triplicate samples were derived from the fluidized bed reactor 200 g colonized sand, from the trickling filter 36 cm
2
of colonized PVC, from the digestion basin 3 ml sludge and from the water in the treatment system 2 l. Dry weight and total
phosphorus content of the organic matter present in the samples were determined. Total phosphorus in each of the treatment compartments was calculated based on
the following information: total sand dry weight in fluidized bed reactor, 800 g; total surface area of PVC in trickling filter, 240 m
2
; total sludge dry weight in digestion basin, 8.8 kg; total water volume in treatment system, 1000 l.
2
.
5
. Analytical procedures Inorganic nutrients were determined in GFC Whatman, UK filtered samples.
Total ammonia NH
3
and NH
4 +
was determined as described by Scheiner 1976, nitrite according to Strickland and Parsons 1968 and nitrate was measured with
the Szechrome NAS reagent Ben Gurion University, Applied Research Institute or, in laboratory batch experiments, with a specific nitrate electrode Radiometer,
Denmark amplified with a pH meter Radiometer, model: PHM92. Inorganic orthophosphate phosphate throughout the text in filtered samples and total
phosphorus organic, particulate and inorganic orthophosphate in unfiltered sam- ples was determined with the ascorbic acid method described by Golterman et al.
1978.
Oxygen and temperature were measured with a YSI model 57 temperatureoxy- gen probe Yellow Springs Instruments, USA.
Bacterial dry weight and dry weight of organic matter obtained from the various components of the treatment system were determined after overnight drying of the
samples at 105°C. Protein was determined according to Lowry et al. 1951 with bovine serum albumin as standard.
3. Results
3
.
1
. Inorganic nitrogen and phosphorus concentrations in the experimental treatment system
Despite the closed-mode of operation and daily feed supply, inorganic nitrogen and phosphate concentrations in the experimental system determined in samples
obtained from the trickling filter basin, did not accumulate over the 210 days of operation Fig. 2. Oxygen was at saturation in the trickling filter basin while it was
undetectable in the digestion basin and fluidized bed reactor not shown. The pH of the system fluctuated between 6.9 and 7.6 not shown.
Due to the relatively short retention times, differences between inlet and outlet concentrations of inorganic nitrogen and phosphate in each of the treatment
compartments Fig. 3 were often below the analytical limit of detection. However,
Fig. 2. Ammonia, nitrite, nitrate and phosphate concentrations in the experimental treatment system during 210 days of operation.
Fig. 3. Removal positive values or production negative values of ammonia, nitrite, nitrate and phosphate by the fluidized bed reactor F.B.R., trickling filter T.F. and digestion basin D.B. over the
experimental period.
it is evident that ammonia was removed in the trickling filter and the fluidized bed reactor and produced in the digestion basin. Nitrate was produced in the trickling
filter, removed in the fluidized bed reactor and removed or produced in the digestion basin. The fluidized bed reactor removed phosphate while it was removed
or produced in the trickling filter and digestion basin.
A qualitative analysis revealed that samples derived from the fluidized bed reactor were high in phosphorus content as compared to samples from other
treatment compartments Table 1. Absolute values of total phosphorus were highest in the trickling filter and much lower in the digestion basin, fluidized bed
reactor and in the water body of the system Table 1. The total phosphorus load of the system was 576 g P 144 feeding days × 4 g P. We estimated that during the
experimental period 16 g P was removed with water exchange and 49 g P with the wasted biomass from the fluidized bed column. Therefore, the total expected
phosphorus content of the system over the experimental period was 511 g P. Based on results presented in Table 1, this implies that as much as 91 9 8 of the added
phosphorus was retained within the trickling filter, 0.7 9 0.1 in the fluidized bed reactor, 4.7 9 1.9 in the digestion basin and 2.2 9 0.5 in the water of the system.
3
.
2
. Phosphate accumulation by denitrifying and nitrifying consortia A denitrifying consortium derived from the fluidized bed reactor was incubated
under laboratory conditions in the presence or absence of nitrate Fig. 4. Phos- phate uptake took place in the presence of nitrate whereas after depletion of nitrate
from the medium, phosphate was released. In the presence of nitrate, the consor- tium assimilated ammonia and phosphate at a molar NP ratio of 1.9. Taking into
account that the molar NP ratio of bacterial biomass varies from 5 to 16 Brock and Madigan, 1991, it can be concluded that in the presence of nitrate, phosphate
is assimilated in excess of the metabolic requirements of the bacteria comprising the consortium.
A nitrifying consortium derived from the trickling filter was incubated in the laboratory under aerobic nitrifying and anoxic denitrifying conditions. Anoxic
incubation was conducted in the presence of acetate. Under aerobic conditions Fig. 5A, ammonia was nitrified to nitrate while phosphate concentrations in the
medium increased gradually. Trickling filter material was rich in organic matter. Degradation of this organic matter and ammonification of nitrogenous organic
compounds to ammonia probably explains the deficit observed between nitrate production and ammonia consumption. Incubation of trickling filter material in the
absence of oxygen resulted in a decrease of nitrate and phosphate concentrations in the medium Fig. 5B.
Table 1 Absolute and relative phosphorus content 9 S.D. in different compartments of the experimental
treatment system after 210 days of operation Compartment
Total phosphorus g Phosphorus in organic matter mgg dry weight
Fluidized bed reac- 118.5 9 5.3
3.5 9 0.3 tor
Trickling filter 464.7 9 40.0
18.6 9 5.2 2.4 9 1.1
Digestion basin 24.2 9 9.7
11.0 9 2.6 Water body
9.0 9 1.5
Fig. 4. Changes in nitrate and phosphate concentrations during batch incubation of a denitrifying consortium derived from the fluidized bed reactor. Arrow indicates time of nitrate addition.
3
.
3
. Phosphate accumulation by denitrifying isolates All three isolates assimilated phosphate in excess under denitrifying conditions
i.e. in the presence of nitrate and released phosphate when nitrate became depleted Fig. 6. Molar NP ratios of the three different isolates at the end of the
denitrifying period were 0.4, 2.1 and 1.3 for P. aeruginosa, P. denitrificans and Pseudomonas sp. JR12, respectively.
3
.
4
. Phosphate accumulation and nitrate remo6al in the fluidized bed reactor The performance of the fluidized bed reactor on selected days throughout the 210
days experimental period are presented in Table 2. It is shown that the denitrifying consortium present in this reactor, assimilated ammonia and phosphate at a molar
NP ratio ranging from 0.5 to 2.4; i.e. phosphate accumulation by this consortium was in excess of the metabolic requirements. With undetectable low inlet concentra-
tions of nitrate and nitrite days 45 and 46, phosphate was released as indicated by the negative phosphate removal values. The latter observation points to the fact
that only under denitrifying conditions, i.e. in the presence of nitrate, the denitrify- ing consortium was capable of phosphate uptake in excess of metabolic
requirements.
Finally, the significance of anoxic treatment fluidized bed reactor and digestion basin with respect to phosphate removal was demonstrated by disconnecting this
stage from the aerobic trickling filter treatment stage Fig. 7. Without supply of nitrate-rich water, levels of nitrate dropped rapidly in the digestion basin. Under
the resulting anaerobic conditions, biologically-stored phosphorus was released as can be seen from the rapid increase in phosphate concentrations in the medium.
130
Y .
Barak ,
J .
6 an
Rijn Aquacultural
Engineering
22 2000
121 –
136
Table 2 Daily removal rates per surface area of carrier of inorganic nitrogen and phosphate by the fluidized bed reactor and calculated ratios between ammonia
and phosphate removal at selected sampling days
a
Day from start of operation NH
4
mmol m
− 2
day
− 1
PO
4
mmol m
− 2
day
− 1
NH
4
PO
4
NO
3
mmol m
− 2
NO
2
mmol m
− 2
day
− 1
day
− 1
53 0.6
43 −
92 274
31 44
0.5 66
167 32
− 64
− 0.2
2 −
10 45
46 70
− 53
− 1.3
21 1.1
20 79
333 2.5
320 135
60 24
85 36
1.4 108
86 230
50 134
485 −
124 36
26 1.4
148 0.8
392 56
− 14
68 153
571 −
10 36
15 2.4
a
Negative values indicate accumulation.
Subsequent reconnection of the water supply was followed by a nitrate increase and a phosphate decrease in the anoxic treatment stage.
4. Discussion
