A two-year comprehensive advanced oxidation processes (AOPs) pilot test was completed for a Singapore waterworks in 2011–2013. This study focused on oxidative removal of spiked organic contaminants with ozone and ozone-based AOPs (ozone application together with hydrogen peroxide, which is necessary for AOPs). The ‘optimized H2O2 dosage’ test philosophy was verified during the test period – keeping the residual ozone at 0.3 mg/L in the water for disinfection purpose by minimizing the H2O2 dosage. This study also monitored the bromate concentration in both ozone- and AOP-treated water, and all the samples reported below the laboratory detection limit (<5 µg/L), which is also lower than the WHO Guidelines for Drinking Water Quality (<10 µg/L). For comparison, a low pressure UV-based AOP test was conducted in the final stage of the study. The electrical energy per order (EEO) value is compared with ozone- and UV-based AOPs as well. The results indicated that ozone-based AOP with an optimized hydrogen peroxide dosage could be the most energy efficient option for this specific water matrix in terms of most selected compounds.

INTRODUCTION

Public utilities in tropical areas face the challenges of seasonal occurrences, variation in taste and odor compounds, and/or other emerging organic contaminants in their source waters. That leads to the application of treatment processes other than the conventional ones (Agus et al. 2011; Xu et al. 2011). Advanced oxidation processes (AOPs) are one of the recommended approaches. The key is to produce highly reactive hydroxyl radicals (OḢ) that can react rapidly with most organic compounds (von Gunten 2003a, 2003b). To study the feasibility of full-scale AOP application, Xylem and PUB have jointly conducted this comprehensive pilot study to obtain first-hand data on treating actual effluent from water treatment plant, which helps to evaluate the options. The test results will help decision makers upgrading plant design or emergency measurements to handle potential organic compound issues in the future.

MATERIALS/METHODS

The pilot plant (Wang et al. 2012) was manufactured and supplied by Xylem Water Solutions Herford GmbH, Germany (Figure 1). It is designed for a water flow of 5–20 m3/h and hydraulic retention times (HRTs) of up to 10 minutes. Ozone, UV, and hydrogen peroxide dosages are all adjustable at a predetermined water flow rate. In total, 19 organic compounds were spiked into the AOP inlet water for this study. They comprised two taste and odor (T&O) compounds (i.e. 2-methylisoborneol (MIB) and geosmin), four artificial sweeteners (e.g. sucralose), four endocrine disrupting chemicals (EDCs) (e.g. E1, E2, and EE2), eight pharmaceuticals and personal care products (PPCPs) (e.g. diclofenac and ibuprofen), and one industrial solvent (i.e. 1,4-dioxane) as listed in Table 1. During the pilot testing, the selected compounds were mixed in cocktail solution and spiked into water at the same time. Grabbed water samples were analyzed at the PUB laboratories (Zhang et al. 2006; Hu et al. 2008; Wu et al. 2008; He et al. 2012).

Table 1

List of organic compounds in this study

No. Contaminants Group 
2-Methylisoborneol (MIB) Taste and odor (T&O) 
Geosmin  
17a-Ethinylestradiol (EE2) Endocrine disrupting chemicals (EDC) 
17ß-Estradiol (E2)  
Estrone (E1)  
Bisphenol A (BPA)  
Acetaminophen Pharmaceuticals and personal care products (PPCP) 
Diclofenac  
Ibuprofen  
10 Triclosan  
11 Carbamazepine  
12 N,N-Diethyl-metatoluamide (DEET)  
13 Salicylic acid  
14 Trimethoprim  
15 Acesulfame K Artificial sweeteners 
16 Cyclamate Na  
17 Saccharin  
18 Sucralose  
19 1,4-Dioxane Industrial solvent 
No. Contaminants Group 
2-Methylisoborneol (MIB) Taste and odor (T&O) 
Geosmin  
17a-Ethinylestradiol (EE2) Endocrine disrupting chemicals (EDC) 
17ß-Estradiol (E2)  
Estrone (E1)  
Bisphenol A (BPA)  
Acetaminophen Pharmaceuticals and personal care products (PPCP) 
Diclofenac  
Ibuprofen  
10 Triclosan  
11 Carbamazepine  
12 N,N-Diethyl-metatoluamide (DEET)  
13 Salicylic acid  
14 Trimethoprim  
15 Acesulfame K Artificial sweeteners 
16 Cyclamate Na  
17 Saccharin  
18 Sucralose  
19 1,4-Dioxane Industrial solvent 
Figure 1

Schematic diagram of containerized AOP pilot plant.

Figure 1

Schematic diagram of containerized AOP pilot plant.

SOURCE WATER QUALITY

Table 2 is a summary of the inlet water characteristics during the pilot tests. Depending on the operating mode at the waterworks, the inlet water for each test trial was either sand filtered, or a mixture of sand filtered water and MF membrane permeate.

Table 2

Characteristics of inlet water to the pilot system

Water Parameter Unit Range 
pH – 6.2–7.0 
Temperature °C 29–31 
Free chlorine mg/L 0–0.4 
Conductivity µS/cm 231–326 
UVT254nm cm−1 90–92% 
Total alkalinity (as CaCO3mg/L 9–23 
Ammonia (as N) mg/L 0–0.03 
Phosphate (as PO4mg/L <0.08 
TOC mg/L 1.9–3.4 
Bromide mg/L 0–120 
Water Parameter Unit Range 
pH – 6.2–7.0 
Temperature °C 29–31 
Free chlorine mg/L 0–0.4 
Conductivity µS/cm 231–326 
UVT254nm cm−1 90–92% 
Total alkalinity (as CaCO3mg/L 9–23 
Ammonia (as N) mg/L 0–0.03 
Phosphate (as PO4mg/L <0.08 
TOC mg/L 1.9–3.4 
Bromide mg/L 0–120 

RESULTS/DISCUSSION

Results from ozone and ozone-based AOP testing

The test data shows that 10 compounds, including six PPCPs including carbamazepine and diclofenac, and all four EDCs can be effectively degraded by dosing with 2 mg/L ozone and 5 minutes contact time. For the other nine compounds including two T&O, two PPCPs (Figure 2 left), four artificial sweeteners (Figure 2 right), and the industrial solvent, higher ozone dosing rates or ozone-based AOP are necessary to achieve a removal rate as high as 90%.

Figure 2

Removal efficiencies achieved with ozone and, separately, with ozone-based AOP with 5-minute HRT (left: T&O compounds and PPCPs, right: artificial sweeteners).

Figure 2

Removal efficiencies achieved with ozone and, separately, with ozone-based AOP with 5-minute HRT (left: T&O compounds and PPCPs, right: artificial sweeteners).

Figure 2 shows some of the treatment results, with a fixed, 5-minute HRT. Under ozone-based AOP conditions, the generation of OḢ radicals was promoted and the removal efficiencies of the compounds increased significantly in comparison to those achieved with the use of ozone alone.

Figure 3 shows some of the treatment results with HRTs between 0.4 and 10 minutes. It clearly shows that in the ozone-based AOP case, faster and more aggressive reactions took place. Overall, 3 mg/L ozone-based AOP with only 2.8 minutes HRT achieved an equivalent or higher removal efficiency than 5 mg/L ozone only with 10 minutes HRT.

Figure 3

Removal efficiencies achieved by ozone (on its own) and ozone-based AOP with from 0.4- to 10-minute HRTs, (a) MIB and (b) 1,4-dioxane.

Figure 3

Removal efficiencies achieved by ozone (on its own) and ozone-based AOP with from 0.4- to 10-minute HRTs, (a) MIB and (b) 1,4-dioxane.

Results from UV and UV-based AOP testing

Figure 4 shows some of the treatment results with UV and UV-based AOP. It is clear that, UV-based AOP is much more effective in removing the organic compounds studied than UV alone. To achieve removal efficiencies comparable to ozone/ozone-based AOP, the UV and hydrogen peroxide dosages required were above 8,000 J/m2 and 10 mg/L, respectively.

Figure 4

Removal efficiencies of achieved by UV alone and UV-based AOP (left: T&O compounds and PPCPs, right: artificial sweeteners).

Figure 4

Removal efficiencies of achieved by UV alone and UV-based AOP (left: T&O compounds and PPCPs, right: artificial sweeteners).

Comparison of electrical energy per order (EEO)

To get a first idea about the energy consumption required to remove some of the chemical species studied, we calculated the electrical energy per log removal (EEO) for removing 90% (i.e. 1-LOG) of representative compounds, i.e. MIB, geosmin, and 1,4-dioxane.

‘3 mg/L ozone + 0.8 mg/L H2O2’ and 5 mg/L ozone only were chosen for the EEO calculation for the ozone-based processes; and a UV dose rate of 8,742 J/m² with H2O2 concentrations of 5 and 10 mg/L, respectively, was used for the EEO calculation for the UV-based processes. The calculations are based solely on the power consumptions of the ozone generator and UV lamps in the pilot system. The EEO results indicate that ozone-based AOP is a more energy efficient solution for the oxidation of organic compounds in this specific water matrix – it should be noted that the vertical scales in Figures 5(a) and 5(b) differ by a factor of 10.

Figure 5

EEO from ozone- and UV-based processes in the pilot system, (a) EEO from ozone/ozone-based AOP with 10-minute HRT and (b) EEO from UV-based AOP.

Figure 5

EEO from ozone- and UV-based processes in the pilot system, (a) EEO from ozone/ozone-based AOP with 10-minute HRT and (b) EEO from UV-based AOP.

For full-scale ozone AOP plant design, depending on the scope of the components, e.g. types of feed gas, ozone generator, cooling system, injection system, H2O2 dosing station, etc., the full-scale EEO value could be up to twice that of the pilot plant. For a full-scale UV AOP plant design, it is recommended that a computational fluid dynamics calculation is carried out in order to achieve a more energy efficient design than that of the pilot UV reactor. In general, even though the differences between the full- and pilot-scale systems were not taken into account, an ozone-based AOP will probably be more energy efficient than UV AOP in full-scale applications.

CONCLUSIONS

For the specific water matrix studied, ozone and ozone-based AOP, with their relatively low energy consumptions and hydrogen peroxide utilization rates, are superior to UV-based AOP in the degradation of the compounds selected.

Throughout the tests, no bromate formation was detected in the treated water. Thus, for this specific water source, both ozone and ozone-based AOP have been shown to be appropriate. The only question is determining the treatment goal in order to select between ozone only and ozone-based AOP processes in real applications. If organic contaminant removal is the treatment goal, an ozone-based AOP will definitely be superior as compared to the use of ozone only. If the conventional ozone disinfection goal has higher priority than the removal of organic contaminants, the ozone only operation might be superior to an ozone-based AOP. In this study, an effort has been made to find a balance between the two philosophies: the combination of appropriate ozone and low hydrogen peroxide usage with certain shortened reaction times could be a suitable option for utilities. This idea could be further supported by a follow-up disinfection effect study under optimized ozone-based AOP conditions.

SCALE-UP RECOMMENDATIONS

As the optimized ozone and hydrogen peroxide dosages for the specific water matrix were obtained in this pilot-scale test, a preliminary cost estimate could be made for a waterworks intending to install a full-scale ozone or ozone-based AOP application. Table 3 shows the preliminary CAPEX and OPEX estimations at a water flow of 7,600 m³/h, which is also the water flow of one treatment line at Choa Chu Kang Waterworks.

Table 3

Cost estimate for scaled-up ozonation and ozone-based AOP systems

Process design Option 1 ozone Option 2 ozone-based AOP 
Treatment goal Remove MIB by over 90% 
Treatment capacity (m³/h) 7,600 
Hydraulic retention time (HRT) (min) 10 
Ozone dosage (g/m³) 
H2O2 dosage (g/m³) – 
OPEX 
 Est. water flow (m³/year) 66,576,000 
 Est. oxygen cost (SGD/year) 719,741 431,844 
 Est. energy cost (SGD/year) 999,337 573,959 
 Est. H2O2 cost (SGD/year) – 286,277 
 Est. spare parts cost (SGD/year) 10,000 10,000 
 Est. annual OPEX (SGD/year) 1,729,077 1,302,080 
CAPEX 
 Est. capital costs (SGD) 3,300,000 2,200,000 
 Est. depreciation time (year) 10 10 
 Est. annual interest rate (%/year) 
 Est. annual CAPEX (SGD/year) 448,364 298,910 
Process design Option 1 ozone Option 2 ozone-based AOP 
Treatment goal Remove MIB by over 90% 
Treatment capacity (m³/h) 7,600 
Hydraulic retention time (HRT) (min) 10 
Ozone dosage (g/m³) 
H2O2 dosage (g/m³) – 
OPEX 
 Est. water flow (m³/year) 66,576,000 
 Est. oxygen cost (SGD/year) 719,741 431,844 
 Est. energy cost (SGD/year) 999,337 573,959 
 Est. H2O2 cost (SGD/year) – 286,277 
 Est. spare parts cost (SGD/year) 10,000 10,000 
 Est. annual OPEX (SGD/year) 1,729,077 1,302,080 
CAPEX 
 Est. capital costs (SGD) 3,300,000 2,200,000 
 Est. depreciation time (year) 10 10 
 Est. annual interest rate (%/year) 
 Est. annual CAPEX (SGD/year) 448,364 298,910 

Figure 6 illustrates the calculation results from Table 3 to enable comparison between the options. The economic advantage of ozone-based AOP is further proved in terms of achieving the same T&O removal goal.

Figure 6

Comparison of CAPEX, OPEX, and footprint of treatment systems using ozone alone and ozone-based AOP.

Figure 6

Comparison of CAPEX, OPEX, and footprint of treatment systems using ozone alone and ozone-based AOP.

The following recommendations can be made on the basis of the results of the pilot study:

  • MIB is recommended for use as an indicator compound to set the treatment goal(s) for this specific water source under tropical climate conditions. This study has shown that 90% (i.e. 1-LOG) can be a suitable and realistic goal for treatment processes.

  • For a waterworks like Choa Chu Kang Waterworks, if there is an existing ozone contactor with more than 10 minutes contact time, ozone only with a dosage of 5 mg/L shall be efficient to control MIB, or in other words to control T&O. However, with the addition of hydrogen peroxide, the OPEX on site can be reduced.

  • For an upgrading design of new waterworks without existing ozone contactor, ozone-based AOP design can be a good option. In this case, the designed ozone dosage can be reduced to 3 mg/L and the designed contact time can be reduced to 5 minutes or even shorter.

ACKNOWLEDGEMENTS

We thank Xylem Water Solutions and PUB, Singapore's National Water Agency for financial and professional contributions to this study.

Thanks to Mr Yongjun Xiao, Ms Xiaoqing Qian, Dr Jingming Wu, Mr Ruikang Hu, Dr Gao Pingping from labs of PUB for quantitative and sensory analysis, and important advice on the analysis of all water samples.

Special thanks to Mr Robin Wong for setting up this research collaboration and Ms Adeline Choo for local support from Xylem Singapore.

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