The environmental footprint of a membrane bioreactor treatment process through Life Cycle Analysis

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In the last decade, MBRs have attracted a great deal of attention for the treatment of both  In addition, the occurrence of contaminants of emerging concern, including 48 pharmaceuticals (i.e. licit and illicit drugs) and personal care products in treated wastewater 49 and receiving waters is an issue which concerns conventional wastewater treatment. Drugs' 50 removal during CAS treatment occurs through various mechanisms, including 51 biodegradation (biotic process, mainly by bacteria and fungi), and abiotic transformations 52 (e.g. hydrolysis and sorption to biomass or suspended solids) (Cirja et al., 2008). 53 Biodegradation of drugs in CAS systems ranges from almost no biodegradation to high 54 biodegradation, depending on the type of microcontaminant and its biodegradability, but it 55 is far from complete biodegradation (Ternes et al., 2004). On the other hand, MBRs hold a 56 environmental vector at the design/construction phase of a WWTP, in order to optimize 87 the whole system from an environmental point of view (Page et al., 2011). 88 To the best of our knowledge, LCA has been applied to MBR systems only in a few cases 89 for treating urban wastewater (Tangsubkul et Pretel et al. (2013). 115 The LCA results revealed the importance of maximizing the recovery of nutrients, and thus 116 reducing the 'eutrophication' impact category by up to 50%, as well as the recovery of 117 audiences and the latter to identify the impact categories (midpoint) and the areas of 146 protection (endpoint) that are affected by the MBR pilot unit. 148 The functional unit of this study is directly linked to the effective treatment of urban 149 wastewater and the removal of a specific antibiotic compound (i.e. sulfamethoxazole 150 (SMX)). Therefore, the functional unit that was chosen is the "effective treatment of 1 m 3 151 of urban wastewater". It has to be noted that the effluent quality parameters that were 152 achieved at the optimum operational conditions were the removal of at least 67% of 153 effluents' COD (residual COD equal to 40 mg/L) and 82% of SMX (Table 1) 158 In Figure 1, the system boundaries of the MBR pilot unit are presented. These include the 159 construction materials, the MBR equipment, the treated effluent, as to its qualitative and 160 quantitative chemical characteristics, land use, other system outputs to the environment, 161 such as airborne emissions (i.e. from acidification and greenhouse gases (GHG)), the 162 transportation for construction and operation of the unit within the country, where it is 163 installed, and the effluent storage tank. The influent primary treatment (i.e. screening) and 164 the solid sludge waste (i.e. screened grit, removed solids) were not considered within the 165 scope of this LCA study and hence are not included in the system boundaries. This is 166 because a cradle-to-gate approach was used, i.e. the final disposal/reuse of the treated 167 effluent is outside of the system boundaries. The reason is that the route of the effluents' 168 disposal/reuse can affect the overall sustainability of the MBR system and therefore its 169 inclusion would make results valid for the specific route. For example, conventionally- needs and in theory reducing the total environmental footprint. On the other hand, if these 173 effluents are directly released into a freshwater ecosystem they may impose stresses on the 174 'eutrophication' impact category, increasing thus the total environmental footprint, while 175 marine ecosystems are less sensitive to the eutrophication potential than the freshwater 176 ecosystems (e.g. rivers, lakes, etc.). Therefore, the route of disposal, as well as the local 177 conditions and technology used (e.g. piping, pumps, electricity mix, etc.) can have an effect 178 on the total environmental footprint, but this depends on too many local and specific 179 assumptions, which can limit the overall applicability of the results. Similarly, sludge 180 treatment and disposal were not considered within the system boundaries. Solid sludge 181 waste is the main by-product of the MBR pilot unit and as such it could be examined by a 182 separate LCA study. Moreover, different methods to manage the sludge exist, each one 183 with its own limitations, considerations and specific assumptions, and therefore each 184 method is expected to have its own environmental footprint. As a result, including a sludge 185 management scheme in this case study would limit the general relevance of the results 186 obtained by the present study.

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The electricity mix of Cyprus consists of 92.5% from oil, 5.6% from wind power, 1.1% 255 from photovoltaic systems and 0.8% from biomass (Electricity Authority of Cyprus, 2015).

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Data from SimaPro's LCI databases were used to model the aforementioned mix.

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For each of the two stages of the MBR pilot unit (i.e. pre-aeration stage and MBR stage), 301 a thorough LCI was performed, followed by a full LCA, in order to assess the 302 environmental impacts of each stage and identify their main hotspots. Finally, the two 303 stages were modeled together, in order to assess the total environmental footprint of the 304 entire MBR pilot unit. The results of IPCC 2013 impact assessment method, for a timeframe of 100 years, are 307 presented herein. For the functional unit chosen in this case study, which is the effective 308 treatment of 1 m 3 of urban wastewater, the total CO2-eq emissions of the MBR pilot unit are 309 amount to 4.65 kg CO2-eq/m 3 , while the contribution of each parameter (e.g. energy 310 consumption, pumps, membranes, maintenance activities, etc.) of the system to the total 311 GHG emissions is given in Figure 2. attributed to two main reasons: (i) the local energy mix, which is heavily depended on fossil fuels, and (ii) the overall low contribution to the total CO2-eq emissions of the equipment 320 and materials used for the construction of the unit. As far as the energy consumption is 321 concerned, the use of oil accounts by itself for 95.5% of the total CO2-eq emissions, while 322 wind power, biomass and solar energy are responsible for 0.1%, 0.4% and 1%, respectively 323 ( Figure 2(b)). The small contribution of the latter is attributed to the facts that these are 324 renewable energy sources and as such have a minimal environmental impact, and they only 325 contribute by a very small percentage to the local electricity mix. Moreover, 0.6% (or 0.029 326 kg CO2-eq/m 3 ) is attributed to the submerge membrane units, 0.8% (or 0.038 kg CO2-eq/m 3 ) 327 to the pre-fabricated tank (manufacturing procedure and production material (i.e. stainless 328 steel)), while the maintenance activities of the unit contribute 0.85% to the total CO2-eq 329 emissions. It has to be noted that the airborne emissions and the land use of the MBR pilot 330 unit have a few orders of magnitude lower CO2-eq emissions, compared to the energy 331 consumption, and thus they are considered as negligible. This is attributed to the fact that 332 airborne emissions, which are mainly direct CO2-eq emissions, were assumed to be 333 biogenic, having thus a neutral impact on the environment. In addition, the use of chemicals 334 for membrane cleaning and prevention of membrane fouling has a negligible contribution 335 to the total environmental impact, due to the small amounts used and their low 336 environmental impacts (e.g. NaOCl). Moreover, the pumps, the aeration diffuser, the air 337 feeding and the pipes exhibit a very low contribution (<0.1%) to the total CO2-eq emissions. 338 It is noted that the latter refers to the environmental impact of the material production of 339 the above mentioned equipment.   96.5% operational and 3.5% construction phase). 441 In terms of the total environmental footprint, the MBR pilot unit was found to yield low, 442 but still important, environmental impacts. Thus, alternative scenarios to improve its 443 sustainability were examined.   When ReCiPe results are aggregated into a single score, the total environmental footprint 501 of S1 is 74.7 mPt, instead of 442 mPt in the conventional scenario. Thus, a substantial 502 reduction, about 83%, is achieved by adopting solar energy. Moreover, the damage 503 category 'human health' is affected the most, followed by the 'resources' and 'ecosystems'.   has a slight effect, less than 1% reduction, on the overall sustainability of the MBR pilot 529 unit. It has to be noted again that it was assumed that EPDM membranes would have the 530 same treatment performance as the membrane made by chlorinated polyethylene. which is in accordance with the findings of this study (88% reduction of GHG emissions).

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When Grid 2 is used, the total GHG emissions are slightly elevated, compared to the 546 conventional scenario, and amount to 5.70 kg CO2-eq/m 3 . This increase is attributed to the 547 nature of this grid (Grid 2), which is depended on lignite, a less environmentally friendly 548 choice compared to oil used in Grid 1 (Theodosiou at al., 2014). When Grid 3 is used, a 549 reduction of about 26% compared to the conventional scenario (Grid 1), is observed, which 550 is mainly attributed to the use of natural gas, a more environmentally friendly solution than 551 oil (Theodosiou at al., 2014), and to the higher contribution of renewable energy sources.

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Moreover, the effect of nuclear power, which is not a renewable source, was examined by 553 using Grid 4 as input. In this case, a sharp reduction (84%) on the total GHG emissions is 554 observed, since only 0.73 kg CO2-eq/m 3 are emitted, but this is still higher than that emitted 555 in the case of Grid 6 (0.556 kg CO2-eq/m 3 ). When Grid 5 is used, then the MBR pilot unit 556 achieves the highest sustainability, since the total GHG emissions are only 0.25 kg CO2-557 eq/m 3 . Hydropower is the most environmentally friendly energy source and thus a reduction 558 of about 94.5% is observed on the total GHG, compared to the conventional scenario (Grid 1), and 50% compared to Grid 6. A comprehensive overview of the total GHG emissions 560 per energy mix for the treatment of 1 m 3 of urban wastewater by the MBR pilot unit is 561 presented in Figure 6. As shown in Figure 6, the higher environmental footprint of solar 562 energy, when compared to hydroelectricity, is attributed to the energy and materials 563 required for PV system's module production (Fthenakis et al., 2008).

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When the ReCiPe impact assessment method was used, then the results differed, since not 565 only the total environmental footprint was found to be affected by the type of each energy 566 mix but also the scores of the impact and the damage categories varied significantly. In 'ecosystems' and 'resources') and the contribution of each energy mix is presented. As 578 observed, the damage category that is mainly affected by the MBR pilot unit is the category 579 'human health' followed by 'resources'. This is attributed mainly to the airborne emissions 580 from fossil fuel extraction and electricity production by the different energy mixes used, 581 while also air-and water-borne emissions from the same procedure mainly affect the 582 damage category 'ecosystems'.

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As far as the total aggregated environmental footprint is concerned, Grid 1, Grid 2, Grid 3,