Aquaculture Waste Water Treatment and Reuse by Wind Driven Re

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Aquaculture wastewater treatment and reuse bywind-driven reverse osmosis membrane

technology: a pilot study onCoconut Island, Hawaii

Gang Qina, Clark C.K. Liub,*, N. Harold Richmanc,James E.T. Moncurd

aEnvironmental Science and Engineering Program, University of California, Los Angeles,

P.O. Box 951772, CA 90095, USAbDepartment of Civil and Environmental Engineering and Water Resources Research Center,

University of Hawaii at Manoa, 2540 Dole Street, Honolulu, HI 96822, USAcHawaii Institute of Marine Biology, University of Hawaii, P.O. Box 1346, Kaneohe,

HI 96744, USAdDepartment of Economics and Water Resources Research Center, University of Hawaii at Manoa,

2540 Dole Street, Honolulu, HI 96822, USA

Received 11 March 2004; accepted 10 September 2004

Abstract

Nitrogen in aquaculture wastewater may cause many environmental problems to the receiving

water. To protect its pristine coastal water, the State of Hawaii established stringent water quality

limits for aquaculture wastewater. Effluents from aquaculture facilities in Hawaii generally exceed

these limitssometimes by one to two orders of magnitude. Development of cost-effective treatmenttechnology would be one of the most important factors for a profitable aquaculture industry in

Hawaii. Furthermore, recirculating of aquaculture wastewater is highly desirable for environmental

protection and resource conservation. To achieve these goals, a wind-driven reverse osmosis (RO)

technology was developed and applied for the removal of nitrogenous wastes from the culture water

of tilapia on Coconut Island, the home of the Hawaii Institute of Marine Biology, University of

Hawaii at Manoa. A conventional multi-blade windmill is used to convert wind energy directly to

hydraulic pressure for RO membrane operation. Aquaculture wastewater passing through the RO

www.elsevier.com/locate/aqua-online

Aquacultural Engineering 32 (2005) 365378

* Corresponding author. Tel.: +1 808 956 7658; fax: +1 808 956 5014.

Email address: [email protected] (Clark C.K. Liu).

0144-8609/$ see front matter # 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.aquaeng.2004.09.002


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membrane is separated into permeate (freshwater) and brine (concentrated wastewater). The

permeate is recirculated to the fish tanks, while the brine is collected for possible treatment or

reuse. As a result, no wastewater discharge is made to the ambient coastal water. Testing results

indicated that the prototype wind-driven RO system can process and recycle freshwater at a flux of228366 L/h, depending on wind speed. The nitrogen removal rate ranges from 90% to 97%, and the

recovery rate of the RO membrane is about 4056%. A preliminary cost analysis shows that the

production of 1.0 m3 permeate from aquaculture wastewater would cost US$ 4.00. Further study will

focus on the reuse of concentrates and on further enhancement of cost-effectiveness.

# 2004 Elsevier B.V. All rights reserved.

Keywords: Reverse osmosis; Aquaculture wastewater; Water reuse; Nitrogen removal

1. Introduction

Aquaculture has been a fast-growing industry because of significant increases in

demand for fish and seafood throughout the world. Total aquaculture production (including

aquatic plants) in 2000 was 45.7 million tonnes by weight and US$ 56.5 billion by value

(FAO, 2002). Aquaculture is growing more rapidly than any other segment of the animal

culture industry.

Concerns are evoked about the possible effects of aquaculture wastewater both on

productivity inside aquaculture ponds and on the ambient aquatic ecosystem. Nitrogenous

compounds (ammonia, nitrite, and nitrate) are considered major contaminants in

aquaculture wastewater. Ammonia is the principal nitrogenous waste produced by most

fishes. Short-term exposure offishes to a high concentration of ammonia causes increased

gill ventilation, hyperexcitability, loss of equilibrium, convulsions, and then death (Smart,

1978; Thurston et al., 1981). Chronic exposure of fishes to a lesser concentration of

ammonia causes tissue damage, decrease in reproductive capacity, decrease in growth,

increase in susceptibility to disease (Thurston et al., 1984, 1986), and even death (Randall

and Wright, 1987). Nitrite is a naturally occurring intermediate product of the nitrification

process. A major cause of nitrite toxicity is the oxidation of blood hemoglobin iron to its

ferric state, forming methemoglobin (Bodansky, 1951). High levels of methemoglobin in

fish cause the blood to turn brown and sometimes result in hypoxia and death of the fish.

The nitrate ion (NO3

) is the most oxidized form of nitrogen in nature, relatively non-toxicto fishes. However, when nitrate concentrations become excessive and other essential

nutrient factors are present, eutrophication and associated algae blooms can become a

serious environmental problem (Russo and Thurston, 1991).

Aquaculture systems, which incorporate wastewater treatment and effluent reuse

facilities, are rapidly being developed because they have the advantage of minimal water

input and wastewater discharge while allowing full control of the culture environment

(Midlen and Redding, 1998; Van Rijn, 1996). Biological treatment has been considered the

most feasible approach for enabling water reuse in these treatment systems (Metcalf et al.,

1991). The forms of aquaculture wastewater treatment systems may vary, but generally

they can be classified into two categories: biofilters and constructed wetlands. Of the two,biofilters are most commonly used. Many studies have been conducted to examine the

aquaculture wastewater treatment efficiency of different types of biofilters, including

G. Qin et al. / Aquacultural Engineering 32 (2005) 365378366


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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; Ng et al., 1996; Twarowaska et al., 1997). However, the disadvantages of

biofilters are also obvious, including excessive sludge production, unstable performance,and nitrate accumulation. So, research on new methods for aquaculture wastewater

treatment is under way.

One of the most advanced techniques for wastewater treatment and reuse is the reverse

osmosis (RO) process, which is widely used to produce potable water from brackish water

and seawater to reclaim contaminated water sources and to reduce water salinity for

industrial applications (Asano, 1998). But the application of RO membrane for aquaculture

wastewater treatment has been largely limited. One of the major problems is the energy

cost during the membrane filtration process. In order to reduce the energy cost for RO

membrane operation, researchers have turned to renewable energy sources for solutions.

RO desalination using wave (Hicks et al., 1989), solar (Abdul-Fattah, 1986), and wind

energy (Robinson et al., 1992; Liu et al., 2002) were investigated.

The major purpose of this study is to present the technical feasibility of implementing a

wind-driven RO process for aquaculture nitrogen removal, along with a preliminary cost

estimate. As part of this assessment, field experiments with a prototype wind-driven RO

system to treat and recirculate aquaculture wastewater were conducted on Coconut Island,

home of the Hawaii Institute of Marine Biology (HIMB), University of Hawaii at Manoa.

2. Materials and methods

2.1. Aquaculture wastewater

Both seawater and freshwater fish culture is established for research purposes at HIMB.

Monitoring data for discharge quality indicate that the major water quality limits are

related to nitrogenous compounds (including total nitrogen, ammonia nitrogen, nitrite, and

nitrate nitrogen). So, aquaculture nitrogen removal is the main focus of this study.

A cylindrical fish tankfilled with 2.3 m3 water was used in the study. From January to

March 2002, approximately 80 tilapia (Oreochromis niloticus) were raised in the tank and

maintained in freshwater (25 28

C). The stocking density was estimated at 5 kg/m3

, andthe tilapia were fed twice daily with ProForm (Agro Paci.c, Chilliwack, BC, Canada) to

satiation, or approximately 23% of body weight per day. During the test, part of the tank

water flowed into the wind-driven RO system through a bypass pipe. The inlet of the bypass

pipe was located 0.2 m above the bottom of the tank and covered with fine mesh to

minimize solids intake. The water was then treated by the wind-driven RO system and sent

back into the tank.

2.2. Wind-driven RO system

In 1997, a wind-driven RO system for brackish-water desalination was constructed onCoconut Island (Migita, 1999; Liu et al., 2002). This pilot system was later modified for

aquaculture wastewater treatment. Fig. 1 is a schematic of the system.

G. Qin et al. / Aquacultural Engineering 32 (2005) 365378 367


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Aquaculture wastewater from the fish tankflowed by gravity into the storage tank buried

underneath the base of the multi-blade windmill (9.1 m tower with a 4.3 m diameter

wheel). As the blades of the windmill rotated, a deep-well piston pump (7.0 cm in diameterwith a 25.4 cm stroke) in the storage tank pumped the wastewater to a pressure stabilizer,

which was a 284 L (75-gallon) hydro-pneumatic pressure tank with an inside diameter of

56.2 cm, an outside diameter of 57.2 cm, and a height of 114.3 cm. The outlet solenoid

valve of the pressure stabilizer remained closed to allow the water pressure inside it to build

up. When the pressure reached a specific value (483 kPa in the test), the solenoid valve

opened. Wastewater then passed through a cartridge filter (prefilter) with a 5 mm nominal

removal rate and into the RO membrane module. A spiral wound membrane (FILMTEC1

XLE-4040), manufactured by Applied Membranes Inc. (Vista, CA), was used in this study.

Made of thin-film composite polyamide, it had an active surface area of 7.6 m2, fitting into

a 0.1 m (4 in.) in diameter and 1.02 m (40 in.) long stainless steel pressure vessel. Thedesign operating pressure range was 3501200 kPa. This particular membrane was chosen

because it could be operated at relatively low pressure provided by the wind-driven system.

The working pressure through the RO module was controlled between 483 and 758 kPa by

means of a subsystem composed of a Campbell Scientific CR10S datalogger, a pressure

sensor, and solenoid valves. The RO membrane divided the wastewater into two parts:

permeate (clean product water) and brine (concentrated wastewater). The permeate was

sent back to the fish tank while the brine flowed to the storage tank for further treatment.

2.3. Experimental design

The experimental design involved two parts: system operation and nitrogen analysis.

System operation indicated that the whole wind-driven RO system for aquaculture


Fig. 1. Schematic of wind-driven reverse osmosis system for aquaculture wastewater treatment.


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wastewater treatment was connected and operated on Coconut Island according to the

schematic in Fig. 1. Six tests were carried out for system operation. Two tests lasted for 2 h,

three for 4 h, and one for 12 h. Three flow sensors, one pressure sensor, and one wind

anemometer were installed in the pilot system and connected to a data logger to record flowrate, operating pressure, and wind speed data for system operation analysis. Flow sensor 1

measured the flow rate coming out of the fish tank. Flow sensor 2, which was installed right

after the pressure stabilizer, measured the inflow rate to the RO membrane. Brine rate was

measured by flow sensor 3. Permeate flow rate was not measured during the experiment.

Instead, it was calculated by subtracting the brine flow rate from the inflow rate according

to mass balance. Wind speed is another important parameter for the system operation. One

wind anemometer was installed to record the wind speed data.

Membrane fouling and scaling might present another problem for the system operation.

However, the primary scope of the study limited us from further investigating the potential

changes in membrane characteristics over time, and membrane cleaning was not conducted

between experiments.

Water samples were collected periodically from permeate and brine (every 30 min for

the 2 and 4 h tests; every 60 min for the 12 h test) and sent to the Environmental

Engineering Laboratory on the University of Hawaii at Manoa campus for nitrogen

analysis. Ammonia nitrogen (NH3-N) was determined using the ammonia-selective

electrode method (4500-NH3 D) listed in Standard Methods for the Examination of Water

and Wastewater (APHA et al., 1995). Nitrate nitrogen (NO3-N) was measured using the

Hach Method 8192 based on the cadmium reduction method (Hach Company, 1999).

Nitrite nitrogen (NO2

-N) was determined using the Hach Method 8507 based on thediazotization method (Hach Company, 1999). Both of the analytical techniques for nitrate

nitrogen were approved by the US Environmental Protection Agency.

3. Results and discussion

3.1. Technical feasibility

Six experimental tests were conducted on Coconut Island from February to March 2002.

The technical feasibility of the wind-driven treatment system was evaluated in terms ofpermeate production and nitrogen removal.

3.1.1. Permeate production

The average wind speed during the experiments varied from 2.9 to 6.0 m/s. The wind-

driven RO system was able to operate and produce permeate under all these wind speeds.

Figs. 2 and 3 show the permeate flow rate under two different wind conditions. Both figures

show that the system was able to produce permeate even under mild wind conditions (at

about 3.0 m/s). As noted in Fig. 2, several system shutdowns occurred when the wind speed

dropped below 2.0 m/s. Therefore, having a wind speed of approximately 3.0 m/s was

necessary to prevent unnecessary system shutdowns.Fig. 4 shows the effect of wind speed on permeate production from the RO membrane.

The average permeate flux was 228 L/h when the average ambient wind speed was 2.9 m/s.



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The rate increased to 366 L/h when the average ambient wind speed was 6.0 m/s. The windspeed data are correlated well with the permeate flux by a linear regression curve,

expressed by the following empirical equation:

Q 44:8U 99:7 (1)where Q is the permeate flux (L/h) and U the wind speed (m/s).

The permeate produced using the RO process is defined in Eq. (2):

Qp AmKDP Dp (2)where Qp is the permeate flux through the membrane, Am the membrane surface area, Kthe

permeation coefficient, (P the hydraulic pressure differential across the membrane, and

(p the osmotic pressure differential.Using the same RO membrane, both Am and K would remain unchanged during

experiments. (p is generally a constant, although a small increase could occur due to the


Fig. 2. Permeate flow rate scheme under mild wind condition (average wind speed = 2.9 m/s).


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Fig. 3. Permeate flow rate scheme under strong wind condition (average wind speed = 6.0 m/s).

Fig. 4. Effect of wind speed on permeate production.


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increased total dissolved solids concentration in recycled brine. The permeate production

is, therefore, determined principally by (P, or roughly the operating pressure P produced by

the wind in the wind-driven RO system. Fig. 5 shows the relationship between wind speed

and pressure in the stabilizer under typical continuous operation. The operating pressure

increases almost linearly with the wind speed, as shown in Fig. 5. As a result, the linear

relationship between wind speed and permeate production can be explained.

3.1.2. Nitrogen removal

Nitrogen analysis was carried out to monitor the quality of the permeate. Samples offishtank water, permeate, and brine were taken periodically for laboratory analysis.

Two common criteria used in RO desalination, rejection rate and recovery rate, were

also used in this study to determine the RO membrane performance for nitrogen removal.

The recovery rate, or the percentage of permeate produced from feed water, is determined

using Eq. (3):

YQp

Qf100% (3)

where Yis the recovery rate percentage, Qp the permeate flux or volume, and Qf the feed

water flux or volume.The nitrogen removal rate is determined using Eq. (4):

R 1 Cp

Cf

100% (4)

where R is the rejection rate percentage, Cp the concentration of the permeate (NH4-N,

NO3-N, or NO2-N), and Cf the concentration of the feed water.

The nitrogen analysis for all six experiments showed that the nitrogen removal rate

ranged from 90% to 97% and that the RO membrane recovery rate ranged from 39.2% to

57.5%. Moreover, the analysis showed that the ammonia concentrations in permeate

remained below 0.04 mg/L. Nitrite and nitrate concentrations in permeate wereundetectable. All these data indicate the permeate produced from the RO system was

suitable for maintaining the tilapia in the tank.


Fig. 5. Relationship between wind speed and pressure under typical continuous operation.


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3.1.3. General evaluation of system performance

In total, six experiments were carried out to test the performance of the wind-driven RO

system for nitrogen removal under different wind conditions (Table 1). The operation time

for first five experiments ranged from 2 to 4 h. The purpose of these experiments was tostudy the technical feasibility of the system, including permeate production under different

wind conditions and nitrogen removal by RO membrane. A sixth experiment was

conducted for 12 h to test the durability of the system. During this experiment, brine was

discharged every 4 h to keep the quality of the permeate suitable for reuse. The amount of

brine discharged for each 4-h cycle was equal to the volume of the pressure stabilizer

(284 L). The results of the six experiments are presented in Table 2.

From the data in Table 2, we know that the wind-driven RO system can treat and reuse

aquaculture wastewater at an average wind speed of 3.0 m/s or higher. The system can

generate freshwater from aquaculture wastewater at a flow rate of 228366 L/h, depending

on wind speed. Generally speaking, the greater the wind speed, the more the production of

freshwater. It is recommended that this kind of wind-energy application be located in areas

where 3.0 m/s or higher wind speed is available. About 7084% of aquaculture wastewater

can be recycled, decreasing the freshwater supply to the fish tank by 7084% and the

wastewater discharge volume to 1630%. From 90% to 97% of ammonia and nitrate

nitrogen in the fish tank effluent is removed by the system, and the nitrogen level in

permeate remains low enough for tilapia reuse. The 12-h experiment also shows the

possibility of continuous operation for the wind-driven RO system. Thus, the system is

technically feasible for aquaculture nitrogen removal.

3.2. Preliminary cost estimate

A preliminary cost estimate was conducted after the technical feasibility of the wind-

driven RO system was confirmed by field experiments. The cost analysis was based on the

system performance presented in Table 1 and the following assumptions:

system operation: round-the-clock; production capacity: 304 L/h at an average wind speed of 4.0 m/s (7.3 m3/day); system availability: 75% (including days when wind speed is too low to run the system);


Table 1

Experimental tests description

Experiment number Date Operation timea Sampling intervals Wind speedb (m/s)

1 5 February 2002 2 h Every 30 min 3.0





6 19 March 2002 12 h Every 60 min 5.2

a The total system operation time included the time for pressure accumulation, producing permeate and brine,

and membrane clean up. Here, the operation time indicates only the time for producing permeate and brine.b The average value from anemometer readings.


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Table 2

Wind-driven reverse osmosis system performance for six experiments conducted at Coconut Island, Hawaii

Experiment number

1 (date: 5

February 2002)

2 (date: 12

February 2002)

3 (date: 14

February 2002)

4 (date: 21

February 2002)

Wind speed (m/s) 3.0 0.7 2.9 0.7 3.5 0.9 3.5 1.0 Operation time (h) 2 4 4 4

Average inflow rate (L/h) 406 402 536 462

Average brine rate (L/h) 173 174 264 218

Average permeate rate (L/h) 233 228 272 244 Wastewater recycle rate (%)a 70.7 83.2 83.5 83.1

RO membrane recovery rate (%) 57.5 56.7 50.61 52.94

Nitrogen removal rate (%)e 96 93 90 97

Ammonia concentration (mg/L) UDb


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yearly production capacity: 2000 m3/year. But in real situations, the production capacitydepends highly on wind conditions. The actual yearly production capacity may vary

from 1500 to 2200 m3/year;

electricity requirements: about 500 kWh/year for system control and other purposes; electricity price: US$ 0.05/kWh (from US Department of Energy); the capital cost of RO membrane: US$ 325 for M-T4040ULP membrane manufactured

by FILMTEC Corporation;

lifetime: 10 years for the system and 5 years for the RO membrane; interest rate: 6%. The figure is commonly used for computing the capital recovery factor

in treatment cost analysis;

labor cost: one man working 10 h/week at US$ 10/h; land requirement: negligible; consumption of chemicals: US$ 0.13/m3 permeate (Abdel-Jawad et al., 1999).

Table 3 shows the cost estimate for a wind-driven RO system for the removal of

nitrogenous waste from tilapia culture at a design capacity of 2000 m3 permeate output per

year.

Table 3 shows that producing 2000 m3 of permeate per year from aquaculture

wastewater using a wind-driven RO system costs US$ 8005. The unit cost is US$ 4.00/m3

of permeate. As the actual production rate may vary from 1500 to 2200 m3/year with regard

to variability of wind speed, similarly, the cost of producing 1 m3 of permeate may vary

from US$ 3.64 to US$ 5.34. This cost is much higher than reported figures of large-scale

wastewater treatment systems using membrane technology (Glueckstern et al., 2000;


Table 3

Cost estimate for a wind-driven reverse osmosis system

Capital

cost US ($)

Average annual

cost US ($)

Fixed costs

Dempster 30 ft. windmill with 14 ft. zzdiameter wheel and pump 7000 951

Pressure stabilizer 1000 136

RO membrane and pressure vessel 325 77

Campbell scientific data logger 2000 272Sensors (pressure and flow sensors) 1500 204

Miscellaneous items: prefilter, PVC pipes,

fittings and valves, electrical components,

storage tanks, paint, concrete, wood, etc.

2000 272

Construction 3000 408

Subtotal 16825 2320

Variable costs

Labor 5200

Chemicals 260

Electricity 25

Maintenance 200Subtotal 5685

Total 16825 8005


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Redondo, 2001; Van Hoof et al., 1999). The high price is due to the small scale of the wind-

driven RO system, for which the labor cost represents about 65% of the total annual cost

(Table 3). If the system scale is enlarged, it is believed that the unit cost would decrease

substantially. There are two ways to increase the system capacity while keeping the labor

cost unchanged. One is to increase the number of wind-driven RO systems. The other is to

increase the number of RO membranes driven by one windmill. Table 4 gives a cost

comparison for producing freshwater using different windmill and RO membrane

combinations. The table is based on the assumptions presented above as well as the

following assumptions:

One person can take care of up to three windmill systems at the same time without

increasing the salary. One windmill can provide the pressure for the operation of two RO membranes. Two RO membranes can be arranged in a serial or parallel manner. The product rate can

be almost doubled using two RO membranes driven by one windmill.

The combinations in Table 4 show that the production capacity of the system can be

increased by using a system with multiple windmills and membranes. With such com-

binations, the unit cost will be much lower.

In a large conventional RO desalination plant (producing 1150 m3 of permeate using

electricity for high-pressure pumping), the cost of energy represents about 23% of the total

unit water production cost (Abdel-Jawad et al., 1999). But for wind-driven RO systems, thewind energy is free and only a small amount of electricity is needed for system control. The

cost of electricity for the system driven by wind energy would only be about 0.3% of the

total unit water production cost, according to the estimate given in Table 3. Although

producing water using the wind-driven RO system seems too expensive in the current

situation, this technology has great promise once the system scale can be upgraded.

4. Conclusions

Since HIMB on Coconut Island faces serious high nutrient levels in its freshwateraquaculture effluents, especially nitrogenous substances, a wind-driven RO system was

designed and tested for aquaculture nitrogen removal. The study shows that the wind-


Table 4

Cost comparisons for producing freshwater using different windmill and reverse osmosis membrane combinations

Combination Number of

windmills

Number of

RO membranes

Product rate

(m3

/year)

Total annual

cost (US $/year)

Unit cost

(US $/m3

permeate)1 1 1 15002200 8005 3.645.34

2 1 2 30004400 8342 1.902.78

3 2 1 30004400 10810 2.463.60

4 2 2 60008800 11484 1.441.91

5 3 1 45006600 13615 2.063.02

6 3 2 900013200 14626 1.111.62


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