Subsequent reconnection of the water supply was followed by a nitrate increase and a phosphate decrease in the anoxic treatment stage.
4. Discussion
Information on phosphorus dynamics in recirculating fish culture systems is scarce. As determined in more conventional fish culture systems, phosphorus
recovery values by fish vary from 10 to 30 of the phosphorus added in the feed Avnimelech and Lacher, 1979; Boyd, 1985; Schroeder et al., 1991. Of the released
phosphorus, roughly 20 is in the soluble form while the remainder is present in
Fig. 5. Changes in ammonia , nitrite , nitrate
and phosphate concentrations during aerobic A and anoxic B batch incubation of a bacterial consortium derived from the trickling filter.
Anoxic incubation was conducted in the presence of acetate.
Fig. 6. Changes in nitrite , nitrate and phosphate
concentrations during batch incubation of: A P. aeruginosa, B P. denitrificans and C Pseudomonas sp. JR12. Arrow indicates time of
nitrate addition.
Fig. 7. Changes in nitrate and phosphate concentrations in the digestion basin after uncoupling nitrate off and reconnecting nitrate on the water supply from the trickling filter to the digestion basin.
the organic sludge Bodvin et al., 1996. Chen et al. 1996 estimated that the total phosphorus in aquaculture sludge is as high as 1.3 of the total solids. Increased
environmental concern associated with phosphorus discharge has stimulated re- search on phosphorus reduction in aquaculture systems. Most of the studies in this
field have dealt with decreasing phosphorus inputs by increasing the dietary phosphorus availability Rodehutscord et al., 1994; Rodehutscord and Pfeffer,
1995. Phosphorus released into the culture systems is generally left untreated and discharged with the organic solids and effluent water.
In the present study it is demonstrated that crude denitrifying consortia and denitrifying isolates are capable of phosphorus storage in excess of their metabolic
requirements. By Niesser staining and electron microscopy we found that this excess phosphorus is stored by these bacteria as polyphosphate Barak and van
Rijn, in press. As mentioned in the introduction section, the presence of denitrifying poly-P organisms in EBPR processes used for wastewater treatment has
recently been demonstrated. Information on these organisms is mainly derived from mass balance studies on changes in inorganic nitrogen and phosphorus in crude
sludge samples exposed to alternating aerobicanoxic conditions or from similar balances on enrichment cultures Kuba et al., 1997. From these studies it can be
concluded that PHA-mediated polyphosphate synthesis in the aerobic or anoxic treatment step is a common trade among all the organisms thought to be involved
in the phosphorus dynamics in these systems. Unfortunately, this evidence is circumstantial since, so far, efforts to obtain pure, axenic cultures of either
denitrifying or non-denitrifying poly-P organisms have been unsuccessful Mino et al., 1998.
Based on the information available on poly-P organisms in EBPR processes we assume that these bacteria are different from the denitrifying organisms responsible
for polyphosphate accumulation in the present study. This assumption is based on the fact that the denitrifying organisms present in the fluidized bed reactor in this
study were shown to be capable of polyphosphate synthesis under permanent anoxic conditions. Further evidence for this assumption was obtained from recent
studies in our laboratory. Studies on a number of denitrifying strains isolated from the fluidized bed reactor revealed that: a polyphosphate synthesis and denitrifica-
tion were conducted by organisms incapable of producing PHA; and b in organisms capable of producing PHA, polyphosphate synthesis was not coupled to
PHA and glycogen degradation Barak and van Rijn, in press.
A characterization of the phosphate dynamics in the experimental treatment system revealed that, although most of the phosphorus accumulated in the trickling
filter, active removal of phosphate was highest in the fluidized bed reactor. The phosphorus content of organic matter attached to the sand particles in the fluidized
bed reactor was as much as 11.8 of the dry weight. Similar values were reported for polyphosphate accumulating organisms in wastewater treatment plants Degre´-
mont Ltd., 1991. The high total phosphorus content of the trickling filter sludge can be explained as follows. As our experimental set-up contained no mechanical
filtration stage, water led into the trickling filter was rich in organic matter. It may be assumed, therefore, that much of the sloughed denitrifying biomass from the
fluidized bed reactor was captured in the trickling filter. This, together with the anoxic areas within the trickling filter resulting from the high organic load, may
have resulted in a considerable accumulation of denitrifiers in the trickling filter. Evidence for this assumption was provided by the observed denitrification potential
of the trickling filter material upon batch incubation under anoxic conditions Fig. 5B. It is interesting to notice that aerobic incubation of the trickling filter
material resulted in a release of phosphorus into the surrounding medium Fig. 5A. A possible explanation for this observation is that phosphorus was released
due to carbon limitation of the denitrifying organisms since no external carbon was added. We obtained similar results phosphorus release under conditions of carbon
limitation in batch experiments with denitrifying isolates not shown.
5. Conclusions
