Lifecycle cost analysis of a new reverse osmosis concentrate management system using brackish diatoms for enhanced freshwater recovery

Date

2022-12

Authors

Roy, Emon

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Abstract

World population increase and climate change call for an urgent need for an alternative water source. Brackish and recycled water (also known as reclaimed water) are considered possible alternative sources. Brackish groundwater desalination and potable reuse of recycled water often require reverse osmosis (RO) to remove undesirable impurities and produce freshwater. However, the challenges of the treatment process are high capital expenditure (CAPEX) and operation and maintenance expenditures (OPEX), along with the availability of brine disposal methods, especially for inland communities. To increase the freshwater recovery and reduce concentrate volume, incorporating an additional stage of RO (secondary RO) after the existing stages (primary RO) is desirable. However, a high concentration of silica, calcium and other inorganic scalants in the primary RO concentrate (ROC) may cause frequent scaling in the secondary RO. A novel diatom-based photobiological treatment of ROC can be introduced after the primary RO to treat the concentrate and solve the scaling problem for the secondary RO. Comprehensive bench-scale research works have been conducted in our laboratory to determine the conditions necessary to operate and maintain a photobioreactor (PBR). Although the technical feasibility of the photobiological treatment has been demonstrated along with bench-scale experiments, no research has been performed to propose this method for an industry-scale implementation. The goal of this research was to construct a detailed life cycle cost analysis (LCCA) model by exploring several parts, including designing a hypothetical industry-scale PBR and secondary RO, estimating the quantities of additional freshwater recovery, energy and chemical use, by-products production, and disposal cost reduction. In this research, a hypothetical one million gallons per day (MGD) industry-scale PBR-secondary RO facility was proposed and designed to enhance freshwater recovery and reduce concentrate disposal costs. In this hypothetical facility, concentrate from a brackish groundwater treatment plant would be treated as part of the photobiological treatment with a brackish water diatom Gedaniella flavovirens Psetr3 and the photobiologically treated water would be sent to the secondary RO. Two different light sources for the diatoms [namely, sunlight and light-emitting diode (LED)] could be used for the photobiological treatment of primary ROC. In the sunlight system, the photobioreactors would be inside greenhouses, whereas in the LED system, the photobioreactors would be inside a warehouse. A hydraulic retention time (HRT) of 1.5 days was selected, according to previous lab-scale experiments conducted in our laboratory. In addition, scenarios assuming 1.0 and 1.5 days of HRTs were also discussed in this study. Freshwater production was optimized by RO configuration and membrane selection. Energy recovery device installation was also considered for the secondary RO. Chemical dosages for antiscalant and cleaning solutions were calculated for the secondary RO facility. For the photobiological treatments, nutrient dosage was also calculated. In the photobiological treatment, the diatoms precipitate calcium and produce cellular biomass made of silica and organics, which might be valuable by-products to offset the introduction of the new concentrate management process. Silica and calcite would generate revenue to offset the investment cost for the plant set-up, and biogas production from the diatom biomass could partially offset the power requirement of the proposed secondary RO facility. Bioresources production rate, along with the revenue from the bioresources was discussed in this research. Additional freshwater recovery would also be additional revenue of the system. For the LCCA modeling, all the components of the PBR-secondary RO facility were listed along with their specifications. A net present value analysis was performed to consider the time value of money that would translate the future cash follows into today’s dollars. A break-even point analysis was also performed to determine the year when the project would start making revenue. Based on the LCCA, the sunlight system was more revenue generator than the LED system. CAPEX for the sunlight and LED systems for the 1.5-day HRT scenario were $17.1M and $30.8M, respectively, whereas the OPEX was $1.0M and $3.6M, respectively. HRT played a significant role in determining the most economically feasible scenario. CAPEX was reduced by 24% and 43% by the 1.0- and 0.5-days HRT scenarios in comparison to the 1.5-days HRT scenario for the sunlight system. OPEX was reduced by 6% and 12% by the 1.0- and 0.5-days HRT scenarios in comparison to the 1.5-days HRT scenario of the sunlight system. The significant difference between the sunlight and LED system in terms of CAPEX was caused by the construction of a warehouse, LED lighting installation as a light source, as well as the installation of a heating, ventilation and air conditioning system instead of an evaporative cooling system for the LED system. The high OPEX for the LED system compared to the sunlight system is caused mainly by the high electricity cost to run the LED lights. Freshwater production costs for all the scenarios of sunlight and LED systems were determined to understand how the cost of producing fresh water from the proposed PBR-secondary RO facility would compare with the existing conventional water treatment methods and alternative water sources. Freshwater production costs for the 1.5 days HRT scenario of the sunlight system would be $5.33 without any grant, and with a 30% grant on the CAPEX the production cost would be $2.04. For the 0.5 days HRT scenario of the sunlight system, the production costs reduced to $2.49 and $0.02 for without and with a 30% grant on the CAPEX, respectively. Freshwater production costs for the 1.5 days HRT scenario of the LED system with 1.5 days HRT scenario $31.12 and $22.59 for without grant and with considering a 30% grant on the CAPEX, respectively. For the 0.5 days HRT scenario of the LED system, the production costs reduced to $13.90 and $9.77 for without and with a 30% grant on the CAPEX, respectively. Freshwater production cost comparison indicates the economic advantage of the sunlight system over the LED system. Freshwater production costs from brackish water typically ranges between $1.49 ̶$2.49, and for seawater, the cost ranges between $3.00 ̶$9.00. Direct potable reuse costs $1.7 ̶$2.84 to produce freshwater. Comparing freshwater production costs of the sunlight and LED systems with alternative sources, it can be said that the sunlight system could be a promising ROC treatment system if the HRT could be reduced, whereas the LED would not be a feasible solution. Due to high CAPEX and OPEX, the LED system did not show any break-even point for any of the scenarios within the project lifetime of 20 years. However, there was break-even points for the sunlight system after 18, 15, and 7 years for 1.5-, 1.0- and 0.5-days HRT scenarios when a 30% grant was considered on the CAPEX. Additionally, the sunlight system also showed break-even point after 13 years with the 0.5 days HRT scenario with no grant consideration. Further research is needed to propose the shortened HRTs to an industry-scale ROC treatment system.

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Keywords

lifecycle cost analysis, reverse osmosis, diatom

Citation

Roy, E. (2022). Lifecycle cost analysis of a new reverse osmosis concentrate management system using brackish diatoms for enhanced freshwater recovery (Unpublished thesis). Texas State University, San Marcos, Texas.

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