FI101292B - Method for removing water from biologically treated sewage sludge - Google Patents

Description

101292 METHOD FOR REMOVING WATER FROM BIOLOGICALLY TREATED Sludge

The invention relates to a method for removing water from a biologically treated treatment sludge, in which peat is mixed into the sludge, after which the sludge is pressed against a filter to separate the water part therefrom. The invention relates in particular to the treatment of sludges from sewage treatment plants, but is also suitable for use, for example, in the treatment of fibrous sludges 10 from the forest industry.

The sludge that separates at the wastewater treatment plant requires post-treatment because it cannot be discharged into nature as such. According to the prior art, peat and other litter can be mixed into the slurry and an attempt can be made to compost the mixed mass. So-called open composting has increased recently, but it is by no means the final solution. Construction of the fields is expensive and the maintenance workload is high, and nutrient emissions to the air cannot be controlled. The quality variations of the 20 "finished" products produced by open composting are large. There is also a problem in many places with space constraints, it is not possible to place fields of the required size at close distances, which results in transport costs.

25 FI patent application 895032 discloses a method for mixing peat and sludge, which gives the sludge a less environmentally harmful mass. Here, however, the consumption of peat is very high, typically 1.6 times the amount of sludge. Sludge digestion is also not the final solution, as the digested sludge still requires litter, which increases the amount of pulp to be treated. FI patent publication 51338 discloses the benefit of preparation using digested sludge. Its dry. the substance content must be greater than 10%. Here, the sand material and possibly peat are mixed into the slurry and the mixture 35 is subjected to compression. In patent application 790856, the sludge is pressed through a peat filter, from which the filter material is used as a soil improver. Excess moisture is squeezed out of the saturated filter material.

U.S. Patent No. 10,102,292, 4,659,472, discloses a method of treating sludge from a wastewater treatment plant to provide a compostable mixture. The slurry is mixed with finely ground sawdust, peat or bark as a carbon carrier, after which the mixture is pressed with a filter belt press to separate the water therefrom. The mixing takes place pneumatically, which promotes digestion. The mixture is then passed to composting vessels. The use of a flocculant, a suitable polymer, prior to compression is essential to the process.

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The object of the invention is to provide a simpler method than before for removing water from a slurry so that the amount of pulp to be further processed is actually reduced. The characteristic features of the invention are set out in the appended claims. Mixing should be done gently to avoid breaking the cellular structure of the peat. Peat, here especially coarse surface peat, has many functions. It acts as an internal filter, increases friction in screw compression and provides a suitable amount of carbon for composting. The operation of the invention is probably based on the fact that the fibers of the slurry partially enter the cellular structure of the peat, whereby the pressure is able to release water from them. In other words, the compressive strength of the slurry is increased in this way. The peat is mixed very little in relation to the total amount of slurry, since the dosing of the dry matter of the peat is 35 to 65% to 25% of the dry matter of the slurry. Dosing also depends on the quality of the slurry. For example, fibrous sludges from the forest industry may require different dosing than sewage sludges from treatment plants. In any case, the amount of peat is in the range of 1 to 100% of the amount of sludge.

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The invention will now be described with reference to the accompanying figure,. which schematically shows the method according to the invention.

The sludge to be treated is fed to a mixing vessel 3, into which a suitable amount of surface peat, for example curd, is dispensed from silo 2. Although the dosing varies according to the quality of the sludge, the dosing at the same location can be empirically proportioned to dry matter. The solids content of the peat should be between 50 and 80 kg / m3 ‘3 101292

Mixing of sludge and peat must be done carefully so that the internal cellular structure of the peat is not broken. Possibly, a pneumatic mixer known from the above-mentioned US publication can be used. In experiments, a slowly rotating paddle mixer 4 has proven to be good. The mixing takes several minutes, even in a small batch.

The mixed mixture is fed to a continuous screw press with a filter jacket, for at least four hours, preferably immediately. As the screw press, 10 models "Tasster FA" manufactured by, for example, the French Neyrtec can be used, which is schematically shown in the figure by reference number 6.

In the press, the screw 7 pushes the mass into the cylinder 8, which is perforated around its part surrounding the jacket 9. The compression takes place against a pressure plate 10, which releases the compressed mass at the end of the cylinder into the discharge channel 11. The water part exits through the jacket 9 together 12.

In the test compression, only at a back pressure of 3-4 bar, 32% of the dry matter content of the biological sludge of the wastewater treatment plant (not digested) was reached, which had a dry matter content of 2.3%. Peat with a moisture content of 60% was mixed so that the dry matter of the peat was taken from 50% of the dry matter of the slurry.

25 The resulting dry end product is a very stable, slightly odorous, soil-like mass that is easy to process by incineration, composting or other means.

With the help of the flocculant, the water part is brought out brighter, the dry-30 substance content is only 0.01 to 1%. At the same time, the amount of peat must be slightly increased to increase friction. At the treatment plant, the water part can be: · recirculated. Without the flocculant, the dry matter content rises here to about 1%.

The compression pressure is preferably between 3 and 30 bar. The dry matter content is made to rise between 45 and 50%, possibly even higher.

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For example, the Pieksämäki city treatment plant generates 5,000 m3 of 15% thick sludge annually. With the method according to the invention, the amount of sludge remains at 3000 m3 and its dry matter content is 37%.

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The sludge is pretreated biologically. As such, the method can also be used for non-organic sludges.

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Claims (3)

1. A method for removing water from biologically treated sludge, in which peat is mixed in the sludge, after which the sludge is pressed against a filter to separate the water part therefrom, characterized in that - the peat used is a somewhat roughened peat, dry content 50 - 80 kg / m3 and this is portioned in the sludge so that the dry matter of the peat is 35 - 65% by weight of the dry matter of the sludge, 10. and that the mixing takes place without destroying the structure of the peat with pressure-free paddle mixer or the corresponding slow-moving mechanical mixer, - and that the pressing is carried out within at least four hours, immediately after mixing, with a continuous screw press, which is provided with a filtering jacket. Process according to claim 1, characterized in that a flocculant is mixed in the pulp before pressing, e.g. a suitable polymer. 20 Method according to claim 1 or 2, characterized in that the pressing pressure is in the range 3-30 bar.