Journal of Hazardous Materials 266 (2014) 10–18
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Effect of coagulation on treatment of municipal wastewater reverse osmosis concentrate by UVC/H2 O2 Muhammad Umar, Felicity Roddick ∗ , Linhua Fan School of Civil, Environmental and Chemical Engineering, RMIT University, GPO Box 2476, Melbourne, 3001 Victoria, Australia
h i g h l i g h t s • • • • •
Alum coagulation is an effective pre-treatment for UVC/H2 O2 treatment of high salinity ROC. Comparable DOC in samples but different coagulation success due to different nature of organics. Comparable mineralization obtained for two different ROCs with UVC/H2 O2 only treatment. UVC/H2 O2 treatment led to increased biodegradability with and without coagulation. Significant reduction in energy consumption obtained after pre- and biological post-treatment.
a r t i c l e
i n f o
Article history: Received 16 August 2013 Received in revised form 24 November 2013 Accepted 5 December 2013 Available online 12 December 2013 Keywords: Coagulation Alum UVC/H2 O2 Reverse osmosis concentrate Biodegradability
a b s t r a c t Disposal of reverse osmosis concentrate (ROC) is a growing concern due to potential health and ecological risks. Alum coagulation was investigated as pre-treatment for the UVC/H2 O2 treatment of two high salinity ROC samples (ROC A and B) of comparable organic and inorganic content. Coagulation removed a greater fraction of the organic content for ROC B (29%) than ROC A (16%) which correlated well with the reductions of colour and A254 . Although the total reductions after 60 min UVC/H2 O2 treatment with and without coagulation were comparable, large differences in the trends of reduction were observed which were attributed to the different nature of the organic content (humic-like) of the samples as indicated by the LC-OCD analyses and different initial (5% and 16%) biodegradability. Coagulation and UVC/H2 O2 treatment preferentially removed humic-like compounds which resulted in low reaction rates after UVC/H2 O2 treatment of the coagulated samples. The improvement in biodegradability was greater (2–3-fold) during UVC/H2 O2 treatment of the pre-treated samples than without pre-treatment. The target DOC residual (≤15 mg/L) was obtained after 30 and 20 min irradiation of pre-treated ROC A and ROC B with downstream biological treatment, corresponding to reductions of 55% and 62%, respectively. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Increasing use of reverse osmosis (RO)-based processes in water and wastewater treatment has led to significant attention being paid to the treatment of the resultant RO concentrate (ROC). The successful rejection of various inorganic and organic contaminants by RO membranes results in their elevated (3–4-fold) concentrations in ROC. The addition of chemicals (antiscalants, biocides and acids) further complicates the situation as they can change the character of the organic and inorganic pollutants and can influence the chemical equilibrium of the dissolved constituents [1]. The genotoxicity of a ROC was investigated in a recent study by Tang et al. [2] using the SOS umu method and found that it ranged between 500 and 559 g 4-NQO (4-nitroquinoline-1-oxide)/L, which was much
∗ Corresponding author. Tel.: +61 3 9925 2080; fax: +61 3 9639 0138. E-mail address: [email protected] (F. Roddick). 0304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2013.12.005
higher than for the RO influent (105–160 g 4-NQO/L). Therefore, discharging the ROC to the environment can pose serious toxicological and environmental risks. Some facilities have made treatment of ROC mandatory prior to its discharge. For example, in Brisbane (Australia), the Bundamba advanced wastewater treatment plant which contributes purified recycled water to the Western Corridor Recycled Water Scheme, the largest recycled water scheme in Australia and the third largest advanced water treatment project in the world, is required to treat ROC and monitor nutrients and metal concentration prior to its discharge to the Brisbane river [3]. In addition to minimizing the environmental impacts, economically profitable reuse applications can help to offset the costs of treatment processes [4]. Due to the successful application of UVC/H2 O2 process in drinking water treatment and wastewater polishing after advanced treatment (e.g., RO permeate), its use in the treatment of ROC has recently been investigated [5–7]. The process has been reported to reduce the concentration of organic matter as well as improve
M. Umar et al. / Journal of Hazardous Materials 266 (2014) 10–18 Table 1 Characteristics of ROC samples.
a
11
higher for ROC A. All experiments were carried out in duplicate and average results are reported.
Parameter
ROC Aa
ROC Bb
DOC (mg/L) COD (mg/L) pH Colour (Pt.Co) Chloride (mg/L) TDS (mg/L) A254 (/cm) SUVA (L/mg/m) Alkalinity (as CaCO3 , mg/L) Conductivity (mS/cm)
32.5 155 7.4 137 8875 17,400 0.6 1.9 450 27.5
37.5 105 8.3 158 8060 16,140 0.68 2 410 22.3
Collected on 17 April, 2012. b Collected on 26 June, 2012.
the biodegradability. In addition, UVC/H2 O2 treatment caused no toxicity (Microtox assay) and slightly reduced the trihalomethane formation potential when combined with biological treatment [6]. However the process is considered energy intensive. Pre-treatment can potentially reduce irradiation time and thus energy requirements, and can facilitate the subsequent UVC/H2 O2 treatment by improving UV transmittance (UVT). Some studies [8–10] have reported coagulation as an individual process for the treatment of ROC but none of these investigated sequential coagulation-UVC/H2 O2 treatment. Furthermore, the ROC samples used in those studies were of markedly different characteristics, particularly in terms of salinity. The ROC in the present investigation was high in salinity (22–27 mS/cm). Coagulation for the removal of humic substances in saline (marine) water conditions [11–14] has been suggested to occur differently than in low salinity water in terms of colloid destabilization and removal as the high ion content can affect chemical hydrolysis and metalhydroxide solubility reactions. The present study was carried out to investigate the effect of sequential coagulation using alum and UVC/H2 O2 treatment on a high salinity ROC with a view to reducing the irradiation time to produce a target residual DOC concentration of ≤15 mg/L. Alum was chosen due to its wide use in water treatment, less impact on pH and lower cost than iron-based coagulants. The most appropriate coagulant dosage (as Al3+ ) and pH were established to enable comparison of the efficiency of the UVC/H2 O2 process with and without pre-treatment. The change in biodegradability was investigated by determining the biological dissolved organic carbon (BDOC) concentration, and fluorescence excitation–emission (EEM) spectra and liquid chromatography-organic carbon detection (LC-OCD) were used to track the changes in the organic components of the ROC. A preliminary estimate of the energy requirements was made to find appropriate conditions in terms of process efficiency and cost effectiveness.
2.2. Alum coagulation Alum stock solution was prepared using Al2 (SO4 )3 ·18H2 O (Chem-Supply, Pty Ltd., Australia). A range of alum concentrations (1–6 mM as Al3+ ) was tested to find the best coagulant dosage. Coagulation was conducted with a laboratory jar test apparatus (Phipps and Bird, PB-700) using 2 L ROC samples. The samples were rapidly mixed for 2 min at 250 rpm followed by slow mixing for 30 min at 30 rpm and subsequent settling for 2 h before taking supernatant for analyses. Desired pH value was adjusted using 1 M H2 SO4 or 1 M NaOH. 2.3. Irradiation conditions Irradiation was conducted using an annular reactor (working volume 900 mL) with a centrally mounted UV lamp [16]. The average irradiated area was 464 cm2 , and the path length was 1.94 cm. UVC irradiation ( = 254 nm) was provided by a 39 W UV lamp (Australian Ultra Violet Services, G36T15NU). The average fluence rate of the lamp was 8.91 mW/cm2 . After the addition of H2 O2 (3 mM), the samples were mixed and aerated by humidified air in the reactor and irradiated for various contact times. The H2 O2 dosage was selected based on initial tests conducted using a range of dosages (1–6 mM). For comparison, the pH of the ROC samples subjected to UVC/H2 O2 only treatment was adjusted to 5. 2.4. Analytical methods DOC was determined using a TOC analyser (Sievers model 5310C) in in-line mode to purge inorganic carbon. Absorbance was determined using a double beam scanning UV–vis spectrophotometer (Unicam UV2). Colour was measured with a Hach DR 4000 spectrophotometer at 455 nm in Platinum Cobalt (Pt.Co) units. Fluorescence excitation–emission matrix spectra were determined with a Perkin Elmer LS-50B luminescence spectrometer. The biodegradability of the organics was evaluated as BDOC using the assay introduced by Joret and Levi [17] and modified by Volk et al. [18]. The concentration of residual H2 O2 was estimated using Merckoquant® peroxide test strips. To avoid the interference of H2 O2 in COD and DOC measurement, the residual H2 O2 was quenched by using the enzyme catalase (from Aspergillus niger, Calbiochem® ) as described elsewhere [19]. Molecular size distribution was determined using liquid chromatography with organic carbon detection (LC-OCD) at the Water Research Centre of the University of New South Wales (Sydney, Australia) with a DOC-Labor LC-OCD Model 8 with a Toyopearl TSK HW-50S column, using a phosphate buffer of pH 6.4 as the mobile phase.
2. Materials and methodology 3. Results and discussion 2.1. Collection and characterization of ROC 3.1. Effect of pH and dosage on coagulation Two grab samples of ROC (Table 1) were collected from a wastewater reclamation facility at a local municipal wastewater treatment plant (WWTP), and analyzed according to standard methods [15]. In the treatment process at the WWTP, the raw sewage is screened and de-gritted and sent to intermittently decanted extended aeration (IDEA) bioreactors where it is treated in a cycle of aeration, settling, and decant. The IDEA reactor effluent is then treated using a combination of ultrafiltration and RO. Both samples were high in salinity and total dissolved solids (TDS) concentration, and overall, the bulk characteristics of both ROC samples were comparable except for pH, COD and conductivity which were
Alum is generally considered effective at pH 5–6 [20] and the effect of these values on ROC A and B was investigated using 1.5 mM Al3+ to confirm the best pH. Coagulation efficiency was significantly greater at pH 5 than at pH 6 (Table 2) for both samples and therefore pH 5 was chosen for further investigation. A range of concentrations (1–6 mM as Al3+ ) was then tested for ROC A to find the best dosage at pH 5. The reduction of DOC increased with increasing coagulant dosage up to 3 mM (22%) whereas COD reduction remained fairly similar above 1 mM (Fig. 1a). The reduction of colour and A254 increased with increasing
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M. Umar et al. / Journal of Hazardous Materials 266 (2014) 10–18
Fig. 1. Normalized concentration of (a) DOC and COD, (b) colour and A254 of ROC A; and (c) DOC and COD, (d) colour and A254 of ROC B (pH 5).
coagulant dosage up to 3 mM (Fig. 1b). Colour reduction increased from 45% to 67% when the Al3+ dosage was increased from 1.5 to 3 mM. The reduction of A254 was 22% for 1.5 mM and it almost doubled (43%) with doubling the dosage to 3 mM. Due to the significant increase in the reduction of colour and A254 on increasing the Al3+ dosage from 1.5 to 3 mM, both dosages were selected for subsequent UVC/3 mM H2 O2 treatment of ROC A. Based on the results for ROC A, 0.5–3 mM Al3+ was selected to find the best dosage for ROC B at pH 5. Increasing reduction of DOC and COD occurred with increasing coagulant dosage, however, the increment in the reduction was lower after 1.5 mM (Fig. 1c). Similarly the increment in the reduction of colour and A254 was lower after 1.5 mM, i.e., only 10–12% additional reduction occurred when the dosage was increased to 3 mM (Fig. 1d). UVC/3 mM H2 O2 treatment was therefore carried out on ROC B after 1.5 mM Al3+ pre-treatment. 3.2. UVC/H2 O2 treatment with and without coagulation UVC/H2 O2 only treatment of ROC A gave a DOC reduction of 25% after 60 min (Fig. 2a). Although greater DOC reduction was obtained for 3 mM Al3+ than 1.5 mM Al3+ , it was comparable for both dosages after 60 min, i.e., 36% and 38%, respectively (Fig. 2a). The reduction of DOC after 20 min irradiation of the pre-treated sample was Table 2 Effect of pH on coagulation (1.5 mM Al3+ ) of ROC A and ROC B. % reduction
DOC COD Colour A254
ROC A
ROC B
pH 5
pH 6
pH 5
pH 6
16 17 40 22
10 11 31 16
29 29 47 34
17 16 25 18
greater than after 60 min without pre-treatment, showing the contribution of coagulation in removing a portion of the organic matter. The reduction of DOC by UVC/H2 O2 treatment during the first 10 min (Fig. 2a) was greater for the raw sample (10%) than for the 1.5 mM (4%) and 3 mM Al3+ (2%) pre-treated samples. The difference in the reduction of DOC for 1.5 and 3 mM Al3+ pre-treated sample was greatest (6%) after 30 min but reduced to 3% after 60 min, indicating little benefit of the higher coagulant dosage. The trend for the reduction of COD was different with a large reduction during the first 10 min for the raw and pre-treated samples which is attributed to the greater removal of UV-labile COD. Little difference in the reduction was observed for 1.5 and 3 mM Al3+ pre-treated samples at all irradiation times (Fig. 2b), which was consistent with the DOC results. The trend for colour and A254 reduction was markedly different for the raw and pre-treated samples (Fig. 2c and d). Compared with steady decrease of colour and A254 with increasing irradiation during UVC/H2 O2 only treatment, slower initial reductions followed by significantly larger reductions occurred for the pretreated samples. The colour was unchanged during first 10 min irradiation but then decreased significantly for 1.5 mM (30%) and 3 mM (48%) during the next 10 min. Similarly, the reduction of A254 was much greater between 10 and 20 min, i.e., 36% and 32% for 1.5 and 3 mM Al3+ , respectively, compared with in the first 10 min (3–17%). Although 3 mM Al3+ coagulation led to markedly greater colour and A254 reduction than 1.5 mM, the reduction after 20 min irradiation was fairly similar for both these dosages (Fig. 2c and d). As the results obtained for the two Al3+ dosages with subsequent UVC/H2 O2 treatment showed little difference after 20 min irradiation, 1.5 mM Al3+ was chosen as the most effective dosage in terms of removal performance with minimal chemical consumption. As the most effective dosage and pH were similar for the samples, these conditions were used for comparison of the treatment performance for ROC A and ROC B.
M. Umar et al. / Journal of Hazardous Materials 266 (2014) 10–18
13
Fig. 2. Normalized concentration of (a) DOC, (b) COD, (c) colour and (d) A254 of ROC A after UVC/H2 O2 treatment with and without pre-treatment.
3.3. UVC/H2 O2 treatment with and without coagulation of ROC A and ROC B The data for ROC A shown in Fig. 2 has been re-plotted in Fig. 3 to enable direct comparison with ROC B. Coagulation was more effective for ROC B than for ROC A, particularly for the removal of DOC. The reduction of DOC was similar for both samples during the UVC/H2 O2 only treatment after 60 min (Fig. 3a). The reduction for pre-treated ROC B was lower (8%) than ROC A (19%) after 60 min with most of it occurring in the first 20 min, i.e., 11% and 6%, respectively, showing the recalcitrant nature of the remaining organic compounds as a sufficient residual of H2 O2 (≥25 mg/L) was present. Although the overall DOC reductions were fairly similar after 60 min, the difference in the trend of reduction indicates the different nature of the organic content of the samples which was not evident from the initial DOC and SUVA values (Table 1). The trend in the reduction of COD was different for the two ROC samples during UVC/H2 O2 treatment with and without pretreatment (Fig. 3b). The reduction was greater for ROC A (46%) than ROC B (37%) after 60 min UVC/H2 O2 only treatment. For ROC A, a large decrease (32%) occurred after 10 min followed by a slow reduction compared with the gradual decrease for ROC B. Significant reduction (25%) of COD after the first 10 min irradiation of pre-treated ROC A shows that coagulation did not remove UV-labile COD. The reduction of COD was similar (51%) for both ROC samples after 60 min irradiation of the pre-treated samples (Fig. 3b). The reduction of colour and A254 was high for both samples during UVC/H2 O2 only treatment (Fig. 3c and d) unlike the gradual reduction for raw ROC, the trend in the reduction of colour and A254 was markedly different when pre-treated ROC A was subjected to the UVC/H2 O2 treatment whereas it was fairly similar for ROC B. Very little reduction of colour occurred in the first 10 min for the pre-treated samples, however, it had increased markedly after 20 min, comparable with that at 60 min irradiation without pre-treatment.
Enhanced reduction of colour and A254 during UVC/H2 O2 treatment of the pre-treated samples was attributed to the greater breakdown of the chromophores due to improved UVT after coagulation (40–43% from initial values of 21–23%). The large reduction of colour and A254 but low DOC reduction implies that decolourization occurred due to the breakdown of the chromophore bonds with the major fragments or bulk of the original molecules remaining intact, i.e., they were not mineralized. It has been shown that complete decolourization can occur with little reduction of TOC and COD [21]. The large reductions in SUVA (Fig. 3e) indicate the loss in aromaticity and UV-absorbing functional groups of the ROC and their transformation to non/less UV-absorbing compounds. The initial higher reduction of DOC of the raw sample was attributed to the breakdown of the preferentially targeted compounds (humic-like) whereas the slower initial DOC reduction of the pre-treated sample during UVC/H2 O2 treatment was attributed to the preferential removal of the same compounds by coagulation. Coagulation mainly removes large molecular weight (MW) compounds such as humics and these are also preferentially targeted by the HO• generated during UVC/H2 O2 treatment [22]. Nonetheless, greater reduction of DOC was observed after UVC/H2 O2 treatment with pre-treatment than without pre-treatment. Taking into account the fairly similar initial DOC and salt concentrations of both samples, the effectiveness of the coagulation appears to be a function of nature of the organic content. Faster and greater reductions of colour and A254 occurred during UVC/H2 O2 treatment of the pretreated sample such that their values after 20 min irradiation of pre-treated ROC A, and after 30 min for ROC B, were almost similar to or lower than after 60 min UVC/H2 O2 only treatment. 4. Kinetics The reduction of DOC, COD, colour and A254 were modelled for the first 30 min of irradiation as first order kinetics according to Eq. (1). The data was processed using the GRG nonlinear solving
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M. Umar et al. / Journal of Hazardous Materials 266 (2014) 10–18
Fig. 3. Normalized concentration of (a) DOC, (b) COD, (c) colour, (d) A254 and (e) SUVA reduction after UVC/H2 O2 treatment with and without coagulation for each ROC.
method using Microsoft Excel Solver, and then the fit to pseudo first-order kinetics determined. The reliability of this approach has been confirmed in the literature [23]. ln
C t
Co
= −kt
(1)
where k is the reaction rate constant (min−1 ) and Co and Ct are the concentrations at irradiation time 0 and t, respectively. By plotting ln(Ct )/ln(Co ) versus time (t), the values of k were obtained and are given in Table 3. The reduction of DOC, COD, colour and A254 followed a pseudo first-order reaction for the first 30 min for both samples. The reduction of DOC, colour and A254 occurred at fairly similar rates for ROC A and B for UVC/H2 O2 only treatment, however the rate of COD reduction was lower for ROC B (Table 3). The data correlated well with acceptable R2 values (≥0.85) except for COD reduction for ROC A. The lower R2 value (0.74) for COD suggested only a moderate fit to the model for the reduction of COD. The rates of DOC reduction were lower after pre-treatment for both the samples due to the removal of a considerable fraction of the large
molecular weight compounds (humics) that were preferentially targeted by the UVC/H2 O2 treatment. However, the rates for COD reduction were similar to that of UVC/H2 O2 only treatment for each sample and there was a stronger correlation for ROC B (R2 = 0.96) than ROC A (R2 = 0.76). Markedly higher rates of colour and A254
Table 3 Constants to model the loss of DOC, COD, colour and A254 after 30 min UVC/H2 O2 treatment with and without coagulation. Treatment
ROC A
ROC B −1
R
k (min−1 )
R2
0.006 0.02 0.02 0.02
0.85 0.74 0.94 0.97
0.006 0.01 0.02 0.03
0.88 0.99 0.97 0.98
0.005 0.02 0.09 0.05
0.93 0.76 0.85 0.94
0.004 0.01 0.03 0.03
0.93 0.96 0.80 0.99
Parameter
k (min
UVC/H2 O2 only
DOC COD Colour A254
1.5 mM Al3+ + UVC/H2 O2
DOC COD Colour A254
)
2
M. Umar et al. / Journal of Hazardous Materials 266 (2014) 10–18
reduction were observed after UVC/H2 O2 treatment of the pretreated ROC A, whereas they were comparable for ROC B after 30 min UVC/H2 O2 treatment with and without pre-treatment.
5. Fluorescence excitation–emission matrix spectra Fluorescence excitation–emission matrix (EEM) spectra provide a “fingerprint” of the types of organics in water and wastewater [24]. The impact of UVC/H2 O2 treatment with and without coagulation (1.5 mM Al3+ ) was investigated using the fluorescence regional integration (FRI) technique [24]. The DOC of each sample was adjusted to 10 mg/L to avoid the inherent quenching effect. The difference between FRI results was
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Effect of coagulation on treatment of municipal wastewater reverse osmosis concentrate by UVC/H2 O2 Muhammad Umar, Felicity Roddick ∗ , Linhua Fan School of Civil, Environmental and Chemical Engineering, RMIT University, GPO Box 2476, Melbourne, 3001 Victoria, Australia
h i g h l i g h t s • • • • •
Alum coagulation is an effective pre-treatment for UVC/H2 O2 treatment of high salinity ROC. Comparable DOC in samples but different coagulation success due to different nature of organics. Comparable mineralization obtained for two different ROCs with UVC/H2 O2 only treatment. UVC/H2 O2 treatment led to increased biodegradability with and without coagulation. Significant reduction in energy consumption obtained after pre- and biological post-treatment.
a r t i c l e
i n f o
Article history: Received 16 August 2013 Received in revised form 24 November 2013 Accepted 5 December 2013 Available online 12 December 2013 Keywords: Coagulation Alum UVC/H2 O2 Reverse osmosis concentrate Biodegradability
a b s t r a c t Disposal of reverse osmosis concentrate (ROC) is a growing concern due to potential health and ecological risks. Alum coagulation was investigated as pre-treatment for the UVC/H2 O2 treatment of two high salinity ROC samples (ROC A and B) of comparable organic and inorganic content. Coagulation removed a greater fraction of the organic content for ROC B (29%) than ROC A (16%) which correlated well with the reductions of colour and A254 . Although the total reductions after 60 min UVC/H2 O2 treatment with and without coagulation were comparable, large differences in the trends of reduction were observed which were attributed to the different nature of the organic content (humic-like) of the samples as indicated by the LC-OCD analyses and different initial (5% and 16%) biodegradability. Coagulation and UVC/H2 O2 treatment preferentially removed humic-like compounds which resulted in low reaction rates after UVC/H2 O2 treatment of the coagulated samples. The improvement in biodegradability was greater (2–3-fold) during UVC/H2 O2 treatment of the pre-treated samples than without pre-treatment. The target DOC residual (≤15 mg/L) was obtained after 30 and 20 min irradiation of pre-treated ROC A and ROC B with downstream biological treatment, corresponding to reductions of 55% and 62%, respectively. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Increasing use of reverse osmosis (RO)-based processes in water and wastewater treatment has led to significant attention being paid to the treatment of the resultant RO concentrate (ROC). The successful rejection of various inorganic and organic contaminants by RO membranes results in their elevated (3–4-fold) concentrations in ROC. The addition of chemicals (antiscalants, biocides and acids) further complicates the situation as they can change the character of the organic and inorganic pollutants and can influence the chemical equilibrium of the dissolved constituents [1]. The genotoxicity of a ROC was investigated in a recent study by Tang et al. [2] using the SOS umu method and found that it ranged between 500 and 559 g 4-NQO (4-nitroquinoline-1-oxide)/L, which was much
∗ Corresponding author. Tel.: +61 3 9925 2080; fax: +61 3 9639 0138. E-mail address: [email protected] (F. Roddick). 0304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2013.12.005
higher than for the RO influent (105–160 g 4-NQO/L). Therefore, discharging the ROC to the environment can pose serious toxicological and environmental risks. Some facilities have made treatment of ROC mandatory prior to its discharge. For example, in Brisbane (Australia), the Bundamba advanced wastewater treatment plant which contributes purified recycled water to the Western Corridor Recycled Water Scheme, the largest recycled water scheme in Australia and the third largest advanced water treatment project in the world, is required to treat ROC and monitor nutrients and metal concentration prior to its discharge to the Brisbane river [3]. In addition to minimizing the environmental impacts, economically profitable reuse applications can help to offset the costs of treatment processes [4]. Due to the successful application of UVC/H2 O2 process in drinking water treatment and wastewater polishing after advanced treatment (e.g., RO permeate), its use in the treatment of ROC has recently been investigated [5–7]. The process has been reported to reduce the concentration of organic matter as well as improve
M. Umar et al. / Journal of Hazardous Materials 266 (2014) 10–18 Table 1 Characteristics of ROC samples.
a
11
higher for ROC A. All experiments were carried out in duplicate and average results are reported.
Parameter
ROC Aa
ROC Bb
DOC (mg/L) COD (mg/L) pH Colour (Pt.Co) Chloride (mg/L) TDS (mg/L) A254 (/cm) SUVA (L/mg/m) Alkalinity (as CaCO3 , mg/L) Conductivity (mS/cm)
32.5 155 7.4 137 8875 17,400 0.6 1.9 450 27.5
37.5 105 8.3 158 8060 16,140 0.68 2 410 22.3
Collected on 17 April, 2012. b Collected on 26 June, 2012.
the biodegradability. In addition, UVC/H2 O2 treatment caused no toxicity (Microtox assay) and slightly reduced the trihalomethane formation potential when combined with biological treatment [6]. However the process is considered energy intensive. Pre-treatment can potentially reduce irradiation time and thus energy requirements, and can facilitate the subsequent UVC/H2 O2 treatment by improving UV transmittance (UVT). Some studies [8–10] have reported coagulation as an individual process for the treatment of ROC but none of these investigated sequential coagulation-UVC/H2 O2 treatment. Furthermore, the ROC samples used in those studies were of markedly different characteristics, particularly in terms of salinity. The ROC in the present investigation was high in salinity (22–27 mS/cm). Coagulation for the removal of humic substances in saline (marine) water conditions [11–14] has been suggested to occur differently than in low salinity water in terms of colloid destabilization and removal as the high ion content can affect chemical hydrolysis and metalhydroxide solubility reactions. The present study was carried out to investigate the effect of sequential coagulation using alum and UVC/H2 O2 treatment on a high salinity ROC with a view to reducing the irradiation time to produce a target residual DOC concentration of ≤15 mg/L. Alum was chosen due to its wide use in water treatment, less impact on pH and lower cost than iron-based coagulants. The most appropriate coagulant dosage (as Al3+ ) and pH were established to enable comparison of the efficiency of the UVC/H2 O2 process with and without pre-treatment. The change in biodegradability was investigated by determining the biological dissolved organic carbon (BDOC) concentration, and fluorescence excitation–emission (EEM) spectra and liquid chromatography-organic carbon detection (LC-OCD) were used to track the changes in the organic components of the ROC. A preliminary estimate of the energy requirements was made to find appropriate conditions in terms of process efficiency and cost effectiveness.
2.2. Alum coagulation Alum stock solution was prepared using Al2 (SO4 )3 ·18H2 O (Chem-Supply, Pty Ltd., Australia). A range of alum concentrations (1–6 mM as Al3+ ) was tested to find the best coagulant dosage. Coagulation was conducted with a laboratory jar test apparatus (Phipps and Bird, PB-700) using 2 L ROC samples. The samples were rapidly mixed for 2 min at 250 rpm followed by slow mixing for 30 min at 30 rpm and subsequent settling for 2 h before taking supernatant for analyses. Desired pH value was adjusted using 1 M H2 SO4 or 1 M NaOH. 2.3. Irradiation conditions Irradiation was conducted using an annular reactor (working volume 900 mL) with a centrally mounted UV lamp [16]. The average irradiated area was 464 cm2 , and the path length was 1.94 cm. UVC irradiation ( = 254 nm) was provided by a 39 W UV lamp (Australian Ultra Violet Services, G36T15NU). The average fluence rate of the lamp was 8.91 mW/cm2 . After the addition of H2 O2 (3 mM), the samples were mixed and aerated by humidified air in the reactor and irradiated for various contact times. The H2 O2 dosage was selected based on initial tests conducted using a range of dosages (1–6 mM). For comparison, the pH of the ROC samples subjected to UVC/H2 O2 only treatment was adjusted to 5. 2.4. Analytical methods DOC was determined using a TOC analyser (Sievers model 5310C) in in-line mode to purge inorganic carbon. Absorbance was determined using a double beam scanning UV–vis spectrophotometer (Unicam UV2). Colour was measured with a Hach DR 4000 spectrophotometer at 455 nm in Platinum Cobalt (Pt.Co) units. Fluorescence excitation–emission matrix spectra were determined with a Perkin Elmer LS-50B luminescence spectrometer. The biodegradability of the organics was evaluated as BDOC using the assay introduced by Joret and Levi [17] and modified by Volk et al. [18]. The concentration of residual H2 O2 was estimated using Merckoquant® peroxide test strips. To avoid the interference of H2 O2 in COD and DOC measurement, the residual H2 O2 was quenched by using the enzyme catalase (from Aspergillus niger, Calbiochem® ) as described elsewhere [19]. Molecular size distribution was determined using liquid chromatography with organic carbon detection (LC-OCD) at the Water Research Centre of the University of New South Wales (Sydney, Australia) with a DOC-Labor LC-OCD Model 8 with a Toyopearl TSK HW-50S column, using a phosphate buffer of pH 6.4 as the mobile phase.
2. Materials and methodology 3. Results and discussion 2.1. Collection and characterization of ROC 3.1. Effect of pH and dosage on coagulation Two grab samples of ROC (Table 1) were collected from a wastewater reclamation facility at a local municipal wastewater treatment plant (WWTP), and analyzed according to standard methods [15]. In the treatment process at the WWTP, the raw sewage is screened and de-gritted and sent to intermittently decanted extended aeration (IDEA) bioreactors where it is treated in a cycle of aeration, settling, and decant. The IDEA reactor effluent is then treated using a combination of ultrafiltration and RO. Both samples were high in salinity and total dissolved solids (TDS) concentration, and overall, the bulk characteristics of both ROC samples were comparable except for pH, COD and conductivity which were
Alum is generally considered effective at pH 5–6 [20] and the effect of these values on ROC A and B was investigated using 1.5 mM Al3+ to confirm the best pH. Coagulation efficiency was significantly greater at pH 5 than at pH 6 (Table 2) for both samples and therefore pH 5 was chosen for further investigation. A range of concentrations (1–6 mM as Al3+ ) was then tested for ROC A to find the best dosage at pH 5. The reduction of DOC increased with increasing coagulant dosage up to 3 mM (22%) whereas COD reduction remained fairly similar above 1 mM (Fig. 1a). The reduction of colour and A254 increased with increasing
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M. Umar et al. / Journal of Hazardous Materials 266 (2014) 10–18
Fig. 1. Normalized concentration of (a) DOC and COD, (b) colour and A254 of ROC A; and (c) DOC and COD, (d) colour and A254 of ROC B (pH 5).
coagulant dosage up to 3 mM (Fig. 1b). Colour reduction increased from 45% to 67% when the Al3+ dosage was increased from 1.5 to 3 mM. The reduction of A254 was 22% for 1.5 mM and it almost doubled (43%) with doubling the dosage to 3 mM. Due to the significant increase in the reduction of colour and A254 on increasing the Al3+ dosage from 1.5 to 3 mM, both dosages were selected for subsequent UVC/3 mM H2 O2 treatment of ROC A. Based on the results for ROC A, 0.5–3 mM Al3+ was selected to find the best dosage for ROC B at pH 5. Increasing reduction of DOC and COD occurred with increasing coagulant dosage, however, the increment in the reduction was lower after 1.5 mM (Fig. 1c). Similarly the increment in the reduction of colour and A254 was lower after 1.5 mM, i.e., only 10–12% additional reduction occurred when the dosage was increased to 3 mM (Fig. 1d). UVC/3 mM H2 O2 treatment was therefore carried out on ROC B after 1.5 mM Al3+ pre-treatment. 3.2. UVC/H2 O2 treatment with and without coagulation UVC/H2 O2 only treatment of ROC A gave a DOC reduction of 25% after 60 min (Fig. 2a). Although greater DOC reduction was obtained for 3 mM Al3+ than 1.5 mM Al3+ , it was comparable for both dosages after 60 min, i.e., 36% and 38%, respectively (Fig. 2a). The reduction of DOC after 20 min irradiation of the pre-treated sample was Table 2 Effect of pH on coagulation (1.5 mM Al3+ ) of ROC A and ROC B. % reduction
DOC COD Colour A254
ROC A
ROC B
pH 5
pH 6
pH 5
pH 6
16 17 40 22
10 11 31 16
29 29 47 34
17 16 25 18
greater than after 60 min without pre-treatment, showing the contribution of coagulation in removing a portion of the organic matter. The reduction of DOC by UVC/H2 O2 treatment during the first 10 min (Fig. 2a) was greater for the raw sample (10%) than for the 1.5 mM (4%) and 3 mM Al3+ (2%) pre-treated samples. The difference in the reduction of DOC for 1.5 and 3 mM Al3+ pre-treated sample was greatest (6%) after 30 min but reduced to 3% after 60 min, indicating little benefit of the higher coagulant dosage. The trend for the reduction of COD was different with a large reduction during the first 10 min for the raw and pre-treated samples which is attributed to the greater removal of UV-labile COD. Little difference in the reduction was observed for 1.5 and 3 mM Al3+ pre-treated samples at all irradiation times (Fig. 2b), which was consistent with the DOC results. The trend for colour and A254 reduction was markedly different for the raw and pre-treated samples (Fig. 2c and d). Compared with steady decrease of colour and A254 with increasing irradiation during UVC/H2 O2 only treatment, slower initial reductions followed by significantly larger reductions occurred for the pretreated samples. The colour was unchanged during first 10 min irradiation but then decreased significantly for 1.5 mM (30%) and 3 mM (48%) during the next 10 min. Similarly, the reduction of A254 was much greater between 10 and 20 min, i.e., 36% and 32% for 1.5 and 3 mM Al3+ , respectively, compared with in the first 10 min (3–17%). Although 3 mM Al3+ coagulation led to markedly greater colour and A254 reduction than 1.5 mM, the reduction after 20 min irradiation was fairly similar for both these dosages (Fig. 2c and d). As the results obtained for the two Al3+ dosages with subsequent UVC/H2 O2 treatment showed little difference after 20 min irradiation, 1.5 mM Al3+ was chosen as the most effective dosage in terms of removal performance with minimal chemical consumption. As the most effective dosage and pH were similar for the samples, these conditions were used for comparison of the treatment performance for ROC A and ROC B.
M. Umar et al. / Journal of Hazardous Materials 266 (2014) 10–18
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Fig. 2. Normalized concentration of (a) DOC, (b) COD, (c) colour and (d) A254 of ROC A after UVC/H2 O2 treatment with and without pre-treatment.
3.3. UVC/H2 O2 treatment with and without coagulation of ROC A and ROC B The data for ROC A shown in Fig. 2 has been re-plotted in Fig. 3 to enable direct comparison with ROC B. Coagulation was more effective for ROC B than for ROC A, particularly for the removal of DOC. The reduction of DOC was similar for both samples during the UVC/H2 O2 only treatment after 60 min (Fig. 3a). The reduction for pre-treated ROC B was lower (8%) than ROC A (19%) after 60 min with most of it occurring in the first 20 min, i.e., 11% and 6%, respectively, showing the recalcitrant nature of the remaining organic compounds as a sufficient residual of H2 O2 (≥25 mg/L) was present. Although the overall DOC reductions were fairly similar after 60 min, the difference in the trend of reduction indicates the different nature of the organic content of the samples which was not evident from the initial DOC and SUVA values (Table 1). The trend in the reduction of COD was different for the two ROC samples during UVC/H2 O2 treatment with and without pretreatment (Fig. 3b). The reduction was greater for ROC A (46%) than ROC B (37%) after 60 min UVC/H2 O2 only treatment. For ROC A, a large decrease (32%) occurred after 10 min followed by a slow reduction compared with the gradual decrease for ROC B. Significant reduction (25%) of COD after the first 10 min irradiation of pre-treated ROC A shows that coagulation did not remove UV-labile COD. The reduction of COD was similar (51%) for both ROC samples after 60 min irradiation of the pre-treated samples (Fig. 3b). The reduction of colour and A254 was high for both samples during UVC/H2 O2 only treatment (Fig. 3c and d) unlike the gradual reduction for raw ROC, the trend in the reduction of colour and A254 was markedly different when pre-treated ROC A was subjected to the UVC/H2 O2 treatment whereas it was fairly similar for ROC B. Very little reduction of colour occurred in the first 10 min for the pre-treated samples, however, it had increased markedly after 20 min, comparable with that at 60 min irradiation without pre-treatment.
Enhanced reduction of colour and A254 during UVC/H2 O2 treatment of the pre-treated samples was attributed to the greater breakdown of the chromophores due to improved UVT after coagulation (40–43% from initial values of 21–23%). The large reduction of colour and A254 but low DOC reduction implies that decolourization occurred due to the breakdown of the chromophore bonds with the major fragments or bulk of the original molecules remaining intact, i.e., they were not mineralized. It has been shown that complete decolourization can occur with little reduction of TOC and COD [21]. The large reductions in SUVA (Fig. 3e) indicate the loss in aromaticity and UV-absorbing functional groups of the ROC and their transformation to non/less UV-absorbing compounds. The initial higher reduction of DOC of the raw sample was attributed to the breakdown of the preferentially targeted compounds (humic-like) whereas the slower initial DOC reduction of the pre-treated sample during UVC/H2 O2 treatment was attributed to the preferential removal of the same compounds by coagulation. Coagulation mainly removes large molecular weight (MW) compounds such as humics and these are also preferentially targeted by the HO• generated during UVC/H2 O2 treatment [22]. Nonetheless, greater reduction of DOC was observed after UVC/H2 O2 treatment with pre-treatment than without pre-treatment. Taking into account the fairly similar initial DOC and salt concentrations of both samples, the effectiveness of the coagulation appears to be a function of nature of the organic content. Faster and greater reductions of colour and A254 occurred during UVC/H2 O2 treatment of the pretreated sample such that their values after 20 min irradiation of pre-treated ROC A, and after 30 min for ROC B, were almost similar to or lower than after 60 min UVC/H2 O2 only treatment. 4. Kinetics The reduction of DOC, COD, colour and A254 were modelled for the first 30 min of irradiation as first order kinetics according to Eq. (1). The data was processed using the GRG nonlinear solving
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Fig. 3. Normalized concentration of (a) DOC, (b) COD, (c) colour, (d) A254 and (e) SUVA reduction after UVC/H2 O2 treatment with and without coagulation for each ROC.
method using Microsoft Excel Solver, and then the fit to pseudo first-order kinetics determined. The reliability of this approach has been confirmed in the literature [23]. ln
C t
Co
= −kt
(1)
where k is the reaction rate constant (min−1 ) and Co and Ct are the concentrations at irradiation time 0 and t, respectively. By plotting ln(Ct )/ln(Co ) versus time (t), the values of k were obtained and are given in Table 3. The reduction of DOC, COD, colour and A254 followed a pseudo first-order reaction for the first 30 min for both samples. The reduction of DOC, colour and A254 occurred at fairly similar rates for ROC A and B for UVC/H2 O2 only treatment, however the rate of COD reduction was lower for ROC B (Table 3). The data correlated well with acceptable R2 values (≥0.85) except for COD reduction for ROC A. The lower R2 value (0.74) for COD suggested only a moderate fit to the model for the reduction of COD. The rates of DOC reduction were lower after pre-treatment for both the samples due to the removal of a considerable fraction of the large
molecular weight compounds (humics) that were preferentially targeted by the UVC/H2 O2 treatment. However, the rates for COD reduction were similar to that of UVC/H2 O2 only treatment for each sample and there was a stronger correlation for ROC B (R2 = 0.96) than ROC A (R2 = 0.76). Markedly higher rates of colour and A254
Table 3 Constants to model the loss of DOC, COD, colour and A254 after 30 min UVC/H2 O2 treatment with and without coagulation. Treatment
ROC A
ROC B −1
R
k (min−1 )
R2
0.006 0.02 0.02 0.02
0.85 0.74 0.94 0.97
0.006 0.01 0.02 0.03
0.88 0.99 0.97 0.98
0.005 0.02 0.09 0.05
0.93 0.76 0.85 0.94
0.004 0.01 0.03 0.03
0.93 0.96 0.80 0.99
Parameter
k (min
UVC/H2 O2 only
DOC COD Colour A254
1.5 mM Al3+ + UVC/H2 O2
DOC COD Colour A254
)
2
M. Umar et al. / Journal of Hazardous Materials 266 (2014) 10–18
reduction were observed after UVC/H2 O2 treatment of the pretreated ROC A, whereas they were comparable for ROC B after 30 min UVC/H2 O2 treatment with and without pre-treatment.
5. Fluorescence excitation–emission matrix spectra Fluorescence excitation–emission matrix (EEM) spectra provide a “fingerprint” of the types of organics in water and wastewater [24]. The impact of UVC/H2 O2 treatment with and without coagulation (1.5 mM Al3+ ) was investigated using the fluorescence regional integration (FRI) technique [24]. The DOC of each sample was adjusted to 10 mg/L to avoid the inherent quenching effect. The difference between FRI results was
