5. Solids separation technology
5
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1
. Choice of unit process Several types of particle separators, or clarifiers, are commercially available for
integration into an intensive aquaculture treatment system and are capable of accepting the pre-concentrated wastes from tanks or ponds. Not all are suitable for
treating aquaculture wastewater that has not been pre-treated. Several authors have reviewed particle separators, including Wheaton 1977, Huguenin and Colt 1989,
Landau 1992, Lawson 1994, Chen et al. 1994a and Cripps and Kelly 1996 Fig. 2.
It is generally more feasible to remove the solids and thus the nutrients and BOD associated with them in high flow – low concentration commercial aquacul-
ture wastewaters, than to treat the dissolved fraction using some form of filter bed. The exception to this is where a large proportion of the wastewater is to be reused,
such as in intensive recirculation systems, or where discharge legislation is restric- tive. Solids separation technology can be conveniently divided into mechanical and
gravitational methods. The most popular method of mechanical particle separation is by the use of screens. Traditionally, the aquaculture industry in Europe and
North America have used sedimentation for first stage particle separation. The low residence times, and hence high overflow rates associated with intensive systems
makes sedimentation, as a first stage technique, inefficient Cripps and Kelly, 1996; Summerfelt, 1998. It may however prove a suitable process for the clarification of
lower flow rates, such as the sludge flow produced by a screen separator.
Fig. 2. Particle sizes removed by different solids separation processes adapted from Chen et al., 1994a.
5
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2
. Microscreens Ma¨kinen et al. 1988 reported the use of a triangle filter which is a static screen
inclined so that the primary flow through the screen also serves to carry the separated particles to the waste trough. Its current lack of widespread use may be
due to capacity limitations, though no data have been found to verify this. Rotating
microscreens are
an alternative
to primary
sedimentation Tchobanoglous and Burton 1991 and so have been more commonly installed at
farms in recent years. These usually comprise a fine mesh screen often 60 to 200 m
m pore size in the form of a rotating drum or disc through which the wastewater is passed. Particles held back on the mesh are backwashed or scraped, to a waste
collection trough. Rotating microscreens are especially suited to applications where blockage is likely Wheaton, 1977, and so are used in fish farms because of the
large flow of wastewater which must pass through the screen and the small screen pore size which is required to separate out the solids.
Cripps and Kelly 1996 reviewed commercially available rotating microscreens and concluded that the majority currently used in aquaculture were developed for
the treatment of drinking water. The near perpendicular flow of water through the screens was designed to stop the particles rather than remove them. Setting the
screen at a gradient of about 30° to the water surface towards the water flow, such as is achieved using band or belt ‘filters’, had the potential to gently remove
particles, with minimal damage, out of the primary waste stream.
Rotating screens can have a substantial backwash sludge water flow that usually requires further thickeningdewatering see below. The backwashing however can
be initiated automatically with little intervention required. Pore size, rotation speed and backwash flow can be adjusted to the application.
In order to minimise the quantity of backwash water required and maximise the solids concentration, advanced units can be operated intermittently. As particles
build up on the static screen, a filter mat becomes established. The back pressure increases and a level indicator is used to initiate the rotating and backwashing
operation when the water height difference across the screen exceeds a pre-deter- mined value. For special operations, such as high fat content wastes, warmhot
water can be used for backwashing. When discharging to a marine recipient or recycle system, sea-water can be used for backwashing. Little of the salt is retained
within the sludge that may be later applied to farm land Bergheim et al., 1998.
Usually, wastewater flows into a drum microscreen through one of the ends of the drum. It then passes out through the walls of the drum, which are composed of
the filter mesh. The backwash jets are located out of the water on the outside of the drum, so that collected particles are washed into a trough inside the drum
Huguenin and Colt, 1989. These are directed out of the drum through one of the open ends.
Wheaton 1977 described an alternative design of drum screen which acted more like a water mill and was more suited to applications with large debris-type
particles. In this device, the effluent is passed through the screen from the outside to the inside of the drum, whilst the particles are carried over the drum to a
collection channel. It is important to ensure that the hydraulic capacity and pore size is matched to the characteristics of the effluent to be treated Cripps and Kelly,
1996. The treatment efficiency of a Hydrotech drum filter has been tested by several
workers including Ulgenes 1992a, Ulgenes and Eikebrokk 1992 and Twarowska et al. 1997. During extensive tests conducted by the Norwegian Hydrotechnical
Laboratory, the treatment efficiency of a 60-mm pore size drum screen varied considerably within the ranges SS 67 – 97, TP 21 – 86 and TN 4 – 89.
Again, efficiency was found to vary proportionally with the waste effluent concen- tration. Twarowska et al. 1997 however achieved lower solids removal rates of
36.5 using the same type of 60-mm pore size screen. Clearly then the efficiency of such screens is dependent on the characteristics of the effluent and hence, and
pre-treatment techniques applied.
Unpublished information has been located that describes a design of drum screen with axially positioned troughs located on the inside of the drum. These troughs
were designed to catch large particles, such as uneaten pellets or faeces, and deposit them in the backwash trough. Such a design would appear to be a useful method
to quickly and gently transport large particles to waste, but no operational data to verify this has been located.
Rotating disc screens are composed of a flat circular disc of microscreen material held approximately perpendicular to the primary wastewater flow. Large screens are
required in high flow-rate situations, such as is common in aquaculture, but these are difficult to rotate, so a sequence of screens with pore sizes decreasing down-
stream e.g. 200 and 60 mm may be required.
The main effluent flow travels through an axial flow screen perpendicular to the plane of the screen, which is disc-shaped. Commonly, backwash water jets extend
across the full radius of the downstream side of each screen. The waste, which accumulates on the screen during a rotation cycle, is then collected in a trough on
the upstream side. The sludge streams from multiple screen units are usually combined prior to disposal or further treatment Cripps and Kelly, 1996.
Several workers Liltved, 1988; Liltved and Hansen, 1990; Bergheim et al., 1991; Ulgenes, 1992b; Bergheim et al., 1993a,b have tested the treatment efficiency of a
commercially available Unik disc microscreen. Similar to the drum screen results, treatment efficiency estimates using this unit vary considerably, both due to
variations in effluent quality and characteristics, and with the pore size of the screens chosen Table 1.
Ulgenes 1992b testing 250- and 120-mm pore screens together achieved a wide range of SS removal efficiencies of 16 – 94, whilst Bergheim et al. 1991 achieved
an average 40 suspended dry matter SDM removal using 350- and 60-mm pore size screens. Liltved 1988 obtained even lower TS removal efficiencies of 20, but
this was using large pore size screens of 1600 and 600 mm. During tank washing operations, this value was however increased to greater than 80. The low removal
efficiency could be attributed to the low particle concentration in the effluent and the large pore sizes of screens employed.
Table 1 Published removal efficiencies using Unik disc screens
Removal efficiency Reference
Screen pore size Coarse
TP Fine
TN SS
18–65 1–49
120 Ulgenes 1992b
250 16–94
1600 20–80
Liltved 1988 600
350 \
40 60
7–30 Bergheim et al. 1991
\ 40
These results indicate that screen pore size should be chosen to suit the applica- tion and that the choice should be based on the characteristics of the wastewater to
be treated. The capacity of a drum screen is proportional to its length and its diameter, while
the capacity of a disc screen is limited by the diameter Wheaton, 1977. Drum microscreens are therefore not as capacity limited as disc screens. In practice
however, at high flow rates, such as those in aquaculture applications, several disc or drum units are operated in parallel. This also allows for a unit to be out of
operation, for repair or maintenance.
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3
. Sedimentation The use of sedimentation in aquaculture has been reviewed by Wheaton 1977
pp. 505 – 513, Lawson 1994, Cripps and Kelly 1996 and Summerfelt in press. Sedimentation is the process by which settleable suspended solids, that have a
greater density or specific gravity than water, can settle out of suspension and so be separated from the main flow.
It is gravity, in the absence of other confounding influences, that causes particu- late waste matter to sink. The settling velocity is controlled by the viscosity of the
fluid water and the diameter of the particle if the particle is assumed spherical, as described by Stokes’ Law. The physical properties of sedimentation have been
described by Weber 1972, Wheaton 1977, Gregory and Zabel 1990 and Lawson 1994.
There are four types of sedimentation: type 1, discrete; type 2, flocculent; type 3, hindered or zone; and type 4, compression settling Gregory and Zabel, 1990. The
type of sedimentation that occurs in a vessel is dependent on the concentration of the particles and their interaction with each other. The low particle concentrations
of aquaculture wastes, that have not been pre-treated, usually settle discretely without interacting with each other Chesness et al., 1975.
Only in pre-concentrated aquaculture backwash water or sludge is there likely to be flocculent or hindered settling, as described by Bergheim et al. 1998. The
processes of flocculation, whereby waste particles combine, either through natural collision or attraction, or are induced artificially, can assist settlement by increasing
particle size and settling velocity.
In view of the large number of parameters affecting sedimentation and the complicated physical characteristics of the wastewater, it is common to conduct
site-specific analyses of settling prior to planning a settling basin for a specific application.
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. Settling basins Settling tanks for aquaculture applications are usually designed to have a plug
flow Wheaton, 1977 in which turbulence and resuspension are minimised Sum- merfelt, in press, though both circular and rectangular settling basins are common
Lawson, 1994. A basin will have three design functions. Any failure in one of these functions will impair the performance of the tank and, if serious, destroy the
effectiveness of the process almost completely Weber, 1972. These are: to effec- tively remove SS, leaving a clear effluent; to collect and discharge the settled sludge;
to provide a thickened sludge with minimal volume. In practice however, especially at the high flow rates encountered in aquaculture applications, short-circuiting
movement of inlet water directly to the outlet without mixing in the basin, turbulence and resuspension scouring, by the water flow, of settled material off the
bottom can occur, so that it becomes difficult to collect waste concentrations of less than 10 mg l
− 1
Henderson and Bromage, 1988. In ideal sedimentation tanks, four zones can be identified: inlet, settling, sludge
and outlet Wheaton, 1977; Lawson, 1994. Baffles and an outlet weir are often incorporated to promote quiescent conditions. Whilst settling efficiencies are inde-
pendent of basin depth Hazen’s Law, hence overflow rates the maximum flow through a basin that will still allow a given particle to settle are calculated in terms
of volume of effluent per unit area per h m
3
m
− 2
h
− 1
, the tank must have sufficient depth to collect the sludge and to minimise the cross-sectional area
through which the wastewater flows. An exception to this is the use of inclined plate or tube settlers, in which settling depth is minimised to a few centimetres.
Various design criteria have been proposed specifically for aquaculture applica- tions. To avoid the scouring of settled solids and the risk of turbulence reducing
settling velocity, Henderson and Bromage 1988 recommended that flow velocities should not exceed 4 m min
− 1
, though preferably 1 m min
− 1
. Warrer-Hansen 1982 adopted more conservative guidelines of 1.2 – 2.4 m min
− 1
. Within wastewater treatment, it is more common to quote design recommenda-
tions in terms of retention time or overflow rate. Mudrak 1981 recommended a retention time of longer than 30 min, which was in agreement with the findings of
Henderson and Bromage 1988 who calculated the retention times at 16 farms. Overflow rates of about 1.5 – 3.0 m
3
m
− 2
h
− 1
would seem to produce adequate settlement of aquaculture solids, with Mudrak 1981 reporting 1.7, Warrer-Hansen
1982 2.4 and Bergheim et al. 1998 1.0 – 2.7 m
3
m
− 2
h
− 1
. The use of coagulants to assist the agglomeration and hence faster settlement of
small particles was not considered economically viable by either Chesness et al. 1975 or Cripps 1994.
Sedimentation basins of various designs are common throughout the industry. They range in design from simple ponds dug downstream of the farm, to compact
second stage cones, or advanced basins incorporating automatic sludge removal and flow manipulation Tchobanoglous and Burton, 1991. Their main advantage is
that spare ponds or tanks can be adapted for this use. Despite their widespread use, the practical problems inherent in their operation limit their application within
aquaculture wastewaters so that they are, in any form, rarely suitable for the treatment of the primary effluent from land-based, flow-through facilities, because
of inadequate flow dynamics and sludge removal problems. Though particle settlement velocities of aquaculture wastes are sufficiently fast to allow the use of
sedimentation as a means of separation, flow rates from farms are high. This can lead to various flow dynamics problems including: insufficient residence time to
allow the particles time to settle out; scouring of settled particles off the bottom; and short circuiting of influent water direct to the outflow. The use of sedimenta-
tion is not inherently wrong. It is the application to which the operation is applied that is often inappropriate. Flow rates of the primary untreated effluent are high,
but sludge flows from treatment devices, such as screening, are far lower, commonly less than 1 of the primary flow. This sludge almost always requires further
thickening. Sedimentation is one of the most suitable methods to accomplish this. Sedimentation therefore is appropriate for the localised i.e. within tank pre-con-
centration of wastes, and for second stage de-watering of separated sludge within a multi-stage treatment system Cripps and Kelly, 1996. It is not suitable for
clarifying the untreated main wastewater flow from a farm.
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5
. Bead filters Bead filters or expandable granular biofilters EGBs can function as both
mechanical and biological filters Chen et al., 1993a,b and because of this they have been used for recycle systems. Chen et al. 1993a,b claimed that the filter
offered both high hydraulic loading rates and removal of particles smaller than 100 m
m. Chen et al. 1993a,b, 1994a, Wheaton et al. 1994a and Malone et al. 1998
described the functioning of bead filters. Buoyant, inert, 3- to 5-mm diameter polyethylene beads retained within the filter housing are fluidised as the wastewater
is up-flowed through the bead bed. Suspended particles are either strained out or deposited on the bead surface. Flow to the filter is then stopped and the beads are
aggressively backwashed. The bed is expanded to remove the retained particles from the system. Solids are then allowed to settle to the bottom of the filter
chamber, where they are discharged to waste. The volume of backwash water produced using this method is said to be 1 – 5 of that produced by comparable
sized sand filters, whilst filtration flux rates can be as high as 0.5 – 1.5 m
3
m
− 2
bead surface min
− 1
Chen et al., 1994a. The dual function of the filter, whilst advanta- geous within recycle system, may lead to problems of optimising operation for both
biological and physical processes simultaneously. Wheaton et al. 1994b confirmed this by noting that bead filters achieve satisfactory solids removal at loading rates
of 80 kg feed m
− 3
beads, whilst the nitrification capacity is reached at about 24 – 32 kg feed m
− 3
beads.
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6
. Flotation The removal of fine solids smaller than about 50 mm from wastewater is difficult.
Sedimentation rates for these particles are slow and the flow capacities of micros- creens with such small pore sizes are low. In flow-through systems, these are less of
a problem because they do not represent a large fraction of the discharged solids, either by weight or volume Cripps, 1995. These sized particles do however tend to
build up in recirculating systems Timmons, 1994 and hence need to be removed polished. Foam fractionation can be used to remove these small particles. Whilst
Timmons 1994, referring to bubble and surfactant interface theory, expected foam fractionation to remove only particles smaller than about 30 mm, Chen et al. 1992
showed that the mean particle size distribution in the wastewater and the foam were similar at 10.6 mm, indicating that a wide range of sizes were being separated.
Descriptions of the process of foam fractionation, also known as flotation, protein skimming or air stripping, are given by Gregory and Zabel 1990, Lawson
1994, Timmons 1994 and Summerfelt in press. The wastewater is passed downwards through a contact chamber. Bubbles, produced near the bottom of the
chamber float upwards against the wastewater flow. Surface-active particles become attached to these bubbles so that the density of the bubble – solids aggregates is less
than water. They rise to the surface, the bubbles break and the associated surface-active material is released into the foam. Should a stable foam build up, this
can be collected over a weir and discharged to waste. Ozone gas can be used in foam fractionation to remove fine particulate organics Otte and Rosenthal, 1979;
Williams et al., 1982.
Flotation is dependent on bubble diameter, solids concentration, air-to-water ratio, surface chemistry of the solids, and the surfactant concentration in the water
Huguenin and Colt, 1989; Summerfelt, in press. Wheaton 1977 considered that foam systems function better in seawater than freshwater and cited Dwivedy 1973
who showed that it was even possible to remove bacteria from water using foam fractionation.
In order to remove particles with the minimum of mechanical disruption that would cause them to be sloughed off the bubbles, and to increase the bubble
surface contact area available, it is normally the aim to produce small bubbles about 10 mm, even though they have less buoyancy than larger bubbles \ 100
m m. Even these small bubbles, singly or in groups, should be capable of removing
the small particles and compounds that foam fractionation is aimed at. Hence, the method used to produce the bubbles is important. The two main types of bubble
generation that can be applied to aquaculture systems are dispersed and dissolved air. Dissolved air injection is achieved by the manipulation of pressure. Weber
1972 and Gregory and Zabel 1990 make a case that the dispersed air systems produce large, inefficient bubbles. Air is injected into the system using some form
of diffuser such as a venturi constriction. Nevertheless these have been shown to be suitable for aquaculture applications Weeks et al., 1992; Chen et al., 1994b,c.
6. Secondary sludge thickening, stabilisation and disposal
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