Life Cycle Assessment of urban wastewater reuse with ozonation as tertiary treatment: A focus on toxicity-related impacts
Introduction
According to the European Environment Agency, Spain is considered as a water-stressed country (European Environment Agency, 2005). The distribution of the resource is very heterogeneous and some Spanish regions, namely the Mediterranean region and the Balearic and Canary Islands suffer from water scarcity, mainly due to agriculture and tourism. Agriculture alone used 17,808 hm3 in 2004, which represents more than 80% of total water use in Spain (Insituto Nacional de Estadística, 2008). The authorities have discarded large river transfers as a solution for water supply in water-deficient areas, and consider instead desalination and wastewater reuse as the main technological options to prevent water shortage. At present there are more than 700 operative desalination plants in Spain, with a production capacity above 800,000 m3/day. According to official foresights, desalination capacity will be increased, leading to additional 621 hm3/year (Ministerio de Medio Ambiente, 2007), this means a threefold increase as compared to current desalination capacity. Concerning wastewater reuse, national statistics show that 1 hm3/day, or 6.6% of the treated wastewaters, was reused in 2004 (Instituto Nacional de Estadística, 2008), although additional 133 hm3/year are expected to be reused in drought-prone regions in the near future (Ministerio de Medio Ambiente, 2007). Nevertheless, the overall potential for wastewater reuse is much higher: according to Hochstrat et al. (2006), Spain is the European country with the highest reuse potential, with a maximum of 2000 hm3/year, a figure an order of magnitude above the current situation.
Among the reasons why wastewater reuse has not received appropriate attention up to date, the potential effects in human health and the environment of trace contaminants, such as priority pollutants, pharmaceuticals and personal care products, etc., must be highlighted. Some of these compounds show little biodegradability, thus entering the environment via treated effluents from sewage treatment plants. For this reason, effective tertiary treatment technologies are needed in order to ensure that reclaimed wastewater is safely used. Available technologies for wastewater reclamation include from simple sand filtration until advanced oxidation processes and reverse osmosis. The choice of the most appropriate technology or combination of technologies will depend on the quality requirements and expected application of the reclaimed water.
Ozonation is a well established technology for water treatment, especially drinking water, and it has been the focus of attention in literature in the last few years as an option for advanced wastewater treatment (Pera-Titus et al. 2004). Ozone is an expensive oxidant, but its ability to mineralize organic matter, alone or in combination with hydrogen peroxide, may be attractive for wastewater reuse purposes (Rodríguez et al., 2008).
Although water reuse strategies are intended to address the problem of water scarcity, measures taken to solve this problem must not come at the price of aggravating other environmental problems, such as human health or global warming. In this context, the Life Cycle Assessment (LCA) methodology (Guinée et al., 2002) offers a holistic approach for environmental assessment, in which problem shifting is avoided, since impacts in different places and moments in time can taken into account. Up to date few LCA studies have focused on wastewater reuse, and in most cases, they have focused on energy and material requirements of the process, whereas toxicity related to trace elements in wastewater has not been heeded. Stokes and Horvath (2006) assessed water supply systems in energy terms, including desalination, imports, and recycling, being the latter the environmentally preferable option. Ortiz et al. (2007) compared the environmental impact of several membrane technologies for wastewater reclamation, including the indirect toxicity contribution from energy and infrastructure. Tangsubkul et al. (2005) assessed membranes and stabilisation ponds as reclamation technologies, including toxicity of trace pollutants in biosolids management, which appear to be an environmental hotspot, although the actual pollutants assessed are not shown. With regard to ozonation, it has been included in several LCAs of wastewater treatment (Nijdam et al., 1998, Pillay et al., 2002, Muñoz et al., 2005, Muñoz et al., 2006a, Muñoz et al., 2007), in which the fate of trace pollutants was excluded. On the other hand, Wenzel et al. (2008) assessed ozonation, sand filtration, and membranes, taking into account the toxicity of some priority and emerging pollutants in wastewater, but in the context of advanced treatment without subsequent wastewater reuse. In this work we aim to assess the life-cycle environmental impact of urban wastewater reuse for agricultural purposes, putting special emphasis on the potential toxicity of priority and emerging pollutants present in the effluents to be reused. The tertiary treatments assessed are ozonation and ozonation in combination with hydrogen peroxide, whereas desalination is chosen as the reference technology for water supply in a no-reuse scenario.
Section snippets
Experimental
LCA has been applied on the basis of bench-scale ozonation experiments and analytical work carried out with the effluent from a wastewater treatment plant (WWTP) in Alcalá de Henares (Madrid). This WWTP applies a physical pre-treatment, primary settling, secondary treatment by means of activated sludge with nitrogen removal, and secondary settling, after which the effluent is discharged to a river.
Goal and scenarios assessed
The goal of this case study is to assess the environmental advantages and drawbacks of urban wastewater reuse in agriculture, mainly focusing on toxicity-related impact categories. For this purpose, the following scenarios have been included:
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No reuse: this scenario represents the situation in most Spanish WWTPs, in which treated wastewater is discharged to a natural water stream after secondary treatment.
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Direct reuse: this scenario involves reusing the treated effluent from the WWTP, but
Toxicity modelled with USES-LCA
Fig. 3 shows the life-cycle impact scores obtained for each scenario with the multimedia fate, exposure and effects model USES-LCA. From a freshwater ecotoxicity point of view (Fig. 3a), the worst scenario is not reusing wastewater, since all the pollutants in the WWTP effluent end up in the aquatic environment. If wastewater is instead reused, the impact is up to 2 orders of magnitude lower, due to the fact that the aquatic ecosystem no longer receives this pollution load, which is transferred
Conclusions
LCA has been used to compare different scenarios involving wastewater reuse for agricultural purposes in Spain, with special focus on environmental impacts related to ecotoxicity and human toxicity. Two alternative characterisation methods have been used to model toxicity of chemical substances, namely USES-LCA and EDIP97. Four alternative scenarios have been assessed: wastewater discharge plus desalination supply, wastewater reuse without tertiary treatment, wastewater reuse after applying a
Acknowledgements
The authors thank the Spanish Ministry of Education and Science for its financial assistance through the Consolider-TRAGUA project (contract CSD2006-44) and CTM2007-65544/TECNO.
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2022, Science of the Total EnvironmentCitation Excerpt :Tertiary treatment becomes increasingly necessary to provide a water quality that conforms to the highest standard of European and French reuse regulations. The range of technologies for reclaimed water disinfection for irrigation include from simple UV system (Carré et al., 2017) until advanced oxidation processes (Muñoz et al., 2009; Arzate et al., 2019) and membrane filtration (UF, NF, reverse osmosis) (Carré et al., 2017; Roth et al., 2022). Thus, three representative disinfection technologies were investigated: a mild-treatment (UV disinfection), chemical treatment (chlorination), and an advanced treatment (UF filtration).