Method for dewatering sludges

Liquid purification or separation – Processes – Making an insoluble substance or accreting suspended...

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210723, 210738, 210748, 210749, 210770, 210243, 204164, C02F 124, C02F 136, C02F 148

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active

058274324

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to a method for removing water from sludges, such as sludges from community sewage treatment plants, sludges of the wood-processing and food manufacturing industries or the like biosludges, the method comprising the following subsequent steps: coagulate the dissolved substances therein, i.e. transform them into particulate solid matter; the particulate solid matter in the sludge; and of a belt filter, a screw press or by centrifuging.
When treating waste waters various sludges are produced which can, mechanically concentrated, be used as such, be composted, be used for soil improvement or be combusted directly or after drying.
In Finland, at the end the 1980, 50% of the sludges from community sewage treatment plants was spread over fields, 25% was used for building of parks and gardens and 25% was transported to dumps. Of the sludges of the wood processing industry only approximately half of it was effectively used (combusted) and the other half went to dump tips. The total amount of the sludges from the communities and the wood processing industry is nearly 0.6 million tons sludge calculated as dry matter.
To deposit sludges as such at dumping areas causes big transport and dumping cost, due to the space needed and the hazardous effluents to the water system along with the runoff and seepage waters. In Germany, for instance, regulations are being drafted, according to which only the ash which is produced when sludges are combusted is allowed to be transported to the dumps. As the environmental regulations are made stricter, the use of sludges as such on the fields will decrease everywhere from what it is now. The regulations being drafted in the Netherlands will totally put an end to the use of sludge on cultivated lands in 1996.
The trend in Europe seems to be that the combustion of sludges is becoming more common. The ash material produced by the combustion is tipped at dumping areas. By the present-day dewatering methods, however, the dry matter content of the sludge, 15-30% for instance with the sludge qualities of the community sewage treatment plants and the industrial biosludges, is so low that auxiliary fuel, such as oil, gas or peat, is needed in the combustion. A trouble-free combustion of sludge without any additional fuel requires, depending on the ash content, a dry matter content level of 40-50%, which is difficult to reach with most sludges by mechanical methods only.
Regardless of the final placement of the sludges, a higher dry matter content than that of today will be required in the future. At the present, even when for instance composting sludges, additives have to be used. The amount of additives can be reduced or they can be completely omitted, if the sludge can be dewatered to a dry matter content of 35-40%.
By using higher pressures, the sludge could be pressed to a higher dry matter content. A factor setting limits to the increase of the pressure is, however, in many cases the low compression strength of the sludge, i.e. that the sludge is not dewatered but disintegrates and goes directly through the pressing wires when the pressure is increased. Some sludge passes through the wires even at pressures as low as 1 bar, whereas the flocs in other sludges are more resistant against pressure. The strength or resistance of the flocs is an essential factor when sludges are dewatered by pressing.
Addition of various chemicals improves the strength of the flocs. It is, however, necessary to establish a good contact between the chemicals and the solids in the sludge in order to achieve the desired effect. The layer of water which is firmly bound to the surface of the solid particles hampers the formation of tight and strong flocs. Removal of this layer of water in the coagulation stage would make it possible to produce a tighter and stronger floc structure.
The quality and the water retention properties of the sludges vary greatly, even at the same plant and often even daily. The following factors, inter alia, have an in

REFERENCES:
patent: 4260488 (1981-04-01), Condolios
patent: 4340488 (1982-07-01), Toth et al.
Publication of National Board of Waters, Finland; 1992: Forest industry's waste sludges.
Jarvinen, Saunamaki: Forest industry's treatment of sludge in active sludge plants, "Vesitalous" (Water management) issue Jan. 1990.
Report No. 124/1977 of National Board of Waters, Finland: Studies on Conditioning and Mechanical DeWatering of Waste Water Sludge.
Chemical Abstract: 91(14)112076q (Undated).
Chemical Abstract 104(12)94792a 1986.
Chemical Abstract 104(18)155213m 1986.
Chemical Abstract 105(26)231867r 1986.
Chemical Abstract 98(14)113065s Undated.
Chemical Abstract 105(4)2397g Undated.
Chemical Abstract 110(10)81908s Undated.

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