the detected DBPs in the finished water in Table S3, with the maximum of 91.86 μg/L, exceeding the drinking water quality standard (80 μg/L) in the USA and Taiwan. CF concentration was the highest among all the detected DBPs in the finished water, in the range 3.96–44.12 μg/L. HAN4 concentration (3.45–23.25 μg/L) in the finished water was much lower than that of THM4, but higher than those reported in Melbourne, Australia (2–7 μg/L) (Simpson & Hayes 1998) and Japan (1.6–3.2 μg/L) (Kawamoto & Makihata 2004), which can be attributed to the different raw water quality. HK2 (ND–0.07 μg/L) and TCNM (ND–0.35 μg/L) were hardly formed in Yangtze River raw water, but they were detected in finished water. The main form of HK2 in finished water was 1,1,1-TCP, which may due to the application of pre-chlorination. Weinbert et al. (2002) reported that when pre-chlorination is applied, 1,1,1-TCP formation will increase. Formation of TCNM could possibly be owing to the application of ozonation (Weinberg et al. 2002). Other DBP species (tetrachloromethane (PCM), tetrachloroethylene (PCE) and trichloroethylene (TCE)) were hardly formed during the treatment process, and their concentrations ranged from ND to 1.61 μg/L (Tables S3 and S4) in raw water and slightly increased after the treatment process with the similar trend of the total DBP concentrations possibly caused by post-chlorination.
Figure 2(a) and 2(b) display the seasonal variation of DBP concentrations in the finished water from the DWTP, which varied greatly with season. Generally speaking, in spring and summer, DBP levels are higher (82.33 and 117.28 μg/L) than those in autumn and winter (41.10 and 78.47 μg/L), which can be attributed to higher DOC levels in spring and summer (1.98 and 2.46 mg/L, respectively, in Table S1). Higher DOC concentration represents more NOM existing in the raw water and DBP formation will increase as NOM concentration increases (Ates et al. 2007). As shown in Figure 2(a), THM4 dominate in DBP concentrations in the four seasons so that the seasonal variance trend of total DBPs was consistent with that of THM4, with the highest concentration (117.28 μg/L) in summer and the lowest concentration (41.10 μg/L) in autumn, which is related to water temperature and residual chlorine in water. Water temperature has a significant impact on DBP formation. CF formation increases with increasing temperature (Zhang et al. 2013), and is the dominating specie in THMs. However, in two reservoirs (Balçova and Tahtali) in Turkey (Ates et al. 2007), it was reported that THM formation was higher in winter than in summer. Therefore, different source water quality parameters and seasonal variation greatly affect THM formation. A previous study found that increase of bromide concentration leads to increased formation of all halogenated brominated DBPs, which means bromide concentration has an important role in DBP formation (Kristiana et al. 2017). In this study when bromide concentration increased in spring and winter, Br-THMs were a larger proportion of THM4.
As for seasonal variations of HAN4, HK2 and TCNM, it was discovered that these three kinds of DBPs had a similar seasonal variance trend to that of THM4 and total DBPs (high in summer and low in autumn and winter), which could also be contributing to the DOC levels. When bromide concentration increased in spring and winter (Table S1), DBAN formation increased remarkably, and DBAN and DCAN became the main contributors (Table S4), while in summer, DCAN was the dominant specie, which may be due to the high chlorine dosage applied at the DWTP and low bromide concentration in the raw water during that period. The nitrogen in the TCNM originated mainly from dissolved organic matter (DOM) and the formation will have been enhanced by ozonation (Yang et al. 2012). In summer, algal bloom will have caused the increase of DOM, which contains nitrogen, so the concentration of TCNM increased greatly.
Figure 2(b) displays the seasonal variation of THM speciation in the finished water from the DWTP. CF concentration increased as temperature increased in summer. In contrast to CF, Br-THMs gradually increased as temperature decreased, especially in spring and winter, in which Br-THMs accounted for 68.5% and 88.4% of the total THMs respectively.
CF concentration (3.96–44.12 μg/L) in the finished water accounts for the highest proportion of THM4, followed by BDCM、DBCM and BF. This study found that in spring and winter, Br-THMs increased with increasing bromide concentration. As shown in Figure 2(c), when bromide concentration reached 129.50 μg/L in winter (Table S1), DBCM (35.5%) and BF (30.6%) became the main contributors to THM4. In summer and autumn, bromide was hardly detected, BF was barely generated, and CF accounted for 45.4% and 52.8% of THM4 respectively. As shown in Figure 2(d), when bromide concentration reached the maximum value in winter, DCAN, BCAN and DBAN contributed to 10.7%, 42.8% and 46.4% of the HAN4 concentration respectively, and brominated HANs made up 89.3% of the HAN4, but TCAN was not detected. When bromide concentration was low in summer and winter (Table S1), DBAN consisted of 0.0% and 1.1% of HAN4, which were much lower than that in spring and winter, but DCAN consisted of 88.7% and 75.3% of HAN4 in summer and autumn. Dihalogenated acetonitrile was the major species of HANs during chlorination (Liu et al. 2018), so DCAN became the dominant species when bromide concentration was low. It can be concluded that bromide had an obvious impact on DBP speciation.
Many studies on the seasonal variation of THMs have pointed out that THM concentrations were generally higher in summer due to the presence of more organic matter in water caused by microbial activity (Rodriguez et al. 2003), whereas a recent study (Ates et al. 2007) in Turkey reported that of 29 surface water sources, 16 had higher THM formation potential in winter, and six reached their highest formation potential in spring and autumn. In İzmir, the highest HAN concentrations were measured in spring while the lowest ones were measured in summer and autumn. The seasonal trend for THMs (spring > winter > autumn > summer) in this study is similar to the trend for HANs (Baytak et al. 2008). Therefore, although the trend in the seasonal variance of DBP concentrations may be similar in one area, there is no general trend in different areas, which can be caused by the geographical location of water sources, NOM characteristics in the water, and treatment processes applied at DWTPs.
Variation of DBP concentrations through the treatment processes in four seasons
Due to the application of pre-chlorination, the variation of DBP concentrations through the water treatment process is also worth attention. Figure 3 shows the DBP concentration after different units at the DWTP in each season. DBP concentrations reached their highest value in summer and their lowest in autumn. DBP concentration increased significantly after pre-chlorination, especially in spring (increased 57.66 μg/L) and summer (increased 72.55 μg/L), which may be caused by the higher NOM concentration (DOC and TN in Table S1) and chlorine dosage in these two seasons. DBP concentrations kept increasing in the sequential sedimentation process except in spring, which means that residual chlorine continued reacting with organics in the water. Filtration and BAC processes had a slight effect on the removal of DBPs, and the removal efficiency by ozone was also inconspicuous in spite of its strong oxidizing ability. In summer and autumn, the ozonation process decreased DBPs by 1–9%, which is similar to a previous study (Deeudomwongsa et al. 2017), but in spring DBP concentrations increased less than 5%, which could be regarded as error. In spring, summer and autumn, DBP concentrations decreased 15–20% after the BAC process due to the adsorption capacity of activated carbon (Qian et al. 2018) and their biodegradability. As post-chlorination was applied after filtration, significant increase of DBP concentrations in the finished water was observed in all seasons. Although ozonation and BAC have a slight reduction in DOC (Figure S1) and the DBPs produced by pre-chlorination, there is still a certain amount of DOC in the water that can be used as DBP precursors. After ozonation and BAC, the DOC with low molecular weight (<1 kDa) took up a greater proportion (Xu et al. 2007) and had higher formation potential for some DBPs (Zhou et al. 2015). In addition, the chlorine dose after the BAC unit (>1.0 mg/L) was much higher than that in pre-chlorination (0.5 mg/L). As result, DBP concentrations in the water increased after post-chlorination. In spring, the pH value of raw water (8.22–8.38) was higher than in the other three seasons (Table S1 and Figure S1(a)). Based on previous studies, THM formation can be enhanced under alkaline conditions in chlorination (Hong et al. 2013), and the highest coagulation efficiency by aluminum sulfate usually occurs at pH 8–9 (Wei et al. 2015). This can be used to explain the highest DBP concentration in raw water and the most obvious DBP removal by sedimentation in spring (Figure 3).
Figure 4 compares the distribution of THM and HAN speciation after each water treatment process in the DWTP during spring in which raw water contains high bromide concentration. In spring, four kinds of THM accounted for similar proportions and increased dramatically after pre-chlorination. The concentration and distribution of THM species remained stable in the sequential sedimentation, filtration, ozonation and BAC processes. DBCM and BF increased rapidly and became the dominant contributors to THMs, while CF concentration increased slightly (increased 4.70 μg/L) after pre-chlorination. In spring, DBCM and BF increased again in the finished water because of post-chlorination. The water purification process had limited effect in bromide removal (Kristiana et al. 2011), therefore, high bromide concentration in spring (70.19 μg/L) led to high Br-THM formation in spring, which will pose more threat to public health. As for HAN4, three kinds of HAN4 (DBAN, DCAN, BCAN) account for similar proportions and increased significantly after post-chlorination while TCAN was hardly detected during the whole treatment process. The concentration of HAN4 decreased slightly and the distribution of HAN species remained stable in the sequential sedimentation, filtration, and ozonation processes. BAC has a good removal efficiency for HAN4, after which HAN4 was hardly detected. However, HAN4 increased again after post-chlorination, which is the same as THM4.
From the above analysis, the water produced in this DWTP was under certain risk in spring, summer and winter. Other pretreatment methods such as pre-ozonation and potassium permanganate pre-oxidation might be suitable to deal with high organic pollution during summer time. As in winter and spring, water treatment plants could increase the monitoring frequency of water quality parameters (including DOC, bromide etc.) in response to sudden water pollution accidents. The previous study (Liu et al. 2018) found that with the increase of chlorine the concentration of THM and HAN concentration would increase but with further increase of chlorine THM would increase slightly and HAN would decrease. Chlorine application rates need to be determined to meet the requirements of residual chlorine and to control the total DBP concentrations. Moreover, advanced treatment processes in this DWTP had limited effect on the control of DBPs. While DOC concentration has a close relationship with DBP formation, the DOC removal efficiency needs to be improved, such as by adding coagulant to enhance coagulation, and other additional treatment units, such as membrane process, can be applied to deal with water pollution.
THMs were the major DBPs identified in the finished water of a full-scale DWTP using Yangtze River water in East China. THM concentrations varied seasonally, with higher concentrations in spring and summer than those in autumn and winter, as did total DBP concentrations because of higher temperature and higher DOC concentrations. HANs, HKs and TCNM were usually detected at levels ranging from below the detection limit (<1 μg/L) to several μg/L in all seasons. As bromide concentration increased in spring and winter, Br-THMs became the predominant species, which should be worth attention because the adoption of ozonation in this DWTP may form carcinogenic bromate in the finished water. DBP concentrations increased dramatically after pre-chlorination, and the advanced treatment processes in this DWTP showed limited DBP removal efficiency. But in spring, the subsequent treatment after pre-chlorination showed an obvious DBP removal efficiency uniquely due to the pH conditions, even though the finished water still contained a considerable amount of DBPs after post-chlorination. Therefore, the chlorine dosage needs to be adjusted and other advanced treatment processes such as nano-filtration and reverse osmosis can be evaluated to enhance the removal of DBP precursors before post-chlorination in future research.
This study was supported in part by National Natural Science Foundation of China (Nos. 51778444, 51678354 and 51478323), National Major Science and Technology Project of China (No. 2017ZX07207004), State Key Laboratory of Pollution Control and Resource Reuse Foundation (No. PCRRK16005) and the Ministry of Science and Technology in Taiwan (MOST-107-2221-E-992-008-MY3) and the Shanghai Sailing Program (No. 18YF1406000).