Liquid purification or separation – Processes – Including controlling process in response to a sensed condition
Patent
1995-08-18
1997-03-18
Popovics, Robert J.
Liquid purification or separation
Processes
Including controlling process in response to a sensed condition
210770, 210771, 210101, 210178, 34360, 34368, 34379, 34397, 34575, 34576, 34586, 34578, 110245, C02F 1112
Patent
active
056119356
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a method and a device for treating sludge, especially sewage sludge, whereby dry sludge is mixed into mechanically predewatered thick sludge, the mixture is dried to form dry sludge, and the dry sludge is cooled before being transported away.
Such a sludge drying serves to condition the sludge primarily originating in water (sewage) treatment plants such that later on, optionally after intermediate storage, it can be disposed of, for example, by combustion in a refuse incinerator or as fertilizer in agriculture.
The sewage sludge, in general, contains an amount of dry substance of approximately 2 to 5 mass %. The mechanical predewatering process increases the dry substance contents to approximately 20 to 30 mass %. This thick sludge cannot be subjected directly to the actual drying process. Indeed, the drying process requires a dry substance contents of approximately 50 to 80 mass %. Therefore, the thick sludge is mixed with already dried sludge. During drying the dry substance contents of the mixture is increased to approximately 95 mass %. During this process the temperature of the dry sludge increases to approximately 120.degree. C. In general, this temperature must be lowered before transporting the dry sludge to approximately 50.degree. to 70.degree. C. because otherwise an unacceptably high risk of self ignition is present.
European patent application 0 458 221 describes a method and a device of the aforementioned kind. Here, the temperature of the dryer is controlled by adjusting the mixture ratios between the thick sludge and the dry sludge. For this purpose, the gas temperatures of the recirculating gas is determined and fed into the control device. The latter controls the metering device and thus the amount of dry sludge to be admixed.
It has been found that the conventional sludge treatment needs improvement with respect to economic efficiency.
Accordingly, it is an object of the invention to provide a possibility for sludge treatment with increased efficiency.
SUMMARY OF THE INVENTION
The method for treating sludge by predewatering to form thick sludge and drying to form dry sludge according to the present invention is primarily characterized by:
a) mechanically predewatering thick sludge to produce predewatered thick sludge;
b) mixing the predewatered thick sludge with non-cooled dry sludge to produce a sludge mixture;
c) measuring the temperature of the sludge mixture to be dried;
d) determining the dry substance contents of the dry sludge based on the measured temperature;
e) compensating fluctuations in the dry substance contents of the dry sludge by controlling the amount of recirculated dry sludge to be mixed with the predewatered thick sludge as a function of the temperature of the sludge mixture to be dried;
f) drying the sludge mixture to produce dry sludge; and
g) recirculating a portion of the dry sludge to step b).
The method further includes the step of adding dust, produced during the sludge treating process, to the predewatered thick sludge or the sludge mixture.
The step of drying preferably includes the step of employing a fluidized bed dryer and wherein, after drying, the dry sludge contains dust of a particle size of less than 500 .mu.m in an amount of less than 20 mass % for use in the step of adding dust.
Advantageously, the method further includes the step of removing dust from the circulating dryer medium of the fluidized bed dryer for use in the step of adding.
Expediently, the method further comprises the step of recirculating, after removing dust therefrom, a portion of the exhaust vapor, produced during drying, into the fluidized bed dryer as a circulating dryer medium for use in the step of adding dust.
The method may also comprise the step of cooling the remaining portion of dry sludge, wherein the step of cooling includes the steps of employing a fluidized bed cooler and removing dust, for use in the step of adding, from the circulating cooler medium of the fluidized bed cooler.
In another embodiment of the i
REFERENCES:
patent: 4330411 (1982-05-01), Florin et al.
patent: 4560527 (1985-12-01), Harke et al.
patent: 5419267 (1995-05-01), Raiko
Brauweiler Helmut
Bublies J org
M oller Burkhard
Rupert Ivan
Thiemann Heinz
Popovics Robert J.
Steag Aktiengesellschaft
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