Process and Apparatus for the Incineration of Aqueous Sewage Sludge

This invention relates to the disposal of aqueous sewage sludge and, more particular, to a process and apparatus for the incineration of such sludge.

The disposal of aqueous sewage sludge such as from community sewage treatment plants has become a steadily increasing problem in present day living. Among the various approaches employed for disposing of such sludge, the most suitable approach from a hygienic as well as from an economical standpoint has been found to be incineration of the sludge. Sewage sludge as discharged from modern sewage treatment plants generally is a highly aqueous suspension of both solid and putrid matter, generally having an initial water content of approximately 92%. When disposing of the sludge by incineration, the sludge must first be de-watered to complete dryness before a combustion of the sludge takes place. Various methods of de-watering aqueous sludge using mechanical, thermal or chemical means, are known in the art, but complete drying of the sludge, at least in the final stages thereof, is most effectively carried out by thermal treatment of the wet sludge in a suitably heated vaporizer so as to evaporate all of the water from the sludge.

Such thermal drying of the sludge prior to its incineration produces an exhaust gas comprising the air taken taken to the dryer, some malodorous gases and the water vapor evaporated from the wet sludge. Such exhaust gas can be effectively deodorized by heating it to high temperatures on the order of 700 - 800.degree. C., which is generally accomplished by conducting the exhaust gas into the combustion chamber of the incinerator used for incinerating the sludge, whereby the exhaust gas is deodorized by heating with the aid of the hot combustion gases in the incinerator. For maximum efficiency in carrying out such a procedure, it would be highly desirable to keep the quantity of exhaust gas taken to the incinerator for deodorization at as low a level as possible so as to minimize the quantity of heat required in the incinerator for effecting deodorization of the exhaust gas.

It is therefore an object of the present invention to provide a process for the incineration of aqueous sewage sludge such as from community sewage treatment plants, having improved efficiency as compared with the prior practiced processes.

Another object of the invention is to provide a process for the incineration of aqueous sewage sludge including a thermal drying of the sludge prior to its incineration and a deodorization of the exhaust gas resulting from such thermal drying by heating of such exhaust gas, wherein the amount of heat required for deodorizing the exhaust gas is minimized by minimizing the quantity of exhaust gas required to be deodorized.

A further object of the present invention is to provide a process for the incineration of aqueous sewage sludge as described in the preceeding object, wherein the water vapor present in the exhaust gas resulting from the thermal drying of the sludge prior to its incineration, is eliminated from the exhaust gas prior to its deodorization.

Still another object of the present invention is to provide an apparatus for efficiently and economically carrying out the process described in the preceeding objects.

The above and other objects are achieved in accordance with the present invention wherein aqueous sewage sludge such as from community sewage treatment plants is first mechanically de-watered, for example, by filtration, to obtain a wet sludge having a high solids content, which is thereafter substantially completely dried by indirect heating in a heated drier, thereby producing in the drier a dried sludge and a water vapor-containing exhaust gas. The dried sludge is thereafter conducted from the drier to the combustion chamber of a solid refuse incinerator where it is incinerated. The water-containing exhaust gas from the drier is conducted through a condenser, whereby the water is condensed and removed from the exhaust gas to produce a substantially dried exhaust gas, which is thereafter conducted into the combustion chamber of the incinerator used for incinerating the sludge, where the exhaust gas is deodorized by heating with the aid of the hot combustion gases in the incinerator. By removing the water vapor from the exhaust gas prior to its deodorization, the quantity of the exhaust gas taken to the incinerator for deodorization is minimized, thereby greatly reducing the quantity of heat required in the incinerator for effecting the deodorization of the exhaust gas.

The process and apparatus of the present invention and the advantages thereof will be better understood from the following detailed description thereof when considered in conjunction with the accompanying drawing whose single FIGURE illustrates an operational layout for carrying out the process of the present invention for the incineration of aqueous sewage sludge.

Referring now to the FIGURE, an apparatus for effecting the incineration of aqueous sewage sludge is shown as comprising a rotary-drum filter 1, a drier 2, a condenser 3, a solid refuse incinerator 4, an electrical precipitator 5 and a boiler 6. The rotary-drum filter 1 is of standard construction, having an aqueous sludge inlet at 7 and a wet sludge outlet at 8. The drier 2 is a conventional indirect drier. An example of such an indirect drier is the Myren multicoil drier manufactured by Myrens Verksted A/S, Oslo, Norway, having a stationary shell and a heating fluid passage 9 in the form of several tube coils connected to a rotating shaft. The heating fluid passage has a heating fluid inlet at 10 and a heating fluid outlet at 11. Additionally, the drier has a secondary heating gas inlet 13 through which a heating gas may be introduced into direct contact with the sludge in the drier. The drier 2 is also provided with a wet sludge inlet at 15, a dried sludge outlet at 16, and an exhaust gas outlet at 17. The condenser 3 is provided with an exhaust gas inlet at 18, an exhaust gas outlet at 19, and a condensate outlet at 20.

The incinerator 4 is of standard construction and is provided with a charging shaft 21, a combustion grate 22, which generally is of the mechanical type, an uncooled combustion chamber 23 having refractory brick work, a clinker discharge opening 24, and a hot combustion gas outlet duct 25. The outlet duct 25 leads through the electrical precipitator 5 for removing from the hot combustion gases any solid particles that may be suspended therein, and thereafter leads out to the atmosphere. Optionally, as shown in the drawings, downstream of the electrical precipitator 5, the outlet duct 25 may be provided with an outlet opening at 26, connected by conduit 27 to the heating gas inlet 13 of the drier. The quantity of hot combustion gases effluent optionally diverted in this manner for heating the drier 2, will generally be relatively small, with the drier 2 being heated either entirely or at least primarily by means of steam generated in boiler 6 by heat from the incinerator and led through conduit 28 to the heating fluid inlet 10 of the heating passage 9 of the drier 2. Such steam will be discharged as condensate from the heating fluid outlet 11 of the primary heating coil 9 and any hot combustion gases effluent will be discharged through the exhaust gas outlet 17 together with exhaust gases from the sludge.

The apparatus further includes a conduit 29 leading from the wet sludge outlet 8 of the rotary-drum filter 1 to the wet sludge inlet 15 of the drier 2, a conduit 30 leading from the dried sludge outlet 16 of the drier 2 to the charging shaft 21 of the incinerator 4, a conduit 31 leading from the exhaust gas outlet 17 of the drier 2 to the exhaust gas inlet 18 of the condenser 3, and a conduit 32 leading from the exhaust gas outlet 19 of the condenser 3 to an exhaust gas inlet at 33 in the combustion chamber 23 of the incinerator 4.

In operation, aqueous sewage sludge such as from a community sewage treatment plant and having a water content of about 92%, is brought through the inlet 7 into the rotarydrum filter 1 where it is mechanically de-watered to obtain a wet sludge having a high solids content. The wet sludge is led from the outlet 8 of the filter 1 through conduit 29 to the wet sludge inlet 15 of the drier 2. As described above, the drier 2 is being heated either entirely or at least primarily by means of steam generated in boiler 6 and conducted by conduit 28 to the heating fluid inlet 10 of the heating passage 9, the steam leaving as condensate at the heating fluid outlet 11 of the heating passage 9. In the drier 2, the wet sludge is indirectly heated by the steam or other heating fluid and substantially completely dried so as to produce in the drier a dried sludge and an exhaust gas comprising the air brought into the drier, some malodorous gases and the water vapor evaporated from the sludge. The dried sludge leaving the drier at the outlet 16 is transported, preferably pneumatically, through the conduit 30 to the charging shaft 21 of the incinerator 4. The dried sludge passes through the charging shaft 21 to form a layer on the combustion grate 22 at which point it is ignited and burnt out in known manner, the non-combustible clinker being discharged through the opening 24. In addition to sludge, the incinerator may also be normally operating on household refuse. The hot combustion gases in the combustion chamber 23 pass through the outlet duct 25 and the electrical precipitator 5 so as to remove therefrom any solid particles that may be suspended therein, and are thereafter discharged to the atmosphere. Optionally, as described above, a small quantity of the combustion gases may be diverted through outlet opening 26 of the outlet duct 25 and through conduit 27 to the heating gas inlet 13 of the drier 2 for secondarily heating the drier. The combustion gases so diverted are discharged through the exhaust gas outlet 17.

The water vapor-containing exhaust gas produced in the drier 2 leaves the drier through the exhaust gas outlet 17 and is conducted through the conduit 31 to the exhaust gas inlet 18 of the condenser 3. In the condenser the water vapor is condensed and removed from the exhaust gas and is discharged as condensate through outlet 20. The substantially dried exhaust gas leaving the outlet 19 of the condenser is conducted through the conduit 32 to the exhaust gas inlet 33 in the combustion chamber 23 of the incinerator 4. In the combustion chamber, the exhaust gas is heated with the aid of the hot combustion gases therein, and is thereby effectively deodorized.