Liquid purification or separation – Processes – Making an insoluble substance or accreting suspended...
Reexamination Certificate
2000-10-27
2003-02-04
Popovics, Robert (Department: 1724)
Liquid purification or separation
Processes
Making an insoluble substance or accreting suspended...
C210S729000, C210S767000, C210S770000, C210S800000, C210S808000
Reexamination Certificate
active
06514418
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a process for dewatering sludges, preferably municipal sewage sludges in sewage treatment with rapid pressure filters, particularly chamber filter presses and membrane filter presses, using water-soluble polyelectrolytes as flocculation aids in sludge conditioning.
In addition to agricultural utilization, the obligatory disposal of sewage sludges is effected by thermal treatment of the sewage sludge with combustion or degassing and gasification, and predominantly via landfill.
Sewage sludges represent compositions including exceedingly high levels of water, so that both in landfill and in thermal processes, the solids content of the sludge is increased by at least one mechanical dewatering stage, preferably using screen belt presses, centrifuges or pressure filtration. Particularly in improving such dewatering on filter presses, the sludge preconcentrated by thickening has to be conditioned by means of an additional treatment. To this end, the sewage sludge is flocculated using organic, polymeric flocculation aids, preferably high molecular weight cationic polyelectrolytes, and subsequently subjected to dewatering in a batchwise procedure using chamber filter presses or membrane filter presses.
According to EP-B-19176, dewatering can be performed in two steps. Initially, the flocculated sludge is subjected to dewatering in a screening drum or on separate screening belts and collected in an intermediate container, subsequently fed into the chamber filter press using a piston membrane pump or an eccentric screw pump, and filtrated under a pressure of up to 15 bars. Owing to the previous dewatering, this process is remarkable for its shorter batch times in the chamber filter press. However, this advantage is compensated by the measures and time required for the first step of the process.
In gfw-wasser/abwasser 126 (1985), pp. 124-130, V. Zees, T. Clausdorf and G. Gerardts make the supplementary statement that following sludge conditioning and removal of supernatant water, only limited dewatering of the flocculated sludge in the chamber filter press is possible because no sufficient stability of the flocs is achieved for this purpose.
According to K. J. Thomé-Kozmiensky, Klärschlammentwässerung, TK-Verlag K. J. Thomé-K., Neuruppin, 1998, p. 266, polymer conditioning of sewage sludge for dewatering on filter presses is advantageous only in those cases where high demands with respect to shear strength are not made.
Therefore, a dewatering process is known according to EP-A-151,747 wherein the flocculent is metered to the conveyed amount of sludge as a function of sludge density and the sludge thus pretreated is fed directly into the chamber filter press. With pressure times of 3 hours, a dewatered sewage sludge having a dry matter content of 30-40 wt.-% is achieved in the press discharge.
However, this procedure involves the disadvantageous process that the water which is difficult to remove from the sludge flocs is not predominantly withdrawn during pressure filtration and also, those amounts of water in the flocculated sludge mixture already made free of turbidities by flocculation undergo filtration through a developing layer having a filtering effect and compacting under pressure exposure. Consequently, this dewatering process on chamber filter presses which is widely used for sewage sludges involves considerable technical and economical drawbacks.
DE-OS-38 20 110 describes a process wherein the conditioning of sewage sludge is performed using at least two organic polymer flocculation aids having low and high molecular weights, respectively. Again, partial dewatering of the flocculated sludge is possible upstream the chamber filter press.
According to WO 88/03048, previous dewatering is effected on screen belts arranged in a story-like fashion, by means of which the sludge is conveyed to a sludge silo without the use of pumps.
Furthermore, a process is known according to U.S. Pat. No. 4,861,492, wherein sewage sludge is flocculated using polymeric flocculation aids, sedimented, and subsequently held in a rest phase without mechanical load for at least 2 minutes. Thereafter, the supernatant waste water is decanted, and the thickened sludge is subjected to dewatering in a rapid pressure filter with additional flocculant aid. This process requires additional equipment-related input for decanting, as well as time and means to stabilize the flocculated state.
In another process, according to Chem. Ing. Techn. 66, No. 9 (1994), pp. 1222, 1223, the sewage sludge previously added with flotation coal and ashes is subjected to a pressure flocculation in a pressure-resistant flocculation reactor; the flocculated sludge is to reach the filter press without further mechanical stress, and minimum filter drag throughout the feeding cycle is to be ensured.
In 44
th
Purdue Industrial Waste Conference Proceedings, 1990, Lewis Publishers, Inc., Chelsea Mich. 48118, 1989 (1990), pp. 513-518, J. T. Shah reports on the possibilities of optimizing the dewatering process on chamber filter presses by stepwise pressure increase during the dewatering process, thus enabling an increase of the filter cake solids content.
DE 93 07 712 U1 describes a special design of a sludge dewatering press wherein the pressurized air used in secondary pressing subsequently is utilized to blow out the dewatered sludge. By using this sludge dewatering press, the high consumption of pressurized air is intended to be reduced.
DE 93 20 903 U1 describes a filtering means for liquids, particularly swimming pool water, where the liquid to be filtrated is sucked out of a container over filtering elements with precoat filter layer. The design of this filtering means is intended to improve the cleaning operation, i.e., removal and disposal of the consumed filtering aid, and reduce the amount of water required for this purpose.
DE 36 17 519 A1 describes a process and a device for removing residual impurities from a pre-clarified potable liquid, wherein a filter press having precoated layers of filtering aids is used. Here, the structure of the regeneratable layer of filtering aids is regarded as crucial to the invention, precoating with said layer of filtering aids being effected at a specific flow rate. Filtration in the filter press is performed under pressure as usual.
DE 41 19 167 A1 describes a method of determining the characteristics and parameters essential for the sizing and dimensioning of filter presses, as well as a device for performing said method. It is a measuring procedure wherein a hydrostatic pressure corresponding to the previous filtration pressure is applied in the filter chamber, which hydrostatic pressure is continuously monitored in order to detect the pressure drop characteristic of completion of the filtration process.
U.S. Pat. No. 5,275,740 describes a method and a device for controlling the sludge pressure in a filter press and, in particular, for controlling the final pressure of the stepwise and automatic filtration cycles in order to have simultaneous optimization of the press feeding process and to maintain a constant pump pressure. In this case as well, filtration is effected in the press under pressure and in a way previously known from prior art.
JP 07124600 A describes a process and a device for the treatment of organic sludges, wherein the organic sludge initially is flocculated using an organic polymer flocculant and subsequently subjected to dewatering on a filter press using gravity filtration, i.e., solely by the hydrostatic pressure of sludge supplied to the filter press.
Depending on type and amount of organic and mineral components, municipal sludges exhibit diverse, frequently varying properties affecting the dewatering behavior, particularly during flocculation of dispersed solids by means of organic flocculation aids and during filtration. Particularly in the dewatering of municipal sludges on chamber filter presses, the individual operations of the process therefore have to be optimized and balanced. In addition to selecting suita
Bartscherer Josef
Chramenkov Stanislav Vladimirovitch
Hartan Hans Georg
Held Winfried
Lobanov Fedor Ivanovitch
Popovics Robert
Stockhausen GmbH & Co. KG
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