These slides describe my current Doctoral research in Pressure Retarded Osmosis, where I demonstrate the applicability of the technology to desalination utilities in Tampa, Florida. While still in it's early stages, I hope to continue this project throughout my Doctoral Program. This talk was presented at the 2018 American Ecological Engineering Society Meeting in Houston, Texas.

1. Alternative Usage Cases for
Pressure Retarded Osmosis Power
Generation
Joshua Benjamin, Dr. Mauricio Arias, Dr. Qiong Zhang
University of South Florida
18th Annual Meeting of the American Ecological Engineering Society
6/12/2018
1

2. OVERVIEW
• Theory – Pressure Retarded Osmosis
• Natural Salinity Gradients
– Seawater-Riverwater
– Viability
• Desalination
– Variable Gradients
– Process Optimization
– Source Comparison
– Future Work
2

3. OSMOTIC PROCESSES
a) Osmotic Equilibrium
a) 𝐽 𝑤 = 0
b) Reverse Osmosis
a) 𝐽 𝑤 = 𝐴 ∆𝑃 − ∆𝜋
c) Forward Osmosis
a) 𝐽 𝑤 = 𝐴 ∆𝜋
d) Pressure-Retarded Osmosis
a) 𝐽 𝑤 = 𝐴 ∆𝜋 − ∆𝑃
Image modified from
Saito et al., 2012
3

4. BASIC PROCESS
Image adapted from Skilhagen, 2010
• Image for a Riverwater – Seawater
System
4

5. GLOBAL POTENTIAL
Image from Le Mantia et al., 2011
5
• Estimated maximum free energy of mixing is 0.61
kWh/m3 of freshwater (for freshwater-seawater
mixing)1
• Global flow for rivers flowing into the ocean =
1.2E6 m3/s
• If fully harnessed, could create 2 TW, or ~13% of
current world energy consumption
• More realistically  124.8 GW (0.8%) can be
safely and realistically harnessed2
– This assumes 10% of each river’s average discharge is
used, with a 40% conversion efficiency

6. ISSUES
• Concentration Polarization • Membrane Fouling
Image from Achilli et al., 2009
Image from Thelin et al., 2013
6

7. ISSUES
• While the estimated free energy of mixing is 0.61 kWh of free
energy per m3 of freshwater (for SW/RW mixing)3,
– Due to CP, process inefficiencies, and fouling, net specific energy for
SW/RW is estimated at < 0.22 kWh/m3  not economically viable3
• Solution lies in treating RO Concentrate  higher salinity =
higher energy density
– Net specific energy for RO concentrate is estimated at 0.66 kWh/m3.
• Energy Recovery instead of Energy Generation
7

8. DESALINATION
• Highly energy intensive process
– RO between from 2.4 kWh/m3 to 8.5 kWh/m3 fresh product
water4
• Due to high cost, cannot be implemented in areas
where it is needed
– Cape Town, South Africa – tentativeness to build full scale RO
plant due to potential “white elephant” effect5
• Issues with Brine disposal
– Can contain biocides and anti-scaleing additives, high salt
content can negatively affect DO levels
• Can both potentially be solved through PRO energy
recovery
Image from goodfreephotos.com
8

9. RESEARCH QUESTION
Primary Question:
How viable is PRO as an energy recovery scheme in desalination
processes?
Note: viable means economically, socially and environmentally feasible
compared to other pre-existing technologies based on the overall
project lifecycle.
9

10. OBJECTIVES
• Primary Objective: Maximize Net Power
• Can be done by selecting membrane material, configuration, and
feed water quality that:
– Minimizes membrane fouling
– Minimizes process energy consumption
– Maximizes long-term energy production
𝐸𝑅𝑅 = 𝑃𝑟𝑖𝑐𝑒 𝐸𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑖𝑡𝑦 𝑊𝑃𝑅𝑂 − 𝑊𝑃𝑢𝑚𝑝 − 𝑊𝑃𝑟𝑒𝑡𝑟𝑒𝑎𝑡𝑚𝑒𝑛𝑡
10

11. FUNDAMENTAL EQUATIONS
• W: Membrane Power Density (W/m2)
• 𝐽 𝑤: Water Flux (L m-2 h-1)
– A: Water permeability of the active layer (L
m-2 h-1 bar-1)
– B: Salt permeability of the active layer (L m-
2 h-1)
– 𝐶 𝑉𝑎𝑛′ 𝑡: Van’t Hoff factor (bar kg g-1)
– 𝐶 𝐷: Draw solution concentration (g/kg)
– 𝐶 𝐹: Feed solution concentration (g/kg)
– k: Mass transfer coefficient (L m-2 h-1)
– S: Structural parameter (µm)
– D: Diffusion Constant (m2 s-1)
𝑊 = 𝐽 𝑤∆𝑃 = 𝐴 ∆𝜋 − ∆𝑃 ∆𝑃
11
𝐽 𝑤 = 𝐴 𝐶 𝑉𝑎𝑛′ 𝑡
𝐶 𝐷 exp −
𝐽 𝑤
𝑘
− 𝐶 𝐹 exp
𝐽 𝑤 𝑆
𝐷
1 +
𝐵
𝐽 𝑊
exp
𝐽 𝑤 𝑆
𝐷
− exp −
𝐽 𝑤
𝑘
− ∆𝑃

12. SITE INFORMATION
12
Tampa Bay Seawater Desalination
Plant
• Proposed in 1997, began full
operation in 2007
• Operates primarily during dry
season
• Treats up to 44 MGD of seawater
per day
• Generates up to 25 MGD of
drinking water, and 19 MGD of
RO Concentrate
• Mixes RO Concentrate with 1.4
BGD of saltwater discharge from
nearby Big Bend Power Plant Image from Tampa Bay Water

13. POTENTIAL GRADIENTS
• RO Concentrate and Seawater
– Can be conducted on-site
– Assume 100% capture rate and 50:50 mixing
ratio, can harness 19 MGD of RO
Concentrate
• RO Concentrate and Treated Wastewater
– Wastewater would need to be transported
from nearby Howard F. Curren AWWTP
– Assume 50:50 mixing ratio, can harness 19
MGD of RO Concentrate
• Treated Wastewater and Seawater
– Could occur at Howard F. Curren AWWTP,
energy transmitted to Tampa Bay
Desalination Plant
– Can harness up to 52.27 MGD of WW
13
Tampa

14. BASE ENERGY GENERATION DIFFERENCES
Draw (salinity in ppt) Feed (salinity in ppt) Max Energy Density
Range (W/m2)
Water Flux Range*
(L/m2h)
Operating Pressure
Range* (bar)
RO Concentrate (66) DI water (0) 10.05 – 33.92 15.27 – 51.09 23.7 – 23.9
RO Concentrate (66) TW water (0.01) ** 10.04 – 33.89 15.25 – 51.05 23.7 – 23.9
RO Concentrate (66) Saltwater (35) 0.76 – 3.54 2.40 – 11.00 11.4 – 11.6
Saltwater (35) DI water (0) 3.32 – 11.06 9.65 – 31.09 12.4 – 12.8
Saltwater (35) TW water (0.01) ** 3.31 – 11.04 9.63 – 31.31 12.4 – 12.7
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– DI stands for Deionized water, TW stands for treated waste
– *Note: These ranges indicate the water flux and hydraulic pressure at the maximum energy density
– **Note: can also be interpreted as fresh due to similar salinity range

15. FOULING AND PRETREATMENT
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Fouling
• Fouling can reduce overall membrane energy
density by 45-80% 6, 7
• Fouling is material dependant – TFC vs CTA
• Osmotic backwashing can bring performance
back up to 80% of original6
Pretreatment
• Ideal pretreatment scheme is nanofiltration
combined with low-pressure RO
• Energy of pretreatment typically around 0.2
kWh/m3 (8)
Image from She et al., 2017

16. TRANSMISSION AND GENERATION
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Transmission
• Energy expended in transmission is 0.005
kWh/m3-km(4)
– To transport from HFCAWWTP to TBDP
0.1175 kWh/m3 would be used
Generation
• Pelton Turbine vs Rotary Pressure Exchanger
(RPE)
– Pelton Turbine has 70-85% efficiency, would be used at
WWTP to generate electricity3
– RPE has 98% efficiency, can directly transfer pressure
back into system
Image from Energy Recovery Inc.

17. OVERALL POTENTIALS
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2.91
0.96
1.99
0.35
0.32
0.66
0.82
0.35
1.00
0.10
0.12
0.33
0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50
SW/WW
ROC/SW
ROC/WW
E (net) kWh/m3
P (net) MW
E (gross) kWh/m3
P (gross) MW
ROC/WW
• Highest net overall
• Would require 158 4000 ft2
membranes
ROC/SW
• Lowest net Power
• Would require 733 4000 ft2
membranes
SW/WW
• Highest gross overall
• Would require 709 4000 ft2
membranes

18. FUTURE WORK
• Experimental work
necessary to generate
ideal process design
– Use reactor to isolate
individual parameters
– Test promising scenarios
using process model
– Replicate scenarios using
bench scale reactor for
model validation
18

19. POSSIBLE CONFIGURATIONS
Recovery Technique
Membrane
Type/Configuration
Feed Pretreatment
Feed Solution
Draw Solution RO Concentrate
Saltwater Wastewater
None Disinfection
TFC/Hollow
Fiber
TFC/Spiral
Wound
Pressure
Exchanger
Pelton Turbine
CTA/Hollow
Fiber
CTA/Spiral
Wound
Nanofiltration
Low-Pressure
RO
DI Water
121 Possible Configurations 19

20. ACKNOWLEDGEMENTS
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• USF Watershed Sustainability Group
• USF MASS (Modeling Assessment of
Sustainable Systems) Group
• Aleix Cebrian

21. REFERENCES
1. La Mantia, F., Pasta, M., Deshazer, H. D., Logan, B. E., and Cui, Y. (2011) Batteries for efficient energy
extraction from a water salinity difference. Nano letters, 11(4), 1810–3.
2. Helfer, F. and Lemckert, C. (2015) The power of salinity gradients: An Australian example. Renewable and
Sustainable Energy Reviews, 50, 1–16.
3. Sarp, Li, and Saththasivam, J. (2016) Pressure Retarded Osmosis (PRO): Past experiences, current
developments, and future prospects. Desalination, 389, 2–14.
4. Plappally, A. K. and Lienhard, J. H. V. (2012) Energy requirements for water production, treatment, end
use, reclamation, and disposal. Renewable and Sustainable Energy Reviews, 16(7), 4818–4848.
5. Oliver, D. W. (2017) Cape Town water crisis: 7 myths that must be bust. The Conversation.
6. Yip, N. Y. and Elimelech, M. (2013) Influence of natural organic matter fouling and osmotic backwash on
pressure retarded osmosis energy production from natural salinity gradients. Environmental Science &
Technology.
7. Thelin, W., Sivertsen, E., Holt, T., and Brekke, G. (2013) Natural organic matter fouling in pressure
retarded osmosis. Journal of Membrane Science, 438, 46–56.
8. Achilli, A., Prante, J. L., Hancock, N. T., Maxwell, E. B., and Childress, A. E. (2014) Experimental results
from RO-PRO: a next generation system for low-energy desalination. Environmental science &
technology, 48(11), 6437–43.
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