then a factor of 2 – 5 times greater than without oxygenation. Fivelstad and Binde 1994 found that a water flow below 0.16 – 0.20 l kg
− 1
min
− 1
reduced growth and caused tissue damage in freshwater fish. The concentration of carbon dioxide in
freshwater should not exceed 10 – 20 mg l
− 1
in freshwater Colt and Watten, 1988. A reduced water consumption, often by combining recirculation and addition of
oxygen, is a means to improve the utilisation of the water supply and to reduce the discharged effluent load because of improved treatment efficiency Cripps and
Bergheim, 1995.
3
.
4
. Variation in solids loading Several parameters have been found to influence the waste load, and hence cause
variability in the quality of the wastewater. Kelly et al. 1994 found that the waste quantity discharged from a fish farm increased with temperature. Foy and Rosell
1991b however showed that the proportion of nutrients in the particulate fraction increased with temperature. These nutrients were not then becoming more soluble
with increasing temperature as may have been expected. At sites where water temperature changes throughout the year, care must be taken at the planning stage
of development, to ensure that the capacity of treatment facilities will be adequate to consistently produce the required discharge or recirculation water quality.
Changes in ionic composition, dissolved nutrient concentration and physical parameters, such as flow rate and scouring, also influence the partitioning of
nutrients between the dissolved and particulate phases, as reviewed by Hamann et al. 1990. Again changes in these parameters will form design criteria.
Intermittent solids loading increases can occur as a result of intermittent tank cleaning operations Kelly et al., 1997, or from unit processes that function
irregularly, such as back-pressure activated rotating micro-screens see below. Studies also indicate the advantage of continuous pre-treatment to concentrate
wastes Twarowska et al., 1997. Bergheim et al. 1998 further provided evidence that if the waste solids concentration is increased, then the efficiency of the
clarification processes will increase see below.
4. Pre treatment
4
.
1
. Ad6antages The main parameter determining flow-through tanks is the need for adequate
gaseous exchange, as described above. Solids handling is normally a lower priority that has to be accommodated into an already designed and managed tank or pond.
The three main advantages to preparing i.e. concentrating solids prior to clarifica- tion processes are an improved culture environment, an increased treatment effi-
ciency and a reduction in required treatment capacity.
4
.
2
. Tank hydrodynamics and culture en6ironment Numerous studies have indicated the advantages of maintaining a ‘clean’ culture
environment, with the minimum of waste matter present that could lead to an increase in the incidence of diseases Klontz et al., 1985; Braaten et al., 1986,
andor an increase in stress as a result of sub-optimal water quality Pickering, 1981; Rosenthal et al., 1982; Klontz et al., 1985; Braaten et al., 1986. In particular,
environmental gill disease, which is a chronic, non-infectious condition that occurs under intensive culture, is thought to be caused directly by the accumulation of
suspended solids andor un-ionised ammonia Burrows, 1964. Braaten et al. 1986 propose that it is these physicalchemical parameters, including prolonged exposure
to organic particles, that cause the gill lesions, rather than micro-organisms that are secondary invaders. In order to optimise culture production, it is therefore impor-
tant to remove solids quickly and efficiently from culture facilities.
Various studies have been conducted into the effects of tank hydrodynamics on suspended particle movement, such as feed distribution Backhurst and Harker,
1989, self-cleaning and efficient use of available water Watten and Beck, 1987; Westers, 1991; Cripps and Poxton, 1992; Yoo et al., 1995, and periodic cleaning
Stabell, 1992. This information, combined with a knowledge of the water quality requirements of different species e.g. Alabaster and Lloyd, 1980; Wickins, 1981;
Poxton and Allouse, 1982, has lead to the development of tank designs in which solids transport and separation are important design criteria e.g. Boersen and
Westers, 1986; Watten, and Johnson, 1990; Wagner, 1993.
Various devices have been proposed or used for moving particles within tanks or ponds. Such devices include: the juxtapositioning of inflow and outflows; vanes; or
screens. Inflow direction and distribution, with outflow location and size, can be used: to create high energy andor quiescent zones, as described by Cholette and
Cloutier 1959, Rosenthal et al. 1982, and Burley and Klapsis 1985; for the effective transport of particles to the effluent e.g. Cobb and Titcomb, 1930;
Burrows and Chenoweth, 1955, 1970; Watten and Johnson, 1990; or for transport to a quiescent collection point Westers, 1991.
4
.
3
. Treatment efficiency and capacity Kelly et al. 1997 showed a large variation in effluent solids loading as a result
of farm management regimes such as intermittent tank cleaning. The greater loads increased the efficiency i.e. proportion of the total waste load removed by any
screen pore size with which the effluent was clarified using micro-screens see below. Cripps 1995 proposed that the reason for such an effect may be the
build-up of a filter cake that would restrict the passage of particles smaller than the nominal pore diameter, as described by Hamann et al. 1990.
A similar effect was described by Bergheim et al. 1998, who found that the settling efficiency of an aquaculture sludge sedimentation chamber increased from
about 58 at about 1 mg SS min
− 1
to nearly 90 at 18 mg SS min
− 1
at the same flow rate. Such an effect is likely to be due to an increase in the probability of
particles contacting and coalescing in more concentrated suspensions, and so increasing their mass, thereby settling faster, as described by Tchobanoglous and
Burton 1991 pp. 220 – 240. Solids concentrations from culture vessels that have been increased in order to
increase treatment efficiency, can be discharged either as a continuous stream of a lesser flow than the primary effluent, or as a discontinuous stream of short
duration – high flow pulses. In either case, the total mass flow of the wastes should be approximately similar to the flow with no pre-treatment, however the total mass
flow of the wastes and water in the pre-treated effluent will be substantially less than with no pre-treatment. Hence, facilities for treating pre-treated wastes can be
reduced in size, because of the reduced capacity required to treat a smaller volume of waste. This should result in savings in both capital and operational costs at the
treatment stage.
4
.
4
. Pre-treatment technology Technology used to pre-concentrate solid wastes prior to treatment can, as
proposed above, be classified into equipment or procedures that produce an intermittent plug, or continuous flow, of high solids content waste.
The need to expel waste solids from a tank in an intermittent plug can arise from the accumulation of material, either deliberately or accidentally. Solids can be
deliberately deposited within a section of a tank that is either close to the effluent andor partitioned from the culture stock, e.g. particle traps described by Westers
1991. Accidental deposition can arise as a result of sub-optimal tank flow management. Intermittent tank cleaning is the traditional means of removing solids
from a tank. Wastes that have accumulated are scrubbed and then flushed out of the tank by increasing the inflow rate and lowering the water level. This method is
not as good as continual removal because it can lead to a stressed stock and poor water quality in the tank. Rather than allowing this plug to exit the farm in the
main effluent, it may be stored in separate holding facilities to allow treatment devices sufficient time to function at a lower hydraulic load than if treatment of the
primary flow was required.
A preferential alternative to flushing is the use of a combined concentrator and separate waste solids outlet, as reviewed by Cripps and Bergheim, 1995. Particle
concentrators, though sometimes expensive, are becoming more popular. They are devices at the tank outlet which assist the settlement and consolidation of solids.
This concentrated waste is removed from the tank periodically, or preferably continuously, through an outlet which is separate from the primary flow Fig. 1.
This sludge outlet is then led to the treatment device. The primary flow commonly does not require treatment for solids removal.
Eikebrokk and Ulgenes 1993 described a system which combined a within-tank particle concentrator with a separate outlet and sludge dewatering unit whirl
separator. The particle-enriched outlet flow was 5 – 6 of the total flow, thus pre-concentrating particles by a factor of 20 assuming approximately 80 of the
particles were trapped. An overall system removal of 71 SS, 38 TP and 14
TN was estimated. The dry matter content of the dewatered sludge from the whirl separator was about 14.
Ma¨kinen et al. 1988 studied the effluent P budget in tanks with two separate outlets: a ‘bottom effluent’ and a main ‘surface effluent’ with flow rates of 10 and
90 of the total flow, respectively. Without tank flushing, the average P concentra- tion in the effluent from the tank bottom was more than double the surface
concentration. Using a stationary microsieve removing particles from the bottom effluent, the overall P treatment efficiency was 46, incorporating the flushing load.
A commercially available particle concentrator system has recently been further developed which is a combination of a Norwegian Ecofish tank separator system
and a US recycle system Twarowska et al., 1997. This system combined both a specially designed particle trap that separated excess feed pellets from faecal wastes
so that feeding could be more closely monitored, and collectors for sludge and dead fish removal.
Fig. 1. Within-tank pre-treatment and separate particle treatment system courtesy of Aqua Optima- Eco-tankEco-trap
®
.
5. Solids separation technology
