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Ammonia removal from aquaculture water by means of fluidised technology
ELSEVIER
Aquaculture
139 ( 1996) 55-62
Ammonia removal from aquaculture water by means of fluidised technology W.J. Ng a,*, Kevin Kho a, S.L. Ong a, T.S. Sim b, J.M. Hoc ” Department of Civil Engineering, National University of Singapore, Kent Ridge, Singapore 0511, Singapore ’ Department of Microbiology, National University of Singapore, Kent Ridge, Singapore 0511. Singapore ’ Primary Industries Enterprise, 100 Beach Road, 31-00 Shaw Towers, Singapore 0718, Singapore Accepted
14 August 1995
Abstract bed bioreactor was used to treat aquaculture effluent intended for recycling. r.Maximum NH,-N removal was 82% of influent concentration when loading was between 0.201 and 0.397 g mm2 day-‘; at this level, effluent NH,-N did not exceed 0.05 mg 1-l. A bench-scale
fluidised
The reactor was loaded with ammonia-nitrogen (NH,-N) from 0.025 to 5.675 g me2 day-
Kqvwords: Aquaculture
effluent; Ammonia;
Bioreactor;
Fluidised bed
1. Introduction
Biological wastewater treatment processes are commonly used to renovate water in closed recirculating systems in aquaculture. As ammonia is the primary toxic waste of fish, most of these processes are targeted at nitrification. The activated sludge process has been successfully used in this role. However, on the basis of equivalent space requirements, attached growth systems have often been found superior to suspended growth systems such as activated sludge (Liao and Mayo, 1974). Rakocy and Allison ( 1981) successfully used an attached growth system, the trickling filter, to culture Oreochromis aureus in a closed recirculating system. Manci and Quigley ( 198 1) showed that water quality was better in a recirculating system (for Percafluuescens) with an upflow biological filter than one with a trickling filter and settler. Kruner and Rosenthal ( 1983) had earlier also noted the poorer performance of the trickling filter when
presented with increasing ammonia and nitrite loadings. Clogging was, however, a recurring problem with the upflow filter (Hilge and Rakelmann, 1984). * Corresponding
author
Elsevier Science B.V. SSDIOO44-8486(95)01153-6
56
W. J. Ng et al. / Aquaculture
139 (1996) 55-62
The fluidised bed bioreactor is in many ways a hybrid between the attached and suspended growth systems. Paller and Lewis ( 1988), using granular activated carbon, achieved ammonia removal rates of 414-1004 g mm3 day- ‘. The system was used to support a culture of common carp reared at 20°C. The system has, however, not been as extensively investigated as the trickling or upflow filters. This paper reports a study which investigated the fluidised bed bioreactor (FBR) in the treatment of aquaculture effluent intended for recycling.
2. Materials and methods The FBR was fabricated from a 5.4 cm internal diameter plexiglass column of length 201.5 cm. A wire gauze of 0.09 cm* mesh was used to support a bed of activated carbon granules. Beneath the wire gauze was a transition zone (40 cm length) which separated the inlet from the carbon bed. This was to reduce the occurrence of channelling in the latter. Pretreated aquaculture effluent was pumped (67 ml min-‘) from the pretreated effluent tank and this merged with the recycle flow (850 ml mini) from the aeration tank. The total stream entered the reactor from the bottom, while effluent overflowed near the top into the bottom of the aeration tank (2.3 cm diameter with 13 cm of water height) via a 2 cm diameter hose. Within the aeration tank, air was supplied through diffusers at a rate of 2.5 1 min- ‘. A flow equal to the feed flow was allowed to overflow from the aeration tank. This overflow constituted the net treated effluent leaving the system. Fig. I is a schematic diagram of the reactor used. The FBR was located in the laboratory of an aquaculture farm. The ambient temperature in the laboratory ranged from 24 to 28°C. Granular activated carbon (NORIT PKI-3) was first sieved through a 1.18 mm sieve and collected on a 1.OOmm sieve. Sieved carbon was washed with distilled water until the water was clear of carbon fines and then dried at 103°C. The reactor was filled to a depth
AIR
,
r--i
r ‘;
EXPANOEO CARBON BE0
.:: :’ ‘.’ ‘..
WIRE GAUZE
FEE0 PUMP Fig. 1. Schematic
RECYCLE PUMP
diagram of the experimental
fluidised bed reactor.
W.J. Ng et al. /Aquaculture I39 (1996) 55-62 Table 1 Average characteristics
of pretreated
57
water used to prepare FBR feeds
Parameter’
Concentration(mg
NH,-N NO?-N NOT-N PO,-P Alkalinity Hardness Chloride TSS vss BOD COD TKN TP
0.04 0.12 1.22 0.01 60 12 29 15.9 13.7 2.3 27.3 1.7 0.12
1-l)
“TSS, total suspended solids; VSS, volatile suspended solids; BOD, biochemical oxygen demand; TKN, total Kjeldhal nitrogen; TP, total phosphorus.
SD
?I
0.02 0.16 0.53 0.01 11 12 9.7 7.7 6.5 0.6 12.3 8 0.19
64 64 59 53 60 49 60 22 22 26 22 22 25
oxygen demand; COD, chemical
(static bed height) of 49.5 cm but during operation the bed was expanded by 32%. The reactor was seeded with nitrifiers using a commercial preparation (Interbio Ltd., Dublin) suitable for fish culture. Over a 3 day period, 9 g of the bacteria preparation were diluted into 150 1 of feed water daily and this was then pumped into the reactor. The reactor was allowed to stabilise over 3 months. To estimate the amount of bacteria attached to the carbon granules, 1 g of activated carbon granules was withdrawn from the reactor periodically and vigorously agitated in 9 ml of sterile water for 3 min using a vortex mixer, The resulting liquid was then used for plate counts with nutrient agar at 35°C and 48 h incubation (APHA et al., 1985). Liquor from the reactor was also plated for a total plate count. Selected samples were also tested for Nitrosomonas and Nitrobacter using the pour plate method with suitable media and incubated at 26°C over 21 days. Carbon granules were returned to the reactor after use. Characteristics of the pretreated aquaculture effluent from the food fish farm are shown in Table 1. Pretreatment consisted of coagulation, flocculation and clarification. AmmoTable 2 FBR nitrification Run
A B C D E F
at different NH,-N loadings
NH,-N area1 loading(g mm’ day- ‘)
NH,-N + NO,-N converted( g day-‘)
NO,-N produced( g Alkalinity day-‘) consumed(g day-‘)
0.02525 0.06305 0.20057 0.39736 0.89575 5.67522
0.01145 0.03784 0.16170 0.31712 0.47902 0.46397
0.01238 0.04566 0.11997 0.26160 0.50840 0.49615
- 0.02521 0.07578 0.90865 1.88289 2.88155 3.20097
CaCO,
DO consumed (g day-‘)
0.233 0.432 0.553 1.321 1.618 1.834
58
W.J. Ng et al. /Aquaculture
I39 (1996) 5542
nium sulphate was then added to produce different ammonia loadings. The pretreated aquaculture effluent was prepared fresh each day. Total NH,-N concentrations entering the reactor ranged from 0.019 to 4.271 mg 1-l. The area1 loading rates reported in Table 2 were obtained using the surface area of the activated carbon granules. The latter was obtained by estimating the number of granules in the reactor and then assuming each granule to be a sphere of 1.18 mm diameter. The specific area based on the external surface of the spheres was estimated to be 622 m-* rnp3. Samples for analysis were collected from the feed line just after the feed tank and from the aeration tank over 7-day periods for each experimental condition. Samples collected were centrifuged at 4°C for 10 min at 12 000 rev min.’ to determine dissolved parameters. Samples which could not be analysed immediately were stored at 4°C. An autoanalyser (Technicon TRAACS 800) was used to determine the concentrations of NH,-N, NO*-N (nitrite-nitrogen), N03-N (nitrate-nitrogen), Pod-P (orthophosphate) , hardness, alkalinity and chloride. Chemical oxygen demand (COD), 5-day biochemical oxygen demand (BOD,), total phosphorus (TP), total suspended solids (TSS), volatile suspended solids (VSS) and turbidity were determined in accordance with standard methods (APHA et al., 1985). Total Kjeldhal nitrogen (TKN) was determined by first digesting a sample in an alkaline solution of potassium peroxidisulphate in an autoclave for 30 min and then testing for nitrates on the autoanalyser. To determine the relationship between this value and the value obtained with standard methods, values so obtained were compared with TKN values determined according to standard methods (APHA et al., 1985) on the same samples.
3. Results The relationship relating the autoanalyser-obtained using the standard methods is:
TKN results to the results determined
Standard Method TKN = 0.9 1 X Modified Method TKN The number of samples (n) was 57, the correlation coefficient (2) 0.78, and the data range obtained was 0.8-4.1 mg 1-l Modified Method TKN. The TKN values reported in this paper were corrected using the factor obtained. In addition to the above and since many fish farmers in southeast Asia do not have easy access to laboratory testing facilities, a correlation between TSS and an easily monitored parameter such as turbidity would facilitate water quality monitoring. TSS values were correlated to turbidity readings and the following relationship obtained: TSS = 0.84 X turbidity The data range obtained was 5-60 mg l- ’ TSS (n = 64, ? = 0.87). Turbidity was measured in Formazin turbidity units (FTU) . The treatment performance in terms of nitrification is shown in Table 2. In all, six loading rates, identified as runs A-F, were investigated. A curve relating NH,-N removal to loading rate is shown in Fig. 2. NH4-N removal increased to a maximum of 82% when the loading reached 0.201 g me2 day-‘. This performance was maintained up to a loading of 0.397 g m ~’ day - ‘, after which it declined. Effluent NH4-N did not exceed 0.05 mg 1-l except
W.J. Ng et al. /Aquaculture
01
0
139 (1996) 55-62
59
I
I
1
I
I
1
2
3
L
5
LOAOlNG (g NH&-NlmZ/dayl Fig. 2. NH,-N removal at various NH,-N loadings.
-L
0
I
I
I
I
I
1
2
3
1
5
6
LOADING (g NH&-N/m*/dayl Fig. 3. Alkalinity
01
0
consumption
at various N&-N loadings.
I
I
I
I
I
1
2
3
1
5
LOADING 1g Fig. 4. DO consumption
J
6
NH&-Nlm’ldayl
at various NH,-N loadings.
when loading exceeded 0.397 g rnF2 day-‘. NH4-N levels were about 3.90 mg 1-l at 5.68 g m - 2 day - ’ loading. Data relating alkalinity and dissolved oxygen (DO) consumption to NH,-N conversion and loading are shown in Table 2 and Figs. 3 and 4, respectively.
60
W.J. Ng et al. /Aquaculture
Table 3 Microbiological
counts for FBR
Run
Total plate count
139 (1996) 55-62
Pour plate count Nitrosomonas
A B C D E F
Nitrobacter
AC”
MLb
AC
ML
AC
ML
25500 24000 22500 22000 58000 77500
2400 15100 5400 1200 8400 8200
NA NA NA NA 15000 28000
NA NA NA NA 1480 1400
NA NA NA NA 160000 520000
NA NA NA NA 2100 1300
“Activated carbon. hMixed liquor. Values given are the viable count. NA, not available.
Microbiological tests were intended to confirm that there were more viable colonies on the activated carbon samples than in the reactor’s mixed liquor (Table 3). This may be seen in the carbon granules pour plate counts for Nitrosomonas and Nitrobacter which were higher than the corresponding mixed liquor counts. A comparison between the total plate and pour plate counts suggested that nitrifiers dominated the microbial population in the reactor. With regard to the other pollutants present in the water (Table 4)) N02-N removal ranged between 7.6 and 55.9% showing a maximum removal rate between loadings of 0.013 and 0.026 g NO,-N m-* day-’ during Runs C and D. Total phosphorus removal was between 4.9 and 12.2%. TSS, COD and TKN removals were 11.8-18.2%, 5/l-11.3%, and 0.76.4%, respectively.
Table 4 Pollutant removal by the FBR Run
A B C D E F
NH,-N
TKN
NO*-N
TP
TSS
In*’
Out”
R
In
Out
R
In
Out
R
In
Out
R
In
Out
R
0.019 0.04 0.152 0.30 0.676 4.271
0.010 0.018 0.026 0.053 0.290 3.899
45.8 63.0 82.8 82.6 57.1 8.7
0.0045 0.0062 0.0100 0.0201 0.0230 0.0100
0.0039 0.0056 0.0044 0.0109 0.0212 0.0071
11.9 10.3 55.9 45.6 7.6 28.9
1.21 2.1 1.97 3.4 4.27 9.28
1.20 2.04 1.84 3.33 4.16 9.15
0.7 3.3 6.4 2.9 2.6 1.3
0.028 0.061 0.039 0.045 0.044 0.033
0.027 0.057 0.036 0.039 0.039 0.030
4.9 7.9 6.7 12.2 12.0 11.5
6.03 13.34 8.65 10.34 12.00 12.76
5.32 11.51 7.22 8.46 10.12 11.16
11.8 13.8 16.5 18.2 15.6 15.6
YZoncentrations (mg l- ‘) in the combined flow of feed and recycle to the FBR. “Concentrations (mg I-‘) in the effluent from the FBR. ‘Percentage removal.
W.J. Ng et al. /Aquaculture
139 (19%) 55-62
61
4. Discussion Apart from assimilation and cell synthesis, a large portion of the feed NH4-N was nitrified. The latter becomes progressively more dominant as the microbial population established itself in the reactor. In the FBR studied, this was obvious when the amount of N03-N formed was compared with the amount of NH,-N and NO,-N removed. NO,-N was included in the calculations because it was noted that a small portion of the feed’s NH,-N was converted to NO*-N even before it entered the reactor. Table 2 shows the N03-N formed, which almost equalled the NH,-N and NO*-N converted. Note that N02-N in the treated effluent was generally below 0.040 mg 1-l. Nitrification occurring in the FBR would have resulted in a drop in the feed’s alkalinity as it went through the reactor. Haug and McCarty ( 1972) had estimated that 7.13 kg of bicarbonate alkalinity (as CaC03) was required to neutralise the hydrogen ions produced during the oxidation of 1 kg of NH4-N. This study suggested that when the reactor reached maximum nitrification, approximately similar quantities of alkalinity were required (Fig. 3). Before maximum N03-N production was observed, DO consumed per unit mass of N03-N produced (Fig. 4) was much higher than the 4.W.6 expected. Alkalinity consumption was, however, lower than expected (Haug and McCarty, 1972). There was probably an ammonia substrate limitation at the lower loadings. DO, instead of being used primarily for nitrification, could then have been used for intrinsic cell maintenance activities. The latter would not have reduced alkalinity in the reactor markedly. The large difference between the plate counts of the FBR liquor and carbon granules suggested that the activated carbon granules were good supports to nitrifying bacteria. Numbers of the latter on the granules easily exceeded even the total numbers which were suspended in the reactor’s liquor. This is particularly useful because activated carbon granules have very large surface areas for cell attachment compared with other types of support media and would facilitate the development of high rate and compact reactor systems. Although the results indicated it was possible to convert substantial quantities of NH,N, removal of phosphorus had not been as good. This was not unexpected as the observed bacterial yield in the reactor had not been high and as such substantial phosphorus removal via microbial assimilation was not expected. Acknowledgements This project was supported by a grant (No. BM/86/ 14) from the National Science and Technology Board with funds from the Ministry of Trade and Industry, Singapore. The authors also acknowledge the support provided by Primary Industries Enterprise and the Environmental Engineering and Microbiology Laboratories, NUS and are indebted to Leslie Cheung and George Tay, PPD for their advice on fish husbandry. References APHA, AWWA and WPCF, 1985. Standard Methods for the Examination Public Health Association, Washington, DC, 1134 pp.
of Wastewater,
16th edn. American
62
W.J. Ng et al. /Aquaculture
139 (19%)
55-62
Haug, R.T. and McCarty, P.L., 1972. Nitrification with submerged filters. Water Pollut. Control Fed., 44: 20862102. Hilge, V. and Rakelmann, U.V., 1984. Laboratory scale experiments on the nitritication of fish tank effluent in a fixed film bed reactor. Spec. Publ. No. 8, Research on Aquaculture, European Mariculture Society, Bredene, Belgium, pp. 55-66. Kruner, G. and Rosenthal, H., 1983 Efficiency of nitrification intricklingfilters using different substrates. Aquacult. Eng., 2: 49-68. Liao, P.B. and Mayo, R.D., 1974. Intensified fish culture combining water reconditioning and pollution abatement. Aquaculture, 3: 61-85. Manci, W.E. and Quigley, J.T., 1981. Determination of opera&g parameter values for water reuse in aquaculture. Bio-Engineering Symposium for Fish Culture. Fish Culture Section, American Fisheries Society, pp. 97-103. Paller, M.H. and Lewis, W.M., 1988. Use of ozonation and fluidized bed biofilters for increased ammoniaremoval and fish loading rates. Prog. Fish Cult., 50: 141-147. Rakocy, J.E. and Allison, R., 198 1, Evaluation of a closed circulating system for the culture of tilapia and aquatic macrophytes. Bio-Engineering Symposium for Fish Culture, Fish Culture Section, AmericanFisheries Society, pp. 296-307.
Aquaculture
139 ( 1996) 55-62
Ammonia removal from aquaculture water by means of fluidised technology W.J. Ng a,*, Kevin Kho a, S.L. Ong a, T.S. Sim b, J.M. Hoc ” Department of Civil Engineering, National University of Singapore, Kent Ridge, Singapore 0511, Singapore ’ Department of Microbiology, National University of Singapore, Kent Ridge, Singapore 0511. Singapore ’ Primary Industries Enterprise, 100 Beach Road, 31-00 Shaw Towers, Singapore 0718, Singapore Accepted
14 August 1995
Abstract bed bioreactor was used to treat aquaculture effluent intended for recycling. r.Maximum NH,-N removal was 82% of influent concentration when loading was between 0.201 and 0.397 g mm2 day-‘; at this level, effluent NH,-N did not exceed 0.05 mg 1-l. A bench-scale
fluidised
The reactor was loaded with ammonia-nitrogen (NH,-N) from 0.025 to 5.675 g me2 day-
Kqvwords: Aquaculture
effluent; Ammonia;
Bioreactor;
Fluidised bed
1. Introduction
Biological wastewater treatment processes are commonly used to renovate water in closed recirculating systems in aquaculture. As ammonia is the primary toxic waste of fish, most of these processes are targeted at nitrification. The activated sludge process has been successfully used in this role. However, on the basis of equivalent space requirements, attached growth systems have often been found superior to suspended growth systems such as activated sludge (Liao and Mayo, 1974). Rakocy and Allison ( 1981) successfully used an attached growth system, the trickling filter, to culture Oreochromis aureus in a closed recirculating system. Manci and Quigley ( 198 1) showed that water quality was better in a recirculating system (for Percafluuescens) with an upflow biological filter than one with a trickling filter and settler. Kruner and Rosenthal ( 1983) had earlier also noted the poorer performance of the trickling filter when
presented with increasing ammonia and nitrite loadings. Clogging was, however, a recurring problem with the upflow filter (Hilge and Rakelmann, 1984). * Corresponding
author
Elsevier Science B.V. SSDIOO44-8486(95)01153-6
56
W. J. Ng et al. / Aquaculture
139 (1996) 55-62
The fluidised bed bioreactor is in many ways a hybrid between the attached and suspended growth systems. Paller and Lewis ( 1988), using granular activated carbon, achieved ammonia removal rates of 414-1004 g mm3 day- ‘. The system was used to support a culture of common carp reared at 20°C. The system has, however, not been as extensively investigated as the trickling or upflow filters. This paper reports a study which investigated the fluidised bed bioreactor (FBR) in the treatment of aquaculture effluent intended for recycling.
2. Materials and methods The FBR was fabricated from a 5.4 cm internal diameter plexiglass column of length 201.5 cm. A wire gauze of 0.09 cm* mesh was used to support a bed of activated carbon granules. Beneath the wire gauze was a transition zone (40 cm length) which separated the inlet from the carbon bed. This was to reduce the occurrence of channelling in the latter. Pretreated aquaculture effluent was pumped (67 ml min-‘) from the pretreated effluent tank and this merged with the recycle flow (850 ml mini) from the aeration tank. The total stream entered the reactor from the bottom, while effluent overflowed near the top into the bottom of the aeration tank (2.3 cm diameter with 13 cm of water height) via a 2 cm diameter hose. Within the aeration tank, air was supplied through diffusers at a rate of 2.5 1 min- ‘. A flow equal to the feed flow was allowed to overflow from the aeration tank. This overflow constituted the net treated effluent leaving the system. Fig. I is a schematic diagram of the reactor used. The FBR was located in the laboratory of an aquaculture farm. The ambient temperature in the laboratory ranged from 24 to 28°C. Granular activated carbon (NORIT PKI-3) was first sieved through a 1.18 mm sieve and collected on a 1.OOmm sieve. Sieved carbon was washed with distilled water until the water was clear of carbon fines and then dried at 103°C. The reactor was filled to a depth
AIR
,
r--i
r ‘;
EXPANOEO CARBON BE0
.:: :’ ‘.’ ‘..
WIRE GAUZE
FEE0 PUMP Fig. 1. Schematic
RECYCLE PUMP
diagram of the experimental
fluidised bed reactor.
W.J. Ng et al. /Aquaculture I39 (1996) 55-62 Table 1 Average characteristics
of pretreated
57
water used to prepare FBR feeds
Parameter’
Concentration(mg
NH,-N NO?-N NOT-N PO,-P Alkalinity Hardness Chloride TSS vss BOD COD TKN TP
0.04 0.12 1.22 0.01 60 12 29 15.9 13.7 2.3 27.3 1.7 0.12
1-l)
“TSS, total suspended solids; VSS, volatile suspended solids; BOD, biochemical oxygen demand; TKN, total Kjeldhal nitrogen; TP, total phosphorus.
SD
?I
0.02 0.16 0.53 0.01 11 12 9.7 7.7 6.5 0.6 12.3 8 0.19
64 64 59 53 60 49 60 22 22 26 22 22 25
oxygen demand; COD, chemical
(static bed height) of 49.5 cm but during operation the bed was expanded by 32%. The reactor was seeded with nitrifiers using a commercial preparation (Interbio Ltd., Dublin) suitable for fish culture. Over a 3 day period, 9 g of the bacteria preparation were diluted into 150 1 of feed water daily and this was then pumped into the reactor. The reactor was allowed to stabilise over 3 months. To estimate the amount of bacteria attached to the carbon granules, 1 g of activated carbon granules was withdrawn from the reactor periodically and vigorously agitated in 9 ml of sterile water for 3 min using a vortex mixer, The resulting liquid was then used for plate counts with nutrient agar at 35°C and 48 h incubation (APHA et al., 1985). Liquor from the reactor was also plated for a total plate count. Selected samples were also tested for Nitrosomonas and Nitrobacter using the pour plate method with suitable media and incubated at 26°C over 21 days. Carbon granules were returned to the reactor after use. Characteristics of the pretreated aquaculture effluent from the food fish farm are shown in Table 1. Pretreatment consisted of coagulation, flocculation and clarification. AmmoTable 2 FBR nitrification Run
A B C D E F
at different NH,-N loadings
NH,-N area1 loading(g mm’ day- ‘)
NH,-N + NO,-N converted( g day-‘)
NO,-N produced( g Alkalinity day-‘) consumed(g day-‘)
0.02525 0.06305 0.20057 0.39736 0.89575 5.67522
0.01145 0.03784 0.16170 0.31712 0.47902 0.46397
0.01238 0.04566 0.11997 0.26160 0.50840 0.49615
- 0.02521 0.07578 0.90865 1.88289 2.88155 3.20097
CaCO,
DO consumed (g day-‘)
0.233 0.432 0.553 1.321 1.618 1.834
58
W.J. Ng et al. /Aquaculture
I39 (1996) 5542
nium sulphate was then added to produce different ammonia loadings. The pretreated aquaculture effluent was prepared fresh each day. Total NH,-N concentrations entering the reactor ranged from 0.019 to 4.271 mg 1-l. The area1 loading rates reported in Table 2 were obtained using the surface area of the activated carbon granules. The latter was obtained by estimating the number of granules in the reactor and then assuming each granule to be a sphere of 1.18 mm diameter. The specific area based on the external surface of the spheres was estimated to be 622 m-* rnp3. Samples for analysis were collected from the feed line just after the feed tank and from the aeration tank over 7-day periods for each experimental condition. Samples collected were centrifuged at 4°C for 10 min at 12 000 rev min.’ to determine dissolved parameters. Samples which could not be analysed immediately were stored at 4°C. An autoanalyser (Technicon TRAACS 800) was used to determine the concentrations of NH,-N, NO*-N (nitrite-nitrogen), N03-N (nitrate-nitrogen), Pod-P (orthophosphate) , hardness, alkalinity and chloride. Chemical oxygen demand (COD), 5-day biochemical oxygen demand (BOD,), total phosphorus (TP), total suspended solids (TSS), volatile suspended solids (VSS) and turbidity were determined in accordance with standard methods (APHA et al., 1985). Total Kjeldhal nitrogen (TKN) was determined by first digesting a sample in an alkaline solution of potassium peroxidisulphate in an autoclave for 30 min and then testing for nitrates on the autoanalyser. To determine the relationship between this value and the value obtained with standard methods, values so obtained were compared with TKN values determined according to standard methods (APHA et al., 1985) on the same samples.
3. Results The relationship relating the autoanalyser-obtained using the standard methods is:
TKN results to the results determined
Standard Method TKN = 0.9 1 X Modified Method TKN The number of samples (n) was 57, the correlation coefficient (2) 0.78, and the data range obtained was 0.8-4.1 mg 1-l Modified Method TKN. The TKN values reported in this paper were corrected using the factor obtained. In addition to the above and since many fish farmers in southeast Asia do not have easy access to laboratory testing facilities, a correlation between TSS and an easily monitored parameter such as turbidity would facilitate water quality monitoring. TSS values were correlated to turbidity readings and the following relationship obtained: TSS = 0.84 X turbidity The data range obtained was 5-60 mg l- ’ TSS (n = 64, ? = 0.87). Turbidity was measured in Formazin turbidity units (FTU) . The treatment performance in terms of nitrification is shown in Table 2. In all, six loading rates, identified as runs A-F, were investigated. A curve relating NH,-N removal to loading rate is shown in Fig. 2. NH4-N removal increased to a maximum of 82% when the loading reached 0.201 g me2 day-‘. This performance was maintained up to a loading of 0.397 g m ~’ day - ‘, after which it declined. Effluent NH4-N did not exceed 0.05 mg 1-l except
W.J. Ng et al. /Aquaculture
01
0
139 (1996) 55-62
59
I
I
1
I
I
1
2
3
L
5
LOAOlNG (g NH&-NlmZ/dayl Fig. 2. NH,-N removal at various NH,-N loadings.
-L
0
I
I
I
I
I
1
2
3
1
5
6
LOADING (g NH&-N/m*/dayl Fig. 3. Alkalinity
01
0
consumption
at various N&-N loadings.
I
I
I
I
I
1
2
3
1
5
LOADING 1g Fig. 4. DO consumption
J
6
NH&-Nlm’ldayl
at various NH,-N loadings.
when loading exceeded 0.397 g rnF2 day-‘. NH4-N levels were about 3.90 mg 1-l at 5.68 g m - 2 day - ’ loading. Data relating alkalinity and dissolved oxygen (DO) consumption to NH,-N conversion and loading are shown in Table 2 and Figs. 3 and 4, respectively.
60
W.J. Ng et al. /Aquaculture
Table 3 Microbiological
counts for FBR
Run
Total plate count
139 (1996) 55-62
Pour plate count Nitrosomonas
A B C D E F
Nitrobacter
AC”
MLb
AC
ML
AC
ML
25500 24000 22500 22000 58000 77500
2400 15100 5400 1200 8400 8200
NA NA NA NA 15000 28000
NA NA NA NA 1480 1400
NA NA NA NA 160000 520000
NA NA NA NA 2100 1300
“Activated carbon. hMixed liquor. Values given are the viable count. NA, not available.
Microbiological tests were intended to confirm that there were more viable colonies on the activated carbon samples than in the reactor’s mixed liquor (Table 3). This may be seen in the carbon granules pour plate counts for Nitrosomonas and Nitrobacter which were higher than the corresponding mixed liquor counts. A comparison between the total plate and pour plate counts suggested that nitrifiers dominated the microbial population in the reactor. With regard to the other pollutants present in the water (Table 4)) N02-N removal ranged between 7.6 and 55.9% showing a maximum removal rate between loadings of 0.013 and 0.026 g NO,-N m-* day-’ during Runs C and D. Total phosphorus removal was between 4.9 and 12.2%. TSS, COD and TKN removals were 11.8-18.2%, 5/l-11.3%, and 0.76.4%, respectively.
Table 4 Pollutant removal by the FBR Run
A B C D E F
NH,-N
TKN
NO*-N
TP
TSS
In*’
Out”
R
In
Out
R
In
Out
R
In
Out
R
In
Out
R
0.019 0.04 0.152 0.30 0.676 4.271
0.010 0.018 0.026 0.053 0.290 3.899
45.8 63.0 82.8 82.6 57.1 8.7
0.0045 0.0062 0.0100 0.0201 0.0230 0.0100
0.0039 0.0056 0.0044 0.0109 0.0212 0.0071
11.9 10.3 55.9 45.6 7.6 28.9
1.21 2.1 1.97 3.4 4.27 9.28
1.20 2.04 1.84 3.33 4.16 9.15
0.7 3.3 6.4 2.9 2.6 1.3
0.028 0.061 0.039 0.045 0.044 0.033
0.027 0.057 0.036 0.039 0.039 0.030
4.9 7.9 6.7 12.2 12.0 11.5
6.03 13.34 8.65 10.34 12.00 12.76
5.32 11.51 7.22 8.46 10.12 11.16
11.8 13.8 16.5 18.2 15.6 15.6
YZoncentrations (mg l- ‘) in the combined flow of feed and recycle to the FBR. “Concentrations (mg I-‘) in the effluent from the FBR. ‘Percentage removal.
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4. Discussion Apart from assimilation and cell synthesis, a large portion of the feed NH4-N was nitrified. The latter becomes progressively more dominant as the microbial population established itself in the reactor. In the FBR studied, this was obvious when the amount of N03-N formed was compared with the amount of NH,-N and NO,-N removed. NO,-N was included in the calculations because it was noted that a small portion of the feed’s NH,-N was converted to NO*-N even before it entered the reactor. Table 2 shows the N03-N formed, which almost equalled the NH,-N and NO*-N converted. Note that N02-N in the treated effluent was generally below 0.040 mg 1-l. Nitrification occurring in the FBR would have resulted in a drop in the feed’s alkalinity as it went through the reactor. Haug and McCarty ( 1972) had estimated that 7.13 kg of bicarbonate alkalinity (as CaC03) was required to neutralise the hydrogen ions produced during the oxidation of 1 kg of NH4-N. This study suggested that when the reactor reached maximum nitrification, approximately similar quantities of alkalinity were required (Fig. 3). Before maximum N03-N production was observed, DO consumed per unit mass of N03-N produced (Fig. 4) was much higher than the 4.W.6 expected. Alkalinity consumption was, however, lower than expected (Haug and McCarty, 1972). There was probably an ammonia substrate limitation at the lower loadings. DO, instead of being used primarily for nitrification, could then have been used for intrinsic cell maintenance activities. The latter would not have reduced alkalinity in the reactor markedly. The large difference between the plate counts of the FBR liquor and carbon granules suggested that the activated carbon granules were good supports to nitrifying bacteria. Numbers of the latter on the granules easily exceeded even the total numbers which were suspended in the reactor’s liquor. This is particularly useful because activated carbon granules have very large surface areas for cell attachment compared with other types of support media and would facilitate the development of high rate and compact reactor systems. Although the results indicated it was possible to convert substantial quantities of NH,N, removal of phosphorus had not been as good. This was not unexpected as the observed bacterial yield in the reactor had not been high and as such substantial phosphorus removal via microbial assimilation was not expected. Acknowledgements This project was supported by a grant (No. BM/86/ 14) from the National Science and Technology Board with funds from the Ministry of Trade and Industry, Singapore. The authors also acknowledge the support provided by Primary Industries Enterprise and the Environmental Engineering and Microbiology Laboratories, NUS and are indebted to Leslie Cheung and George Tay, PPD for their advice on fish husbandry. References APHA, AWWA and WPCF, 1985. Standard Methods for the Examination Public Health Association, Washington, DC, 1134 pp.
of Wastewater,
16th edn. American
62
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Haug, R.T. and McCarty, P.L., 1972. Nitrification with submerged filters. Water Pollut. Control Fed., 44: 20862102. Hilge, V. and Rakelmann, U.V., 1984. Laboratory scale experiments on the nitritication of fish tank effluent in a fixed film bed reactor. Spec. Publ. No. 8, Research on Aquaculture, European Mariculture Society, Bredene, Belgium, pp. 55-66. Kruner, G. and Rosenthal, H., 1983 Efficiency of nitrification intricklingfilters using different substrates. Aquacult. Eng., 2: 49-68. Liao, P.B. and Mayo, R.D., 1974. Intensified fish culture combining water reconditioning and pollution abatement. Aquaculture, 3: 61-85. Manci, W.E. and Quigley, J.T., 1981. Determination of opera&g parameter values for water reuse in aquaculture. Bio-Engineering Symposium for Fish Culture. Fish Culture Section, American Fisheries Society, pp. 97-103. Paller, M.H. and Lewis, W.M., 1988. Use of ozonation and fluidized bed biofilters for increased ammoniaremoval and fish loading rates. Prog. Fish Cult., 50: 141-147. Rakocy, J.E. and Allison, R., 198 1, Evaluation of a closed circulating system for the culture of tilapia and aquatic macrophytes. Bio-Engineering Symposium for Fish Culture, Fish Culture Section, AmericanFisheries Society, pp. 296-307.