|The sludge resulting from the purification of industrial and civil processes is produced by biological-oxidative treatment brought about thanks to the presence and regeneration of facultative micro-organisms. The biological film formed separates from the support and, together with the sewage, passes in the form of flakes to the drainage, where it is decanted during the final sedimentation phase.
Separation of the biological film is facilitated by increased hydraulic load, which can be achieved by recirculating part of the purified sewage, as is done in intensive percolator beds.
The resulting flocculate is output as sludge with very high water content and is sent to a digestor, which eliminates the risks connected with putrefaction of matter. The phenomenon is characterised by the anaerobic decomposition of organic substances in closed tanks, in a slightly alkaline medium. The process, which involves psychrophilic bacteria (at temperatures < 30°C), mesophylic bacteria (30 - 45°C) and thermophylic bacteria (at temperatures > 45°C), must take place at temperatures that are constant as far as possible and with limited temperature variations around 35°C, where the mesophylic bacteria are most active.
COMPACTING THE SLUDGE
The black, digested sludge has a water content of about 87%-90%. It can be dried naturally in layers (from which a solid containing about 55% liquid is obtained); otherwise it can be dehydrated mechanically, after appropriate chemical and heat treatment. Natural drying uses chlorides and sulphates to break up the colloidal structure by coagulation: mechanical drying gives the same result, thanks to heating.
Mechanical dehydration of sludge is usually done using cylindrical-conical centrifuges. This centrifuge has two rotating elements: the cylindrical-conical rotor and the screw conveyor, which rotates slightly faster than the rotor. The sludge enters the conveyor through the hollow co-axial shaft, which has openings, and contacts the rotor where the solids are thrust against the walls by centrifugal force and sent for unloading by the screw conveyor; the clarified liquid flows out from the centre.
Another tested system involves compacting of sludge by mechanical compression in filter presses. The filter consists of a series of plates and frames arranged alternately on supporting guides. When the press is closed, the plates are in contact with each other, connected laterally to the frame, thus forming a series of chambers into which the sludge is pumped under high pressure. The solid particles are held back by the cloth, while the liquid is removed. At the end of the process, the cloths are covered with panels having high solid content.
For both technologies, performance does not usually exceed 70%.
Thickening of the sludge can also be achieved mechanically using innovative screw conveyor systems capable of squeezing out the residual liquid content in the sludge.
This screw conveyor system consists of a contrast cover at the unloading section, which obstructs the flow of the material conveyed. The sludge conveyed by thrusts from the screw conveyor accumulates at the obstruction and is pressed, releasing a certain amount of liquid. This pressing continues until the force exerted on the cover by the compacted sludge exceeds the counter pressure imposed, resulting in unloading of the dehydrated sludge. The sewage obtained during pressing is collected and sent for treatment; the compacted sludge, depending on the type, is sent to the incinerator or used as compost.
FIELDS OF APPLICATION
The thickening of sludge and slurries containing varying solid content is adopted in numerous industrial sectors, apart from the waste treatment sector.
Where organic or inorganic solids are dispersed in a liquid matrix, compacting system is ideal both in terms of economy and process efficiency.
This technique has numerous applications in the food industry: treatment of water from the food preservation process (containing vegetable waste, peel, core, etc.), the sugar industry (containing washing residues, sugar beet tops, etc.), fish and meat processing (containing processing residues including skin, hair, etc.), beverage industry (containing mixtures of squeezed fruit, barley, malt, etc.). These new compacting techniques can be used to great advantage to reduce the percentage of liquid content in the wastes to be sent for disposal and enable appropriate treatment of the liquid current before it is discharged.
It should also be remembered that other sectors such as the chemical industry (treating solid by-products of reactions in the form of slurries); paper industry (treatment of paper mill sludge), glass recycling industry (treatment of water used for washing the bottles and removing the labels) or incinerator sector (in treatment of the aqueous phase coming from the flue gas scrubbing section, for removing ash and other particles) are also concerned with the problem of handling sludge and slurries with high liquid content.
The absence of mechanical parts in contact with the material handled and the construction of components coated with materials that are compatible with food substances, allows the use of these handling techniques not only for handling process fluids containing waste material, but also in the food sector for handling materials such as pureed fruit and legumes, sugar pulp, preserves, creams and pastes.
MECHANICAL TRANSPORT OF MUDDY, ADHESIVE AND VISCOUS MATERIALS AND FIELDS OF APPLICATION
The handling of a different type of "slurry" products poses serious problems for mechanical transport, deriving from the adhesion and inconsistency of the solid material content. The use of normal screw conveyor handling systems with screws fitted with internal shafts involves a high level of adhesion of the material on the surface of the screw, and stretching of material between the helicoid flight and trough (with progressive flooding of the system): these phenomena determine the impossibility of the system to convey the material efficiently.
The use of shaftless screw conveyors (the so-called spiral conveyors) that do not require intermediate or end supports represents an innovative solution for the transport of sludge, adhesives and viscous materials; the reduced surface of the screw, consisting only of the helicoid flight, without centre pipe, limits the phenomenon of adhesion at the origin.
The material is conveyed by the helicoid flight, very thick to allow transmission of high torque, which creeps along in direct contact with the U-shaped channel trough thanks to a non-stick, wear-proof coating.
The principle of screw conveyor transport, based on the friction that develops between the surfaces of the screw conveyor and the material handled, is not affected by these innovations. These help overcome limitations regarding the homogeneity of the materials handled, however, as the risk of blocking the screw conveyor because of insertion of bodies having special shapes between the helicoid flight and trough is reduced to a minimum.
Because of its intrinsic features, this handling system can also be used for the vertical transport of "difficult" materials.