Life Cycle Assessment for Tertiary Wastewater Treatment and Reuse versus Seawater Desalination

Wastewater reuse is now indispensable for meeting the increasing water demand, particularly under conditions of alarming water scarcity, which is now already affecting every continent. The objective of this study is to apply life cycle assessment (LCA) to evaluate the environmental impact and missed opportunity of treating municipal wastewater to tertiary quality and compare it to conventional seawater desalination in the Gulf Corporation Council (GCC) countries, namely: Multistage flash distillation (MSF), multi-effect distillation (MED) and seawater reverse osmosis (SWRO). The study follows the ISO 14040/44 standards and uses a functional unit of 1 Mm3 of tertiary treated effluent (TTE). The modeling concept adopts the cradle-to-gate consequential paradigm. The life cycle inventory is based on fielded data collection, reports, literature and Ecoinvent database processes. The scope includes: infrastructure, grid, materials, energy requirements, chemical additives and sludge disposal; for primary, secondary and tertiary treatment. The life cycle impact assessment is applied on both the characterized and normalized levels using the ReCiPe method. Compared to distillation, TTE exhibits an average reduction of 94% in fossil depletion. For climate change and particulate matter, an average reduction of 79% and 73% can be realized respectively. The large difference is due to energy consumption in desalination, despite that fact that the energy considered is only the allocated portion to distillation in the cogeneration total, using exergy specific power consumption.


Introduction
The Gulf Corporation Council (GCC) countries are of the most vulnerable regions to water stress and water scarcity [1]. The gap between water demand and availability is increasing, over the past decade. Water consumption has increased almost 12-folds, and the population has almost tripled during the same period. The demand for freshwater in the GCC is satisfied by seawater desalination followed by nonrenewable groundwater abstraction [2]. Approximately half of GCC oil production is consumed for and energy and water co-generation. The abundance of fossil fuels at relatively low extraction costs has allowed for more desalination [3]. With the global movement that urges sustainable energy and water production, oil prices are expected to fall within 2030; water and energy are pushed to decouple and decentralize; and use treated wastewater (TWW).
Utilizing TWW closes the water cycle by embracing the cradle-to-cradle philosophy of circular economy (CE) transition, aiming to reconcile sustainable development by giving water additional life cycles. CE and water decouple economic growth from natural resource use [4]. Within water context, IOP Publishing doi: 10.1088/1755-1315/1026/1/012001 2 water recycling shifts wastewater from water category to one of the most untapped valuable solutions to contribute to elevating water scarcity. TWW contributes to food security; as over 20 million hectares of arable land worldwide are reported to be irrigated with TWW [5]. A study by Rezapour, et al. [6] indicates that TWW irrigation can not only be applied to combat water scarcity, but it can also improve soil nutrients level and increase productivity. Due to energy savings and contributing to satisfy demand, TWW reuse contributes to promoting Sustainable Development Goals (SDGs) 6: Clean water and 13: Climate action, SDG 11: sustainability, among others.
This study conducts a Life Cycle Assessment (LCA) to tertiary wastewater treatment and compares it to seawater desalination technologies in the GCC: multistage flash distillation (MSF), multi-effect distillation (MED) and seawater reverse osmosis (SWRO). The results are intended to assist in policy makers for a better utilization TWW. The study is applied to Kuwait but have a wider repercussion.

Country Context
Kuwait is a hyper arid desert climate located on the Arabian Gulf Peninsula, occupies a land area of 17,818 km 2 . The population of Kuwait was approximately 4.3 million in 2020 [7]. Freshwater consumption data were collected from the ministry of electricity and water and renewable energy (MEWRE). Freshwater production has an average yearly increment rate of 3.8% (see Figure 1). The expected production for the next 15 years was forecasted using linear with R 2 = 98%, where the amount of desalinated produced water (P) is P = 4514.8 (year) -9x10 6 in million imperial gallons. Given this forecast, relying on recycling water is indispensable. Especially, with deteriorating environmental indicators and huge impact on the economy [3,8]. Energy and water cogeneration is responsible for considerable GHG emissions [9]. The Kuwait environmental protection authority (KEPA) [10] database indicates that a staggering 82,556.572 Gg of CO2 equivalent is attributed to energy water cogeneration, out of the total 86,336.448 Gg CO2-equivalent emissions of the same year. Around 49% of Kuwait's water demand is met by desalination, 29% by recycled wastewater and 22% by groundwater. The decision was made to exclude treated effluent from all amenity uses and to restrict it to agricultural use to safe crops even if its quality exceeded that required for potable use [11]. Currently, Kuwait has eleven desalination plants (DPs); three SWRO DPs, one operates using MED (cogeneration), while the remaining DPs operate using MSF (cogeneration). As shown in Figure 2, there are seven wastewater treatment plants (WWTPs) in Kuwait: Alriqqa, Um Alhayman, Sulaibiya, Kabd, Wafra, Sabah Alhamed and Alkhiran (pilot plant). Wastewater generated 3 from activities of residential, governmental, commercial, and public areas is collected and then directed to the target treatment plant via dedicated sewer systems [12]. In contrast, stormwater is drained by stormwater network lines and then discharged without treatment to the sea.

Figure 2.
Kuwait wastewater treatment plants' inflows and outflows. The Wafra and Khiran are smaller WWTPs with daily treated effluent of 749 and 232 cubic meters per day respectively [11].
As shown in Figure 3, Kuwait municipal sewage network receives an inflow of 1075.73 Km 3 /d of wastewater and safely treats an average of 90% of household wastewater. The numbers vary from year to year according to floods if any and/or rejection of contaminated inflow due to illegal sewage connections (which hardly takes place nowadays). Approximately half of all wastewater is treated using reverse osmosis (RO) quality, the remainder to tertiary and advanced quality that uses ultraviolet (UV). RO permeate is utilized for irrigating edible crops while tertiary effluent is utilized for landscape and fodder irrigation in addition. Future use of RO permeate will be directed to air conditioning, cooling and oil excavation processes. The infrastructure is under early stages of construction. TWW sludge is not utilized.

Assessing wastewater treatment technologies using LCA
LCA has been applied since the 1990s to assess the environmental impact of different technologies, scenarios, and operation alternatives associated with wastewater and sludge management [13][14][15][16][17][18][19][20][21][22]. To date, more than 100 research papers have been published in this field. Detailed literature reviews comparing different LCA objectives, challenges, methodological choices and results related to wastewater treatment and sludge management can be found in Corominas et al. [23], Yoshida et al. [24], Pradel et al. [25], Gallego-Schmid and Tarpani [26] among others. In addition, LCA has been used to analyze environmental impacts in the field of storm-water management [27]; to determine appropriate solutions in the field of the urban water cycle [28]; to control emitted greenhouse gases [29]; and to identify the environmental impacts of sea water desalination [30]. To evaluate of the environmental sustainability and benefits of rainwater harvesting systems used to control overflows in a combined sewer system.

USA
Anastasopoulou, et al. [34] Presents a comparison using Nano Membrane toilets with different systems of lavatories.
South Africa Petit-Boix, et al. [28] decision makers To determine critical variables and LCA stages of the urban water cycle. Spain

Li, et al. [35] -
To investigate the critical issue of environmental impacts of organic micropollutants in advanced WWTPs for three different wastewater treatment technologies: ozonation, granular activated carbon adsorption and reverse osmosis.

China
Awad, et al. [36] Decision makers Address the economic and environmental benefits of adding a tertiary treatment/sludge treatment unit to primary and secondary treatment processes by using LCA.
[29] -LCA was used in the assessment of the associated impacts from biodiesel production from microalgae feedstock cultivated in two different media: wastewater and fresh water.

India
Guven, et al. [37] -LCA is used to compare two options for upgrading a preliminary WWTP. The first uses a high-rate activated sludge system. The second option adds a food waste process.

Lopes, et al. [38] -
To evaluate the technical aspects and the environmental performance of a WWTP consisting of high-rate algal ponds as an alternative method for the activated sludge secondary treatment unit.

Spain
Zhao, et al. [39] decision makers To investigate the importance of incorporating a regional impact category using LCA in order to reflect the local impact of organic pollution.

China
Pradel and Aissani [40] -To investigate the environmental impacts related to phosphorus recovery from WWTPs. -LCA has also been used to address the environmental impacts as well as the benefits of supplementing, upgrading, and improving treatment processes [32,[36][37][38]. The obtained results indicated that LCA was powerful for optimizing the interaction among the operational parameters, thus facilitating access to the optimum decision [39][40][41][42]. A categorized literature review for LCA and wastewater treatment in accordance to LCA requirements by ISO 14040 [43] is provided in the next sections. The goal of an LCA states the intended application, the reasons for carrying out the study, the intended audience, and whether the results are intended to be used in comparative assertions to be disclosed to the public [44]. Table 1 shows the different goal statements for selected wastewater LCA studies.

The scope and functional units in wastewater LCA
The scope and systems boundaries indicate which aspects of the system are included to enable better comparative analysis and interpretation. The scope in wastewater LCA encompasses different boundaries, including the wastewater treatment, additional water purification, electricity production, the sewer system network, and materials production processes. In most wastewater LCA studies, the system boundaries have excluded end-of-life WWTP demolition due to the lower impact of this stage over other phases of the LCA. A comparison among different wastewater treatment scopes in LCA is shown in Table 2. The functional unit (FU) applied in WWTPs is quantified by the specific volume of wastewater generated or treated as a result of human activities on a daily basis (m 3 /day) [32,36,37]. It is used as a benchmark in the assessment of sewer systems from the generation point to the targeted treatment units [27,28]. 1 MJ of energy produced from biodiesel from fresh water and wastewater Cradle-to-grave: The biodiesel production phases of cultivation, flocculation, centrifugation, extraction, and transesterification.

Life Cycle Inventories and Databases Used for Wastewater LCA
The life cycle inventory (LCI) lists the comprehensive collection of the required data associated with the input and output of materials, energy, and emissions of the system boundary under consideration, including foreground and background data and elementary flows for the scenarios under consideration [45,46]. The vast majority of wastewater LCA studies have used Ecoinvent [27-30, 36, 38, 40]. The USEtox and Ecoinvent databases were used in Li, et al. [35] and Zhao, et al. [39], respectively. Ma, et al. [31] created new LCI processes based on a coal-based power generation plant located in China. Liu, et al. [33] used a Chinese LCI Database. For wastewater LCA, the LCI database is often integrated within commercially available software such as SimaPro [28,34,38] or GaBi [27,30]. Other software programs include Umberto NXT [29], e-Balance, open LCA and others.

Life Cycle Impact Assessment for Wastewater LCA
Life cycle impact assessment (LCIA) is the result of having elementary flows translated into environmental impact scores using an LCIA method. Table 3 shows some of the LCIA methods and impact categories discussed in wastewater LCA in selected studies. Table 3. LCIA methods and categories used in selected wastewater LCA studies

Materials and Methods
The goal is to evaluate the environmental burden of TTE on two levels. The first to analyse the TTW system adhering to the four stages outlined by ISO 14040 [47]. At this level, analysis is conducted using open-loop consequential modelling. On a second level, the scope is expanded to compare TTE to distilled water from MSF, MED and SWRO. In cogeneration systems, water is considered a co-product of an electricity-water cogeneration combined cycle, hence, only the distillation burden is considered using system allocation paradigm. The FU used is 1 Mm 3 of TTE to the specifications found in Table 4. Bai, et al. [32] CML-AI Acidification, eutrophication, human toxicity, photochemical oxidation, global warming, and abiotic depletion of fossil fuels.

System Scope and Boundary
The system boundaries are cradle-to-gate, so all processes, materials, energy requirements and chemical additives through operation are calculated through field visits, report or the literature. Water delivery and disposal are excluded. Electricity generation is modelled to partially supply requisite power to major WWTPs in the GCC. The phases included in the system boundary are provided in the following sections. The system boundary is shown in Figure 4.
Primary treatment is the physical/mechanical process that removes suspended and floating particles from wastewater entering the WWTP [49]. Primary treatment includes screening to screen grit and other suspended solids. Wastewater primary treatment removes 50-60% of the total solids and 20-30% of the BOD [50]. Four bar screens are used to remove large objects and grit from the incoming wastewater. The wastewater then passes through grit chambers to separate inorganic particles/fine materials, oil is removed from the wastewater using scrappers, then the wastewater undergoes an odour control process.
Secondary treatment uses biological processes to digest and dissolve organic pollutants to produce settleable solids [51]. In this stage, microorganisms consume organic matter and then convert it to water, energy and CO2. The process is followed by aeration basins or settling tanks to clarify the influent by removing approximately 85% of its suspended solids and BOD [50,52]. The biological treatment stage uses a vertical loop reactor (VLR), which is operated on the basis of partial ventilation and depends on a lack of oxygen [53]. The wastewater is then moved by gravity to secondary wastewater treatment, which consists of aeration chambers and primary clarifiers. The system includes four aeration tanks. Each aeration tank has two treatment systems: a VLR and return activated sludge (RAS) line. At the VLR, an average denitrification rate of approximately 80% is achieved without the need for internal recycling of sewage water. The oxygen supplying the VLR is supplied by six outside blowers into the aeration tank. The air is blown through diffusers at the upper end perforated with holes to form bubbles on the surface of the aeration tank. The suspended solids are reduced to no more than 15 mg/l using a peripheral feeding process using a hydraulic distribution system and sludge removal pipes at the bottom of the VLR [53]. The liquid from the aeration tanks passes to the clarifier and remains there for eight hours. The type of clarifier used is a "rim flow clarifier", which has a depth of six meters with an internal diameter of 45 m to accommodate maximum flow with a capacity of 270 K m 3 /d. The flow inlet is at the centre, while the outlet is along the periphery for the centre feed clarifier. A concentric baffle spreads and distributes the discharge evenly in the radial direction. The resultant active sludge is continuously recycled to the aeration tank, where it mixes with incoming wastewater to feed bacteria and maintain the required food to microorganism (F/M) ratio. The surplus activated sludge (SAS) goes to sludge treatment [22]. Tertiary treatment eliminates over 95% of all impurities from sewage. Tertiary treatment upgrades conventional secondary treatment by removing additional pollutants, residual suspended solids, phosphorus and nitrogen from secondary TWW [50]. The effluent discharge is distributed using a special chamber for purification using twenty-four rotating disc filters with a 100 m 2 effective filtration area per unit. The disc filters have a size of 10 microns (3.93x10 -4 inches). The effluent is disinfected using an UV system. A four-channel UV system is used. The chlorination step is used for disinfection, colour removal and odour control. The effluent is treated at a concentration to achieve between 0.5 and 1.0 mg/l residual chlorine.
Excess sludge is thickened to reduce its volume. The system under study includes three operating units. A polymer preparation unit (PPU) is used for additional thickening and flocculation [53]. Eight aerobic digesters use gravity belts that carry sludge for dewatering to form a sludge cake. This sludge cake is landfilled [22].

Life Cycle Inventory
The LCI is built in accordance with the system boundaries described earlier using processes from Ecoinvent version 3.0. The chemical additives used are found in Table 5. A new process was created for the UV disinfection system based on Lee,et al. [55]. The disinfection system with a flow rate of 100 K m 3 /day is based on a 20-year lifetime. The electrical energy required for tertiary treatment is 0.39452 kWh/m 3 . The electricity production process adopts the high voltage from Ecoinvent based on data found in Al-Shayji and Aleisa [56] and Aleisa and Heijungs [3] using an energy mix of heavy fuel oil, diesel, crude oil and natural gas (NG). The landfilling for sludge and grit disposal facility is designed for biogenic waste from the Ecoinvent database version 3.0. It has a design capacity of 1.8 million m 3 volume with a 30-year lifetime. It is equipped with a leachate and landfill gas collection system [22]. The energy for the cogeneration alternatives are based on energy allocation to the distillers using exergy analyses of combined cycle arrangements and performance ratios obtained from Wakil Shahzad, et al. [57], Shahzad, et al. [58], Gude [59] and Ihm, et al. [60]. The types and amounts of chemical additives per cubic meter of desalinated water were obtained from local desalination plants [61] and from Al-Shayji and Aleisa [56].

Life Cycle Impact Assessment
The LCIA phase is conducted according to ReCiPe 2016 (H) V1.03 on the midpoint [62,63] to determine the adverse effects on the environmental impact categories: climate change (CH) expressed in kg CO2-eq to air, fossil fuel depletion (FD) in kg oil-eq, metal depletion (MD) in kg Cu-eq, human toxicity (HT) in kg 1,4-DCB-eq to urban air, and particulate matter formation (PM) in kg PM2.5-eq to air; these categories are considered the most critical in irrigation applications, which are the end use of TTE. The results are discussed in terms of both their characterized and normalized values, as are the process contributions and inventory substances with the most environmental impact.

Results
The LCIA phase was performed according to ReCiPe 2016 (H) V1.03 to determine the adverse effects on the environmental impact categories. The analyses are conducted on two levels: First to identify hotspots within tertiary wastewater treatment, the second compares TTE to desalination.

LCIA of Tertiary Effluent
The characterized results shown in Figure 5Figure 6 are the relative environmental impact for each process with respect to midpoint impact categories. Infrastructure has the most significant impact on HT and MD. Generated electricity impacts CH, and PM.   Figure 6, in normalized values, the categories that are most affected include: MD, FD and CH. Substances high in manganese, iron and nickel affect the MD category, while high oil and NG substances also affect the FD category. The normalized elementary flows or inventory results represent the significant number of substances that contribute to the impact categories. Using a cut-off of 1%, the five substances with the greatest impact are fossil depletion, carbon dioxide, dinitrogen monoxide fossil, fossil methane and biogenic methane.  Figure 7 shows the normalized environmental impact of TTE to that of distilled water from MSF, MED and SWRO. Compared to distillation, TTE exhibits an average reduction of 94% in FD. For CH and PM, an average reduction of 79% and 73% can be realized respectively. The large difference is due to energy consumption in distillation, despite that the energy considered is only the allocated to distillation boilers as opposed to that is used for cogeneration, using exergy specific power consumption (SPC) [60]. Regulations exclude all amenity uses or edible crops' irrigation for TTE even if TWW properties exceeds standards [12]. Nonetheless, this policy has led to lost opportunities in the effective utilization of TTE. Research shows that TTE is a better option than distilled water for crop irrigation, including irrigation of vegetables and fruits consumed raw. This is because TTE contains essential salts and nutrients that are necessary for plant growth. This saves a proportion of the cost of organic and inorganic fertilizers and chemical compounds that are typically added to maximize crop yield. [64]. Finally, research has also linked excessive fertilizer imports to concerns related to food security [65], which is threatened by depletion of the Earth's resources, pollution, and climatic changes [66].  Figure 7. Normalized results comparing TTE with MSF desalination production using ReCiPe Midpoint (H) V1.10 The y-axis in million normalized points

Conclusions
This study conducts a Life Cycle Assessment (LCA) to tertiary wastewater treatment and compares it to seawater desalination technologies in the GCC: MSF, multi-effect distillation MED and SWRO. For the LCA comparison to desalination from cogeneration systems, the energy is based on energy allocation to the distillers using exergy analyses of combined cycle arrangements and performance ratios. The types and amounts of chemical additives per cubic meter of desalinated water were obtained from local desalination plants. The FU used is 1 Mm 3 of TTE meeting the specifications for agricultural use. The LCIA phase was conducted according to ReCiPe 2016 (H) V1.03 on the midpoint with respect to CH, FD, MD, HT, and PM.
The LCIA for comparing TTE to desalination, indicates an average reduction of 94% in FD. For CH and PM, an average reduction of 79% and 73% can be realized respectively. The large difference is due to energy consumption in distillation, despite that the energy considered is only the allocated to distillation boilers as opposed to that is used for cogeneration, using exergy specific power consumption. Although wastewater treatment in the GCC is commendable, the effluent reuse requires additional awareness, supporting legislation, and better applied strategies. Policies to promote TWW reuse remains by far one of the most important factors in addressing water scarcity issues in the GCC. It is a solution to mend the broken water cycle and support water sustainability, which is arguable the biggest challenge facing the GCC in the upcoming decade.