WO1996007620A1

WO1996007620A1 - A method and device for dewatering of sludge

Info

Publication number: WO1996007620A1
Application number: PCT/FI1995/000473
Authority: WO
Inventors: Arto RYHÄNEN
Original assignee: Vapo Oy
Priority date: 1994-09-06
Filing date: 1995-09-06
Publication date: 1996-03-14
Also published as: FI944083A; FI944083A0

Abstract

The object of the invention is a method and equipment for drying sludge, in which method a flocculant is mixed with the sludge and the mixture obtained is treated in a centrifuge in order to separate the water component from the solid substance. Peat, to the amount of 10-30 % of the sludge solid substance, is gently mixed into the sludge before the centrifuge treatment.

Description

A METHOD AND DEVICE FOR DEWATERING OF SLUDGE

The object of the invention is a method and equipment for drying sludge, in which method the sludge is mixed with a flocculant and the mixture obtained is treated in a centrifuge, in order to separate the water component from the solid substance.

Sludges that are environmentally detrimental arise in water treatment plants and in agriculture, among other places. For example, the liquid manure from a piggery cannot be released directly into the natural environment and certainly not into a watercourse.

Several different ways are known of treating sludge to a create a form less harmful to the environment. Sludge has been filtered through a suitable filter, compressed by means of a screw press equipped with a perforated jacket, and treated in a centrifuge. The intention is to separate the solid substance and the water component as completely as possible from one another. In most cases, a flocculant mixed with the sludge is used, this usually improving the result of the separation significantly. Normally, the solids content of the solid substance obtained remains less than 30 %. However, by using a flocculant the water component obtained is fairly clean.

The intention of this invention is to create a new kind of method and equipment, by means of which the centrifuge treatment can be considerably improved over previous ones. The charac¬ teristic features of the method in accordance with the invention is presented in the accompanying Patent Claim 1 and the charac¬ teristic features of corresponding equipment are presented in Patent Claim 7. The invention is based on the surprising obser¬ vation that small additions of peat to the sludge before centri- fuging improve the separation result considerably. The manner of the influence of the peat is not precisely known at this stage. The water component does not directly travel through the quan¬ tity of peat, so that the manner of operation is different to that in, for example, a peat filter. It seems to be that a small quantity of peat solidifies the solids mass in the sludge, which makes possible an improved separation result.

In what follows, the invention is illustrated with the aid of an example, which is shown in the accompanying Figure.

A centrifuge model SKZSNH S530L4, which has a capacity of 8 - 25 m³ sludge per hour and is manufactured by the German Maschinen- fabrik Sangerhausen Gmbh, is advantageously used as the centri- fuge 1. The solids content of the sludge is between 0,5 - 10 %. First of all, the sludge is led to a mixing tank 2 , in which there are paddle mixers 3, by means of which the peat is gently mixed with the sludge. The mixture is led with the aid of pump 4 to the centrifuge,, the flocculant dose being led to the same pipe. The flocculant is dosed with the aid of dosing tank 7 and is pumped to the centrifuge feed pipe by means of dosing pump 8. A clear water component 5 and a noticeably dry solid substance 6 are obtained from the centrifuge 1. Already in preliminary tests the solids content of the final product has been as much as 36 %, which can probably be further improved. Without the addition of peat, the solids content of the final product in tests was only 30 %. The purity of the reject water in par¬ ticular has been improved with the aid of flocculation. The reject water obtained is generally clear. In the mixing tank 2 the peat is dosed as 10 - 20 % of the solids of the sludge. The normal moisture content of peat is 50 % ± 5 %.

The speed of revolution of the centrifuge used can be steplessly adjusted up to 3000 rpm. In the tests a speed of revolution of 200 rpm was used, which gives a circumferential velocity of 55 m/s with a rotor diameter of 530 mm. Most advantageously the circumferential velocity is less than 70 m/s, when the structure of the peat does not break down too early in the centrifuge treatment.

If the final product is to be composted, intermediate grade peat is used, i.e. it is slightly decomposed. If, on the other hand, the final product is to be burned, fuel peat, which is highly decomposed and has a moisture content of about 45 %, is used.

The mixing that takes place in the mixing tank 2 is so gentle that the peat would rise to the surface, if the mixture was allowed to stand. The mixture, which is subject to continual mixing, is, however, led as such to the pipe, and the mixture is not given time to stand before the centrifuge treatment.

Claims

Patent Claims

1. A method for drying sludge, in which method the sludge is mixed with a flocculant and the mixture obtained is treated in a centrifuge (1) in order to separate the water component from the solid substance, characterized in that 10 - 30 % peat to the solid substance is mixed gently into the sludge, before the centrifuge treatment.

2. A method in accordance with Patent Claim l, charac¬ terized in that if a compostable mass is desired, intermediate grade peat, i.e. slightly decomposed peat, is used.

3. A method in accordance with Patent Claim 1, charac- terized in that if a mass suitable for combustion is desired, fuel peat is used.

4. A method in accordance with one of Patent Claims 1 - 3, characterized in that the mixing of the peat and sludge takes place in a tank with the aid of one or more paddle mixers (3) .

5. A method in accordance with one of Patent Claims 1 - 3, characterized in that the mixing of the peat and sludge takes place in a tank with the aid of compressed air.

6. A method in accordance with one of Patent Claims 1 - 5, characterized in that the circumferential velocity of the centrifuge is less than 70 m/s.

7. Equipment intended to implement the method in accor¬ dance with Patent Claim 1, which consists of a centrifuge (1) , and sludge and flocculant feed devices characterized in that the equipment includes a mixing tank (2) for dosing the peat into the sludge and implements, e.g. a paddle mixer (3) or compressed air feed equipment, for gently mixing the peat.