flotation columns have been shown to perform well for solids removal in limited flow-rate recycle systems. The sludge produced by separation technology can be thickened and
stabilised by the addition of lime, to kill pathogenic diseases and restrict putrefaction. The resulting sludge has been spread on agricultural land. © 2000 Elsevier Science B.V. All rights
reserved.
Keywords
:
Solids; Particles; Wastewater; Effluent; Aquaculture; Treatment
1. Introduction
Wastes from aquaculture include all materials used in the process which are not removed from the system during harvesting. The main wastes from aquaculture
operations are uneaten feed, excreta, chemicals and therapeutics, as described by Ackefors and Enell 1990, 1994, Beveridge et al. 1991, Braaten 1992. Dead and
moribund fish, escaped fish and pathogens Bergheim and A , sga˚rd, 1996 can also
be considered forms of waste. The main source of potentially polluting waste either discharged in the farm effluent, or to be made available for reuse within the farm,
is feed-derived Pillay, 1992, such as uneaten feed, undigested feed residues and excretion products.
The feed-derived wastes include components that are either dissolved, such as phosphorus P and nitrogen N based nutrients, or are in the solid phase as
suspended solids, as described by Losordo and Westers 1994. This review will present information relating to the management of particulate wastes in intensive
land-based aquaculture facilities.
The management of solids, through feed design, feeding management and flow regulation, in addition to the eventual removal of solids using separation and sludge
treatment technology, becomes increasingly important as aquaculture systems in- tensify. For recycle systems, in which reused water must be of an adequate quality
to maintain the culture organisms in a healthy and fast-growing condition, it is especially important to remove waste products.
A build-up of solids in an aquaculture system can lead to a decline in culture water quality that will increase the stress on the culture organisms Rosenthal et al.,
1982; Klontz et al., 1985; Braaten et al., 1986. This will be as a result of: a direct impact of the solids on the animals, e.g. through the partial smothering of gills; or
indirectly, e.g. by offering a suitable habitat for the proliferation of pathogenic organisms Braaten et al., 1986; Liltved and Cripps, 1999, and the consumption of
dissolved oxygen as the solids decay Welch and Lindell, 1992.
This review aims to identify and examine realistic aquaculture waste solids management strategies. This will be achieved by the critical analysis of the
components that could comprise a multi-stage treatment system. The aims of waste treatment in general and solids management in particular
differ, depending on whether the intensive culture system is single-pass flow- through, water reuse with little reconditioning, or recycle, as summarised by
Losordo and Westers 1994.
The main reason for treating solids to be discharged from flow-through systems will be to reduce potential negative impacts on the surrounding aquatic environ-
ment. Feed-derived wastes from intensive aquaculture facilities can degrade the environment and conflict with other aquatic resource users. Many authors have
described these impacts, including impacts from solids, such as: Enell and Lof 1983, Ackefors and Enell 1990, Gowen 1991, Gowen et al. 1991, Pillay
1992, Costa-Pierce 1996.
In reuse and recycle systems, solids management will normally be aimed at providing at least an adequate culture water quality. In such cases, solids manage-
ment will often be designed to be combined with other unit processes for the control of dissolved nutrients, biochemical oxygen demand BOD, dissolved gases
O
2
and CO
2
, pH and pathogens. Irrespective of the type of culture system, modern solids management commonly
comprises a series of stages, or unit processes. More advanced systems integrate these so that solids are prepared by one unit process or management regime, to be
better handled at the next stage Cripps and Bergheim, 1995. Hence, solids control stages such as feed management, pre-treatment, primary separation, secondary
solids handling and disposal may comprise an integrated solids management system Alana¨ra¨ et al., 1994; Summerfelt, 1998.
2. Solids characteristics
