1. Introduction
Water quality deterioration due to excessive nutrient loading is of great concern in intensive, recirculating fish culture systems. Often this concern not only relates to
the water quality requirements of the cultured animals but also to the quantity and quality of waste discharge from these systems. The latter concerns arise from more
stringent environmental restrictions and concomitant higher levies on waste discharge.
The principal sources of aquaculture wastes are uneaten feed and excreta. The bulk of this waste is in the particulate form and in recirculating systems this is often
removed in a concentrated form by gravitational or mechanical methods Chen et al., 1994. Dissolved organic and inorganic nutrients, making up a smaller fraction
of the total waste in these systems, are removed with the effluent water. Among the dissolved inorganic nutrients, nitrate, the end product of nitrification, is usually
present at high concentrations in the effluent of recirculating systems. Also phos- phorus effluent concentrations are high due to the fact that much of the phosphorus
added with the feed is unutilized by the fish Rodehutscord and Pfeffer, 1995 and, in addition, due to the lack of appropriate methods for phosphorus removal in
these systems.
Enhanced biological phosphorus removal EBPR from domestic wastewater in activated sludge plants is accomplished by alternate stages in which the sludge is
subjected to anaerobic and aerobic conditions. Under these conditions, phosphorus is released from the bacterial biomass in the anaerobic stage and is assimilated by
these bacteria in excess as polyphosphate poly-P during the aerobic stage. Phosphorus is subsequently removed from the process stream by harvesting a
fraction of the phosphorus-rich bacterial biomass Toerien et al., 1990. Recently, evidence was provided that some of these organisms are also capable of poly-P
accumulation under denitrifying conditions, i.e. with nitrate instead of oxygen serving as the terminal electron acceptor Barker and Dold, 1996; Mino et al.,
1998. As recently reviewed by Mino et al. 1998, studies on poly-P accumulating organisms have revealed the involvement of specific set of metabolic properties
under anaerobic, aerobic and anoxic conditions. Under anaerobic conditions, acetate or other low molecular organic compounds are converted to polyhydrox-
yalkanoates PHA, poly-P and glycogen are degraded and phosphate is released. Under aerobic and anoxic conditions, PHA is converted to glycogen, phosphate is
taken up and poly-P is intracellularly synthesized. Under the latter conditions, growth and phosphate uptake is regulated by the energy released from the
breakdown of PHA.
In previous studies van Rijn et al., 1995; Aboutboul et al., 1996 we showed that discharge of organic sludge and nitrate-rich water from recirculating systems can be
avoided by subjecting the sludge to anaerobic digestion and using the digestion products to fuel denitrifiers for removal of nitrate. Essentially, by means of such
treatment, organic matter is converted to carbon dioxide and inorganic nitrogen to nitrogen gas. In the present study it is demonstrated that by means of this treatment
method a considerable fraction of the added phosphorus is trapped in the denitrify-
ing bacterial biomass. Differences between bacterial phosphate accumulation by denitrifiers in this system and in EBPR systems are discussed.
2. Materials and methods
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