Life-cycle cost analysis of a hybrid algae-based biological desalination – low pressure reverse osmosis system
Introduction
Desalination plays an increasingly important role in meeting the high purity water demand in the coastal areas (Humplik et al., 2011). The total volume of produced desalinated water increased from approximately 25 million m3/d in 2000 to around 95 million m3/d in 2019, and this trend is expected to continue in the future due to the rapid population growth, the higher water demand and effects of climate change (Ahmed et al., 2019; Jones et al., 2019; Shahzad et al., 2019). Although various technologies (Multistage Flash (MSF) (Borsani and Rebagliati, 2005; Fiorini and Sciubba, 2005), Multi-effect Distillation (MED) (Ophir and Lokiec, 2005; Sharaf et al., 2011), electrodialysis (Al-Amshawee et al., 2020; Lee et al., 2002), and membrane distillation (Gao et al., 2019a; b; Warsinger et al., 2015)) have been used for desalination purpose, Reserve Osmosis (RO) currently dominates the desalination market, supplying 69% of the total produced desalinated water with approximately 65.5 million m3/d (Jones et al., 2019).
RO is considered as the state-of-art technique for desalination, but it is an energy intensive process with 3–5 kWh/m3 energy consumption. Although the renewable energy sources have been investigated to drive the RO systems (e.g., solar-driven, wind-driven), they have not been utilized to drive the large desalination plants (Mito et al., 2019). Consequently, the large scale desalination plants are still powered by the conventional energy sources, and the high energy consumption will result in a high greenhouse gas emission (Berenguel-Felices et al., 2020; Jia et al., 2019; Qasim et al., 2019). Additionally, a large amount of brine is produced as the noxious by-product from the RO desalination plant, which could lead to significant environmental and ecological issues (Morillo et al., 2014). Thus, a more environmentally friendly and sustainable desalination technology is highly desired. The utilization of microalgae for desalination started to attract attentions. The salt removal by microalgae is based on biosorption (adsorption) and bioaccumulation (absorption), which is a natural and energy-passive process (Wei et al., 2020). The microalgae also capture CO2 during the photosynthetic process for growth, resulting in a lower greenhouse gas emission. Furthermore, the harvested algal biomass can be used as the raw materials for various high-value products, including biodiesel generation, food additives manufacturing, and bio-gas production (Acién Fernández et al., 2018; Passos et al., 2016; Salama et al., 2017).
As an energy-efficient process, algae-based salt removal shows high potential in desalination application, however, this emerging technology has limitations. Microalgae are vulnerable to the high saline condition, only limited microalgae species can survive in high salinity environments with reduced growth (Shetty et al., 2019). Algae-based desalination could be used for brackish water treatment rather than seawater desalination. Brackish water with lower salinity could benefit the growth of algae. Meanwhile, more algae species could be selected for the brackish water desalination. Furthermore, seawater is only available in the coastal areas, but brackish water is more widely available, leading to more opportunities for algae-based desalination system. Previous studies have also demonstrated that the intracellular sodium concentration of the salt-stressed microalgae is always lower than the sodium concentration in the microalgae culture medium, this is due to the active sodium export mechanism as a part of the physiological and metabolic responses of microalgae to reduce the toxic effect of high sodium concentration (Hagemann, 2011). Wei et al. (2020) have used the microalgae Scenedesmus obliquus to investigate the desalination mechanisms. They found both adsorption and absorption contributed to the salt removal, however, the adsorption process played a more important role and required less reaction time compared to absorption. The desalination efficiency increased when the culture medium salinity increased from 2.8 g/L to 8.8 g/L, and the maximum desalination efficiency achieved by that study was 20%. Sahle-Demessie et al. (2019) have examined desalination potential of Scenedesmus sp. and Chlorella vulgaris. They found that the salt removal increased steadily along the reaction time until day 40 reaching 32% removal efficiency, and the maximum removal efficiency of 36% was achieved at day 85. Other studies (Gan et al., 2016; Moayedi et al., 2019; Yao et al., 2013) have identified the similar phenomenon that the maximum desalination efficiency achieved by algae was in the range of 16% - 33%. To overcome this barrier of limited salt removal capacity of microalgae, multi-stage process is suggested (Sahle-Demessie et al., 2019). When the maximum salt removal is achieved after reacting with the microalgae at the first stage, the effluent flows into the next stage and reacts with the fresh ‘un-saturated’ microalgae again. With multi-stage desalination process, a higher salt removal efficiency can be achieved. Nagy et al. (2017) used a pilot installation to investigate the desalination performance of Scenedesmus. The pilot plant consisted of three parallel treatment trains and each train had three consecutive algae basins (3 stages). The saline water flowed through each basin to remove the salts. The retention time in each basin varied between 7 - 9 days. The total dissolved solids (TDS) removal efficiencies were 52%, 78% and 93% after first, second and third stages, respectively. El Sergany et al. (2019) used the similar pilot installation to investigate the optimum algae dose for algae-based desalination system. They found that with 300 mL/path algae dosage, 38%, 60% and 66% of TDS removal could be achieved after first, second and third stages, respectively. The retention time of each stage was 7 days.
It is obvious that a complete salt removal cannot be achieved even with the multi-stage algae-based desalination system, and its desalination efficiency is lower compared to RO process. However, the ‘fit-for-purpose’ desalinated water could be produced directly from the algae-based desalination system. Certain amount of the salts can be removed from each stage of the algae-based desalination system. The salty water after 3 – 4 stages of treatment may still have high salt concentration, which could not be used for drinking purpose, but it could be potentially utilized for other applications with higher salt tolerance, such as car washing, landscaping, and gardening.
Another alternative approach is to utilize algae-based desalination as the pre-treatment for RO process. The seawater can be firstly treated by the microalgae to reduce its salinity level, afterwards, it can be further treated by RO. Generally, the low pressure RO (LPRO) system has a lower operating pressure and energy consumption but a higher recovery rate compared to the seawater RO system (SWRO), leading to a lower capital expenditure (CAPEX) and operational expenditure (OPEX) (Al-Karaghouli and Kazmerski, 2012).
Various previous studies (Arashiro et al., 2018; Garfí et al., 2017; Linares et al., 2016; Pazouki et al., 2020) have investigated the life-cycle costs for algae-based wastewater treatment systems and SWRO systems, however, to the best of the authors’ knowledge, no life-cycle cost analysis (LCCA) has been undertaken for algae-based desalination system. A better understanding of the life-cycle cost of algae-based desalination system can help us to determine the system's economic viability and implementation strategy.
This study investigates the economic aspects of algae-based desalination system by comparing three different scenarios: (1) a multi-stage microalgae based desalination system; (2) a hybrid desalination system based on the combination of microalgae and RO system; and (3) a RO desalination system. This LCCA is undertaken based on a total expenditure (TOTEX) approach, which takes a holistic view to manage the life-cycle cost of the water infrastructure. Our analysis also takes resource recovery (algal biomass reuse) and possible integration with wastewater treatment into consideration. The sensitivity analysis and uncertainty analysis are also carried out. In addition to the economic aspects, the environmental impacts of different scenarios are discussed.
Although this LCCA will guide researchers and technology early adopters to explore the new research direction and undertake option analysis, it is worthwhile mentioning that RO and algae-based desalination systems have different Technology Readiness Levels (TRLs). RO based desalination technology is fully commercialized with standard operating and maintenance procedures. Its supply chain is mature at industrial scale, from the membrane manufacture to pre-/post-treatment installation. On the contrary, algae-based desalination is at proof of concept phase. The majority of the investigations are based on laboratory experimental study with artificial operating conditions (nutrients, carbon and light), further technology assessment is still required before the full scale implementation.
Section snippets
Scenarios
Three different scenarios are assessed in this study, which include a multi-stage algae-based desalination system, a hybrid desalination system based on the combination of algae-based desalination and LPRO system and a SWRO desalination system. Based on this comparison, a better insight of the financial viability and implementation strategies for algae-based desalination system can be obtained.
Scenario 1: a multi-stage microalgae based desalination system. A medium size plant is assumed for
CAPEX, OPEX and TOTEX comparison
Fig. 2 shows the CAPEX, OPEX and TOTEX analyzed for 3 different scenarios. The OPEX and CAPEX of different system components (algae system and membrane system) for different scenarios are summarized in Tables 4 and 5. Further detailed calculation can be found in Tables S1 – S5 in Appendix A. It is worthwhile mentioning that the revenues obtained from algal biomass reuse for scenarios 1 and 2 are not taken into account for the calculated values shown in Fig. 2. The effect of algal biomass reuse
Conclusions
This study analyzes the economic aspects of algae-based desalination system by comparing the life-cycle costs of three different scenarios: (1) a multi-stage microalgae based desalination system; (2) a hybrid desalination system based on the combination of microalgae and LPRO system; and (3) a SWRO desalination system. It is identified that the CAPEX and OPEX of scenario 1 are significantly higher than those of scenarios 2 and 3, when algal biomass reuse is not taken into consideration. The
Declaration of Competing Interest
All the authors (Li Gao, Gang Liu, Arash Zamyadi, Qilin Wang, Ming Li) agreed to submit our original research paper titled "Life-cycle cost analysis of a hybrid algae-based biological desalination – low pressure reverse osmosis system" for consideration of publication in Water Research.
This manuscript is not under consideration by any other journal. The authors declare no conflict of competing interest.
Acknowledgments
This work was supported by the National Natural Science Foundation of China [grant no. 51979236]. ML was funded as Tang Scholar by Cyrus Tang Foundation and Northwest A&F University.
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