Removal performance of dissolved organic matters from municipal secondary effluent by different advanced treatment processes and the following disinfection by-products generation rules

doi:10.21203/rs.3.rs-2489620/v1

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Removal performance of dissolved organic matters from municipal secondary effluent by different advanced treatment processes and the following disinfection by-products generation rules

https://doi.org/10.21203/rs.3.rs-2489620/v1

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published 30 Sep, 2023

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Various advanced treatment processes including ultrafiltration (UF), ozonation, enhanced coagulation, and biological aerated filter (BAF) have been applied to reduce dissolved organic matters (DOMs) from the secondary effluent of municipal wastewater treatment plants (MWTPs). In this study, DOMs were characterized and the relationship between DOMs characteristics and DBPs generation was investigated systematically. Results showed that BAF and ozonation processes could significantly affect DOMs characteristics in the treated effluents and the following DBPs generation. UF and enhanced coagulation reduced the production of DBPs by removing large molecular hydrophobic organics. The removal of low molecule DOMs by BAF resulted in a 67.6% reduction in trihalomethanes (THMs) production. Ozonation could oxide large hydrophobic DOMs into small hydrophilic molecules containing aldehyde and ketone groups, leading to 54% increase of chlorinated aldehydes (CHs) and halogenated ketones (HKs). Humic acid (HA) was the main organics type in DOMs and important precursor for THMs and dichloroacetonitrile (DCAN) formation. The generation of trichloromethane (TCM) showed a significant positive correlation (R² = 0.987) with the specific ultraviolet absorbance at 254 nm (SUVA). Large molecule hydrophobic DOMs devoted the most to the formation of carbonaceous disinfection by-products (C-DBPs) and \(\text{N}{\text{H}}_{\text{4}}^{\text{+}}\)-N content was an important factor affecting the generation of nitrogenous disinfection by-products (N-DBPs). These results are important for the optimization of advanced treatment process in MWTPs, and controlling DBPs should consider the removal of low MW hydrophobic DOMs and the reduction of SUVA.

Disinfection by-products

Secondary effluent

Advanced treatment

Dissolved organic matters

Molecular weight.

Disinfection is one of the most important wastewater purification technologies in municipal wastewater treatment plants (MWTPs) (Winward et al. 2008). It has been found that disinfection can inactivate pathogenic microorganisms to prevent the spread of waterborne diseases. However, dissolved organic matters (DOMs) in the secondary effluent from MWTPs can react with disinfectants to produce toxic disinfection by-products (DBPs), which have raised concerns about wastewater quality safety (Richardson et al. 2007). Methods of controlling DBPs include enhancing the removal of precursors before disinfection and developing effective removal techniques after DBPs are generated (Philip et al. 1999). The high cost and complexity of various DBPs removal technologies make them difficult to be used widely in MWTPs (Wu &Wu 2009). Therefore, with the increasing awareness of DBPs and the improvement of wastewater quality standards, some advanced treatment processes such as ultrafiltration (UF), ozonation, enhanced coagulation, and biological aerated filter (BAF) were installed after the conventional secondary biological treatment in MWTPs to remove the precursors of DBPs (Mazhar et al. 2020b). It has been reported that many DBPs precursors had a pore size larger than 10 kDa, making membrane filtration as an accepted method for their removal. The conventional coagulation can remove DOMs with high MW and negative charge density, and has a low removal capacity for small MW DOMs. Therefore, enhanced coagulation has been widely used to promote the removal of DOMs. Sarparastzadeh et al. found that the removal rates of COD, TP, and total suspended substance (TSS) by enhanced coagulation were 38%, 66%, and 69%, respectively (Sarparastzadeh et al. 2007). The removal rate of dissolved organic carbon (DOC) by UF was 16% (Fiksdal &Leiknes 2006). In addition, it was found that ozonation and BAF could remove DOMs through chemical oxidation and biodegradation, and 86.4% and 30% removal rates of COD were achieved (Ahmed et al. 2011, Sallanko &Okkonen 2009).

Indeed, previous studies mainly focused on the removal efficiencies of various advanced treatments on traditional wastewater quality indicators such as DOC, COD, TN, etc. Enhanced coagulation could only remove organic matters with MW > 10 kDa, resulting in only 32% removal rate of DBPs precursors (Pramanik et al. 2015). Although enhanced coagulation was difficult to remove small molecules of DOMs, it can remove some hydrophobic organic matters, resulting in 30% reduction of trihalomethane potential (THMFP) (Zhao et al. 2013). In addition, it was found that UF could diminish the formation of 63% trihalomethanes (THMs) and 38% haloacetic acids (HAAs) by trapping large molecules of DOMs (Ates et al. 2009). It can be seen that BAF and ozonation were more effective than UF and enhanced coagulation in removing DOMs, resulting in higher removal of disinfection by-product formation potential (DBPFP). BAF could effectively degrade DOMs, thus achieving higher removal rate of DBPFP with 70% (Pramanik et al. 2015). Besides, it has been found that ozonation could diminish total organic halide formation potential by 20% (Wert et al. 2007). Although ozonation led to 24% and 37% decrease in the formation of total THMs and HAAs (De Vera et al. 2015), ozonation of aromatic DOMs typically produced ozonation by-products such as carboxylic acids, aldehydes and ketones(Swietlik &Sikorska 2004). After ozonation process, the related abundance of unsaturated structures containing C = O bonds including aldehyde and ketone groups in the hydrophilic fraction increased, leading to an increase of THMFP as well as a significant increase of dichloroacetic acid (DCAA) formation potential (Qi et al. 2018). Advanced treatment processes could influence the characteristics of DOMs and the following generation of DBPs during chlorine disinfection process. However, limited studies pay attention to the comparison of DOMs characteristics by different advanced treatments. There were also fewer studies on the relationship between DOMs characteristics and DBPs generation during disinfection process. The primary purposes of this study were to (i) compare the changes of DOMs characteristics in secondary effluent after four advanced treatment processes, including enhanced coagulation, UF, ozonation and BAF and DBPs formation in the subsequent disinfection; and (ii) investigate the constitutive relationship between DOMs characteristics and DBPs formation.

2.1 Raw wastewater (RW)

Samples of RW were collected from the secondary effluent of a MWTP in Beijing, China. After filtration through 0.45 µm cellulose membrane filters, RW was reserved at 4°C to minimize the constituents change. Table 1 summarized the common water parameters of RW.

Table 1

Common water parameters of RW
Parameter	Unit	Mean value
DOC	mg/L	19.07 ± 1.8
UV₂₅₄	cm^− 1	0.1885 ± 0.025
SUVA	L/ (mg m)	0.984 ± 0.014
COD	mg/L	34.66 ± 4.69
TP	mg/L	0.16 ± 0.02
TN	mg/L	16.0 ± 3.945
\(\text{N}{\text{H}}_{\text{4}}^{\text{+}}\)-N	mg/L	1.18 ± 0.29

2.2 Advanced treatment experiments

2.2.1 Enhanced coagulation

2.5 mL poly aluminium chloride (PAC) suspension (5 g/L) was added into RW (500 mL) with rapid mixing (300 rpm) for 2 minutes. Then, the sample was stirred at low speed (50 rpm) for 30 minutes to allow the growth of the flocs. The supernatant obtained after 2 hours of static sedimentation was the treated effluent of enhanced coagulation.

2.2.2 UF

A dead-end batch ultrafiltration unit (Shanghai Mosu Science Equipment Co., Ltd) was set up to filter RW. The molecular weight cut-off (MWCO) of the used UF membrane (Mosu, China) was 50 kDa. The UF unit was pressurized with nitrogen gas at 0.10 ± 0.05 MPa with stirring.

2.2.3 Ozonation

Gaseous ozone was constantly diffused into RW for 30 minutes using an ozone producer (3S-A5, Beijing Tonglin Technology Co., China). The ozone concentration diffused into the RW was controlled at 10.5 ± 2.0 mg/L.

2.2.3 BAF

BAF was constructed in a polymethyl methacrylate chamber with an internal diameter of 10 cm and height of 30 cm with biological ceramsite as the medium. Activated sludge was inoculated on the biological ceramsite of BAF and additional nutrient sources were provided to foster the rapid growth of biofilm. The removal rate of DOC was stable after 30 days operation, suggesting that the biofilm grew properly on the surface of biological ceramsite.

In this study, the characteristics of DOMs and the content of DBPs were analyzed to simulate the removal effect of different advanced treatments on DOMs and DBPs precursors in secondary effluent during actual treatment.In the following, UF_e, coagulation_e, ozonation_e, and BAF_e stand for the treated effluents of UF, enhanced coagulation, ozonation, and BAF, respectively.

2.3 DOMs characteristics analysis

DOC concentration was determined using a total organic carbon analyzer (Shimadzu, Japan). Concentrations of COD, \(\text{N}{\text{H}}_{\text{4}}^{\text{+}}\)-N, TN and TP were evaluated depending on standard methods stated in the Standard Methods for the Examination of Water and Wastewater (APHA, 2012). The UV absorbance at 254 nm (UV₂₅₄) was measured using a spectrophotometer (UV-16000, Shimadzu). The ratio of UV₂₅₄ to DOC is SUVA. Many nitrogenous organic compounds in the DOMs could react with chlorine to form more toxic nitrogenous disinfection by-products (N-DBPs). Nitrate and ammonia as normal inorganic nitrogen sources are crucial indicators of wastewater quality. In prior study (Zhang et al. 2022), it was found that the contents of nitrate and nitrite had less influence on N-DBPs, while \(\text{N}{\text{H}}_{\text{4}}^{\text{+}}\)-N had more significant influence, so in this study, \(\text{N}{\text{H}}_{\text{4}}^{\text{+}}\)-N and TN were selected as the indicators of affecting N-DBPs formation.

Nonionic Amberlite XAD-8 resins were used to fractionate DOMs into hydrophilic substances (HIS), hydrophobic acids (HOA), and hydrophobic bases (HOB). A 500 mL water sample was pumped through the cleaned XAD-8 resin column at a rate of 10 mL/min. This effluent was named fraction A. The HOB was backflush eluted with 0.1 mol/L HCl. Fraction A was acidified to pH = 2 with HCl and recycled through the XAD-8 column at the rate of 10 mL/min. Fraction that passed through the resin column was HIS fraction. The HOA fraction was backflushed out using 0.1 mol/L NaOH (Malcolm 1992). After elution or recovery, each XAD-8 fraction was conditioned to 500 mL. Then the pH of each fraction was regulated to 7.80 ± 0.5, consistent with the value of RW.

Molecular weights distribution (MWD) was performed by Shimadzu's LC-2010AHF high-pressure size exclusion chromatography (HP-SEC) with UV detection at 254 nm. The wastewater samples were filtered by a 0.22 µm filter membrane and analyzed by Agilent GPC/SEC (PL-aquagel-OH, 8µm, 300×8mm) gel chromatography column. The mobile phase was phosphate buffer: 0.2722 g/L KH₂PO₄, 0.4564 g/L K₂HSO₄∙3H₂O, and 5.8440 g/L NaCl, and the flow rate was 1.0 mL/min.

Three-dimensional excitation and emission matrix (3D-EEM) spectra were obtained by using a luminescence spectrometry (F-4500 FL spectrophotometer, Hitachi, Japan).

2.4 DBPs determination

The wastewater samples were disinfected with 1.0 mg/L NaClO for 30 min and 60 min to determine the amount of DBPs generation, and 10 mg/L NaClO for 72 h to determine DBPFP.

The concentrations of THMs, haloacetonitriles (HANs), haloacetamides (HAcAms), halogenated aldehydes (HAs), and HKs were analyzed by gas chromatograph with an electron capture detector (GC/ECD) (Agilent 7890, USA). The DBPs were extracted from wastewater samples with methyl-tert butyl ether (MTBE). The heating process was as follows: temperature was held at 30°C for 2 minutes and ramped to 40°C with 1°C/minute, then to 205°C with 20°C/minute and held for 4 minutes.

3.1 Influences of different advanced treatment processes on DOMs characteristics

3.1.1 Effects of different advanced treatment processes on wastewater qualities

The wastewater qualities after different advanced treatments were shown in Table 2. DOC, UV_254, and SUVA were extensively used as proxy parameters to present the concentration of DOMs and the amount of aromatic compounds (Hu et al. 2016). Two primary purification mechanisms in the biological treatment process were biodegradation/biotransformation and adsorption (Verlicchi et al. 2012). DOC and UV₂₅₄ were efficiently eliminated during BAF process with removal efficiencies of 60.0% and 52.9%, respectively, suggesting that BAF was effective in reducing DOMs content. In addition, ozonation caused 47.1% decrease of UV₂₅₄, proving that ozonation can effectively remove unsaturated functional groups such as aromatic rings and C = O bonds. The removal rate of TP by enhanced coagulation could reach 90%. In addition, the removal efficiency of \(\text{N}{\text{H}}_{\text{4}}^{\text{+}}\)-N by BAF also reached 69.5%. These results demonstrated that ozonation and BAF can improve the effluent quality, particularly in terms of the reduction of organic matters, while UF and enhanced coagulation processes provide relatively weak removal of DOMs.

Table 2

Changes of wastewater qualities after different advanced treatment processes
	RW	UF_e	Ozonation_e	Coagulation_e	BAF_e
UV₂₅₄ (cm^− 1)	0.189	0.132	0.10	0.14	0.079
DOC (mg/L)	19.07	13.9	10.04	14.57	9.62
SUVA	0.99	0.95	1.03	0.96	0.82
COD (mg/L)	34.66	26.5	24.25	29.35	24.28
TN (mg/L)	16.02	11.7	11.6	11.2	11.8
\(\text{N}{\text{H}}_{\text{4}}^{\text{+}}\)-N (mg/L)	1.18	0.85	0.82	0.63	0.36
TP (mg/L)	0.16	0.13	0.12	0.016	-

3.1.2 Sources and chemical composition changes of DOMs in the treated effluents after different advanced treatments

The ratio of the emission intensity at 450 nm compared to 500 nm for an excitation wavelength of 370 nm is the fluorescence index (FI), and this index can be used to distinguish relative contributions of microbial and terrestrial derived DOMs. The FI value of terrestrial DOMs is below 1.4, while that of microbial DOMs is greater than 1.9 (Fellman et al. 2009, Goldman et al. 2012). In this study, FI values of RW and all types of advanced effluents were greater than 1.9, indicating that the DOMs in RW and all effluents were dominated by microbial products.

The 3D-EEM spectra of DOMs in RW and different treated effluents were shown in Fig. 1. The EEM diagrams of DOMs can be divided into five types which are Region I and Region Ⅱ aromatic proteins (AP) (λ_ex/λ_em=(220 ~ 250)/(280 ~ 380) nm), Region Ⅲ fulvic acid (FA) (λ_ex/λ_em=(220 ~ 250)/(380 ~ 480) nm), Region Ⅳ soluble microbial byproducts (SMPs) (λ_ex/λ_em=(250 ~ 280)/(290 ~ 380) nm) and Region Ⅴ HA (λ_ex/λ_em ≥ 250/(380 ~ 480) nm) (Chen et al. 2003b). As shown in Fig. 1, fluorescent species with a maximum excitation wavelength of 320 nm and a maximum emission wavelength of 400 nm, this component was thought to be associated with high MW aromatic humic substances (Dainard et al. 2015, Mazhar et al. 2020b). Fluorescent substances with maximum excitation wavelengths of 280 nm and 240 nm and maximum emission wavelengths of 400 nm to 450 nm were described as consisting of humic acid-like substances and fulvic acid-like substances (Podgorski et al. 2018). From the results in the Fig. 1, it can be seen that humic acid-like substances and fulvic acid-like substances with the highest fluorescence peaks, indicating that these substances in RW were the main contributors to fluorescence. After ozonation and BAF treatments, the fluorescence intensity in each region was significantly lower than that in RW. It could be interpreted as that ozone can react with the groups with fluorescence response, leading to the reduction of fluorescence intensity. BAF was effective in removing various types of DOMs, but the content of SMPs after BAF treatment was still high. This was probably because the nitrogenous organic matters were converted to the main components of SMPs such as tryptophan and protein (Chen et al. 2003a).

Parallel factor analysis (PARAFAC) is a valuable tool for characterizing and quantifying the fluorescence transformations of DOMs (Jørgensen et al. 2011). The percentage of fluorescence in each region obtained by PARAFAC was shown in Table 3. The area of HA region contributed to more than 50% of the whole areas, indicating that HA was the main DOMs with fluorescent property in all the treated effluents. HA was effectively degraded during BAF process, but FA was not effectively removed (Kang et al. 2015). The composition of DOMs in ozonation_e was partial changed compared with that in RW, and the percentage of HA was significantly reduced. Figure 1 showed that the fluorescence intensity of HA declined significantly after ozonation, indicating that the removal of HA by ozonation was mainly due to the destruction of unsaturated bonds (De Vera et al. 2015).

Table 3

The percentage of fluorescence response
	RW	UF_e	Ozonation_e	Coagulation_e	BAF_e
Ⅰ、Ⅱ	5.5%	6.1%	11.3%	8.8%	5.7%
Ⅲ	12.5%	13.4%	14.0%	13.5%	8.8%
Ⅳ	19.4%	19.3%	19.0%	19.1%	33.3%
Ⅴ	62.7%	61.1%	55.7%	58.7%	57.5%

3.1.3 MWD of DOMs in different treated effluents

The MWD characteristics of DOMs in different treated effluents were shown in Fig. 2. The apparent MW of DOMs was analyzed according to the elution time and can be roughly divided into three intervals: biopolymers (≥ 50 kDa), high molecular HA (3–50 kDa) and small molecular organic matters (≤ 3 kDa) (Umar et al. 2016), corresponding to less than 6 minutes, 6–9 minutes and more than 9 minutes, respectively (Chen et al. 2017). Two primary peaks in the MWD maps of RW and other treated effluents corresponded to high molecular HA (3–50 kDa) and small molecular organics (≤ 3 kDa), with the highest response value in the region of 3–50 kDa. BAF increased the content of DOMs with MW > 50 kDa, and the intensity of the fluorescence peak of SMPs in the 3D-EEM spectrum was relatively high. Therefore, the increase of DOMs with MW > 50 kDa was probably due to the decomposition of senescent microorganisms (Ma et al. 2013).

To further analyze the MWD, the apparent MW was quantitatively characterized in terms of peak area, and the results were shown in Table 4. Ozonation was capable of removing DOMs with MW in the range of 3–50 kDa, with a removal rate of 86.3%, indicating that ozonation can effectively degrade biopolymers and HA. BAF had good removal effect on DOMs with MW < 50 kDa, and BAF was able to remove 32.5% of high molecular HA compared to small molecular organic matters.

Table 4

Peak area of each MW interval
	RW	UF_e	Ozonation_e	Coagulation_e	BAF_e
> 50 k	4.3	0.96	0	0	6.3
3 ~ 50 k	104.6	104.9	51.2	103.1	70.2
3 ~ 50 k	12.2	11.5	7.9	9.5	8.7
< 3 k	25.2	22	23.8	24.6	17.2

3.1.4 Changes of DOMs fractions with different hydrophobicity after different advanced treatment processes

The percentages of DOMs fractions in RW and different treated effluents were shown in Table 5. The distribution of DOMs fractions with different hydrophobicity varied significantly depending on the kind of advanced treatment process. HIS and HOA were the most predominant components in RW, accounting for about 90% of the total DOMs. Compared to the proportion of fractionated components in RW, that in coagulation_e was almost unchanged. These results suggested that PAC did not change the proportion of hydrophilic and hydrophobic substances. UF increased the proportion of HIS fraction and reduced the HA fluorescence peak, suggesting that UF membrane mainly retained the hydrophobic HA with a larger particle size (Shao et al. 2014). Ozonation caused a significant reduction of HOA fraction and an increase of HIS fraction. Combined with the results of Fig. 2, it could be concluded that ozonation broke up large molecules and tended to produce low molecules. Hydrophilic organics were found to be composed of DOMs with relatively lower MW compared to hydrophobic organics (Zhang et al. 2009). Therefore, it could be speculated that ozonation was able to convert large molecular hydrophobic organics into low MW hydrophilic organics (Qi et al. 2018).

Table 5

Distribution characteristics of DOMs fractions with different hydrophobicity
	RW	UF_e	Ozonation_e	Coagulation_e	BAF_e
HIS	48.9%	67.4%	60.6%	48.5%	55.8%
HOA	41.0%	21.6%	13.2%	37.4%	26.8%
HOB	9.0%	11.0%	26.2%	14.1%	12.0%

3.2 Formation and speciation of DBPs during chlorination of RW and different treated effluents

3.2.1 Effects of advanced treatment processes on DBPs generation

The generation of DBPs during chlorination of RW and different treated effluents was shown in Fig. 3. The concentration of DBPs generated after 30 minutes disinfection of RW was 230.4 µg/L. THMs were the most dominant DBPs species, accounting for about 35.9% of the total DBPs, followed by CHs and HKs. The lowest proportion was N-DBPs, accounting for about 25.6%. Among four advanced treatment processes, DOMs in BAF_e exhibited the lowest chlorine reactivity, the concentration of DBPs in disinfected BAF_e was 105.8 µg/L. Compared to RW, the disinfected BAF_e showed 52.9%, 80.8% and 69.3% reduction for DBPs, TCM and BDCM, respectively. After ozonation, the specific THMs formation was reduced by 17.2%. The radicals formed by ozone may activate DOMs and form more DBPs, so the content of TCP increased by 20.3% and the content of CH increased by 69.0% after ozonation. The contents of DBPs generation in RW and different treated effluents increased with the rise of disinfection time. As the disinfection time increased, the disinfected ozonation_e exhibited greater increase of DBPs, with the largest increase of TCP and CH, which were important intermediates of TCM. Therefore, the disinfected ozonation_e had greater increase of DBPs probably due to the increased stability of the transformation products or the formation pathway of THMs was different compared to CH and HKs.

Figure 4 showed the variations of different types of DBPs generated in chlorination of RW and different treated effluents. TCM, BDCM and DBCM were detected in all disinfected effluents, and TCM was the predominate species, comprising about 59.7% of the total THMs. The concentrations of THMs were in the order of ozonation_e > RW > coagulation_e > UF_e > BAF_e, which was consistent with the order of SUVA. This result proved that the formation of THMs had a high correlation with SUVA value (Wang et al. 2021). The formation of THMs correlated well with humic substances (r = 0.805) in EEM (Xu et al. 2021), and the high content of humic acid-like substances in DOMs resulted in the largest percentage of THMs in DBPs. Ozonation was able to convert hydrophobic HA macromolecules into hydrophilic small molecules (Hu et al. 2010) which were easier to react with chlorine to produce BDCM (Farre et al. 2013). Consequently, the BDCM content after chlorination of ozonation_e increased by 6.4%. It was reported that low MW DOMs provided the largest contribution to form C-DBPs including BDCM, DCP, and TCP (Lin et al. 2014). The levels of CHs and HKs in the disinfected ozonation_e were higher than those in the disinfected RW, which may be due to the decomposition of aromatic and aliphatic structures in the hydrophobic fraction by ozonation (Chang et al. 2002). BAF and UF had the most pronounced effect on DCAN generation with 41.4% and 34.3% reduction, respectively. In section 3.1.2, the 3D-EEM fluorescence spectra analysis indicated that UF and BAF had better removal effects for substances in regions V and IV, with BAF showing higher removal efficiency for these substances. These findings indicated that the reduction of hydrophobic HA and amino acid protein-like organisms led to the reduction of DCAN generation. At the same time, the generation of HAcAms remained almost unchanged, probably because SMPs and AP containing high organic nitrogen were important precursors of amide-based DBPs. These precursors were prevalent in SMPs released from damaged aging microbial cells during the BAF treatment.

3.2.2 Effects of DOMs fractions with different hydrophobicity on DBPFP in chlorination of RW and different treated effluents

The DBPFP of each fraction of DOMs was measured and the results were shown in Fig. 5. Generally, hydrophobic substances such as HA were considered to be the primary DOMs fraction that could react with chlorine and generate several types of DBPs (Zhang et al. 2009). Interestingly, in this experiment, the HIS of DOMs had a great potential to form DBPs. It suggested that the hydrophilic part of DOMs may be easier to form DBPs (Marhaba &DoanhVan 2000). The DBPFP of DOMs fractions with different hydrophobicity in chlorination of RW and different treated effluents followed the rule of HI > HOA > HOB.

Figure 6 showed the DBPFP of DOMs fractions with different hydrophobicity in RW and different treated effluents. The HOA and HIS fractions in RW were more likely to generate TCM, while HIS was more likely to react with chlorine to produce BDCM. A similar pattern was observed in the disinfection processes of UF_e and coagulation_e, indicating that hydrophilic organics were easier to react with bromine to form THMs than hydrophobic organics (Hua &Reckhow 2013). The HIS fraction in RW contained more precursors of THMs, but the hydrophobic fraction in ozonation_e contained more precursors of THMs. According to the results of Fig. 6, a significant correlation between THMFP and SUVA value could be found, indicating that the reactivity of DOMs with chlorine to form THMs depended fundamentally on their aromaticity (Ma et al. 2014). The HOA component has higher aromaticity due to the presence of chromophores and aromatic groups, so the THMFP of the HOA component was higher. The hydrophobic fraction of DOMs had high aromaticity, and the enhanced coagulation removes the hydrophobic fraction of humic substances resulting in the reduction of THMs precursors in coagulation_e (Mazhar et al. 2020a). HIS fraction in RW and different treated effluents contained the most TCP precursors. The TCP formation potential (TCPFP) of HIS fraction in ozonation_e accounted for 91.7% of the total TCPFP. This was attributed to the higher reactivity of low MW ozonation by-products containing aldehyde and ketone groups with chlorine (Chang et al. 2002).

3.2.3 Effects of MWD on DBPFP in chlorination of RW and different treated effluents

DOMs with different MW range had different DBPs generation characteristics. Figure 7 showed the DBPFP of different MWD DOMs in RW and different treated effluents. It can be seen that with the decrease of MW, the DBPFP showed a decreasing first and then increasing trend. In the chlorination of RW, DOMs with MW > 100 kDa and MW < 3 kDa had higher DBPFP. Ozonation increased the generation of low MW hydrophilic DOMs, so DOMs with MW < 3 kDa in the ozonation_e contained more CHs and HKs precursors. In RW, DOMs with MW > 100 kDa and MW < 3 kDa had high THMFP. DOMs with MW > 100 kDa had the highest TCM formation potential (TCMFP), contributing 48.6% of TCMFP during chlorine disinfection of RW. About 74.8% of BDCM formation potential was derived from the chlorination of DOMs with MW < 3 kDa. Combined with the conclusion of section 3.2.2, it can be seen that low MW hydrophilic DOMs were more likely to form bromine-containing DBPs (Kitis et al. 2002). BAF could effectively remove the precursors of TCM with MW < 3 kDa and thus reduced TCM production. Both the cleavage of the phenyl structure and the size exclusion effect favor the ozone reduction of macromolecular DOMs to form THMFP (Chen et al. 2017, Huang et al. 2018). Carboxylic acids produced by ozone would react with halogen ions in water, and the carboxyl groups were replaced by halogen atoms and further chlorinated to produce DBPs such as CH and HKs (Liu et al. 2020). Therefore, low MW DOMs in ozonation_e were more likely to generate TCP than that in RW. Chlorination caused cleavage of the side chain of amino acids to produce DCAN, which is further hydrolyzed to HAcAms(Chu et al. 2015). The content of HAcAms precursors with MW < 3 kDa in RW and different treated effluents accounted for about 80% of the total HAcAms precursors, indicating that hydrophilic amino acids with low MW were more reactive to form HAcAms.

3.3 Correlation between DOMs characteristics and DBPs formation

The correlation between the characteristics of DOMs and the generation of DBPs was investigated using redundancy analysis. The closer the distance between different samples indicates less variability between samples. As shown in Fig. 8, the close distance of RW from UF_e and coagulation_e but the far distance from ozonation_e and BAF_e indicated that ozonation and BAF processes significantly changed DOMs characteristics in RW. The content of DOMs with MW < 50 kDa and the percentage of hydrophobic fractions were the main factors controlling the amount of DBPs generation. SUVA showed positive correlations with THMs formation, indicating that aromaticity was an essential characteristic influencing the production of THMs. The HOB fraction was positively correlated to CH, HKs, and DBCM, indicating that increasing HOB fraction could lead to the increase of CH, HKs, and THMs. The content of DOMs with MW < 3 kDa and FA was positively related to the formation of TCM, suggesting that hydrophilic low MW components were the main precursors of TCM.

As shown in Fig. 8 (b), \(\text{N}{\text{H}}_{\text{4}}^{\text{+}}\)-N concentration showed positive correlation with N-DBPs. It has been reported that \(\text{N}{\text{H}}_{\text{4}}^{\text{+}}\)-N could react with chlorine to produce chloramines and provide nitrogen source for the formation of HANs (Kolkman et al. 2015). HAcAms could in turn be generated by hydrolysis of HANs (Huang et al. 2012). In addition, the nitrogen-containing fractions in HA were important precursors of HAcAms.

The primary DBPs in RW were THMs, and the most important factor that determined the reaction of DOMs with chlorine disinfectant to form THMs was the aromaticity of DOMs. The hydrophobic large MW DOMs was also an essential precursor of THMs. BAF and ozonation had higher removal rates for the precursors of TCM. Ozonation could degrade hydrophobic macromolecules such as HA in RW and converted them into low MW hydrophilic precursors of CH and HKs, resulting in a 54% increase in the production of CHs and HKs. \(\text{N}{\text{H}}_{\text{4}}^{\text{+}}\)-N content was the most important factor affecting N-DBPs formation, nitrogen-containing organic matters in hydrophobic HA were main precursors of DCAN, and hydrophilic low MW nitrogen-rich organic matters were easier to generate HAcAms. The results of this study provide a theoretical basis and direction for the removal of DOMs and the control of DBPs. Among the four advanced treatments, BAF has higher removal rate of DBPs precursors. Enhanced coagulation and UF processes were less effective in removing precursors of DBPs. Ozonation and BAF, on the other hand, could significantly affect the characteristics of DOMs and the generation of DBPs in the subsequent disinfection process. BAF decreased the production of DBPs, but ozonation increased the content of HKs and CH precursors.

Acknowledgments

First and foremost, I would like to show my deepest gratitude to my supervisor, Dr. Liqiu Zhang, a respectable, responsible and resourceful scholar, who has provided me with valuable guidance in every stage of the writing of this thesis.

Secondly, I would also like to thank the teachers and students who helped me. They gave me great help during the revision and submission of my thesis.

Last but not least, we gratefully acknowledge funding from the National Natural Science Foundation of China (No. 41977317, 42177051 and 52170021).

Author contribution

Xingyue Li: Writing-Original Draft, Formal analysis, Data curation, Investigation, Methodology, Writing-review & editing, Validation.

Shiqi Liu: Writing-review & editing.

Shiqi Wang: Experiment, Investigation.

Li Feng: Funding acquisition, Supervision.

Peng Gao: Writing-review & editing.

Yongze Liu: Funding acquisition, Supervision.

Liqiu Zhang: Resources, Conceptualization, Supervision.

Funding

This research was financially supported by the National Natural Science Foundation of China (No. 41977317, 42177051 and 52170021).

Data availability

All data generated or analyzed during this study are included in this published article.

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Research involving human participants and/or animals

Not applicable.

Competing interests

The authors declare no competing interests.

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published 30 Sep, 2023

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Removal performance of dissolved organic matters from municipal secondary effluent by different advanced treatment processes and the following disinfection by-products generation rules

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Abstract

Figures

1 Introduction

2 Material And Methods

2.1 Raw wastewater (RW)

2.2 Advanced treatment experiments

2.2.1 Enhanced coagulation

2.2.2 UF

2.2.3 Ozonation

2.2.3 BAF

2.3 DOMs characteristics analysis

2.4 DBPs determination

3 Results And Discussion

3.1 Influences of different advanced treatment processes on DOMs characteristics

3.1.1 Effects of different advanced treatment processes on wastewater qualities

3.1.2 Sources and chemical composition changes of DOMs in the treated effluents after different advanced treatments

3.1.3 MWD of DOMs in different treated effluents

3.1.4 Changes of DOMs fractions with different hydrophobicity after different advanced treatment processes

3.2 Formation and speciation of DBPs during chlorination of RW and different treated effluents

3.2.1 Effects of advanced treatment processes on DBPs generation

3.2.2 Effects of DOMs fractions with different hydrophobicity on DBPFP in chlorination of RW and different treated effluents

3.2.3 Effects of MWD on DBPFP in chlorination of RW and different treated effluents

3.3 Correlation between DOMs characteristics and DBPs formation

4 Conclusions

Declarations

References

Status:

Journal Publication

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