Aquacultural Engineering 22 2000 75 – 85
Biodegradable polymers as solid substrate and biofilm carrier for denitrification in recirculated
aquaculture systems
A. Boley , W.-R. Mu¨ller, G. Haider
Uni6ersita¨t Stuttgart, Institut fu¨r Siedlungswasserbau, Wassergu¨te- und Abfallwirtschaft, Arbeitsbereich Biologie, Bandta¨le
2
, D-
70569
Stuttgart, Germany
Abstract
A simple process for nitrate removal is proposed for its application in aquaculture. Biodegradable polymer pellets are acting as solid substrate and biofilm carrier for denitrifica-
tion. Laboratory experiments with conventional aquaria and fish were used to examine the feasibility and a first evaluation of the process performance in a recirculated aquaculture
system. All over the test-period the fish were in a good condition. Nitrate concentrations in the aquaria with treatment were low compared to the untreated reference system. A further
advantage was the stability of the pH in the units with denitrification whereas pH of the untreated water decreased due to nitrification. © 2000 Elsevier Science B.V. All rights
reserved.
Keywords
:
Water treatment; Recirculating systems in aquaculture; Denitrification; Biodegradable poly- mers; Solid substrates
www.elsevier.nllocateaqua-online
1. Introduction
In aquaculture systems nitrate removal is a problem which has not always found satisfactory solutions in practice. Modern technology of water treatment in recircu-
lating systems consists of solid waste removal, carbon-removal and nitrification, pH and CO
2
control Fig. 1. Consumption of energy and water in those systems can be lowered if the nitrate produced in the aerobic biofilter unit is reduced by a
denitrification step. This diminishes the fresh water addition and the amount and impact of the wastewater.
Corresponding author. Tel.: + 49-711-6855441; fax: + 49-711-6853729. E-mail address
:
angela.boleyiswa.uni-stuttgart.de A. Boley 0144-860900 - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 1 4 4 - 8 6 0 9 0 0 0 0 0 3 3 - 9
Denitrification is defined as the biological nitrate reduction sequence NO
3 −
NO
2 −
N
2
O N
2
. We restrict the discussion to the heterotrophic biological process where organisms gain energy and carbon from organic compounds. A conventional
technique is to add an organic carbon source e.g. ethanol, acetic acid to a denitrification reactor Frick and Richard, 1985; Sto¨ver and Roennefahrt, 1990.
The disadvantage of this treatment process is the need of a close, rather sophisti- cated and costly process control, the risk of overdosing and a deepened knowledge
about the operation of this biological system.
In contrast to conventional treatment units, denitrification with biodegradable polymers presented here is a simple process. Microorganisms use the biopolymer in
form of pellets as biofilm carrier and simultaneously as water insoluble carbon source for denitrification, which is accessible only by enzymatic attack Mu¨ller et
al., 1992; Wurmthaler, 1995.
The scheme in Fig. 2 elucidates the difference between conventional denitrifica- tion and the new process presented here. In conventional denitrification with a fixed
bed reactor a biofilm will grow on the inert carrier and denitrification takes place whenever the water contains NO
3 −
, soluble organic substrate and trace elements. End-products are N
2
, H
2
O, CO
2
and biomass. The new system with biodegradable
Fig. 1. Scheme of a modern recirculated aquaculture system.
Fig. 2. Denitrification processes with different organic substrates.
polymers does not require an external dosing of soluble organic substrate as the polymer itself acts as biofilm carrier and organic carbon source.
Heterotrophic denitrification positively influences the pH of the water. If proteins are metabolized by fish, the end-products of respiration after hydrolysis to amino
acids e.g. glycine are NH
4 +
and HCO
3 −
, which are excreted via gills Eq. 1; Forster and Goldstein, 1969:
NH
2
CH
2
COOH + 1.5 O
2
NH
4 +
+ HCO
3 −
+ CO
2
1 The nitrification equation with biomass formation Wheaton et al., 1994, Eq. 2
indicates the production of protons catalyzed by enzymes of, e.g. Nitrosomonas and Nitrobacter species:
1.021 NH
4 +
+ 1.895 O
2
+ 1.021 HCO
3 −
0.021 C
5
H
7
O
2
N + NO
3 −
+ 1.979 H
2
O + 0.914 CO
2
+ H
+
2 Decreasing pH values have to be coped with by adding, e.g. NaHCO
3 −
. The use of a biodegradable polymer as organic carbon substrate, e.g. PHB, leads
to biomass, carbon dioxide and simultaneous reduction of nitrate to elementary nitrogen. With a yield coefficient of 0.45 g biomassg PHB assumed Heinemann,
1995, the summarized denitrification equation including biomass formation can be given as:
0.494 C
4
H
6
O
2
+ NO
3 −
0.130 CO
2
+ HCO
3 −
+ 0.415 N
2
+ 0.169 C
5
H
7
O
2
N + 0.390 H
2
O 3
The summary equation nitrification and denitrification results in: 1.021 NH
4 +
+ 1.021 HCO
3 −
+ 1.895 O
2
+ 0.494 C
4
H
6
O
2
3.369 H
2
O + 2.044 CO
2
+ 0.415 N
2
+ 0.190 C
5
H
7
O
2
N 4
CO
2
produced can be stripped by aeration. If all the nitrate produced is denitrified, the pH remains constant.
2. Materials and methods
