Journal of Integrative Agriculture2013, 12(8): 1357-1362 August 2013

© 2013, CAAS. All rights reserved. Published by Elsevier Ltd.

doi:10.1016/S2095-3119(13)60541-9

RESEARCH ARTICLE

The Development of a Renewable-Energy-Driven Reverse Osmosis Systemfor Water Desalination and Aquaculture Production

Clark C K Liu

Department of Civil and Environmental Engineering, and Water Resources Research Center, University of Hawaii at Manoa, HI 96822,

USA

Abstract

Water and energy are closely linked natural resources - the transportation, treatment, and distribution of water depends

on low-cost energy; while power generation requires large volumes of water. Seawater desalination is a mature technology

for increasing freshwater supply, but it is essentially a trade of energy for freshwater and is not a viable solution for

regions where both water and energy are in short supply. This paper discusses the development and application of a

renewable-energy-driven reverse osmosis (RO) system for water desalination and the treatment and reuse of aquaculture

wastewater. The system consists of (1) a wind-driven pumping subsystem, (2) a pressure-driven RO membrane desalination

subsystem, and (3) a solar-driven feedback control module. The results of the pilot experiments indicated that the system,

operated under wind speeds of 3 m s-1 or higher, can be used for brackish water desalination by reducing the salinity of

feedwater with total dissolved solids (TDS) of over 3 000 mg L-1 to product water or permeate with a TDS of 200 mg L-1 or

less. Results of the pilot experiments also indicated that the system can remove up to 97% of the nitrogenous wastes from

the fish pond effluent and can recover and reuse up to 56% of the freshwater supply for fish pond operation.

Key words: renewable energy, desalination, pressure-driven membrane processes, aquaculture, fish pond

INTRODUCTION

Existing water desalination processes are based on ei-ther thermal or membrane technology. Because of rapidadvancements of membrane technology, most waterdesalination plants built in the last 30 years in the USAused membrane technology such as reverse osmosis(RO), electrodialysis, and nanofiltration (Glueckstern1995; Liu and Park 2008).

A major problem associated with membrane desali-nation is high energy consumption. To address thisproblem, efforts have been made to develop cost-ef-fective desalination systems that use brackish waterinstead of seawater as feedwater and use renewable

energy instead of electricity to power the system op-eration (Feron 1985; Robinson et al. 1992; Weiner et al.2001; Kershman et al. 2002; Liu et al. 2002). Theosmotic pressure of seawater at a total dissolved solids(TDS) concentration of 35 000 mg L-1 is about 2 700kPa (395 psi). Use of brackish water as feedwater forthe RO desalination process would provide a smallerosmotic pressure and thus, brackish water desalinationwould require smaller applied pressure than seawaterdesalination - the osmotic pressure of brackish water ata TDS concentration of 3 000 mg L-1 is approximately230 kPa (30 psi).

Another major problem associated with membranedesalination is membrane fouling. The fouling problemis caused by the plugging of membrane surfaces by

Received 17 October, 2012 Accepted 10 January, 2013

Correspondence Clark C K Liu, Tel: +1-808-9567658, E-mail: clarkliu@hawaii.edu

1358 Clark C K Liu

© 2013, CAAS. All rights reserved. Published by Elsevier Ltd.

organic and inorganic substances present in thefeedwater. Methods of pretreatment and new foulingresistant membrane materials are being developed bystudying the physicochemical and biological interac-tions between the membrane surface and foulants andanti-fouling agents (Mulder 1996; Liu and Park 2008).

Aquaculture is the fastest growing food productionindustry because of significant increases in demand forfish and seafood throughout the world. Aquaculture isgrowing more rapidly than any other segment of theanimal culture industry. In 2000, the total aquacultureproduction (including aquatic plants) was 45.7 millionmetric tons and valued at US$ 56.5 billion (FAO 2002).Concerns are evoked about the possible effects ofaquaculture wastewater on both productivity in aquac-ulture ponds and on ambient aquatic ecosystem. Nitrog-enous compounds are major contaminants in aquacul-ture wastewater. Ammonia is the principal nitrogenouswaste produced by fishes. Short-term exposure of fishesto high concentration of ammonia causes increased gillventilation, hyper excitability, loss of equilibrium,convulsions, and then death. Chronic exposure of fishesto a lesser concentration of ammonia include tissuedamage, decrease in reproductive capacity (number ofeggs produced, egg viability, delay in spawning), de-crease in growth, and increase in susceptibility to dis-ease (Thurston et al. 1986). Development of cost-ef-fective technology for aquaculture wastewater treatment,especially nitrogen removal, is one of the most importantfactors for achieving a profitable and sustainable aquac-ulture industry in many parts of the world.

Aquaculture wastewater can be treated by usingbiofilters, including trickling filters, submerged filters,rotating media filters, fluidized bed filters and low-density media filters (Jewell and Cummings 1990;Abeysinghe et al. 1996; Hargrove et al. 1996; Nget al. 1996; Twarowska et al. 1997). There are,however, significant drawbacks of using biofilters totreat aquaculture wastewater, including excessivesludge production, unstable performance, and nitrateaccumulation. In this study, after successfully test-ing the use of the renewable-energy-driven RO sys-tem for water desalination, the system was furthermodified and tested as a cost-effective and environ-ment-friendly method for removing nitrogenous wastesfrom culture water of tilapia.

RESULTS AND DISCUSSION

System performance of wind energy conversion

The overall efficiency η of a windmill/pump is definedas the ratio of energy delivered to the water to the avail-able wind energy (Kiranoudis et al. 1997):

(1)

Where ρa=air density (kg m-3), A=rotor swept area (m2),

and U=wind speed (m s-1), ρw=water density (kg m-3),

g=acceleration of gravity (m s-2), Qw=water flow rate

(m3 s-1), and H=hydraulic head (m). Fig. 1 shows theoverall efficiency of wind energy conversion versuswind speed, based on field data collected by this study.

The observed data (dots) are correlated well with aregression curve, which can be expressed by the fol-lowing empirical equation:

(2)

System performance and water desalination

System desalination performance is illustrated inFig. 2. Fig. 2-A shows the flow rate and salinity of thepermeate as a function of wind speed, with a constantfeedwater salinity of 2 500 mg L-1 and a constant oper-ating pressure of 621 kPa (90 psi). The system can beoperated at a wind speed as low as 3 m s-1, a windspeed at which a permeate flow of 0.133 m3 h-1 with a

Fig. 1 Overall system efficiency of wind energy conversion versuswind speed.

The Development of a Renewable-Energy-Driven Reverse Osmosis System for Water Desalination and Aquaculture 1359

© 2013, CAAS. All rights reserved. Published by Elsevier Ltd.

salinity of 146 mg L-1 can be produced. Fig. 2-B showsthe flow rate and salinity of permeate as a function ofoperating pressure, with a constant wind speed of6 m s-1 and a feedwater salinity of 2 500 mg L-1. Underthese conditions, a permeate flow of 0.10 m3 h-1 with asalinity of 175 mg L-1 can be produced under a pres-sure of 517 kPa (75 psi). By increasing the operatingpressure to 862 kPa (125 psi), a permeate flow of 0.20m3 h-1 with a salinity of 110 mg L-1 can be produced.Therefore, the system is relatively sensitive to the op-erating pressure.

A pilot plant of renewable energy driven desalination,to be located on Ewa Beach, Oahu, Hawaii, was de-signed based on experimental data. The pilot plant ofrenewable energy-driven RO desalination was designedby scaling up the testing system, which can be achievedby using 20-ft (6.14-m) windmills and by arrangingmulti-units of windmill/pump and membrane process-ing in parallel and in series (Fig. 3). A cost-analysis indi-cated that the pilot plant can produce freshwater at a rateof 1 285 000 gallons per year, at a cost of US$ 5.40 per1 000 gallons.

System performance of aquaculture wastewatertreatment

The system performances under two categories of windspeed and discharging frequency are shown in Fig. 4.The high operation efficiency zone for water recoveryrate in the system was determined. The recovery ratewas below 65% for the 0-h discharging period thatincreased slowly with wind, which included very strongwinds. In comparison, the water recovery rate for 2-

h, 4-h, and 6-h discharging periods underwent rela-tively faster increases with wind and had a tendency toapproach the apparent maximum value. Among thethree recovery rate curves, 2-h discharging period hasa sharper slope than the others. There was no obviousdifference on the slope changes between the 4-h and 6-h recovery rate curves.

There was an upper limit in increased water recov-ery rate in the system either by increasing the windspeed or by selecting alternatives for the concentrate

Fig. 2 System performance in water desalination. A, wind speed and permeate flow and salinity. B, pressure and permeate flow and salinity.

Fig. 3 Schematic of conceptual design of a renewable-energy-driven RO desalination pilot plant.

1360 Clark C K Liu

© 2013, CAAS. All rights reserved. Published by Elsevier Ltd.

discharging periods. The water recovery rate for the4-h and 6-h modes seemed to reach more than 90%;after approximately 93% water recovery, the systemstabilized with no further recovery rate expected.However, for the 2-h mode, 85% water recovery wasdifficult to attain. Although the 6-h and 4-h modeshad similar performances in the high efficient zone(wind speed >5 m s-1), the 6-h functioned at a slightlyhigher efficiency. This was one of the factors whythe 6-h discharging mode was selected as the opera-tion mode.

CONCLUSION

In this study, a brackish water desalination system wasdeveloped and tested. The system was driven entirelyby renewable energy, which used wind energy to drivethe RO desalination process and used solar energy todrive the system operating control module. With a two-stage wind-driven pumping mechanism, the system cangenerate and maintain two different levels of operatingwater pressure for the pretreatment process and theRO process. The control module operated the systemand allowed continuous operation under varying windspeeds and feedwater salinity.

The system can produce freshwater at a total dis-solved solids (TDS) of less than 200 mg L-1 frombrackish feedwater with a TDS greater than 3 000 mgL-1. Data collected in field experiments showed thatthe system can be operated under mild wind speedsof 3.0 m s-1 or higher, with an average rejection rate

of 94%, and an average recovery ratio of 25%. Acost analysis for a pilot plant indicated that brackishwater desalination was a viable water supply alterna-tive for the Pacific Islands and other remotecommunities.

This study also showed that the renewable-energy-driven RO system was a technically feasible and envi-ronment friendly method of wastewater treatment andreuse in aquaculture production. The system can beoperated at the average wind speed as low as 3.0 m s-1.With an average wind speed of 5.0 m s-1 or more, thesystem worked continuously. It can generate and re-cycle freshwater at a flow rate ranging from 227.8 to366.5 L h-1, depending on the wind speed. Approxi-mately 70 to 84% of aquaculture wastewater can berecycled. The system was capable of removing 90 to97% of nitrogenous waste present in the tilapia cultureeffluents, while the average recovery rate was about40 to 56%.

Agricultural irrigation water use is often associatedwith the problems of (1) the high rate of energyconsumption, especially when long distance water trans-port or deep groundwater pumping is involved; and (2)the excessive salt content in source water. The renew-able-energy-driven RO system developed by this studywas also useful in certain areas where the surface fresh-water was limited, but agricultural and food produc-tions were necessary. This system was evaluated aspart of an irrigation project in South Kona, Hawaii toreduce the salt content in source water and to reducethe pumping cost (Liu 2012). The project feasibilityanalysis indicated that this system can pump 3 milliongallons per day (mgd) of freshwater from a coastalaquifer to irrigate 1 300 acres of coffee and other cropsin the project area.

SYSTEM DEVELOPMENT

System development for water desalination

The prototype renewable-energy-driven RO desalina-tion system developed by the University of Hawaii, USA,on Coconut Island consisted of two subsystems: 1) awind-energy conversion subsystem, and 2) an RO pro-cess subsystem (Fig. 5). In the wind-energy conver-sion subsystem, a pump coupled with a multi-blade

Fig. 4 System performance in aquaculture wastewater treatmentunder varying wind speeds and effluent discharge periods.

The Development of a Renewable-Energy-Driven Reverse Osmosis System for Water Desalination and Aquaculture 1361

© 2013, CAAS. All rights reserved. Published by Elsevier Ltd.

windmill pressurized the feedwater. The overall sys-tem inputs were wind and feedwater, and the systemoutputs were permeate and brine. A control moduledata, which consisted of several acquisition and feed-back control devices, was developed (Liu 2009).Feedwater, which was pressurized by a wind pump,flows into a pressure stabilizer that reduced large fluc-tuations of pressure and flow rate. The relatively stablefeedwater flowed out of the stabilizer and then passedthrough a pretreatment unit before entering the ROmembrane.

A 4.3-m (14-ft) diameter multi-blade windmill in-stalled on a 9-m-tall tower drives a piston pump witha 275-mm (11-inch) stroke and 980-cm3 effectivedisplacement. Both the windmill and piston pumpwere manufactured by Dempster Inc. A stabilizerwas used to maintain a steady feedwater flow byreducing excessive fluctuastion of the pressure andthe flow rate. The stabilizer developed by the Uni-versity of Hawaii was similar to hydro-pneumaticpressure tank with a 0.3-m3 inner volume; it yieldeda mean hydraulic detention time of about 30 min,under design conditions. An ultra low-pressure RO

membrane (M-T4040ULP), manufactured by AppliedMembrane Inc., was used. The effective surfacearea of a single RO unit was 7.40 m2 (80 ft2). Thissystem can provide dual water pressure for pretreat-ment at 172-374 kPa (20-50 psi) and for RO pro-cessing at 517-724 kPa (75-105 psi).

System development for treatment and reuse ofaquaculture wastewater

Aquaculture wastewater passing through the RO mem-brane was separated into permeates (freshwater) andbrine (concentrated wastewater). The permeate wasre-circulated to the fish tank while the brine is divertedto a duckweed pond for further treatment and reuseFig. 6. The system can generate and recycle freshwa-ter at a flow rate that ranged from 227.8 to 366.5 L h-1,depending on the wind speed. The nitrogen removalrate ranged from 90 to 97% and the recovery rate ofthe RO membrane was about 40 to 56%. Further studywill focus on increasing the system capacity while re-ducing the unit cost making the new technology moreaffordable.

Fig. 5 Schematic of renewable-energy-driven RO system for water desalination.

1362 Clark C K Liu

© 2013, CAAS. All rights reserved. Published by Elsevier Ltd.

AcknowledgementsThis study was supported in part by the U.S. Departmentof the Interior Bureau of Reclamation (USBR) through aresearch grant (04-FG-81-1062). This is contributed paperWRRC-CP-2013-07 of the Water Resources ResearchCenter, University of Hawaii at Manoa, Honolulu. Anyopinions, findings, and conclusions in this publication arethose of the authors and do not necessarily reflect theview and policies of the USBR.

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(Managing editor SUN Lu-juan)

Fig. 6 Schematic of renewable-energy-driven RO system for aquaculture wastewater treatment and reuse.