An Assessment of Community-Scale Electrodialysis Desalination Systems and Improved Scale Mitigation through Pulsed Operation
Globally, rapidly developing water scarcity issues stress the need for efficient desalination practices. This thesis demonstrates electrodialysis (ED) as a potentially cost-effective high recovery desalination technology for brackish water desalination and presents novel methods of increasing system...
| Main Author: | |
|---|---|
| Other Authors: | |
| Format: | Thesis |
| Published: |
Massachusetts Institute of Technology
2023
|
| Online Access: | https://hdl.handle.net/1721.1/147281 https://orcid.org/0000-0001-8194-4502 |
| Summary: | Globally, rapidly developing water scarcity issues stress the need for efficient desalination practices. This thesis demonstrates electrodialysis (ED) as a potentially cost-effective high recovery desalination technology for brackish water desalination and presents novel methods of increasing system recovery through pulsed operation.
A high-recovery (80%) pilot electrodialysis reversal (EDR) system was designed, built, and tested against a typical low-recovery (40%) commercial reverse osmosis (RO) system in India. Cost and energy consumption breakdowns were calculated and presented for both systems to determine the suitability of these technologies for different use cases. Pumping energy was identified as the primary source of energy consumption of both systems, and a set of recommendations were made to reduce it, including operational changes and more careful pump selection. Additionally, sensitivity analyses were performed to determine the effects of increasing water and power costs. In situations sensitive to upfront costs, the commercial RO system’s significantly lower capital cost is appealing despite its higher operational costs. The pilot EDR system’s lower energy and feed water consumption make it a promising cost-effective option as power and water costs increase, most notably in situations with higher or varying target salinities.
The maximum water recovery ratio of brackish water electrodialysis systems is limited by scale formation, defined as the attachment of inorganic compounds to membrane surfaces. In an electrodialysis system, selective transport of ions through membranes results in the formation of concentration polarization (CP) near membrane surfaces which worsens membrane scaling and energy dissipation in the process. The application of a non-stationary pulsed electric field can be an effective approach for suppressing CP; however, the performance of pulsed eletrodialysis (PED) heavily relies on the appropriate selection of pulsing parameters. The effects of these parameters on desalination rate and energy consumption were first investigated in the absence of scale precipitating components. The theoretical and experimental results indicate that pulsed operation postpones the limiting condition in ED, allowing for higher input voltages. The energy savings gained from suppressing concentration polarization through PED compensate for the inefficiencies introduced due to the longer desalination time, resulting in specific energy consumption approximately similar to that of CED. This parametric understanding of PED provides the guidelines required to tune the process according to the desired desalination objectives.
To assess the scale mitigating benefits of PED, a series of consecutive nine-day batch experiments using synthesized brackish water with high scaling propensity were performed. Pulsing parameters were selected using insight gained from the aforementioned parametric understandings, and two different pulsing frequencies (0.5 and 5 Hz) were examined to evaluate the effects of pulse/pause durations on various steps of salt formation kinetics. The extent of membrane scaling and the type of formed salt crystals were identified by observing the evolution of system pressure drop and energy consumption over time, combined with microscopic and spectroscopy analyses of membranes extracted from the stack at the end of each nine-day experiment. The observed results indicate that membrane scaling decreased in pulsed operation compared to conventional ED. Low-frequency pulsing was effective in decreasing the transport rate of scale precipitating ions to the concentrate channel. Finally, a novel hybrid pulsed-conventional operation is theorized, with the intent of leveraging the benefits of pulsing early in a batch to control supersaturation of the boundary layers and switching to conventional operation later in a batch to minimize desalination time. |
|---|