Grantee Research Project Results
Final Report: Ultrasonic PVDF Reduces Membrane Fouling for Efficient Water Reuse
EPA Contract Number: 68HERC21C0036Title: Ultrasonic PVDF Reduces Membrane Fouling for Efficient Water Reuse
Investigators: Benjamin, Mark
Small Business: Pure Blue Tech Inc.
EPA Contact: Richards, April
Phase: I
Project Period: March 1, 2021 through August 31, 2021
Project Amount: $100,000
RFA: Small Business Innovation Research (SBIR) - Phase I (2021) RFA Text | Recipients Lists
Research Category: SBIR - Water , SBIR - Clean and Safe Water , Small Business Innovation Research (SBIR)
Description:
Membranes are the gold standard for water treatment and reuse. However, membrane fouling inhibits flux, wastes energy, requires cleaning and replacement, and disables concentrate recirculation. Pure Blue Tech has developed ultrasonic PVDF transducers embedded within membrane systems to reduce fouling. This innovation has potential to transform the economics of membrane operations, wastewater treatment, and water reuse in new-build and retrofit installations for municipal and industrial customers.
We have developed the innovation of integrating PVDF transducer into layers of the membrane system. Ultrasonic wave energy is non-intrusively applied to the operating membrane, preemptively mitigating fouling before and while it occurs by disrupting cake layer formation, organic and biological fouling, particle agglomeration, salt and silica precipitation, and concentration polarization. The value of this innovative PVDF-ultrasound approach is to reduce attenuation, power intensity, system complexity, and costs relative to traditional ultrasound attempts that used thick lead zirconium titanate (PZT) transducers.
Summary/Accomplishments (Outputs/Outcomes):
Three objectives for the project were set, and each was accomplished.
Objective 1: Design, fabricate, and test a small, prototype crossflow membrane system with integrated PVDF transducers.
Objective 2: Evaluate effect of PVDF-generated US energy on membrane performance (contaminant rejection and membrane fouling by humic acid) under a range of operating conditions (geometry, US frequency).
Objective 3: Conduct preliminary, desktop economic analysis for use of PVDF transducers in commercial-scale plate-and-frame or spiral-wound modules.
Conclusions:
Two distinct versions of a dozen PVDF US transducers were manufactured and tested. Each version was sputtered with a different heavy metal. These are referred as Transducer 1 and Transducer 2. We machined two cells in a 6-cell crossflow test rig to create a cavity adequate for the 0.009” and 0.014” thick transducers photographed below.
Crossflow cell tests were preconditioned with 500 mg/L NaCl solution at 200 psi, then operated during runs with DI + 500ppm NaCl + 100ppm humic acid (organic foulant). Complete test procedures are provided in the full final report.
Figure 1.Pure Blue Tech's PVDF US transducer.
In typical operation of conventional RO membrane systems, a system cleaning step is initiated when the normalized flux declines by 15% from the starting value. Applying that criterion to our tests, the intervals between cleaning steps under different operating conditions would be as shown in the table below.
For the simulated plate and frame system, after applying the same criterion to the results, the intervals between cleaning steps under different operating conditions would be 300 minutes for Transducer #2, US Frequency #2; this equates to a 16.7% fouling reduction compared to the control with mesh, and 71.4% reduction without. The mesh spacer added in this simulated plate and frame setup changed the acoustic resonance of the system, geometries within the system, and provided some turbulence for fouling reduction effect, thus the noted different in baselines. The ultrasound system was not optimized for this simulated plate and frame effect with the modified fluid dynamics, thus the relatively lesser benefit.
Figure 2. This graph shows performance data for Pure Blue Tech Inc's PVDF transducers in various test systems.
Test Condition | Cleaning Interval (min) | Fouling Reduction |
---|---|---|
No ultrasound | 100 | N/A |
Transducer 1, Frequency 1 | 150 | 33% |
Transducer 2, Frequency 1 | 300 | 66.7% |
Transducer 2, Frequency 2 | 350 | 71.4% |
Transducer 2, Frequency, Half power | 270 | 63.0% |
Transducer 2, Frequency 2 on/off | 240 | 58.3% |
This table shows performance data for Pure Blue Tech Inc's PVDF transducers in various test systems.
Key Technical Conclusions:
1. PVDF transducers sputtered with material #2 more effectively reduce membrane fouling compared to PVDF transducers sputtered with material #1 (Graph 1). Furthermore, the second metal has a much longer lifetime. This difference could lead to huge economic benefits for our application, as metal #1 is >10 times more expensive than metal #2 for sputtered transducers.
2. Frequency #2 ultrasound outperformed Frequency #1 and Frequency #3 in our test systems (Graphs 2 and 3). Further investigation at frequencies less than Frequency #1 is warranted, as the tradeoffs in power consumption and transducer longevity gained from the lower frequency might make the lower frequency economically attractive.
3. Fouling reduction benefits decrease by only 12% when power is reduced by 50%. Further investigation of system performance with reduced or intermittent US is warranted.
4. Simulation of a plate-and-frame system reduced the fouling rate in both control (no US) and active (with US) test systems (Graph 3). The magnitude of the fouling reduction attributable to ultrasound in these systems was less than in the previous tests. Engineering design in Phase II will address this difference with the objective of maximizing US benefit with membrane element components included, such as permeate carrier, feed spacer, and core permeate tube.
SBIR Phase II:
Ultrasonic PVDF In Commercial Membrane Systems Reduces Fouling for Efficient Water Reuse | Final ReportThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.