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Desalination 202 (2006) 2430

Presented at the conference on Wastewater Reclamation and Reuse for Sustainability (WWRS2005), November

811, 2005, Jeju, Korea. Organized by the International Water Association (IWA) and the Gwangju Institute of

Science and Technology (GIST).

0011-9164/06/$ See front matter 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.desal.0000.00.000

A wind-driven reverse osmosis system for aquaculturewastewater reuse and nutrient recovery

C.C.K. Liua*, W. Xiaa, J.W. ParkbaDepartment of Civil and Environmental Engineering and Water Resources Research Center,

University of Hawaii at Manoa, 2540 Dole Street, Honolulu, HI 96822, USATel. +1 808 956-7658; Fax +1 808 956-5014; email: [email protected]

b

Department of Civil Engineering, Hanyang University, Seoul 133-791, South Korea

Received 31 July 2005; accepted 23 December 2005

Abstract

A wind-driven reverse osmosis system for aquaculture wastewater treatment, developed at the Universityof Hawaii at Manoa, was tested at the experimental facilities on Coconut Island, Oahu, Hawaii. With thistechnology, a fish tank becomes a closed aquaculture production system with zero waste discharge. The permeate(freshwater) from the system can be used as the freshwater supply for fish culture, while the brine (concentrated

wastewater) can be further processed into fish feed by a duckweed-covered reactor.

Keywords: Aquaculture wastewater; Reuse; Reverse osmosis

1. Introduction

A prototype wind-driven reverse osmosis(RO) system was constructed in 19971998 by

University of Hawaii researchers. The system was

successfully tested for brackish water desalinationduring 19982001 [1]. The system can be oper-

ated at a moderate wind speed. At an average

wind speed of 5 m/s, brackish feedwater at a totaldissolved solids concentration of 3000 mg/L

and at a flow rate of 13 L/min can be processed.The average rejection rate was 97%, and the

average recovery ratio was 20%. The energy

efficiency of 35% is comparable to the typicalenergy efficiency of well-operated multivaned

windmills.

Later, research was conducted on the application

of this system for the removal of nitrogenous

wastes from the culture water of Oreochromis

niloticus (tilapia). Nitrogenous wastes can cause

many environmental problems to the receivingwater. To protect its pristine coastal water, the*Corresponding author.

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C.C.K. Liu et al. / Desalination 202 (2006) 2430 25

state of Hawaii established stringent water quality

limits for nitrogen in freshwater aquaculture waste-

water effluent. The ammonia nitrogen (NH3-N)

concentration in effluent discharged into thecoastal water must be less than 20 mg/L, andthe concentration of nitrite and nitrate nitrogen

(NO3 -N + NO2

-N) must be less than 35 g/L.

At present, effluent discharged from most

aquaculture facilities in Hawaii exceeds theselimits. Thus the development of cost-effective

nitrogen-removal technology is essential for

establishing a sustainable and profitable aquac-

ulture industry in Hawaii.

Test results of the wind-driven system, whichseparates the aquaculture wastewater passing

through the RO membrane into permeate (fresh-

water) and brine (concentrated wastewater),

indicated that freshwater can be processed andrecycled [2]. As for the brine, it must be further

processed before being discharged into the envi-

ronment or, preferably, before being reused. In

this study, a duckweed-based tank system was

developed for further brine processing.

The use of aquatic macrophytes such as

duckweed for wastewater treatment has drawngreat attention worldwide in recent years [35].Duckweed-based treatment of municipal waste-

water has been studied at laboratory-, pilot-, and

full-scale levels [69]. These studies indicated

that, in addition to direct duckweed uptake,nitrogenous wastes are removed in a duckweed

treatment system by the biological activities of

bacteria and other microorganisms suspended in

the water column [10].

Although the ability of duckweed treatmentsystems for nitrogen removal has been well

documented, none of the previous studies inves-

tigated the reaction kinetics in a duckweed reactor

with ammonia nitrogen of less than 1.0 mg/L.The performance of a duckweed-covered reactor

at a low nutrient level is therefore the focus of

this study.

Windmill RO Module

Pressure

Tank

Prefilter

Water Supply

Post-treatment

Flow/Pressure Sensor

Photovoltaic System

Ground

Surface

Piston

Pump

Brackish

Water Tank

Solenoid Valve

Solar Energy

Computer Center

Electric Power Data Transportation Line

Check Valve

Data

Logger

Permeate

Brine

Fig. 1. Wind-driven reverse osmosis system.

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26 C.C.K. Liu et al. / Desalination 202 (2006) 2430

2. Development of a wind-driven reverse

osmosis system in Hawaii

In 1998, construction of the prototype wind-

driven RO brackish water desalination system

was completed and the system successfully tested

on Coconut Island. The system was part of a

research effort aimed at desalinating coastalbrackish water for use as an alternative freshwater

supply by Pacific island communities [1]. It com-

bines fully developed windmill and membrane

technology and modern control theory.

Fig. 1 is a schematic of the prototype system.Feedwater, which is pressurized by a wind pump,flows into a pressure stabilizer that reduces large

fluctuations of pressure and the flow rate. A rela-

tively stable flow of feedwater from the stabilizer

(a)

(b) (c) (d) (e)

Fig. 2. The experimental site and major system components: (a) Coconut Island, Oahu, Hawaii, (b) windmill, (c) stabilizer,

(d) RO module, and (e) control device.

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then passes through a cartridge filter, which is a

pre-treatment unit to remove contaminants, before

entering the RO module. The stabilizer, devel-

oped at the University of Hawaii, is a kind ofhydropneumatic pressure tank with a 0.3-m3 innervolume; it yields a mean hydraulic detention

time of about 30 min under design conditions.

A 4.3-m (14-ft) diameter multi-blade windmill

installed on a 9-m-tall tower drives a piston pumpwith a 275-mm (11-in.) stroke and 980-cm3

effective displacement. Both the windmill and

piston pump were manufactured by Dempster

Inc. (Beatrice, Nebraska, USA).

An ultra-low-pressure RO membrane,M-T4040ULP, manufactured by Applied Mem-

brane Inc. (Vista, CA, USA) was used. The effec-

tive surface area of a single RO unit is 7.40 m2

(80 ft2), and the design operating pressure rangeis 3501200 kPa (50175 psi).

The feedback control is accomplished by a

feedback flow/pressure control device, which is

made of three parallel sets of solenoid/throttlevalves (Fig. 1). Water pressure signals in the

stabilizer are sent through the pressure sensor to

the data logger. The data logger evaluates thesesignals and then sends a command to open one or

more sets of solenoid/throttle valves. This controlmechanism allows the system to operate contin-

uously and efficiently. The feedback control

mechanisms are powered by solar energy, so no

commercial electricity is needed.Coconut Island and four system components

(windmill/pump, pressure stabilizer, RO module,

and data acquisition and control mechanisms)

are shown in Fig. 2.

3. Aquaculture wastewater treatment and

freshwater recovery

Application of the wind-driven RO system

was later extended into aquaculture wastewater

treatment, for which pilot-scale experiments havebeen conducted on Coconut Island since 2000

[2]. Aquaculture wastewater passing through the

RO membrane is separated into permeate (fresh-

water) and brine (concentrated wastewater). Test

results indicate that the prototype system can pro-

cess aquaculture wastewater at flow rates rang-ing from 230 to 370 L/h. The permeate, at aquality suitable for fish production, is recircu-

lated to the fish tank (Fig. 3). The brine is sent

back to the storage tank, where it mixes with the

wastewater from the fish tank (Fig. 3). Occa-sional discharge of brine is necessary, as the

nitrogen concentration in the storage tank builds

up over time.

3.1. Nitrogen concentration in permeateand in brine

Experimental results showed that the ammo-

nia nitrogen concentrations in the permeateremained below 0.02 mg/L (Fig. 4), making the

permeate suitable for use in fish culture. On the

other hand, the ammonia nitrogen concentration

in the brine ranged from 0.40 to 1.20 mg/L, sofurther brine processing is required.

3.2. Freshwater recovery

The rate of freshwater recovery depends onwind speed and the frequency of brine discharge.

Experiments were conducted to study the rate of

freshwater recovery under brine recirculation

periods of 2, 4, and 6 h. Without brine recirculation,the recovery rate would not increase appreciably

with wind speed, and vice versa (Fig. 5).

4. Brine processing and nutrient recovery

Various options to process occasionally

discharged brine were evaluated. Use of the

duckweedSpirodela spp. for nitrogen removalfrom brine, at a relative low concentration of less

than 2 mg/L, was selected (Fig. 3). Compared with

water hyacinths, duckweed provides a smallersurface attachment area for microbial growth.

Duckweed usually forms a dense surface mat

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covering the entire water surface. The mat pro-

vides special treatment system characteristics.In past studies of nitrogen removal by duck-

weeds, three design factors were considered,

namely, nutrient concentration, duckweed reactor

(tank) depth, and surface area of reactor. This

study included duckweed density as an additional

design factor. Four stocking densities were

evaluated to identify the optimal duckweed

Fig. 3. Aquaculture wastewater treatment with water reuse and nutrient recovery.

Fig. 4. Nitrogen concentration in permeate and in brine,

with a 6-h brine discharge frequency.Fig. 5. Freshwater recovery at varying wind speeds and

brine discharge frequencies.

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stocking density for nitrogen removal at low

concentrations (Fig. 6).

In a well-designed reactor, uptake by duckweedwas found to be the major nitrogen-removal

mechanism; other mechanisms such as nitrifica-

tion were relatively unimportant. Experimental

results are shown in Fig. 7. The surface loading

rate in this treatment system depends on the

effective surface area, which is the area covered

by duckweed. Desirable duckweed density, orthe percentage of surface area covered by duck-

weed, must be determined experimentally for

individual systems.

5. Concluding remarks

The wind-driven RO process was applied

successfully for nitrogen removal from aquacul-

tural wastewater. The freshwater produced bythis treatment process can be used by recirculating

it directly back to the fish tanks. The brine

produced by this process can be further treated

by duckweed-covered reactors. The duckweed

can be manufactured into fish feed.

It was demonstrated that duckweed plays animportant role in the N-absorption process under

conditions of both high and low nutrient levels.

The duckweed density should be considered in

the design of the treatment for wastewater with a

low nutrient level. The reaction rates in a duck-

weed-covered reactor can be correlated with

Fig. 6. Duckweed-covered reactors.

Fig. 7. First-order nitrogen-removal coefficients as a

function of surface loading.

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modified surface loading rates, which include

consideration of the duckweed density.

Acknowledgements

This work was supported in part by the U.S.

National Science Foundation (award no. INT00-

90193) and by the U.S. Geological Survey (grantno. 01HQGR0079). This is contributed paper

CP-2006-04 of the Water Resources Research

Center, University of Hawaii at Manoa, Honolulu.

Any opinions, findings, and conclusions in this

publication are those of the writers and do notnecessarily reflect the views and policies of the

U.S. National Science Foundation or the U.S.Geological Survey.

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