3.1 Screening
3.2 Sedimentation
3.3 Separation of
Oil and Grease
3.4 Flotation
Primary treatment is generally understood as the set of operations performed to remove floatable and settling solids. These solids are present in an effluent prior to secondary treatment in which biological and chemical processes are used to remove most of the remaining organic matter. In primary treatment only physical operations such as screening, sedimentation and flotation are used. The treatment used largely depends on the operations being carried out in the fish processing plant and on the requirements for disposal of the wastewater. Sometimes the only requisite is that no solids settle after 10 minutes, in which case simple screening and/or settling tanks with short residence times may be used. With more stringent norms, more elaborate processes such as flotation and biological treatment will be required.
By screening, relatively large solids (0.7 mm or larger) can be removed in a primary treatment facility. This is one of the treatments most commonly used by food processing plants as it quickly reduces the amount of solids being discharged. The simplest configuration is that of flow-through static screens, which have openings of about 1 mm. In some cases, such as streams with fish scales, they may require a scrapping mechanism to minimize clogging. The tangential screens are static but less prone to clogging due to their flow characteristics (Figure 3. 1), since the wastewater flow tends to avoid clogging. Removal rates may vary from 40 to 75 %.
Figure 3.1. Diagram of an inclined or tangential screen
Rotary drum screens have also been used for fisheries wastewaters, although they are mechanically more complex. They consist of a drum which rotates along its axis, and the effluent enters through an opening at one end. Screened wastewater flows outside the drum. The retained solids are washed out from the screen into a collector in the upper part of the drum by a spray of the wastewater.
The screening media used in these devices is generally of stainless material, with openings varying from 0.7 to 1.5 mm. Since the fish solids dissolve in water as time proceeds, it is recommended that the waste streams be screened as soon as possible. By the same token, high intensity agitation of waste streams (such as pumping or flow-through valves) should be minimized before screening or even settling, since they cause breakdown of solids rendering them more difficult to separate.
Both screening and settling are used in fish processing plants, screening being used more frequently in small-scale fish processing plants, together with very simple settling tanks. More elaborated sedimentation devices (discussed in the next section) are used in large-scale factories.
Sedimentation is used to remove suspended solids present in the wastewaters. In fisheries wastewaters these include fish scales, portions of fish muscle and offal, the relative proportions varying with the particular process being used.
Sedimentation is based on the difference in density between the bulk of the liquid and the solid particles, which results in the settling of the solids present. The terms sedimentation and settling are often used interchangeably. This operation is conducted not only as part of the primary treatment, but also in the secondary treatment for separation of solids generated in the biological treatments such as activated sludge or trickling filters. Depending on the properties of solids present in the wastewater, sedimentation can proceed as:
Each case has different characteristics which will be outlined. For discrete settling, calculations can be made on the settling velocity of individual particles. In a settling tank, these move both downwards (settling) and towards the outlet zone with the waterflow (see Figure 3.2). The particles that reach the bottom before the outlet will be separated from the effluent while the rest will pass through the settling tank. The critical velocity (vc) below in which a particle will be carried out of the tank is given by the depth of liquid (d), the volume of the tank (V) and the flow rate (Q):
vc = d / (V/Q)
The ratio of V/Q is also known as the residence time of the liquid in the tank. It is called the overflow rate when vc is expressed in terms of volume of effluent per unit surface area of the tank per unit of time. This case may be present in fisheries wastewaters but is not the most common.
Figure 3.2. Schematics of discrete settling
In the case of a flocculant suspension, the formation of larger particles due to coalescence depends on several factors, e.g., the nature of the particles and the rate of coalescence. A theoretical analysis is not feasible due to the interaction of particles which depends among other factors on the overflow rate, the concentration of particles and the depth of the tank.
A settling column is used to evaluate the. settling characteristics of a flocculant suspension (see Figure 3.3). The same kind of column using only one sampling port can be used to study the discrete settling.
Figure 3.3. Laboratory settling column
The zone (or hindered) settling, which occurs when the particles do not settle independently, is also studied by batch tests. In this case an effluent that is initially uniform in solids concentration (see Figure 3.4), if allowed to settle, will do so in zones, the first of which is that of clarified water (1), below is the interfacial zone (2) in which the solids concentration is considered uniform. In the bottom a compact sludge develops in the so called compaction zone (4). Between (2) and (4), a transition zone (3) generally exists.
Figure 3.4. Diagram of a zone settling process
As time proceeds, the clarified effluent and compaction zones will increase in size while the two intermediates will eventually disappear. In some cases, further compaction may occur. The detailed design procedures for all these cases are beyond the scope of this document, and can be found elsewhere (Metcalf and Eddy Inc., 1979; Ramalho, 1977). The actual configuration of a sedimentation tank can be either rectangular or circular. Rectangular settling tanks (Figure 3.5) are generally used when several tanks are required and there is space constraint, since they occupy less space than several circular tanks.
Figure 3.5. Diagram of a rectangular clarifier
For removal of solids, a series of chain-driven scrapers are used: these span the width of the tank, are regularly spaced and move at 0.5 to 1 m/min. The sludge is collected in a hopper in the end of the tank, where it may be removed by screw conveyors or pumped out. Configurations exist in which the sludge is forced opposite to the flow, as shown here, but concurrent flow of the liquid and solids is also used.
The circular tanks are reported to be more effective. In these, the effluent circulates radially, the water being introduced at the periphery or from the centre. Figure 3.6 shows such a configuration. The solids removal are generally removed from near the centre, for which a slope of 10% is required in the bottom of the tank. The sludge is forced to the outlet by two or four arms provided with scrapers which span the radius of the tank. For both types of flow, a means of distributing the flow in all directions is provided: for centrefed tanks there is a circular well, while for the rim-fed tanks a baffle is usually installed and the effluent enters tangentially. An even distribution of inlet and outlet flows is important to avoid short-circuiting in the tank that would reduce the separation efficiency.
Figure 3.6. Diagram of a rectangular clarifter with centre feed
A critical factor for selection of tank size is the so-called surface-loading rate, generally expressed as volume of wastewater per unit time and unit area of settler (m3 /m2 d). This loading rate depends on the characteristics of the effluent and the solids content, and can be determined from the settling tests described above. The retention time in the settlers is generally in the order of one to two hours, but the capacity of the tanks must be determined taking into account the peak flow rates so that good separation is also obtained in these cases.
Some settling tanks, especially the larger ones, are provided with a mechanism for scum removal, since in biological wastes such as fisheries wastewaters, its formation is almost unavoidable.
In cases of small or elementary settling basins, the sludges can be removed using an arrangement of perforated piping placed in the bottom of the settling tank (Rich, 1980). The pipes must be placed regularly spaced (Figure 3.7), be of a diameter wide enough to be cleaned easily in case of clogging and the flow velocities should also be high enough to prevent sedimentation. These last two requisites are somewhat contradictory and a compromise is usually reached, using pipes of 5 cm in diameter, perforated with holes of 1-1.5 cm in diameter, 1 m apart. Flow in individual pipes may be regulated by valves. This configuration is best used after screening and is also found in biological treatment tanks for sludge removal.
Figure 3.7. Pipe arrangement for sludge removal from settling tanks
An alternative to the above configurations for settling tanks is that of the inclined tube separators (Hansen and Culp, 1967). These separators consist of tubes (although there are alternate designs that use plates close to each other) which are tilted (see Figure 3.8).
The concept is that, when a settling particle reaches the wall of the tube or the lower plate, it coalesces with another particle to give one of larger mass and higher settling rate.
Figure 3.8. Typical configurations for inclined media separators
The media are usually inclined 45°- 60°. They are also commonly used to upgrade existing settling tanks since they have a higher separation rate.
Fisheries wastewaters contain variable amounts of oil and grease which depend on the process used, the species processed, and the operational procedure. To remove oil and grease, gravity separation may be used, provided the oil particles are large enough to float towards the surface and are not emulsified. If this is the case, the emulsion must be first broken, which in fish wastes may be achieved by adjustment of the pH. Heat may also be used but it may not be economical unless excess steam is available. Configurations of gravity separators of oil-water are similar to the inclined tubes or inclined plates separators discussed in the previous section. Variations of an original design of the American Petroleum Institute (1959) have been used in food processing wastes.
Flotation is an operation that removes not only oil and grease but also suspended solids. It is discussed in this section since it is one of the most effective systems for suspensions which contain oil and grease. The most common procedure is that of dissolved air flotation (DAF), in which the waste stream is first pressurized with air in a closed tank. After passing through a pressure-reduction valve, the wastewater enters the flotation tank (Figure 3.9a) where, due to the sudden reduction in pressure, minute air bubbles in the order of 50- 100 microns in diameter are formed. As the bubbles rise to the surface, the suspended solids and oil or grease particles adhere to them and are carried upwards. It is common practice to use chemicals to enhance flotation performance. As with coagulants (discussed later) these aids should preferably be innocuous, since these recovered solids are frequently used in animal feed formulations.
Figure 3.9a. Diagram of a dissolved air flotation system
One alternate design involves the recycling of part (10-30%) of the treated water (Figure 3.9b). All systems contain a mechanism for removing the solids that may settle to the bottom of the flotation tanks, usually by a helical conveyor placed in the conical bottom. The main advantage claimed of DAF systems is the faster rate at which very small or light suspended solids can be removed in comparison with settling.
Figure 3.9b. Diagram of a DAF system with recycle
Performance of DAF systems has been reported to be dependent on several factors, of which one of the most important is the solids concentration; higher solids content usually gives higher removal efficiencies. Other factors affecting the efficiency of the operation are the ratio of air to solids (A/S) defined as the amount of air released after pressure reduction and the amount of solids present in the wastewater. There is usually an optimum A/S which is determined by bench scale tests.
Key factors in the successful operation of DAF units are the maintenance of proper pH (usually between 4.5 and 6, with 5 being most common to minimize protein solubility and break-up emulsions), proper flow rates and the continuous presence of trained operators.
In one case, oil removal was reported to be 90% (Ilet, 1980). In tuna processing wastewaters, the DAF removed 80% of oil and grease and 74.8 % of suspended solids in one case, and a second case showed removal efficiencies of 64.3% for oil and grease and 48.2% of suspended solids. The main difference between these last two effluents was the lower solids usually content of the second (Ertz et al., 1977). Although considered very effective, DAF systems are probably not suitable for small-scale fish processing facilities due to the relatively high cost of a plant which in 1977 was estimated at about US$ 250000 (Anon., 1986).
Another flotation system exists in which air is not dissolved but forced through the wastewater by surface aerators. This system generates air bubbles of larger size than DAF systems and no report exists about its application to fisheries wastewaters.
Figure 3.10. Laboratory scale DAF unit
Prior to the design or selection of a DAF system, it is advisable to carry out laboratory experiments to evaluate its applicability and critical operating factors such as the air to solids ratio, the effectiveness of flocculants and the proper pH. This can be conveniently done in laboratory units such as that shown in Figure 3. 10. In these devices, water with or without chemicals and pH adjustment is introduced, and the pressure raised to the desired value. After mixing to saturate the liquid with air, pressure is released and the liquid flows to a graduated cylinder where time is allowed for separation. The detailed procedures for conducting the evaluations are available elsewhere (Eckenfelder and Ford, 1969).
