|The separation of solid particles in a liquid can be done in several ways, depending on the characteristics of the solid-liquid system to be treated and the desired degree of separation.
In designing an efficient separation method, the most important properties in a two-phase solid-liquid system are:
- average diameter, granulometry range, and density of the solid particles;
- tendency of the particles to form aggregates;
- concentration of solids in the waste water;
- density and viscosity of the liquid;
- presence of oily substances;
- presence of flocculating agents.
This document describes the technologies used for the separation of solid particles suspended in a liquid matrix, which are normally used in the pre-treatment areas of waste water treatment plants and in the cement industry.
TECHNOLOGIES USED IN WASTE WATER TREATMENT PLANTS
Pre-treatment involving the separation of incoherent solid substances of various sizes from the waste water to be treated is known as screening.
Treatment involving the separation of sand and inert matter with a size below a few millimetres (with particular reference to sand with different granulometries, inert matter and gravel) is known as sand-trapping.
Screening treatment was traditionally done using machinery with coarse-graded performance while sand trapping was done with screw systems in settling tanks.
Machines have now been designed to carry out both screening and sand trapping operations using a concept of a separation screen linked to a sloped transportation screw system able to clean the screen and carry off the polluting solids.
The isolation and separation of grease and oils in the waste is done during specific treatment phases.
Pre-treatment of waste at the treatment plant is done to reduce the content of solids (coarse and fine) and oils which, if not removed, cause disturbances in the primary waste treatment phase.
The separation of coarse solids is done by screening; depending on the average size of the suspended particles, a suitable spacing in the screen is chosen. Solids larger than that of the gaps in the screen are held back, as also are particles and organic matter, which are stopped by the filtering cake effect.
To maintain the efficiency of the filtering system and to make sure that the flow of liquid is not reduced by an excessive thickness, the cake is removed regularly, or better still continuously. Screen cleaning is usually mechanical, done by suitable scraping tools (knives or brushes); these devices must clean the surface of the screen and remove any solids attached which, where necessary, will be disposed of or triturated. Often the solids removed from the screen have to be washed in order to remove the organic substances, which could lead to putrefaction.
Unless finely-spaced screens are used (which would, however, require continuous cleaning) the finer solids (such as sand) are removed during the operation known as sand-trapping. The presence of these in the waste water to be treated could cause wear or serious damage in successive sections of the treatment plant.
The waste to be treated is sent to tanks in order to reduce the flow speed and therefore the phenomenon of dragging of the fine waste by the liquid stream. Thus, in relatively still conditions, the solid settles, due to its greater specific weight compared to the fluid.
The sedimentation speed of a spheric particle in a still fluid follows Stokes' law: deviations are due to the non-spherical form of the particles, to the reciprocal influence of particles, and to the presence of flocculating agents that may affect free sedimentation of the particles.
The sizing of the settling tanks must be done bearing in mind that long settling times mean greater separation efficiency, but may allow the sedimentation of organic matter, creating the same handling problems for the sedimented material described above regarding screening.
The relatively still state of the fluid in the sedimenting tanks means not only the settlement of solid particles, but also the upward movement of the substances that have a lower specific weight than the fluid, providing the conditions needed for de-oiling and de-greasing of the treated waste.