Relevant water-parameters in the tanks were examined. Weekly measurements of temperature, pH, oxygen, conductivity, NH
4 +
, NO
2 −
, NO
3 −
, PO
4 3 −
were conducted. Occasionally the dissolved organic carbon DOC concentration was determined.
The water volume added for compensation of evaporation was taken into account. A simple model for evaluating the performance of the denireactors with different
polymer packing has been used after the start-up and beyond the lag-time periods of these units. The influence of the oxygen has not yet been taken into account as
well as NO
2 −
was not included into the model. The NO
3 −
concentration in the aquarium, considered as complete mixed tank, as function of time can be described as follows:
dc
A
dt = Q
D
c
E
− c
A
+ m
NO
3
V
A
5 The lowest concentration in the aquarium c
A0
to be achieved under steady state conditions, i.e. with an effluent concentration of the denireactor c
E
= 0 is deter-
mined by the relation: c
A0
= m
NO
3
Q
D
6 where c
A
is the c
A0
NO
3 −
conc., aquarium tank mgl N-NO
3 −
; c
E
is the NO
3 −
conc., effluent denireactor mgl N-NO
3 −
; m
NO
3
is the daily production of NO
3 −
in system mgday N-NO
3 −
; Q
D
is the recirculation rate = throughput denireactor lh; and V
A
is the water volume of aquarium tank l. The overall volumetric denitrification performance r
DV
in mgL h N-NO
3 −
of a denireactor is given by Eq. 7.
r
DV
= Q
D
c
E
− c
A
V
D
7 r
DV
is the overall volumetric denitrification rate of a denireactor mgl h N-NO
3 −
; and V
D
is the denireactor volume l.
3. Results
Due to a preconditioning of the biofilters, ammonium and nitrite concentrations were low during the whole test-periods 1 and 2. NH
4 +
did not exceed 0.1 mgl N-NH
4 +
, NO
2 −
was below 0.05 mgl N-NO
2 −
after the first day. Temperature was stable in a range of 25.1 – 26.1°C. DOC values increased slowly during the tests,
beginning with 3 – 4 mgl they did not exceed 5 – 7 mgl at the end of the tests. In period 1 NO
3 −
concentrations increased in all four aquaria in a very similar way Fig. 4. In this period and from the reference aquarium system, the daily
production of nitrate could be calculated to 56.1 9 5 mgday N-NO
3 −
. Denitrification with PHB started 8 days after installation of the unit following a
lag-time = period of adaptation of denitrifying microorganisms. The lag-time of PCL and Bionolle was 16 days. It was defined as the point when the steepest
negative slope of the nitrate concentration versus time occurred. This was an indication of the unit to operate at its maximum Fig. 4 and Table 2. For Bionolle
two phases of activity could be observed, an explanation cannot yet be given.
80
A .
Boley et
al .
Aquacultural
Engineering
22 2000
75 –
85
Table 2 Estimated maximal denitrification velocities of tested materials
Specific surface Concentration range 5–40 mgl N-NO
3 −
Temp. °C Solid substrate
Flowrate lh m
2
l Surface related rates mg N-NO
3 −
m
2
h Volumetric rates mg N-NO
3 −
l h
0.4–0.6 1.49
20–25 PHB
7–41 5–28
0.87 20–160
20–25 0.2–0.3
PCL 21–166
0.3–0.6 1.5–10; 12–77
1.3–9; 10.5–67 -Bionolle, period 1
1.22 20–25
and 2
As Fig. 4 shows the theoretical concentration limits about 5 mgl, Eq. 6 have approximately been achieved with PCL and Bionolle at the end of test. Nitrate
concentrations in the effluent of these denireactors were below the detection limit 0.23 mgl N-NO
3 −
. This confirmed our assumptions. In contrast to these results the aquarium with the PHB denireactor reached the
equilibrium already at a concentration of 18 mgl N-NO
3 −
. This decrease of performance = decrease of denitrification velocity can probably be explained by
clogging and short-circuiting of the denireactor due to excess biomass production, which has been observed after the end of period 2.
As the acid neutralizing capacity of the tap-water was low ANC = 1 mmoll, pH values decreased with time, due to nitrification Fig. 5. To prevent extensive
decrease of pH, it was adjusted twice with NaHCO
3
, which was added to the reference aquarium packing with glass beads at days 71 and 100. For the
aquarium with the PHB denireactor NaHCO
3
addition was not necessary because at day 71 denitrification had already started. The start of denitrification immedi-
Fig. 4. Nitrate concentrations in testsystems. Temperature: 25 – 26°C.
Fig. 5. pH in testsystems. Temperature 25 – 26°C. Arrows indicate pH adjustment with NaHCO
3
. After 71 days: Reference, PCL, Bionolle; after 100 days: only Reference.
82
A .
Boley et
al .
Aquacultural
Engineering
22 2000
75 –
85
Table 3 Denitrification velocities in fixed bed reactors with different substrates
Temp. °C Volumetric rate mg
Carrier-material Surface related rate mg N-
Substrate Type of water and in-
Spec. surface stallation
N-NO
3 −
l h
NO
3 −
m
2
h m
2
l 145
97 Wastewater, labora-
Methanol Sand
12 1.5
tory-scale
a
Drinking water plant
b
14–34 10–26
12 Burned clay
Acetic Acid 1.3
0.9 54–66
Drinking water plant
c
Ethanol 12–13
Burned clay 49–59
11 1.55
Tap water, labora- PHB
10 16
PHB tory-scale
d
Tap water, labora- 14
22 15
1.6 PHB
PHB tory-scale
e
1.2 10
Tap water, labora- PCL
15 PCL
13 tory-scale
e
Fluidized bed, aqua- d = 0.3–0.9 mm
Sand Dissolved organic
22.5–27 36
substrates culture system
f
Marine closed aqua- 100
20 Brick granules
Ethanol 2.2
culture system
g a
Hawkins et al., 1978.
b
Partos and Richard, 1984.
c
Jestin et al., 1986.
d
Wurmthaler, 1995.
e
Schick, 1998.
f
Arbiv and Rijn, 1995.
g
Sautier et al., 1998.
Table 4 Estimated costs of substrates for nitrate removal
Price of substrate Substrate
Consumption of substrate kg Costs of denitrification
€kg substrate €kg N-NO
3 −
substratekg N-NO
3 −
Methanol: 1.00
2.08–3.98 2.0–4.0
CH
3
OH 2.0
1.20 2.4
Ethanol: C
2
H
5
OH 3.5
2.40 8.0
Acetic acid: CH
3
COOH 6.6–8.9
PCL C
6
H
10
O
2 n
5.00 1.33–1.77
21.0–37.2 2.1–2.7
10.00 PHB C
4
H
6
O
2 n
Bionolle c 6010 Commercially not available
C
6
H
4
O
2 n
ately may lead to an increase of pH. For the PCL and Bionolle denireactor NaHCO
3
was also added at day 71, because denitrification had not yet started. Later pH increased too, therefore an adjustment was no more required. These
results are compatible with Eq. 4. After both test-periods the fish were in a good condition and no fish died. They
almost doubled their initial body weight all together up to 145 g 9 5 per aquarium.
4. Discussion
