Liquid purification or separation – Processes – Separating
Patent
1995-06-07
1997-05-13
McCarthy, Neil
Liquid purification or separation
Processes
Separating
210609, 118 36, 118308, 118325, 71 12, C02F 1112
Patent
active
056289138
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and an apparatus for producing plant nutritive pellets from the sludge arising from wastewater treatment.
2. Description of the Related Art
The major part of all sludge from wastewater treatment plants is deposited on garbage dumps in spite of the sludge containing valuable plant nutrition. The reason for this is not primarily fear of heavy metals in the sludge but, above all, the difficulties farmers have in dealing with such sludge continuously. Previously, attempts have been made to form the sludge into pellets directly but this has been found impossible, since the mass has too loose a structure, because of its high (70-80%) water content. Getting below this water content percentage has been found to be impossible in the sludge handling process, since most of the water in the sludge is cell-bound. In addition, it is very sticky, resulting in that any pellets formed quickly stick together and form large aggregates. It is almost impossible to dry out these large lumps to a sufficiently low water content for storage. These masses are also an infection hazard, since they always contain infectious organisms and substances, e.g. salmonella bacteria, different viruses and parasite eggs.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a method and an apparatus of the kind mentioned above, where the pellets can be formed directly, while preventing the formation of aggregates; such that there is the possibility of effectively drying the pellets down to a water content of about 10-15%, which is adequate for storage, and simultaneously eliminating the infection hazard during handling. The distinguishing features of the invention have been disclosed in the following claims.
By reason of the invention there has now been provided a method and an apparatus that fulfill the inventive objects in an excellent manner. The following advantages are obtained by the pelletization. By dousing the pellets with a preferably hygroscopic powder they will be stiffer as they are pressed out from the extruder, thus enabling them to maintain their shape. The coating of dry powder also prevents them from sticking together to form aggregates as well as enabling them to be dried more easily and quickly. The dry powder, which can be hygroscopic, rapidly absorbs moisture from the surfaces of the newly formed pellets and this moisture is then rapidly drawn away in the hot, dry atmosphere of the tumbler. As the powder is dried out it loosens from the pellets, enabling it to be recycled in the process. The pellets may then be dried conventionally, e.g. as with cereals, to a water content of about 10-15%, adequate for storage. Pellets produced according to the invention can in this way now be delivered at a time suitable to agriculture, since they are capable of being stored. In addition, they can be spread in the same way as artificial fertilizers and thus take the place of large amounts of the latter. The risk of spreading infection is inhibited by heating the pellets to a temperature of 80.degree. C. for fifteen seconds. By using modern heat exchanger technology, drying and pelletization according to the invention can take place with very low energy requirements.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in more detail with the aid of a preferred embodiment and with reference to the accompanying drawings, where
FIG. 1 schematically illustrates an apparatus for carrying out the method in accordance with the invention, and
FIG. 2 schematically illustrates, to an enlarged scale, a section through the extruder nozzle and openings therein, which are provided with an annular, peripherally slotted jet for blowing out compressed air.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 is a schematic view of an apparatus for producing plant nutritive pellets from the sludge arising from wastewater treatment. Pelletization takes place with the aid of an extruder 1, through which the sludg
REFERENCES:
patent: 4193206 (1980-03-01), Vere Maffet
Kurihara K., Dialog Information Services, Soil Improving Agent Comprises Ground, Digested Sludge Adsorbed or Coated with Dry Powder, File 351, World Patent Index 81-93, Dialog accession No. 003498936, WPI accession No. 82-46901E/23.
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