Removal of Particles
Removal of particles from a water flow is called water treatment or water purification. In aquaculture, removal of particles from a water flow is nec-essary in several places: for the inlet water to the farm; for the outlet water from the farm; or if the water is re-used. The inlet water is treated to avoid too high a concentration of particles reaching the fish. High concentrations will have a negative effect on growth and may increase mortality.1,2 Some par-asites in the water are also of a size that makes it possible to remove them with a particle filter.3 They can therefore be removed from the water before it reaches the farm, or if used on the outlet water a filter could remove them from the water flow before it reaches the recipient water body. Another reason for removal of particles from the inlet water is that the function of other water treatment equip-ment can be affected negatively by the particle content. An example here is the dis-infection plant where a low particle content is required. In the outlet water, particle removal is done to reduce the effect of the outlet water on water conditions in the recipient body.4,5 For re-use systems particle removal is particularly important to avoid accumulation of particles in the system and reduction in fish growth.
The aim of using a filter to remove particles is to extract a certain proportion of particles from the water flow, not all. How much is removed depends on the design and function of the filter. The biggest particles are the easiest to remove, regardless of the chosen method. Before the water flow reaches the filter unit where the particles are removed, it mustbe treated as gently as possible to avoid breaking the particles and reducing their size, so increasing the size of the filter necessary for extraction.2 Gentle handling of the particles includes using low water velocity and having few bends, valves, etc., in the system that create extra turbulence where the particles are flowing. For the same reason, the filter ought to be placed as close to the source as possi-ble; for inlet water this means as close to the water source and for outlet water as close to the produc-tion unit as possible. It is also important to have a sufficient flow to prevent particles settling in the pipes, and leakage of nutrients.
The particles in the water are of various differ-ent forms and numbers. Several methods and defi-nitions are used to define the particle content of the water. Total suspended solids (TSS) is defined as the amount of particles stopped by a special fibre-glass filter with a pore size of 0.45 μm. Total solids (TS) represents the total amount of particles in the water; this quantity can also be expressed as total dry matter (DM).
Particles can also be classified according to size.2Those smaller than 0.001 μm are classified as soluble, 0.001–1 μm as colloidal, 1–100 μm as super-colloidal and larger than 100 μm as settleable. Some nutrients may be totally dissolved in the water, which means that they cannot be re-moved with a particle filter but with other filter types. Examples here are biofilters that remove dis-solved substances, such as NH+4 or NO−3.
When removing particles from the water flow there will also be a reduction in the discharge of nutrients because some are included in the parti-cles. There will also be a reduction in the number of micro-organisms because some are attached to the surface of the particles.
To give some ideas of the different particle sizes, the following can be used for illustration: cocoa and talcum powder 5–10 μm, hair straw 50–70 μm, table salt 90–110 μm. The lower limit for easily identify-ing single particles is around 40 μm.