A study on the optimal hydraulic loading rate and plant ratios in recirculation aquaponic system

A study on the optimal hydraulic loading rate and plant ratios in recirculation aquaponic system
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  A study on the optimal hydraulic loading rate and plant ratios in recirculationaquaponic system Azizah Endut a, * , A. Jusoh b , N. Ali b , W.B. Wan Nik c , A. Hassan d a Faculty of Innovative Design and Technology, University Darul Iman Malaysia, KUSZA Campus, 21300 Kuala Terengganu, Malaysia b Faculty of Science and Technology, University Malaysia Terengganu, 21030 Kuala Terengganu, Malaysia c Faculty of Maritime Studies and Marine Science, University Malaysia Terengganu, 21030 Kuala Terengganu, Malaysia d Institute of Tropical Aquaculture, University Malaysia Terengganu, 21030 Kuala Terengganu, Malaysia a r t i c l e i n f o  Article history: Received 4 June 2009Received in revised form 10 September2009Accepted 10 September 2009Available online 9 October 2009 Keywords: Hydraulic loading rateAquaponicAfrican catfishWater spinach a b s t r a c t The growths of the African catfish ( Clarias gariepinus ) and water spinach ( Ipomoea aquatica ) were evalu-ated in recirculation aquaponic system (RAS). Fish production performance, plant growth and nutrientremovalweremeasuredandtheirdependenceonhydraulicloadingrate(HLR)wasassessed.Fishproduc-tion did not differ significantly between hydraulic loading rates. In contrast to the fish production, thewater spinach yield was significantly higher in the lower hydraulic loading rate. Fish production, plantgrowth and percentage nutrient removal were highest at hydraulic loading rate of 1.28m/day. The ratioof fish to plant production has been calculated to balance nutrient generation from fish with nutrientremoval by plants and the optimum ratio was 15–42 gram of fish feed/m 2 of plant growing area. Eachunit in RAS was evaluated in terms of oxygen demand. Using specified feeding regime, mass balanceequations were applied to quantify the waste discharges from rearing tanks and treatment units. Thewaste discharged was found to be strongly dependent on hydraulic loading rate. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Aquaculture probably the fastest growing food-producing sec-tor, now accounts for almost 50% of the world’s food fish and isperceivedas havingthe greatest potential tomeet the growingde-mandfor aquaticfood. It isestimatedthatat leastanadditional 40milliontonnesofaquaticfoodwillberequiredby2030tomaintainthe current per capita consumption (FAO, 2006).When fish are cultured, only a small proportion of the feed isconverted (25–30%) to useable energy (Rakocy et al., 1993). Thebalance of nutrients is excreted in solid and dissolved fractions.Dissolved nutrients accumulate in recirculation systems with lowwater exchange and high feeding rates to levels which approxi-mate hydroponic nutrient solutions.Recirculation aquaponic system (RAS) is a promising technol-ogy in the integration of fish and hydroponic plant production.The fish water, rich in nutrients is used for plant growth, whilethe plants are used as biofilters for water regeneration. Whilstbiofiltration converts the harmful into the harmless, the endpoint is a buildup of nutrients within recirculation systems, prin-cipally consisting of nitrates and phosphates. Nutrient removalby plants improves the quality of effluent and may enhance fishproduction. The amount of nitrate produced in a fish culture sys-tem is directly proportional to two factors: the amount or den-sity of fish in the system and the amount and protein contentof the food, as different fish species require different proteincontent in their respective diets.Integrated systems use water more efficiently through theinteracting activities of fish and plants. The addition of water to afish tank to satisfy the oxygen requirements depends on the oxy-gen consumption of the fish, the oxygen concentration in the inletwater and the lowest acceptable concentration in the outlet water(Lekang, 2007). Hence effective HLR can be employed to achieveoptimal growth for the fish and plants.The rate of change in nutrient concentration can be influencedbyvaryingtheratioofplantstofish(Rakocyetal.,2006).However,since the relative proportions of soluble nutrients made availableto the hydroponic plants by fish excretion do not mirror the pro-portions of nutrients assimilated by normally growing plants, therates of change in concentration for individual nutrients differ.The disparity inaccumulationor reductionrates of different nutri-ents quickly results in suboptimal concentrations and ratios of nutrients, thereby reducing the nutritional adequacy of the solu-tion for plants. Theoretically, the nutrient content of a diet canbe manipulated to make the relative proportions of nutrients ex-creted by fish more similar to the relative proportions of nutrientsassimilated by plants. With such a diet, there would be an optimal 0960-8524/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.biortech.2009.09.040 * Corresponding author. Tel.: +609 6653519; fax: +609 6673523. E-mail address: Endut).Bioresource Technology 101 (2010) 1511–1517 Contents lists available atScienceDirect Bioresource Technology journal  rationof fishtoplantsandoptimalnutrientsupplementation(Sea-wright et al., 1998).Severalmassbalancemodelshavebeenproposedfrompreviousstudies (Pagand et al., 2000; Papatryphon et al., 2005; Schneideret al., 2005; Mongirdas and Kusta, 2006), from which the totalnitrogen and phosphorus discharges into receiving waters can beestimated.However,mostofthesestudieswereconductedinopensystems. Recently, the incorporation of recirculated fish with veg-etablehydroponicsproductionhasbecomeaninterestingmodeltoprivate sector, aquaculture and environmental scientists (Rakocyet al., 2006; Bakhsh and Shariff, 2007; Endut et al., 2009).Theobjectives of this studywereto(1) determinethe optimumhydraulicloadingrateintermof fishproduction, plantproduction,and nutrient removal, (2) evaluate the optimum plants ratio interm of daily fish feed input to plant growing area, and (3) studythe mass balance of oxygen in achieving sustainable balance be-tween fish and plants. 2. Methods  2.1. Experimental design Therecirculationaquaponicsystem(RAS)utilizedisdepictedinFig. 1. The experimental facility waslocated ina greenhouse of theUniversity of Malaysia Terengganu campus. RAS consisted of afiberglass rearing tank, hydroponic trough (growing bed), sand fil-ter for solid removal, sump system for denitrification unit, waterholding tank and reservoir (pre-aeration). Pipelines made of poly-vinyl chloride were installed to connect the culture tank andhydroponic trough to recirculate the water.Three culture tanks arranged in series were used in the rearingofAfricancatfish( Clarias gariepinus ).Airstones,connectedtoanairblower were installed in the culture tank to supply oxygen for fishculture. Water level in each culture tank was kept at 0.85m deepto maintain the water volume at 1000L. Water lost through evap-oration, transpiration and sludge removal was replenished withwater in the pre-aeration tank. The tank openings were coveredby plastic net (20mm aperture) to hinder the fish jumping out of the tanks. Measurements of temperature, dissolved oxygen (DO)and pH of water samples were performed in situ during the sam-pling process using the YSI multi-probe meter (model YSI 550A)and pH Cyber Scan waterproof, respectively. Water temperatureand pH were in the range of 27.5–28.8 ° C and 5.6–7.3 respectivelyand was acceptable for African catfish.Samplesof  C. gariepinus fingerlings,withaninitialaveragebodyweight of 30–40g were randomly transferred into three replicatefiberglass culture tanks. Water exited and flowed from the culturetank was sprinkled over the vegetables in the hydroponic troughand outflow trickled down to the sump for denitrification process.The components were installed such that the water flowed bygravity, by placing components at appropriate elevation relativeto one another. The water was then pumped vertically to the sandfiltrationtanks for particulate removal. After exitingthe sand filterthe water went directly to water storage tank and was flowed bygravity back to the fish tank. During this study the inflow ratesoftheRASweremaintainedtobeidenticalaspossiblebyadjustingthe gate valves according to the target rate of each trial.Inthefirstexperiment,fivetrialswereconsecutivelyperformedat different hydraulic loadingrates, each operatingfor 35days andcompared with control with no plants. Each treatment was repli-catedthrice.Theexperimentalwasoperatedwithfishforoneweekprior tothe initiationinorder to acclimatethe biofilters as tomin-imizenetnutrientuptakebybacteriaatthebeginningofeachtrial.At the initiation of hydraulic loading rate of 0.64m/day, systemwasflushedandfingerlingsAfricancatfishwereaddedtoeachcul-ture tank up to the treatment biomass. Due to our inability toobtain fish of similar size for trials two and three, fish from theprior trial were pooled and reallocated to the systems for the fol-lowingtrial.Handfeeding,twiceperdayattherangeof2–4%bodyweight/day. Fish were fed with 3.2mm commercial diet floatingpellet (Cargill Company) with 32% protein and 10% moisture. Thefood size was adjusted to compensate for changes in fish size.Water spinachseedlings were planted directlyinto the gravel sub-strate of the hydroponic growing bed at 10cm  10cm spacing.In the second experiment, the effect of seven ratios of plants tofish (2, 4, 6, 7, 8, 9 and 10) was evaluated by manipulating waterspinach stocking rate. The size of the hydroponic trough and sys-temvolume were the same for all ratios, but the daily feeding rateincreased in direct proportion to the fish biomass. African catfishwere stocked into rearing tank as at optimum HLR obtained inthe first experiment. Water samples from outlet of fish tank andoutlet of the hydroponic trough were collected once a week forwater quality determination. At the end of the experiment, allfishes in each tank will be netted, weighed and their individuallengths will be recorded as well as the weight of vegetable. Fig. 1. A: Culture tank, B: hydroponic trough (planted bed), C: hydroponic trough (control bed), D: filter, E: sprinkler, F: sump, G: pump, H: rapidsand filter, I: water storagetank, J: air blower, and K: valves.1512 A. Endut et al./Bioresource Technology 101 (2010) 1511–1517   Ifnutrientflowsaresubsequentlyconnectedfromonesystemtoanotherone, overallsystemnutrientretentionandbalancescanbeestimated.Theoxygenflowsanditsretentionswerecalculatedusingmass balances, based on the concept: output=input À retention.This retention can be expresses as g per kg feed with the feed tothe fish (percent fish nutrient). The nutrient discharges (output)fromthesystemserveasinputintheothersubsequentsystem.Table1illustratesthehydraulicconditionforthisstudy.  2.2. Measurement of growth and yield To assess the overall system performance, data on fish growthand feeding were collected. The feeding data collected include,the feeding rate, amount of feed; number of feeding/day; feedamount per tank per day; total feed per day and feed protein. Fish(10%)weretakenfromtheculturetanktomeasuretheirlengthandbodyweighttoestimatethegrowthrateofthefishandassumedtobe representative of the fish in the tanks. The growth of fish wasalso monitored from the time at stocking up till the harvest time.Fish sampling in juvenile and grow-out systems were done on aweekly basis for survival and average weight.The following production parameters were determined accord-ing to the procedure of Ridha and Cruz (2001): Specificgrowthrate ð SGR  Þ¼ð lnmeanfinalweight À lnmeaninitialweight Þ 100culturedays Feed conversion ratio ð FCR  Þ¼ total weight of dry feed giventotal wet weight gain Plants growth was monitored weekly by measuring the plantheightofallwaterspinachplantsona0.5m 2 plantingareaaccom-paniedbycountingofnumberofshoots.Theplantswereharvestedat height ranging from 45 to 50cm. Each growing trough wascleaned and the biomass of plants was measured and recorded.  2.3. Samples analyses Water samples were takenonce a week fromeach culturetank,influent and effluent of the hydroponic trough, sump, water stor-age tank and inflow of culture tank. The samples were analyzedfor 5-day biochemical oxygendemand(BOD 5 ), total suspendedso-lid (TSS), total ammonium nitrogen (TAN), nitrite nitrogen(NO À 2 —N), nitrate nitrogen (NO À 3 —N) and total phosphorus (TP).Dissolved oxygen, pH, and temperature were also recorded eachtime water was collected. Weekly sampling was carried out be-tween8.30amand9.30amineachsamplingdateandrefrigeratedat4 ° Cinlabeledpolythenebottlesforchemicalanalysis.TheBOD 5 and TSS analyses were performed according to Standard Method(APHA 1998). The TAN, NO À 2 —N, NO À 3 —N and TP measurementswere performed using HACH DR4000 spectrophotometer accord-ing to salicylate, diazotization, cadmium reduction and ascorbicacidmethodrespectively. TheDOandpHof thesampleweremea-sured using DO meter YSI 55A and pH cyber scan waterproof respectively.  2.4. Statistical methods Statistical software of Statistical Package for the Social Sciences(SPSS)Version16andMicrosoftExcelwereusedtocalculatemean,standard deviation, and one-way ANOVA. Differences of meanwere evaluated for significance by the range tests of Tukey HSD(  p 6 0.05) for homogeneous variances (Levene test) and by therange test of Dunnett T3 (  p 6 0.05) for inhomogeneous variances,respectively (Schulz et al., 2003). 3. Results and discussion  3.1. Effect of hydraulic loading rates Specific growth rates (SGRs), feed conversion ratio (FCR) andfishproductiondidnotdiffersignificantlybetweenhydraulicload-ing rates (Table 2). FCR values are in the range of 1.23–1.39. In ourstudy, the same feed is used and the ration is fixed similarly in allculture tanks. Stocking at hydraulic loading rate of 1.28m/daygives the best production performance (Table 2).The FCR recorded (1.23–1.39) is not far above the ideal value of 1.0forcultureofAfricancatfishinrecirculationsystemandFCRva-lue 0.85 reported in the culture of African catfish byEding andKamstra (2001). However the recorded FCR are better than therange 1.1–1.7 reported in recirculation system of African catfishas reported byAkinwole and Faturoti (2007).HLR did not affect growth rate or feed conversion ratio.Plants grew actively in the hydroponic trough and did notidentify any nutritional deficiencies or mineral imbalances. Plant  Table 1 Hydraulic condition for operating the aquaponic recirculation system. Q  (m 3 day À 1 ) HLR  a (mday À 1 ) HRT b for overall system (h)4.6 0.64 4.59.2 1.28 2.313.8 1.92 1.518.4 2.56 1.123.0 3.20 0.9 a HLR, hydraulic loading rate, which is flow rate ( Q  ) divided by total surface areaof the trough. b HRT, hydraulicretentiontime, whichcanbecomputedas(surfacearea  waterdepth  porosity of gravel trough/flow rate).  Table 2 Fish growth, feed conversion factor and water spinach by hydraulic loading rates. HLR  a (m/day) Fish Plant (water spinach)SGR  b (%) FCR  c Production (kg/m 3 ) Growth rate d (cm/day) Production (kg/trough/harvest)0.64 1.80 1 1.27 1 45.2 1 1.75 1 17.63 2 1.28 1.83 1 1.23 1 45.7 1 2.50 3 17.90 2 1.92 1.73 1 1.33 1 44.3 1 2.06 1 17.53 2 2.56 1.73 1 1.34 1 44.1 1 1.90 2 17.03 1 3.20 1.68 1 1.39 1 43.3 1 1.90 2 16.83 1 Values given are mean from triplicate data ( n =3).Mean with the different superscript is significantly different at the p 6 0.05 level. a Hydraulic loading rate calculated as HLR= Q  (flow rate) divided by AW (surface area) of hydroponic trough. b Specific growth rate calculated as SGR=ln final weight (g) À ln initial weight (g)  100days À 1 . c Feed conversion ratio is calculated as FCR=total weight of dry feed give/total wet weight gain. d Growth rate=height of plant divided by day.  A. Endut et al./Bioresource Technology 101 (2010) 1511–1517  1513  production increased as the hydraulic loading rate increased from0.64m/day to 1.28m/day, whereas an increase in the HLR from1.28m/day to 3.20m/day did not result in a higher plant produc-tion. At the end of the growth period (20–28days), the plantsreached the market size at average height of 45–50cm. Wholeplant water spinach growth rate and yields differ significantly be-tween hydraulic loading rates (Table 2).Plant growth rate and productions differ significantly betweenHLR. The growth decreased significantly with increasing in HLR supported the development of aerobic conditions in the hydro-ponic trough and hindered denitrification processes. Nevertheless,low HLR with lower out flowing oxygen contents promoted deni-trification and highest NO À 3 —N elimination is observed in lowerhydraulic loading rate (0.64m/day and 1.28m/day).Average plant productions are 17.63kg, 17.90kg, 17.53kg,17.03kg and 16.83kg for HLR 0.64m/day, 1.28m/day, 1.92m/day, 2.56m/dayand3.20m/day, respectively. The decrease inpro-duction corresponded strongly concludes that insufficient N in theinfluent could be a limiting factor for a further increase in plantproduction. An increasing of HLR might diminish the contact timefor nitrate and denitrifying bacteria, thus decreasing the perfor-manceof hydroponictroughfordenitrification(Endutet al.,2009).SnowandGhaly(2008)evaluatedtheuseofbarleyforthepuri-ficationof aquaculturewastewater ina hydroponicsystemand re-ported the crop yield was significantly influenced by the seedquantity. The major growth-limiting mineral is usually nitrogenandhighestgrowthratesandyieldsaregenerallyseenwhennitro-gen is supplied as combination of ammonium and nitrate.Continuous flow operation of the aquaponic system was initi-atedwithalowHLRof0.64m/day.Themeanvalueandpercentageremoval of waterqualityvariablesat variousHLRare showninTa-ble3.ItisfoundthatremovalpercentageofBOD 5 ,TSS,TANandNi-trite–N increased with increasing in HLR. In contrast to BOD 5 , TSS,nitrite–N and TAN, removal percentage of nitrate–N and TP in-creased with increasing in HLR from 0.64m/day to 1.28m/dayand decreased with increasing in HLR from 1.28m/day to 3.2m/day. Statistically, there were significant differences in all waterquality parameters by HLR (  p <0.05) as shown inTable 3. Thewhole treatment, RAS basically showed effective nutrient removalwith average reduction efficiency range from 47% to 89.5%.Values of TSS, BOD 5 , TAN, nitrite–N, nitrate–N and total phos-phorus in final effluent from this study are in accordance withthe previous studies (Eding and Kamstra, 2001; Schulz et al.,2003; Franco-Nava et al., 2004; Lin et al., 2005; Snow and Ghaly,2008). The optimum hydraulic loading rate can be determined bya compromise between fish and plant productions and removalefficiency.Similar to previous studies (Cottingham et al., 1999; Jamiesonet al., 2003), the improvement in TAN removal is paralleled bythe increase in NO À 3 —N. It can be concluded that the improvementin ammonia removal is due to increased nitrification activity. Theaccumulation of NO À 3 —N in the system indicates that after NH 3 –N is nitrified, subsequent denitrification is limited. Possible factorsthatcouldlimitdenitrificationincludeinadequateresidence/reten-tion time for the sump to denitrify NO À 3 —N, the presence of DO, orlack of available carbon in the system.A number of mechanisms are responsible for the removal of NO À 3 —N from the wastewater. One mechanism for the removal of NO À 3 —N is plant uptake through the root system from the growthmedium. A second mechanism for the removal of dissolved solidsis microbial assimilation. It may also be assimilated by microor-ganisms in the water column or by biofilms associated with theroot mats of plants (Vaillant et al., 2004).Denitrification activity is reduced if available carbon supplieswere low and proceeds only when the oxygen supply was inade-quateformicrobialdemand(Hamlinetal.,2008).Inthisstudy,car-bon availability may have been inadequate to support high levelsof denitrification due to the lack of an established litter layer inthe system. If, on the other hand, the influent wastewater itself isan adequate source of carbon, the lack of denitrification may beattributed to the short hydraulic retention time of the system.  3.2. Plant ratios The percentage removal values of TAN, nitrite–N, nitrate–N, to-tal phosphorus and plant productions at seven ratios of plants tofish are shown inTable 4. There was significant difference inTAN, nitrite–N, nitrate–N and total phosphorus concentrations be-tweenratioofplantstofish.Thepercentageremovalofwaterqual-ity parameters and plant productions increased with increasing inplantsratiostofishuntilthemaximumwasreachedatplanttofishratio of 8, which was equivalent to a fish feeding rate of 15–42g/m 2 plant growing area. Further increasing plant ratios led to con-siderabledecreasesinwaterspinachproduction.Thisstronglycon-cludes that insufficient nitrogen in the influent of hydroponictrough could be a limiting factor for a further increase in plantproduction.  Table 3 Mean values for various parameters of water quality by the RAS. HLR (m/day) Water quality parameterBOD 5 TSS TAN NO 2 –N NO 3 –N TP0.64 Influent (mg/L) 6.7 74.6 12.02 0.58 19.8 17.0Effluent (mg/L) 1.7 23 2.68 0.19 5.8 6.7Percent removal (%) 47.3 1 67 1 64.1 1 67.2 1 62.4 4 50.0 4 1.28 Influent (mg/L) 6.7 74.4 12.04 0.56 20 17.1Effluent (mg) 1.3 21.1 2.23 0.14 5.4 6.3Percent removal (%) 54.5 2 69.5 2 68.4 1 75 2 64.9 4 52.8 5 1.92 Influent (mg/L) 6.8 74.8 12.01 0.56 19.9 16.9Effluent (mg/L) 1.3 19.2 1.94 0.11 6.2 7.0Percent removal (%) 55.4 2 72.3 2 71 2 80.4 2 60.4 2 47.8 1 2.56 Influent (mg/L) 6.9 74.4 11.99 0.57 20 17.0Effluent (mg/L) 1.0 14.2 1.68 0.09 6.6 7.1Percent removal (%) 61.4 2 79 3 73.3 2 84.2 2 58.5 3 47.5 3 3.20 Influent (mg/L) 6.7 73.9 11.98 0.57 20.1 17.1Effluent (mg/L) 0.7 11.2 1.14 0.06 9.7 7.9Percent removal (%) 65.5 3 82.9 4 78.3 3 89.5 2 42.3 1 42.8 2 Different superscript numbers within one column denote statistically significant differences (  p 6 0.05).1514 A. Endut et al./Bioresource Technology 101 (2010) 1511–1517   In the field experiment conducted byRakocy et al. (2006), a ra-tio in the range of 60–100g of fish feed/m 2 of plant growing areawas used for the production of tilapia, lettuce, basil and severalother plants in raft aquaponic system. From our results we con-clude that the technical demand on management, especially thefish and plant species used plays a vital role in the establishmentof the configuration and relative size of integrated system compo-nents.Plantroots,hydroponicstructuresandmediaimprovewaterquality by capturing solids and providing surface area forbiofiltration.  3.3. Removal rate constant  Pollutant removal can be described using first-order kineticmodel (IWA, 2000). Average first-order removal rate constantsfor a specific pollutant are determined by substituting meanhydraulicretentiontime(Table1)andmeaninfluent–effluentcon-centrations of the pollutant (Table 3) into the following equation,and then solving for k . CeCi ¼ exp ðÀ kt  Þ¼ exp k e h w HLR    ð 1 Þ where Ce ,effluentpollutantconcentration(mg/L); Ci ,influentpollu-tant concentration (mg/L); k , first-order removal rate constant(day À 1 ), t  , hydraulic retention time (day); HLR, hydraulic loadingrate (m/day); e , porosity of hydroponic trough (assuming 0.45–0.70); and h w , water depth of trough (m).These results are shown inTable 5. Distinct values of removalrate constants for major pollutants have been reported and wereevaluated using the same methods as this study with the influ-ent–effluent data. The effect of hydraulic loading rate on removalrate constant was further examined by linear regression with log-arithmic scale using the k -HLR data inTable 5. Good correlationswith power function were found between removal rate constantand HLR for TAN as depicted inFig. 2.Removal rate constants for TSS, TAN, NO 2 –N and NO 3 –N, ob-tained from this study and other comparative studies (Schulzet al., 2003; Lin et al., 2005), are found to be proportional tohydraulic loading rate to a power equation. These suggest that re-moval rateconstant wouldbe varieddependingonhydraulicload-ing rate. Efficient removal was always achieved in these studiesunder a wide range of hydraulicloading rate because of lowpollu-tantlevelsofaquaculturewastewater,thusleadingtoHLRcontrol-ling the removal rate constant.  3.4. Oxygen concentration dynamics in RAS components Figs. 3–5show oxygen concentration dynamics in culture tank,influent planted trough and effluent planted trough, respectively.At the beginning of the system operation, the oxygen differenceacross the system components was insignificant because of thelowsystemloading(lowfishbiomass andtherefore lowfeedload-ing).Attheendoftheexperiment,oxygenconcentrationinculturetank, influent planted trough and effluent planted trough thenreached a value of 4.70mg/L, 4.35 and 4.2mg/L, respectively. Thedecreased of oxygen concentration with increasing in culture daydue to the increase of fish biomass and accumulation of organicmatter in the system.  3.5. Oxygen consumption in fish tank Rearedfisharethemainoxygenusershere.Oxygendemandde-pends onmetabolicrate so the oxygenusage is expressed interms  Table 5 First-order removal rate constants at various HLR. HLR (m/day) Removal rate constant k TAN (day À 1 ) k TSS (day À 1 ) k NO À 3 —N (day À 1 ) k NO À 2 —N (day À 1 )0.64 4.45 5.94 4.096 5.911.28 7.63 12.77 8.704 10.531.92 11.54 17.64 12.288 13.352.56 11.99 24.02 16.64 21.593.2 12.12 27.41 17.6 30.24 Fig. 2. The relationship between first-order removal rate constant for TAN ( k TAN )and HLR.  Table 4 Percentage removal of water quality parameters and plant productions in RAS with hydroponic water spinach at seven ratios of plants to fish. Fish to plants ratio (1:  x ) Percentage removal (%) Water spinach  x TAN NO À 2 —N NO À 3 —N TP Production (g WW/m 2 )2 67.26 1 69.00 1 39.00 1 27.45 1 360 1 4 69.93 2 76.60 2 43.32 2 33.48 2 700 2 6 72.96 2 82.74 3 44.58 3 41.55 3 1000 3 7 79.323 90.57 4 51.57 4 48.25 4 1120 4 8 81.673 95.79 5 59.02 5 48.99 5 1160 5 9 85.864 96.53 5 60.20 5 49.01 5 1100 4 10 86.22 4 96.84 5 60.55 5 49.74 4 1000 3 WW, wet weight.Values given are mean from triplicate data ( n =3); treatment with the different superscript within the same column is significantly different at the p 6 0.05 level.  A. Endut et al./Bioresource Technology 101 (2010) 1511–1517  1515
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