Mahdi
Bazri
and
Madjid
Mohseni
*
Department of Chemical and Biological Engineering, University of British Columbia, 2360 East Mall Vancouver, BC V6T 1Z3, Canada. E-mail: madjd.mohseni@ubc.ca; Fax: +1 604 822 6003; Tel: +1 604 822 0047
First published on 16th December 2015
Application of UV/H2O2 process for degradation of micropollutants in surface waters could deteriorate the biological stability of treated water. This is because of the partial oxidation of natural organic matter under the applied UV/H2O2 conditions that in turn leads to an increase in assimilable organic carbon (AOC). To address this issue, alum coagulation was investigated as a NOM pretreatment alternative prior to the UV/H2O2 process in order to improve the treatment efficacy and water quality. A recently developed technique was utilized to rapidly assess the AOC of the treated water at various stages. Alum was effective in removing a substantial portion of large to medium molecular weight organic molecules leading to a considerable reduction in AOC. However, the fractions not removed by coagulation were shown to promote some levels of bacterial regrowth after undergoing subsequent UV/H2O2 treatment. That said, alum pretreatment was found to be an effective strategy for reducing the formation of AOC by 14 to 40% depending on the water used and UV dose applied. The findings of this study are of interest for utilities that already have coagulation in use and seek to comply with more upcoming stringent regulations by incorporating advanced oxidation processes (e.g., UV/H2O2) in their treatment train.
Water impactThe biostability of surface waters potentially deteriorates as a result of the partial oxidation of natural organic matter under UV/H2O2 treatment that is applied to the removal of micropollutants. This research highlights the impact of using coagulation to remove natural organic matter prior to the UV/H2O2 process by utilizing a robust and rapid technique for gauging the changes in the biostability. |
Several processes have been proposed and examined in the literature for the removal of NOM under various water loadings and qualities.10–22 Among the options proposed, coagulation processes such as those using alum, ferric chloride and PACl are well established and commonly applied in large-scale applications.12,13,17,23 Moreover, they could serve as viable pretreatment strategies prior to the UV/H2O2 process because of the recognized ability of coagulants (e.g., alum, ferric chloride, PACl) to remove a considerable portion of medium to high molecular weight NOM, and their relatively straightforward operation.24–27 Therefore, the main objective of this research was to gauge the impact of coagulation (e.g., alum) on the degradation of NOM and its subsequent effect on the biological stability (i.e., AOC) of UV/H2O2-treated water.
Two natural water sources were selected and preliminary coagulation tests were conducted to assess the optimum alum dose for NOM removal. Changes in the physicochemical properties (such as UV254, total organic carbon (TOC) content and NOM molecular weight distribution) of raw, alum-treated, UV/H2O2-treated and alum–UV/H2O2-treated water samples were carefully assessed and monitored. A recently developed AOC bioassay using flow cytometry29 that was previously modified for UV/H2O2-treated waters28 was utilized to quantify AOC in all stages. The findings of this research are potentially of interest for those utilities that already use coagulation processes (e.g., alum, ferric chloride, PACl) and hence could readily implement advanced oxidation processes (e.g., UV/H2O2) in their treatment train to meet more stringent guidelines in future.
CR water | BI water | |||
---|---|---|---|---|
Raw water (control) | Alum-treated | Raw water (control) | Alum-treated | |
UV254 | 52.5% (0.032) | 66.7% (0.010) | 34.4% (0.063) | 52.9% (0.018) |
TOC | 25.9% (0.384) | 18.0% (0.131) | 12.1% (0.58) | 8.7% (0.151) |
Moreover, alum is known to be effective in removing large to medium range molecular weight organics.12,13,39 Therefore, downstream UV/H2O2 treatment was expected to cause larger fractional CNOM reductions due to the absence of larger organics that preferentially react with HO˙.5,35,40 It is noteworthy that lower absolute reductions in UV254 and TOC were observed for the alum-treated water samples as a result of the UV/H2O2 treatment (Table 1). This could be mainly explained by the pseudo-first order reaction of TOC and CNOM with OH radicals generated during the UV/H2O2 process.35 That is, with lower amounts of initial organic matter, lower reaction rates are expected; however, higher UV transmittance would result in a higher amount of generated OH radicals, thereby compensating for the organic concentration term.35
Fig. 1 shows the changes in the apparent molecular weight distribution of alum-pretreated CR water that has undergone various UV/H2O2 treatment extents. As demonstrated, the use of alum alone (i.e., Alum-UV 0) was effective in removing a substantial portion of NOM, mainly larger molecular weight fractions. Consistent with the literature, alum was shown to preferentially remove organics of high to medium molecular weight range.12 Application of UV/H2O2 after alum resulted in further decrease in the AMW of UV-absorbing NOM, up to the UV fluence of 500 mJ cm−2. However, no considerable change was observed upon extending the UV dose beyond 500 mJ cm−2.
A similar observation was recorded for the changes in the NOM molecular weight distribution of BI water as shown in Fig. 2. As is illustrated, alum coagulation eliminated the first large eluting peak, often associated with colloidal organic matter, as well as considerable portions of other organic molecules mainly from the large to medium weight range (i.e., >500 Da). However, further downstream UV/H2O2 treatment resulted in small reductions in the remainder of the chromophoric organic molecules.
Fig. 3 and 4 compare the AOC profiles of raw and alum-treated CR and BI waters under various UV doses. As is shown, UV/H2O2 advanced oxidation resulted in a significant increase in AOC for raw waters. This is because of the partial oxidation and breakdown of (larger) organic molecules (i.e., into smaller ones) as a result of the reaction with OH radicals. Fig. 3 and 4 also depict the ability of the organics not removed via coagulation to promote bacterial growth even though their observed initial AOC values were very low. Interestingly, the AOC of the alum-treated waters still increased (and then plateaued) under UV/H2O2 treatment, supporting the fact that further structural breakdown of NOM molecules (even though mostly of lower molecular weight nature) took place. That said, using the HPSEC technique was not sufficient to capture all the changes in the molecular structure of NOM. Nonetheless, as expected, the absolute increase in AOC was noticeably lower for the waters pretreated with alum. This was because alum removed a considerable portion of large to medium organic molecules which are the most susceptible ones towards reaction with OH radicals. As a result of alum treatment, the assimilable percentage of NOM (i.e., AOC/TOC × 100) decreased from 1.42% and 1.56% (for raw waters) to 0.49% and 0.51% for alum-treated CR and BI waters, respectively. This confirms that a considerably lower assimilable fraction remained after coagulation. The observations made here are also in agreement with the observation of Chong Soh et al. (2008),13 who also found that the remaining NOM fractions after coagulation were able to support bacterial regrowth.
Both alum-treated waters showed greater fractional AOC increase (7 and 12 folds, respectively, under UV/H2O2 treatment) in comparison with the raw waters (5 and 3.5 folds, respectively). Also, as previously shown in Table 1, greater fractional UV254 reduction was observed for the alum-treated waters. Therefore, this can be mainly attributed to the more effective interactions between the OH radicals and smaller organic molecules (i.e., in the absence of high MW OH-scavenging dissolved organics), leading to higher enhancement in the biodegradability of NOM. That is, lower levels of organic matter would result in smaller UV254 absorbance consequently leading to a higher UV absorption rate of H2O2.35,41 Therefore, higher fractional AOC increase of the pretreated waters would be expected as less shielding and scavenging effects of NOM exist. As a result, a more effective number of interactions/reactions between the OH radicals and organic matter would be expected.6,35
The behaviour of the AOC profiles of pretreated CR and BI waters over the course of UV/H2O2 treatment is also noteworthy (Fig. 3 and 4). After the UV fluence of 1000 mJ cm−2, the AOC profiles begin to plateau, indicating a possible equilibrium with respect to the formation of smaller (biodegradable) organics and their subsequent degradation with OH radicals. Moreover, the AOC of the pretreated CR starts to decrease slightly after the UV fluence of 1500 mJ cm−2 (Fig. 3). A likely and plausible explanation is that, at this fluence, the degradation rate of organic molecules was dominant and greater than the rate of formation; hence, an overall decrease in the amount of small biodegradable organic molecules was observed.
Moreover, the AOC of pretreated CR water increased by about 7 times at the UV fluence of 2000 mJ cm−2; however, the AOC of pretreated BI water showed an increase of about 12 times from its initial value with the same level of treatment (Fig. 3 and 4). This could be attributed to the higher organic content (i.e., TOC) as well as the nature of NOM in the source water. That is, differences in the nature and characteristics of NOM in BI and CR waters resulted in different behaviours and responses with respect to their reaction with OH radicals and consequently biodegradability increase under identical AOP treatment. Also, alum-treated BI water contained higher amount of organic molecules in the range of low to medium molecular weights (AMW < 1000 Da) in comparison with alum-treated CR water (Fig. 5). As a result, the remaining organics in BI water were likely more biodegradable to start with and underwent further partial oxidation, leading to a greater amount and percentage of AOC generated during UV/H2O2 treatment (i.e., UV fluence of 2000 mJ cm−2). Supporting either of these hypotheses require further investigations using more analytical techniques such as liquid chromatography equipped with organic carbon detection, use of isolation techniques (e.g., nanofiltration) and also evaluation of different source waters. This will eventually lead to better understanding of the fate of NOM during various treatments and the potential for AOC formation.
Even though the AOC of alum-treated water still increased over the course of the UV/H2O2 process, it is important to note that the final AOC was comparable to that of raw water (with no treatment). This means that the combined treatment strategy did not significantly change the biostability characteristics of the water. This is an important consideration because it indicates that the application of combined alum and UV/H2O2 may not deteriorate the biostability of the treated water to a level that downstream biological treatment (e.g., biological activated carbon) would be required. On the other hand, standalone UV/H2O2 treatment with the resultant significant increase of AOC could not be implemented without the application of downstream biological treatment to remove the generated AOC.
Data obtained from the standalone UV/H2O2 treatment along with those of the combined treatment clearly indicated that the application of alum significantly reduces the concentration of high molecular weight NOM. A lower concentration of NOM, in particular from the higher molecular weight fractions, leads to the more effective use of UV photons and lower competition for the OH radicals in the water matrix. The lower scavenging of UV and OH radicals, in turn, leads to more effective removal of target contaminants (i.e., micropollutants) which are the primary reason for the application of UV-based AOPs. More importantly, this will help to conserve a considerable amount of electrical energy used for delivering the necessary UV fluence (to achieve the degradation of target contaminants).30
One important note to consider is that the findings here would be of interest for those facilities that already have coagulation processes in place. Otherwise, incorporating coagulation (coagulation, flocculation and sedimentation) into a new treatment plant may not be a feasible pretreatment alternative since coagulation is a relatively expensive process.
Overall, application of a pretreatment process (e.g., alum) prior to UV/H2O2 treatment can potentially reduce the risk of deteriorating the biostability while saving a considerable amount of electrical energy to achieve the same level of target contaminant removal.
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