Accepted Manuscript

Feasibility study on the application of advanced oxidation technologies fordecentralised wastewater treatment

Meng Nan Chong, Ashok K. Sharma, Stewart Burn, Christopher P. Saint

PII: S0959-6526(12)00286-7

DOI: 10.1016/j.jclepro.2012.06.003

Reference: JCLP 2933

To appear in: Journal of Cleaner Production

Received Date: 19 February 2012

Revised Date: 4 June 2012

Accepted Date: 5 June 2012

Please cite this article as: Chong MN, Sharma AK, Burn S, Saint CP, Feasibility study on the applicationof advanced oxidation technologies for decentralised wastewater treatment, Journal of CleanerProduction (2012), doi: 10.1016/j.jclepro.2012.06.003.

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Feasibility study on the application of advanced oxidation 1

technologies for decentralised wastewater treatment 2 3 4

Meng Nan Chong1*, Ashok K. Sharma2, Stewart Burn2, Christopher P. Saint3 5 6

1 CSIRO Land and Water, Ecosciences Precinct, 41 Boggo Road, Dutton Park QLD 4102 Australia 7 2 CSIRO Land and Water, Highett VIC 3109 Australia 8 3 SA Water Centre for Water Management and Reuse, University of South Australia, Mawson Lakes SA 5095 9 Australia 10 *Corresponding author: Dr Meng Nan Chong 11 CSIRO Land and Water, Ecosciences Precinct, Dutton Park Queensland 4102 12 Tel: +61 7 3833 5593, Fax: +61 7 3833 5501, Email: Meng.Chong@csiro.au 13 14 15 Abstract 16 17 Advanced oxidation technologies (AOTs) have been widely investigated for their 18 potential application in post-tertiary sewage treatment due to their ease of operation, high 19 efficiency in organic mineralisation, inactivation of pathogens and low formation of 20 disinfection by-products. This study aimed to evaluate the technical, economical and 21 environmental feasibility for applying AOTs at decentralised wastewater treatment. A 22 comprehensive process selection and assessment framework for the application of AOTs 23 in decentralised wastewater systems for wastewater recycling and reuse purposes was 24 developed in this study. Different AOTs were assessed for their suitability to retrofit as an 25 advanced wastewater treatment option to a decentralised wastewater case study plant in 26 South East Queensland, Australia to produce Class A+ recycled water. Among the 27 different AOTs assessed, it was assessed that the H2O2/UV process was the best AOT 28 treatment option, in terms of technical, economic and environmental benefits for treating 29 sewage effluent from the decentralised wastewater system analysed in this case study. It 30 is anticipated that additional data sets from different pilot or full scale AOT plants will 31 allow the performance predictions of AOTs for a wide range of decentralised wastewater 32 treatment applications. Through this study, it is anticipated that the potential future uptake 33 of AOTs as an advanced treatment option will be enhanced, given the comprehensive and 34 detailed technical, economic and environmental analysis presented in the paper. 35 36 37 Key words: Small-scale wastewater systems; water recycling and reuse; techno-economic 38 assessment and advanced oxidation processes. 39

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1. Introduction 1

2 Water scarcity has become an issue worldwide due to various factors such as rapid population growth, 3 booming industrialisation activities and climate change issues (Tjandraatmadja et al., 2009). With growing 4 demand on clean water resources, the traditional methods of supplying water supply for day-to-day non-5 potable activities has to be re-examined to consider alternative water sources such as rainwater, stormwater 6 and treated wastewater effluent (Sharma et al., 2010). A strategy utilising various alternative water sources 7 for non-potable household application, could contribute to a reduction in the reliance on traditional mains 8 water supplied from the grid. This would allow alternative water sources to be used on a “fit-for-purpose” 9 basis which could possibly also reduce the costs associated with centralised water treatment, long-water 10 transport distances, pumping from centralised water treatment plants, pumping energy as well as construction 11 and maintenance of water pipeline infrastructure (Chong et al., 2011). In this context, there are several 12 examples in Australia where water recycling and reuse have been successfully demonstrated on a large-scale 13 to supply alternative water sources for non-potable household application (i.e. toilet flushing, cold tap 14 washing machine and external garden irrigation) (Cook et al., 2009). 15 16 In a push to achieve a more sustainable urban water environment through Integrated Urban Water 17 Management (IUWM), the decentralisation of water sources has gained importance for new greenfield urban 18 residential developments (Sharma et al., 2012). These approaches are considered to be beneficial in reducing 19 the stress and loadings on existing water transport infrastructure and treatment systems, as well as the 20 previously mentioned benefits obtained from water recycling and reuse. Reuse of treated sewage effluent 21 might be more promising because it is not hampered by the temporal and spatial availability of rainwater and 22 stormwater sources. Historically, decentralised wastewater systems were mostly used in semi-urban, rural 23 and remote areas where the provision of centralised sanitation technology is not technically, economically 24 and environmentally feasible (Cook et al., 2009). For example, septic tanks and absorption trenches have 25 been used at a decentralised scale to treat wastewater to meet the necessary environmental discharge 26 obligations. With the advancement of water/wastewater treatment technologies, a suite of water treatment 27 technologies is now available to provide a higher standard of wastewater treatment to enable better end-use 28 application of the treated sewage effluents in an urban environment (Ho et al., 2010). These include 29 technologies such as sand and media filtration, adsorption using granular activated carbon, zeolite or other 30 clay materials, membrane bioreactor, biological nutrient removal processes, chlorination and an emerging 31 suite of oxidation processes, which can be applied based on the treated sewage quality requirements and 32 matching specific end-uses (Ho et al., 2010). 33 34 Advanced oxidation technologies (AOTs) have been widely investigated for their potential application in 35 post-tertiary treatment of sewage, particularly where the source waters contain high concentration of 36 ambiguous, refractory and recalcitrant chemical compounds such as aromatics, pesticides, pharmaceuticals 37 and personal care products, drugs and endocrine disruptors and others (Suárez et al., 2008; Snyder et al., 38 2006; Chong et al., 2010a). The principles of AOTs are based on the in situ generation of reactive oxygen 39 species such as hydroxyl radicals, superoxide anions, ozone and hydrogen peroxide for the oxidation of 40 organic compounds that lead to a complete mineralisation (Chong et al., 2009 & 2010a). The AOTs include 41 ultrasound, photocatalysis, ozonation, Fenton’s chemistry, hydrogen peroxide and others (Choi et al., 2002). 42 The main objectives of utilising AOTs for advanced wastewater treatment include; (1) reduction in the 43 formation potential of disinfection by-products (DBPs); (2) operating condition at ambient temperature and 44 pressure; (3) complete oxidation of organics to innocuous carbon dioxide, water or other harmless by-45 products, rather than phase transferred via conventional adsorption, absorption or stripping treatment options 46 (Chong et al., 2010a). The targeted compounds are usually mineralised when treated with AOTs rather than 47 transferred to other phases, making AOTs a rather interesting suite of treatment technologies for water 48 recycling where (1) high water quality obligations have to be met or (2) the wastewater sources are highly 49 contaminated with recalcitrant compounds. Most of the previous studies have prescribed that the AOTs are 50 ease to operate, highly efficient in organics oxidation and potentially adaptable to a range of wastewater 51 treatment scales and final treated effluent requirements (Chong et al., 2009 & 2010a). To date, however; 52 examples of AOTs for decentralised wastewater treatment are scarce and a proper process selection and 53 assessment framework does not currently exist. 54 55

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In this study, a comprehensive process selection and assessment framework for the application of AOTs in 1 decentralised wastewater treatment systems for water recycling and reuse purposes been developed. A 2 preliminary feasibility evaluation of different AOTs for decentralised wastewater treatment was conducted 3 using the data and information from the open literature. The focus was on AOTs using (1) Ozonation; (2) 4 Fenton and photo-Fenton processes; (3) UV-based photolysis and chemical oxidation processes; and (4) 5 Photocatalytic processes. The technical, economic and environmental feasibility criteria for applying AOTs 6 at decentralised wastewater treatment scale have been assessed for a case study plant in South East 7 Queensland (SEQ). The technical feasibility and suitability of utilising different AOTs for advanced 8 treatment, after membrane bioreactor (MBRs) treatment is justified. Other key processes and applicability 9 constraints such as operation and maintenance, environmental impacts (i.e. sludge production, discharge of 10 treated effluent to environmentally sensitive areas) and physical footprints are also discussed. Additionally, 11 the economics of using different AOTs as advanced wastewater treatment options for decentralised systems 12 have been quantified. Kinetic data on organic degradation using AOTs available in the open literature is used 13 to estimate the treatment cost ($/L) together with a simple engineering cost estimation. Through this study, it 14 is anticipated that the potential future uptake of AOTs as an advanced treatment option will be enhanced, 15 given the comprehensive and detailed technical, economic and environmental analysis presented in the 16 paper. 17

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2. Background Information 1

2 2.1. Advanced oxidation technologies (AOTs) 3 4 With the increased awareness of environmental protection coupled with strong wastewater discharge 5 legislation, the need for green wastewater treatment technology is growing fast. AOTs are considered as an 6 advanced technology for wastewater treatment, considering their reported high destruction efficiency for 7 toxic pollutants that are usually resistant to conventional biological wastewater treatment processes (Laera et 8 al., 2011). Previous studies have reported that the pollutants that are amenable to biological treatments can be 9 oxidised by integrating a post-treatment AOT stage (Fedorak and Hrudey, 1984; Barreiro and Pratt, 1992; 10

Reemtsma and Jekel, 1997). Although the mechanism of AOTs rely on the formation of OH. radicals, the 11 formation pathways might be different under various operating conditions and this may have a strong 12 implication on the respective operating and maintenance issues. Table 1 shows the different AOTs, which are 13 commonly used for wastewater treatment, along with their dominant chemicals or equipment used. 14 15 16 2.2. Case study of decentralised wastewater treatment in SEQ 17 18 In Australia, there are a number of well-known cases where local wastewater is collected, treated and 19 reticulated for non-potable reuse within the household. This is usually achieved through the operation of a 20 small decentralised wastewater treatment plant that serves the particular area or urban development of 21 interest. In this study, a decentralised wastewater treatment plant at Capo di Monte, Mount Tamborine (SEQ) 22 that serves 46 detached and semi-detached residential dwellings and a large community centre was used as a 23 case study for assessing the feasibility of using AOTs as an advanced treatment option. Currently, the 24 decentralised wastewater treatment plant is operating with a hydraulic capacity of 11,000 L/d, and is 25 comprised of a raw sewage primary holding wet well followed by a MBR (with submerged Kubota flat sheet 26 membranes), alum dosing for phosphorus removal, UV disinfection and chlorination. Figure 1 shows the 27 schematic for the decentralised wastewater treatment plant at Capo di Monte. The treated Class A+ effluent 28 is reticulated via a dual reticulation system and is used for toilet flushing and external irrigation at each 29 household. A vegetated buffer zone of 6,000 m2 is available for land application of excess treated wastewater 30 to prevent direct discharge into the local waterway. This feasibility study assessed the type of AOT to be 31 used after MBR treatment to minimise the potential public health and environmental risks in treated effluent. 32 An automatic sampler (ISCO 6700 series) was used in parallel with grab sampling (every 4 h) for wastewater 33 samples up to 3 day. The sampler was programmed to fill 1 L high-density polyethylene containers for 34 individual and daily composite samples. Subsequently, these samples were analysed for suspended solids 35 (SS), total nitrogen (TN), pH, biological oxygen demand (BOD) and chemical oxygen demand (COD) in 36 water qualities. 37 38 39 2.3. Methodology - Process selection and assessment framework 40 41 In order to assess the feasibility of using AOTs as an advanced treatment option in a decentralised 42 wastewater treatment plant, a comprehensive process selection and assessment framework was developed 43 (Nakakubo et al., 2012). Figure 2 shows the assessment framework, which contains 6 major process selection 44 criteria of (1) technical suitability, (2) system robustness, (3) economic, (4) environmental impacts, (5) 45 sustainability and (6) space requirements. This framework was developed based on the scenario that AOTs 46 will be retrofitted as an advanced treatment option for existing wastewater treatment plants (WWTPs). In this 47 instance, this framework was used to guide the process selection for the selected case study. 48 49 In this study, three process selection criteria of technical, economic and environmental feasibility were 50 targeted to give a preliminary review on the AOTs considered for the case study. Other process selection 51 criteria assessed once the suitable AOTs are selected, based on the availability of relevant process inventory 52 data sets to permit a comprehensive evaluation process. For the technical suitability criterion, the AOTs was 53 assessed based on their compatibility to the wastewater characteristics and operating conditions if applied 54 downstream of the wastewater treatment processes. These technical assessments include the evaluation of 55 whether (1) the AOTs can handle the wastewater characteristics (i.e. COD, BOD, nitrogen, phosphorus and 56

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total suspended solids) after the MBR treatment; (2) cope with the use of additive chemicals (i.e. pH 1 correction, alum dosing, chlorination and other oxidants) and (3) meet the process needs for different 2 operating conditions (i.e. temperature and pressure). 3 4 The economic feasibility was assessed by using the first principle engineering cost estimation methods based 5 on the literature data available for the reaction rate constant (k, min-1), base reactor volume (L), unit 6 treatment cost ($/1000 US galloon) and specific energy (kWh/kL) (Mahamuni and Adewuyi, 2010). In this 7 instance, the rate constants for degradation of COD by different AOTs were assumed to be the average value 8 comprised of three common pollutants of phenol, trichloroethylene (TCE) and reactive azo dyes. 9 10

3

)(min)(min)(min)(min

1111

−−−− ++

= DyeTCEPhenolCOD

kkkk (1) 11

12 By assuming a first-order rate equation, the total reaction time required to achieve the anticipated final COD 13 concentration was taken as the hydraulic retention time to size the AOT reactors for this feasibility study. 14 15

tkC

CCOD

A

AO =

ln (2) 16

17

COD

A

AO

kC

C

t)ln(

(min) = (3) 18

19 where k is the first order rate constant (min-1), CAO is the initial COD concentration (mg/L) and CA is the final 20 COD concentration (mg/L), and t is the total reaction time or residence time (min). Subsequently, the AOT 21 reactor size required was estimated based on Eq. (4). 22 23 24 25

min]/)[()[min](Re])[( LrateflowHydraulictimesidenceLvolumeAOTV ννννττττ ×= (4) 26

27 where τ is the residence time (min), V is the AOT reactor volume (L) and ν is the hydraulic flow rate 28 (L/min). The unit treatment cost ($/L) was estimated by using available literature data, taken into account 29 both the capital, operating and maintenance (O&M) costs. The total treatment cost for the AOTs was 30 amortized at a rate of 7% over an effective plant life of 30 years. In this instance, the power relationship 31 known as the six-tenths factor rule was used to estimate the unit treatment costs for each AOT assessed in 32 this study (Peters et al., 2004). This is to make use of the literature available data on the base reactor volume 33 and the corresponding unit treatment cost ($/1000 US galloon) (Mahamuni and Adewuyi, 2010). 34 35

××

=

LgalloonUS

galloonUSttreatmentBase

VV

LttreatmentUnitbase

estimated

412.37851000

1000($)cos

)/($cos6.0

(5) 36

37 38 39 40 41 It should be stressed that the treatment costs estimated and used to assess the economic feasibility of AOTs 42 serves as a preliminary guide towards the selection of the most appropriate AOTs. A detailed validation for 43 the treatment cost should be ascertained once the most feasible AOTs are selected by quotations from the 44

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vendors. Such a validation will enable the development of a cost function that can be used to accurately 1 predict the economic feasibility for the application of AOTs in decentralised wastewater systems, 2 Unfortunately at the moment the cost database for such systems is scarce and incomplete. 3 4 The specific energy for different AOTs was assessed in a similar way to the unit treatment cost estimation as 5 presented in Eq. (6). The available energy intensity data (kWh/kL) in literature with the relevant information 6 on base reactor volume was used for this estimation (Mahamuni and Adewuyi, 2010). 7 8

×

=

kLkWhintensityEnergy

VV

kLkWhenergySpecificbase

estimated ][)/(

6.0

(6) 9

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3. Results and Discussion 2

3 3.1. Comparison between different AOTs for decentralised system 4 5 The integration or retrofitting of AOTs as an advanced wastewater treatment option for existing WWTPs 6 requires the development of proper assessment guidelines, such as the one shown in Figure 2. In this 7 instance, the suggested technical assessments such as the evaluation of whether the AOTs can (1) handle the 8 wastewater characteristics (i.e. COD, BOD, nitrogen, phosphorus and total suspended solids); (2) cope with 9 the use of additive chemicals (i.e. pH correction, alum dosing, chlorination and other oxidants) and (3) meet 10 the process needs for different operating conditions (i.e. temperature and pressure) should be considered and 11 assessed thoroughly. 12 13 The basic understanding of the reaction mechanisms, operating conditions and requirements and any other 14 process application constraints for the four AOTs of interest needs to be reviewed prior to the technical 15

assessments. Table 2 shows a brief overview summary and comparison of the formation mechanisms of OH. 16 radicals in common AOTs, as well as a comparison of their operating conditions, requirements and 17 constraints. From Table 2, it can be observed that ozonation processes operate at ambient temperature and 18 pressure but require strict pH control at alkaline conditions to avoid the formation of conjugate base HO2

- 19 which can strongly affect the concentration of reactive radicals formed (Hoigné, 1998). The presence of 20 such conjugate bases might affect the short lifetime of ozone in alkaline solution (Hoigné, 1998). Although 21

the side formation of H2O2 can also play an enhancement role during the decomposition of O3 to form OH., 22 the detrimental effect of pH on its formation has yet to be ascertained, as H2O2 in nature is a weak acid. The 23 pH for pure H2O2 was reported to be 6.2, but this pH can go as low as 4.5 when diluted at approximately 24 60% (i.e. volume/volume %) (US Peroxide, 2011). Also, if the ozonation is to be retrofitted as an advanced 25 wastewater treatment option for the case study presented, additional equipment such as a reactor vessel and 26 special gas-liquid contactor for ozone sparging and distribution needs to be taken into account (Gogate and 27 Pandit, 2004). 28 29 As for the Fenton or photo–Fenton processes that include the UV/Fe3+-Oxalate/H2O2 process in this instance, 30

their basic mechanism relies strongly upon the formation of OH. radicals via the reaction with the iron 31 species found in wastewater (Neyens and Baeyens, 2003). The benefits of the Fenton based process are that 32 iron is usually present in abundance in wastewater and the H2O2 is easy to handle and is environmental 33 friendly (Andreozzi et al., 1999). The only difference between the three Fenton based processes is that the 34 UV/Fe3+-Oxalate/H2O2 system has the highest quantum efficiency of 1.0 – 1.2, followed by the photo–Fenton 35 process with a low quantum yield of 0.14 (at 313 nm) to 0.017 (at 360 nm) and lastly, the Fenton process 36 (Andreozzi et al., 1999). Other distinction between the Fenton based processes and other AOTs is that the 37 mechanism for the formation of reactive hydroxyl radicals is preferred at a low acidic pH of 2.6 – 2.8. 38 Andreozzi et al. (1999) reported that the use of the UV/Fe3+-Oxalate/H2O2 process provides a higher quantum 39 efficiency due to accessibility to the wider UV spectrum of 200-400 nm, which can generate a continuous 40 Fenton’s reagent at only about 20% the energy required by typical photo-Fenton system. Similarly to other 41 AOTs, the downside for the Fenton based processes might be due to competition between the different 42 hydroxyl derivatives, organic and inorganic substrates found in wastewater. 43 44 The UV–based photolysis and chemical oxidation processes utilise similar reactions but enhance the 45

formation of OH. radicals via the presence of a UV irradiation source. The presence of UV source induces 46

cleavage and formation of OH. radicals from its precursors (i.e. H2O2 or O3). A few examples for AOTs that 47 belong to this class are shown in Table 2. Among the different processes shown, it was interesting to note 48 that the reactivity for O3 decomposition under UV irradiation is much higher than H2O2, where the molar 49 extinction coefficient for O3 is 3600 M-1cm-1 (254 nm) and that of H2O2 240 M-1cm-1 (Andreozzi et al., 50 1999). Moreover, it should be noted that the O3/UV process possesses the combined chemical behaviour of 51 H2O2/UV and O3/UV as H2O2 is produced during the reaction. Other than this, the AOTs were found to 52 follow the standard operating conditions and requirements except for the additional UV irradiation needs. 53 Similarly, the UV–based photolysis and chemical oxidation processes might get competition from other 54

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organic substrates, which might act as inner filters to attenuate the intensity of UV lights used. 1 2 Among the AOTs detailed in Table 2, photocatalysis is an emerging type of heterogeneous AOT process 3

where the mechanism for OH. radicals generation occurs on the solid-liquid interface of the semiconductor 4 particles (Chong et al., 2010a). Usually the solid semiconductor particles such as TiO2 are suspended in the 5 targeted effluent for treatment with the co-presence of UV irradiation with wavelengths below 385 nm. The 6 operating conditions are at ambient temperature and pressure. However, the current application of TiO2 7 photocatalytic technology still faces the challenges of (1) post-separation of semiconductor after water 8 treatment; (2) rapid electron-hole recombination that warrants low quantum efficiency; (3) mass transfer 9 problems with oxygen, light and water pollutants at the solid-liquid interface and (4) the semiconductor 10 catalytic surface might get fouled by different organic pollutants and inorganic ions in wastewater (Chong et 11 al., 2010a & 2010b). A comprehensive review of these issues was discussed previously in Chong et al. 12 (2010). After considering all these operating conditions, requirements and constraints for the different AOTs 13 of interest, it is necessary to assess their technical suitability and applicability to the current case study 14 decentralised wastewater plant in SEQ. 15 16 17 18 19

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3.2. Technical Feasibility 1 2 To assess the technical feasibility of retrofitting AOTs as an advanced wastewater treatment option for the 3 case study, it is important to understand the quality of treated sewage effluent that is supplied to the AOT. 4 This ensures that the AOTs can be retrofitted with minimal modification of both the effluent characteristics 5 and process operating conditions. Figure 3 shows the quality characteristics of the treated sewage effluent 6 from the decentralised wastewater treatment plant in SEQ, obtained by using grab sampling methodology 7 (from N = 6 events). From Figure 3, it can be observed that the statistics presented for five common 8 wastewater parameters of suspended solids (SS), total nitrogen (TN), pH, biological oxygen demand (BOD) 9 and chemical oxygen demand (COD) are quite constant. For each wastewater parameter, the relevant mean, 10 median 25th and 90th percentile concentrations are given. The corresponding concentrations (± S.D.) for the 11 measured wastewater parameters are; suspended solids: 4.5 ±0.5 mg/L; total nitrogen: 11.68 ±1.70 mg/L; 12 pH: 7.78 ±0.17; BOD: 5.83 ±2.04 mg/L and COD: 21.50 ±3.27 mg/L). According to the Australian 13 guidelines for water recycling (NRMMC-EPHC-AHMC, 2006), all of the measured concentrations for these 14 common wastewater parameters are deemed safe, acceptable and are within the current threshold limits for 15 both public health and environmental risks. In the guidelines however, no guideline limit was recommended 16 for COD concentrations. In this instance, the presence of a relatively high COD concentration might present 17 public health and environmental risks owing to the potential presence of effluent organic matters, recalcitrant 18 organic compounds, un-degraded reactive azo dye compounds, pharmaceutical parent compounds or 19 metabolites which are hazardous and might further react with chlorine to form DBPs (Chong et al., 2010c). 20 Detailed monitoring of the individual chemical compounds needs to be undertaken to determine the DBPs 21 formation potential. The real public health risks of using treated sewage effluent to augment mains water 22 demand from a decentralised wastewater plant comes from the potential cross connection of piping between 23 the mains water supply and the third pipe supplying treated sewage effluent. Thus, a final COD concentration 24 limit of 10 mg/L was set in this instance to minimise the potential risks, as well as serving as a treatment 25 target for the subsequent economic evaluation of the different AOTs under consideration. 26 27 From the measured pH values, it was easy to assess which AOTs can be retrofitted without the vast use of 28 additive chemical reagents (for pH correction) as well as alteration of the operating conditions for the 29 existing wastewater treatment train. Since the measured pH values are marginally alkaline, it would be more 30 appropriate to retrofit AOTs that operate well within this alkaline pH regime. From Table 2, from all the 31 reviewed AOTs, it can be observed that only ozonation (O3), ozonation/ultraviolet irradiation (O3/UV), 32 hydrogen peroxide/ultraviolet irradiation (H2O2/UV) and TiO2 photocatalysis would be suitable. Although 33 the Fenton–based treatment processes have been shown to be relatively superior in treatment efficacy, 34 especially for the UV/Fe3+-Oxalate/H2O2 process with high quantum yield and lower operational energy, 35 their application in this case study might be hampered by their low acidity operating requirements. Thus, 36 only the assessed AOTs of ozonation (O3), ozonation/ultraviolet irradiation (O3/UV), hydrogen 37 peroxide/ultraviolet irradiation (H2O2/UV) and TiO2 photocatalysis are considered technically feasible and 38 their treatment cost are estimated in the following section. 39 40 Although the results shown in Figure 3 indicted that as the other parameters (i.e. SS, TN and BOD) are close 41 to the threshold limit of 10 mg/L, there might be some chance of exceeding this licensed concentration. 42 Figure 4 shows the probability of exceedances for the respective wastewater parameters, which was 43 estimated using the measured average (± S.D.) in Monte Carlo simulations. Results showed that there is an 44 84% chance for the TN to exceed the 10 mg/L concentration limit, with a maximum simulated concentration 45 of 17.36 mg/L. This is followed by a 4% chance for BOD to exceed the license concentration of 10 mg/L, 46 with a maximum simulated concentration of 12.60 mg/L. It was found that the decentralised wastewater 47 system is quite efficient in the removal of suspended solids, with little chance of the wastewater parameter 48 exceeding the 10 mg/L concentration threshold. Previously, it has been shown that apart from the removal of 49 organic carbon compounds (i.e. TOC and COD), AOTs are also capable of simultaneously degrading BOD 50 and other nitrogenous compounds (Gogate and Pandit, 2004). Thus, given the stochastic variations in the 51 treated sewage effluent, it is anticipated that the integration of the proposed AOTs can not only minimise the 52 public health and environmental risks but also improve the stability of the system to produce treated sewage 53 effluent with the quality needed by the license requirements. 54 55

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3.3. Economic and Environmental Feasibility 1

2 The treatment economics of AOTs using O3, O3/UV, H2O2/UV and TiO2 photocatalytic processes was 3 assessed based on the methodology described in Section 2.3. The design hydraulic flow rate considered for 4 this study was 11,000 L/day (i.e. the current flow rate of the decentralised wastewater treatment plant). Table 5 3 shows the summary of estimated treatment cost ($/L) of various AOTs assessed for the current 6 decentralised wastewater treatment plant case study in SEQ. From the estimated treatment costs, it is evident 7 that the O3 treatment process was the most economically feasible AOT ($ 0.03/L) for minimising the 8 potential health and environmental risks and in ensuring the quality of the treated sewage effluent for reuse 9 purposes. This was followed by the AOTs of H2O2/UV and O3/UV treatment with estimated treatment costs 10 of $ 0.14/L and $ 0.21/L, respectively. From the economic analysis, the TiO2 photocatalytic treatment 11 appeared to be most expensive AOT option. Similarly, Mahamuni and Adewuyi (2010) also estimated the 12 AOTs treatment costs for O3, O3/UV, H2O2/UV and TiO2 photocatalytic treatment processes based on a much 13 higher design hydraulic flow rate of 1000 L/min (i.e. 1,440,000 L/day). They also found a similar trend of 14 treatment costs in the order of O3 < O3/UV < H2O2/UV < TiO2 photocatalytic processes. 15 16 The O3 treatment process was the more economical AOT treatment option for similar level of COD reduction 17 from 21.5 mg/L to 10.0 mg/L compared with the O3/UV and H2O2/UV treatment options, as the later require 18 higher O&M costs for UV system. This includes higher energy intensity and constant bulb replacement for 19 the UV systems involved (Mahamuni and Adewuyi, 2010). It has been reported that the part replacement 20 costs for UV systems are as high as 45% of the annual electrical power consumption costs (USEPA, 2006). 21 Although the O3 treatment process also involves parts replacement costs for the ozone generator, the total 22 O&M costs is still relatively lower than the AOTs, which combine chemical and UV photolysis systems. For 23 example, the H2O2/UV treatment process requires a metering pump, reservoir storage, the use of H2O2 and 24 part replacement for the UV system. For the TiO2 photocatalytic treatment process, the relatively high 25 treatment costs are incurred because of the cost of TiO2 particles used, parts replacement costs for the UV 26 system, catalyst holder replacements for the catalytic system, as well as issues with the post-separation of 27 semiconductor TiO2 particles after wastewater treatment (Mahamuni and Adewuyi, 2010). 28 29 The specific energy requirement (in kWh/kL) for each AOT was also estimated. This was assessed in 30 accordance with the sustainability criterion detailed in Figure 2. This information is important as it allows the 31 determination of the energy efficiency of the AOTs, as well as their indirect GHG emissions from fossil fuel 32 combustion. Figure 5 shows the estimated specific energy for the different AOTs considered in this study. 33 Results showed that the H2O2/UV treatment process is the most efficient technology with a specific energy of 34

0.23 kWh/kL, owing to the OH. radicals generation via the use of chemical reagents. This was followed by 35 the specific energy for O3/UV and TiO2 photocatalytic treatment processes of 6.15 kWh/kL and 7.09 36 kWh/kL, respectively. The O3 treatment process was the most energy intensive process with a specific 37 energy of 11.93 kWh/kL. The reason for the lower specific energy in O3/UV treatment process compared to 38 the O3 treatment process is due to the higher turnover rate or shorter residence time in the former required to 39 achieve the final COD concentration requirement. When the estimated specific energy was converted to an 40 indirect carbon footprint (i.e.0.9 kg CO2-e/kWh), the indirect GHG emission was estimated to be in the range 41 of 0.20 kg CO2-e/kL (H2O2/UV treatment process) to 10.73 kg CO2-e/kL (O3 treatment process) (Hall et al., 42 2009). It is apparent, however; from Figure 2 that a comprehensive sustainability assessment cannot be made 43 as these estimations were not validated through energy system monitoring, nor did they include fugitive 44 GHG emissions (i.e. methane and nitrous oxide) which add to the overall carbon footprint for different AOTs 45 (Blottnitz and Curran, 2007). Further inventory data sets from different pilot or full scale AOTs are needed to 46 allow for a more accurate and comprehensive sustainability assessment of AOTs for decentralised 47 wastewater applications. However, the costs estimated in this study still present a useful guide on the 48 selection of AOTs based on the technical, economical and environmental criteria. Based on this, the 49 H2O2/UV treatment process was considered to be the best AOTs treatment option that can be retrofitted to 50 the current decentralised wastewater case study plant in an effort to improve the quality of treated sewage 51 effluent for non-potable reuse. 52 53

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4. Conclusion 1

2 This study provided new insight into the application of AOTs for decentralised wastewater treatment, in an 3 attempt to improve the quality of treated sewage effluent. From this feasibility study, it can be concluded that 4 the H2O2/UV treatment process is the best AOT in terms of fulfilling the technical, economical and 5 environmental constraints for advanced wastewater treatment for the water quality existing in the case study. 6 The H2O2/UV treatment process was assessed to be capable of treating the connecting wastewater stream 7 after the MBR process to the quality of licensed requirements. The estimated unit treatment cost for the 8 H2O2/UV option was estimated to be $0.14/L. The H2O2/UV treatment process was the most energy efficient 9 technology with a specific energy of 0.23 kWh/kL, which corresponded to a low indirect GHG emission of 10 0.20 kg CO2-e/kL. At present, the overall GHG footprints are incomplete due to missing information on the 11 fugitive GHG emissions. In this context more inventory data sets from different pilot or full scale AOTs 12 plants are needed to allow accurate and comprehensive assessment of AOTs for decentralised wastewater 13 applications. Until this data can be obtained the costs estimated in this study still present a useful guide on 14 the selection of AOTs based on technical, economical and environmental criteria. In conclusion, the 15 H2O2/UV treatment process was found to be the best AOT treatment option that can be retrofitted to the 16 current decentralised wastewater case study plant in an effort to improve the quality of treated sewage 17 effluent for non-potable reuse. 18

19

20 Acknowledgement 21 The authors would like to thank Ms Shivanita Umapathi for the collection of wastewater samples, and the 22 Urban Water Security Research Alliance’s SEQ Decentralised Systems project for the information on 23 wastewater characteristics. This work was funded by the CSIRO’s Land and Water Divisions Capability 24 Development Theme. 25 26 27 References 28 Andreozzi, R., Caprio, V., Insola, A., Marotta, R., 1999. Advanced oxidation processes (AOP) for water 29

purification and recovery. Catal. Today 53, 51-59. 30 Barreiro, R., Pratt, J.R., 1992. Toxic effects of chemicals on microorganisms. Water Environ. Res. 64(4), 31

632-641. 32 Blottnitz, H.V., Curran, M.A., 2007. A review of assessments conducted on bio-ethanol as a transportation 33

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Choi, H., Al-Abed, S.R., Dionysiou, D.D., Stathatos, E., Lianos, P., 2002. TiO2- based advanced oxidation 38 nanotechnologies for water purification and reuse. Sustain. Sci. Eng. 2, 229-254. 39

Chong, M.N., Lei, S., Jin, B., Saint, C., Chow, C.W.K., 2009. Optimisation of an annular Photoreactor 40 process for degradation of Congo red using a newly synthesized Titania impregnated kaolinite nano-41 photocatalyst. Sep. Purif. Technol. 67, 355-363. 42

Chong, M.N., Jin, B., Chow, C.W.K., Saint, C., 2010a. Recent developments in photocatalytic water 43 technology: a review. Water Res. 44, 2997-3027. 44

Chong, M.N., Zhu, H.Y., Jin, B., 2010b. Response surface optimization of photocatalytic process for 45 degradation of Congo red using H- titanate nanofiber catalyst. Chem. Eng. J. 156, 278-285. 46

Chong, M.N., Jin, B., Zhu, H., Saint, C., 2010c. Bacterial inactivation kinetics, regrowth and synergistic 47 competition in a photocatalytic disinfection system using anatase titanate nanofiber catalyst. J. 48 Photochem. Photobiol. A: Chem. 214, 1-9. 49

Chong, M.N., Ho, A., Gardner, T., Sharma, A., Hood, B., 2011. Assessing decentralised wastewater 50 treatment technologies: correlating technology selection to system robustness, energy consumption 51 and fugitive GHG emission. International Conference on Integrated Water Management, Perth, 52 Western Australia, Australia, 2-5 February 2011 (Lecture No. 28). 53

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East Queensland, Urban Water Security Research Alliance Technical Report No. 13, October 2009, 1 Queensland, Australia. 2

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TABLES Table 1: Different types of AOTs used in wastewater treatment, along with their dominant chemicals or equipment used (Choi et al., 2010).

Process Chemicals or equipment used

Ozonation O3

Fenton and photo-Fenton processes Fe2++H2O2, Fe2++H2O2+UV UV-based photolysis & chemical oxidation processes UV+O3, UV+H2O2, UV+O3+H2O2

Photocatalytic processes Semiconductor (TiO2, ZnO)/UV

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Table 2: A brief overview summary and comparison on the formation mechanisms of HO radicals for common AOTs, as well as their common operating conditions, requirements and constraints.

Process Reaction Mechanism Operating Conditions & Requirements Remarks Constraints

Ozonation (Hoigné, 1998)

Ozonation HO- + O3 → O2 + HO2

- ↔ H2O2 (7)

HO2- + O3 → HO2. + O3.- (8)

HO2. ↔ H+ + O2

.- (9)

O2.- + O3 → O2 + O3

.- (10)

O3.- + H+ → HO3

. (11)

HO3. → HO

. + O2 (12)

HO. + O3 → HO2

. + O2 (13)

Requires high pH conditions (alkaline solutions). Requires special contactor for ozone sparging and distribution. Ambient temperature and pressure.

The reaction fundamentals are based on the ozone chemistry in aqueous alkaline solutions. The side H2O2 formed during ozonation also plays a role in the treatment process. Addition of H2O2 will enhance O3 decomposition with formation

of OH..

Usually the reaction extent is limited by the short life time of ozone in alkaline solutions. Influence of pH is evident as the active species from ozonation is the conjugate base HO2

- whose concentration is strictly dependent upon pH.

Fenton and photo-Fenton processes (Andreozzi et al., 1999)

Fenton

Fe2+ + H2O2 → Fe3+ + OH- + OH. (14)

Fe(OH)2+ hv ↔ H+ + FeOOH2+ (15)

FeOOH2+ → HO2. + Fe2+ (16)

Photo-Fenton

Fe(OH)2+ + hv → Fe2+ + OH. (17)

UV/Fe3+-Oxalate/H2O2

[FeIII (C2O4)3]3- + hv

→ [FeII(C2O4)2]2- + C2O4

.- (18)

C2O4.- + [FeIII (C2O4)3]

3- → [FeII(C2O4)2]2- + C2O4

2- + 2CO2 (19)

C2O4.- + O2 → O2

.- + 2CO2 (20)

Fenton process: Low pH (pH 2.7 – 2.8). Photo-Fenton process: Low pH, UV-Vis wavelength of higher than 300 nm. UV/Fe3+-Oxalate/H2O2: Low pH, UV-Vis wavelength of higher than 200-400 nm and addition of H2O2.

Fenton process: Iron is very abundant in wastewater and hydrogen peroxide is easy to handle and environmentally safe. Photo-Fenton process: Allows the photolysis of Fe3+ complexes allows Fe2+ regeneration. Low quantum of 0.14 (at 313 nm) to 0.017 (at 360 nm). UV/Fe3+-Oxalate/H2O2: Can provide a continuous source of Fenton’s reagent and uses about only 20% of the energy required by typical photo-Fenton system. Quantum yield is 1.0-1.2.

Requires strict pH control and sludge can be formed with related disposal problems. Might compete with hydroxyl derivatives of aromatic pollutants as these absorb the same UV range as H2O2 and Fe3+.

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UV-based photolysis & chemical oxidation processes (Andreozzi et al., 1999)

Mn2+/Oxalic acid/Ozone Mn(III) (AO 2-)n + O3 + H+ � Mn(II) + (n-1)(AO2-) + 2SO2 + O2 +OH. (21) H2O2/UV

H2O2 + hv → 2OH. (22)

O3/UV O3 + hv → O1 (D) + O2 (23) O1 (D) + H2O → H2O2 (24)

H2O2 + hv → 2OH. (25)

Mn2+/Oxalic acid/Ozone: The radical formation mechanism is at pH 4.0. H2O2/UV: Requires UV wavelength of smaller than 280 nm to induce the homolytic cleavage of H2O2. Requires alkaline pH. O3/UV: Requires UV light of 254 nm. Operates efficiently under alkaline pH.

Mn2+/Oxalic acid/Ozone: Is usually used to enhance decomposition of ozone to produce HO radicals. H2O2/UV: The cage effect of water molecules might lower the primary quantum yield to 0.5. The reaction is pH dependent, where the molar extinction coefficient increases with an increase in pH. This might due to the higher molar absorption coefficient of HO2

- at 254 nm, which is 240 M-1 cm-1 O3/UV: The extinction coefficient of O3 at 254 nm is 3600 M-1 cm-1, which is much higher than H2O2. Posses the combined chemical behaviour of H2O2/UV and O3/UV, as H2O2 was produced during the reaction.

H2O2/UV: Small molar extinction of H2O2 of 18.6 M-1cm-1 at 254 nm, and thus only a relatively small fraction of light is exploited. Might get competition from other organic substrates, which might act as inner filters to attenuate the UV light.

Photocatalytic processes (Chong et al., 2010a)

TiO2 Photocatalysis TiO2 + hv � e- + h+ (26) e-

CB � e-TR (27)

h+VB � h+

TR (28) e-

TR + h+VB(h+ TR) � e-

CB + heat (29) (O2)ads + e- � O2

.- (30) OH- + h+ � OH. (31) R-H + OH. � R’. + H2O (32) R + h+ � R+. � Intermediate(s)/ Final Degradation Products (33) O2

.- + OH. � HOO. (34) HOO. + e- � HO2

- (35) HOO- + H+ � H2O2 (36)

UV irradiation in the wavelength <385 nm. Continuous irradiation and aeration to provide agitation for catalysts suspension and electron scavengers (for slurry reactor) and electron scavengers only (for fixed bed reactor). Ambient temperature and pressure. Operates well at pH > PZC (semiconductor),

Can be used to recover some noble metals found in wastewater. Low quantum efficiency as caused by the rapid electron-hole recombination.

Treatment efficiency usually limited by mass transfer problems between the catalyst particles and pollutants found in wastewater. Difficulty in post-separation of catalyst particles after wastewater treatment. Catalytic surfaces might get fouled from different organic pollutants and inorganic ions.

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Table 3: Summary of estimated treatment cost ($/L) of various AOPs for the decentralised wastewater case study plant in SEQ.

Reference Rate Constant (k)

Estimated Residence Time (min)

Base reactor volume (L)

Estimated reactor volume (L)

Base Cost $ (/1000 US galloon)

Estimated Cost ($/L)

For phenol

O3 (2 mg/L) Kidak and Ince (2007) 0.0279 min-1 27.4 0.10 209.58 1.20 0.03 O3/UV Kidak and Ince (2007) 0.0869 min-1 8.8 0.10 67.29 38.65 0.51 H2O2/UV Primo et al. (2007) 0.0524 min-1 14.6 0.75 111.59 308.48 1.64 Photocatalysis Chen and Smirniotis (2002) 0.433 ppm min-1 1.8 0.10 13.50 8648.79 43.36

For reactive azo dye

O3 (12.4 mg/L) Tezcanli-Guyer and Ince (2004) 0.01108 min-1 69.1 1.20 527.74 4.08 0.04 O3/UV Tezcanli-Guyer and Ince (2004) 0.02064 min-1 37.1 1.20 283.30 34.02 0.24 H2O2/UV Fung et al. (2000) 0.0124 min-1 61.7 4.50 471.56 74.61 0.32 Photocatalysis Taicheng et al. (2003) 0.0207 ppm min-1 37.0 0.70 282.48 739.85 7.15

For TCE

O3 (6 mg/L) Nakano et al. (2003) 0.0209 min-1 36.6 0.10 279.78 2.35 0.07 H2O2/UV Hirnoven et al. (1996) 0.4418 min-1 1.7 5.00 13.24 3.33 0.01

For COD

O3 This study* 0.01996 min-1 38.4 0.47 292.95 2.55 0.03 O3/UV This study* 0.05377 min-1 14.2 0.65 108.75 36.34 0.21 H2O2/UV This study* 0.1689 min-1 4.5 3.42 34.63 128.81 0.14

Photocatalysis This study* 0.2269 ppm min-1 3.4 0.40 25.78 4694.32 15.10

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FIGURES

Figure 1: Schematic of decentralised wastewater system at Capo di Monte. RAS flow is the return activated sludge stream.

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Figure 2: A comprehensive process selection and assessment framework for the application of AOTs in decentralised wastewater system for water recycling and reuse purposes.

System Robustness Hydraulic loadings perturbation Wastewater qualities perturbation

Economic Costing Capital expenditures Operating & Maintenance (O&M)

Environmental Impacts

Sludge production Disinfection by-products (DBPs) Odour & Noise End-uses & Receiving environments

Sustainability Energy efficiency Indirect GHG emission (fossil fuel

combustion) Direct GHG emission (fugitive GHG)

Space Requirements Physical footprint Accessibility Plant expansion

Technical Suitability Wastewater characteristics Operating conditions

Process Selection Criteria Key Influencing Factors

⇒ Wastewater samplings ⇒ In-situ sensory measurements

⇒ Reaction kinetics estimation ⇒ Wastewater calibration, validation

& modelling ⇒ Dynamic model simulation &

optimisation

⇒ Engineering cost estimation using percentage of delivered equipment approach

⇒ Detailed costing (if data available) ⇒ Life cycle costing

⇒ System & environmental monitoring

⇒ Survey on end-users’ satisfaction

⇒ Energy metering & monitoring ⇒ Mass & energy balances ⇒ Field measurements on fugitive

GHG

⇒ Wastewater treatment process design

⇒ Land & site assessments

Assessment Methodology & Approaches

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Figure 3: Characteristics of the treated sewage effluent from the decentralised wastewater treatment plant in SEQ.

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Figure 4: Monte Carlo simulations on the probability of exceedances for the respective measured treated wastewater parameters.

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Figure 5: Estimated specific energy for the different AOTs considered in this study.