Liquid purification or separation – Processes – Making an insoluble substance or accreting suspended...
Patent
1990-04-13
1992-11-03
Hruskoci, Peter
Liquid purification or separation
Processes
Making an insoluble substance or accreting suspended...
34 12, 34 14, 34 69, 34 70, 100 71, 210770, 210783, 210202, 2102571, 210262, 210297, 210400, C02F 1114
Patent
active
051604400
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention concerns a procedure for dewatering of sewage sludge, whereby the pumpable sludge and the sludge made pumpable by breaking up solid substances is subjected to chemical pretreatment with addition of flocculation agents; thereafter the sludge is dewatered to form a sludge cake by pressing; said sludge cake is then destined for further utilization or final storage.
The procedure according to the present invention is also directed to raw sludges and digested sludges of local sewage purification plants as well as to industrial sewages.
2. Description of the Prior Art
The present invention starts from the process for dewatering of sewage sludge as described in DE-OS 28 38 899 in which the crumbled and chemically pretreated sludge is directly led to an extractor press, in which it is conveyed by a screw from the bottom to the top, with other words: in which the sludge is pumped up. A chemical pretreatment of the sludge, particularly with flocculants, however, is impaired in its effect if the sludge cake treated in this manner is exposed to pumpage.
On principle, the main aim of the previously described sludge dewatering procedure is to reduce as far as possible the volume of the sludge to economize the volume of deposit as well as the transport and depositing costs and to give the sludge the highest possible shearing resistance in order to guarantee a long time of depositing capability. The best dehydrating units, i.e. the chamber filter presses, actually reach only 32-35% of (dry substance at maximum). It results a relatively high remaining humidity which causes many problems for the depositing capability. Therefore, the development tends to burn the sewage sludge with addition of primary energy, i.e. to destroy the sludge.
However, the sewage sludge is a precious substance and it should not simply be thrown away on dumping places or destroyed, and this with the addition of primary energy.
SUMMARY OF THE INVENTION
It is accordingly the object of the present invention to provide a procedure and a plant with which it will be possible to achieve such high dryness degrees of the sludge that the produced sludge cake may be utilized as source of energy for incineration, for pyrolysis, oil production or as drilling fertilizer.
The solution of this problem represents a procedure, in which the chemically pretreated sludge is subjected to pre-dewatering under conveyance without pumping, and is dewatered by the pressure filtration in a way to form such a hard cake that after breaking up this cake, pieces will be obtained which--with accumulated consistency--maintain their shape, and that the pieces of sludge cake will be post-dewatered e.g., when they are conveyed on drying belts. This solution is based on the knowledge that elimination of water of a sludge quantity is subject to physical laws which however, are different in the various dehydration phases. Therefore, the chemically pretreated sludge is first subjected to pre-dewatering with conveying without pumping, i.e. only by gravity, to obtain dry substance of about 20% (all references to percentages refer to weight percent). Conveying without pumping is very important to avoid destruction of the flakes which have been formed with addition of flocculation agent, so that the sludge may be led to the press in a consistency which allows dehydration up to 50% dry substance.
Dehydration with mechanical pressing is to be carried out in a way that after breaking up the hard sludge cake pieces will be obtained which--when they are accumulated--resist by their nondeformability to a third phase, namely the post-dewatering or drying on drying belts. The pieces of sludge cake should be so hard, as they cannot disintegrate into lumps. Entrance of air into the sludge cake pieces is guaranteed, independently from their accumulation on the drying belts, so that a post-dewatering or post-drying up to approximately 90 to 95% may be achieved.
The high dewatering rate of approximately 50% dry substance by mechanical
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patent: 4192743 (1980-03-01), Bastgen et al.
patent: 4341628 (1982-07-01), Fujinami et al.
patent: 4358381 (1982-11-01), Takeuchi et al.
patent: 4491521 (1985-01-01), Wenske et al.
patent: 4587022 (1986-05-01), Shimizu et al.
patent: 4761895 (1988-08-01), Uyama et al.
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