Impact of UV irradiation on disinfection by-product formation and speciation from post-chlorination of dissolved organic matter

The objective of this study was to investigate the effects of combined low-pressure ultraviolet (UV) irradiation and chlorination on the formation of disinfection by-products (DBPs) from different dissolved organic matter (DOM) as DBP precursors. Commercially available humic acid (HA), extracellular organic matter (EOM) from green algae, cyanobacteria, and diatom, namely Scenedesmus quadricauda (SQ), Merismopedia sp. (Msp), and Phaedactylum tricornutum (PT), were used as the sources of DOM. The DBP formation increased with increasing total residual chlorine; EOM from PT presented the highest formation potential followed by HA, Msp, and SQ. The low dosage of 40 mJ/cm UV irradiation is insignificant to change the DBP formation from HA and SQ; however, it decreased the DBP formation from bromide-containing EOM of PT and promoted the DBP formation from EOM of Msp at various total residual chlorines. The DBP formation of each DOM correlated well with total residual chlorine. The maximum DBP formation potential (DBPFP) reduction of 42.25 and 13.75% for haloacetic acid formation potential (HAAFP) and trihalomethane formation potential (THMFP) was obtained at the UV irradiation dosage of 300 mJ/cm for EOM of PT. However, for the EOM derived from Msp, a maximum increase of 58.1 and 51.1% for HAAFP and THMFP was observed after UV-chlorination.

consequently, produces a variety of DBPs during chlorination. UV radiation at 254 nm is well known to be effective to inactivate waterborne pathogens and degrade organics by the process of photolysis (Shah et al. ).
Being a complex mixture of carbohydrate, sugar, lipid, and protein, DOM is susceptible to be degraded during UV disinfection. The complex mixture of intermediates subsequent to photolysis may cause the formation of harmful products after chlorination. It is reported that UV irradiation can alter the characteristics of DOM, such as molecular weight and hydrophobicity/hydrophilicity (Buchanan et al. ; Chen et al. ). Although it is not significant, an increase of low molecular fraction and chlorine demand was observed after UV treatment (Choi & Choi ) (Zhang et al. a). Several studies were conducted on UV irradiation followed by the chlorination of humic acid (HA) in water (Lee & Hur ; Zhang et al. a, b; Ye et al. ). It was found that UV irradiation did not alter the specific DBP formation potential (DBPFP) significantly based on the DOM characteristics, which indicated that most of the DBP precursors could not be removed by photodegradation (Lee & Hur ; Zhang et al. a). With the increasing events of harmful algal blooms all over the world, a control study on the photolysis of AOM followed by chlorination is required to determine the effect of algal matter on drinking water treatment processes, which is the objective of this study.
In our previous study, the DBP formation of both EOM and IOM of four algae and two cyanobacteria were deter-

Algal cultivation
The three freshwater species, SQ, Msp, and PT, were purchased from the Canadian Phycological Culture Centre (CPCC) at Waterloo University (Waterloo, ON, Canada).
They were cultivated in 2 L conical flasks using a specific medium (high salt medium for SQ, BG-11 for Msp, and F/2 for PT) for each species at 23 ± 2 C with intermittent illumination (3,000 lx) at a light/dark cycle of 16/8 h. All solutions were prepared from reagent-grade chemicals and Milli-Q water, except the medium F/2, which was diluted by synthetic seawater prepared by Instant Ocean sea salt (Instant Ocean Company, USA). The growths of algae were monitored by cell counting using a hemocytometer (LW Scientific, USA) under a microscope (ZEISS, Germany).
Algae and cyanobacterial cultures were harvested during the stationary growth phase based on the previous study (Wang et al. ). HA with an average molecular weight of 39.098 kDa was purchased from Alfa Aesar (Thermo Fisher Scientific, USA).

Effect of total residual chlorine on DBP formation
According to the results of the chlorine demand test presented above, the calculated amount of free chlorine was added into water samples of each DOM for the following DBP formation tests. DBP formation was evaluated for the water samples before and after UV irradiation to evaluate the influence of UV disinfection on the DBP formation under different total residual chlorine after incubation in dark for 2 h at room temperature as the post-chlorination process.
Under the chlorination conditions adopted, the total DBP formation increased with the increasing total residual chlorine as shown in  For the other DOM, DCAA and TCAA were the dominant HAA species for SQ, Msp, and HA with the total HAA formation of 38.23 ± 1.25, 313.56 ± 6.05, and 340.82 ± 9.29 μg/L, respectively, when the total chorine residual was around 1.0 mg/L. It was noted that the DCAA/TCAA from each EOM followed a similar trend that was obtained with extracted HA and fulvic acid from surface water (Hua et al. ). In this study, TCAA formed was less than DCAA, indicating that hydroxyl groups account for a major proportion in the chemical structure of EOM (Reckhow & Singer ). However, HA formed more TCAA than DCAA, which demonstrated that the oxidizable functional groups (e.g. conjugated system) are abundant in the chemical structure of HA; it is reasonable considering its higher SUVA value of 3.18.  It should be noted that TCM was the only THM species formed from SQ, Msp, and HA. TBM (Figure 3(b)) was the most abundant species of THM for bromide-containing water of PT. It was interesting to find that even Msp showed the highest chlorine demand (Figure 2), the DBP formation was much lower than PT and HA. Overall, except SQ, the DBP formation by the other three DOM (i.e., Msp, PT, and HA) always exceeded the DBP limit of 80 μg/L for HAAs and 100 μg/L for THMs as regulated by the guidelines for Canadian drinking water quality (Wang et al. b).
The effect of UV radiation at 40 mJ/cm 2 , a typical dosage used for disinfection, caused a slight decrease in DBP formation after chlorination, although the effect was insignificant ( Figure 3). The comparison of the DBP formation between with and without UV irradiation is shown in Figure 4, and the regression coefficients are presented in Table 1. There was no increase in DBP formation after UV radiation for SQ and HA (Figure 4). This is reasonable as much higher dosage is required for effective photolysis of trace concentration of organics (micropollutants) (Chowdhury et al. ). However, there is a small amount of DBP formation changes for PT and Msp, where both HAA and THM formation decreased for PT, but with a small increase for Msp. Table 1 shows the regression coefficients between each of the HAAs/THMs and total residual chlorine. The strong correlations (0.936-0.999) found between HAAs/THMs and total residual chlorine indicated that experimental conditions were well controlled and total residual chlorine as a variable showed robust correlations with DBP formation from each DOM. Similar results were found in a previous study (Dyck et al. ).

CONCLUSIONS
The investigation of DBPFP of selected DOM with a variation of total residual chlorine and UV irradiation was performed in this study. DBP formation was highly correlated with residual chlorine EOM from PT presented the highest formation potential followed by HA, Msp, and SQ.
The comparison of DBP formation between with and without UV irradiation for each type of DOM indicates that 40 mJ/cm 2 UV irradiation may be insignificant to change the DBP formation from HA and SQ. However, it can decrease the DBP formation from bromide-containing

DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.