Sewage sludge treatment system

Abstract

Gaseous discharge from a pelletizing drier used in the treatment of sewage sludge is partially directed back to a combustion chamber that generates an effluent which is fed to the drier. Volumetric requirements of a gas scrubber and an afterburner are reduced to the volume of gaseous discharge not recycled back to the combustion chamber. A concentrated stream of sewage sludge is mixed with a quantity of dehydrated particulate matter and supplied to a rotary pelletizing drier. Fuel and air undergo a combustion process and are mixed with additional air and part of the gaseous discharge in the combustion chamber which generates a hot gaseous effluent that is directed through the drier. The effluent removes moisture from the mixture of concentrated sludge and dehydrated particulate matter to provide dehydrated particulate sludge and the gaseous discharge. Entrained materials are initially separated from the gaseous discharge by cyclone separators. A gas flow proportioning valve is disposed in a duct system interconnecting the cyclone separators, gas scrubber and combustion chamber for directing a portion of the gaseous discharge back to the combustion chamber.

I claim: 
     
       1. A sewage sludge stream treatment system comprising: a dewatering apparatus for separating water from the sewage sludge stream to provide a concentrated stream;   a mixing unit for combining a quantity of dehydrated particulate matter with the concentrated stream to provide a feedstock having a lower moisture content than the concentrated stream;   a rotary drier having means for tumbling and advancing the feedstock through the drier;   a combustion chamber in which fuel and gases undergo a combustion process generating exhaust gases which are mixed with other gases to produce a hot gaseous effluent, said hot gaseous effluent being directed through the drier to absorb moisture from the feedstock to provide dehydrated particulate sludge and producing a gaseous discharge having entrained materials;   at least one separator operative on said gaseous discharge for removing entrained materials from the gaseous discharge;   fan means operative on said gaseous discharge for inducing flow through the separator;   a first gas flow proportioning valve means operative on the gaseous discharge received from the separator for directing a first of said gaseous discharge to the combustion chamber;   a gas scrubber operative on a second portion of said gaseous discharge having a water flow path through which the gaseous discharge passes for cooling and removing entrained materials from the gaseous discharge;   a second gas flow proportioning valve means operative on said second portion of said gaseous discharge received from the gas scrubber for dividing said second portion into a third portion and a fourth portion, said third portion of said gaseous discharge being directed to the combustion chamber;   an afterburner operative on said fourth portion of said gaseous discharge to decompose noxious components of said fourth portion; and   whereby energy use is reduced by the introduction of the first and third portions of gaseous discharge into the combustion chamber and by reducing the volume of gaseous discharge routed through the gas scrubber to the volume of the second portion which decreases the volumetric requirements of the gas scrubber and by reducing the volume of gaseous discharge routed through the afterburner to the volume of the fourth portion which decreases energy use requirements of the afterburner.   
     
     
       2. The system of claim 1 wherein the temperature and humidity of the hot gaseous effluent is adjusted by changing the volume of said first portion and said third portion by adjusting said first and second gas flow proportioning valve means to obtain a predetermined moisture level in the dehydrated particulate sludge thereby compensating for variation in the composition of the sewage sludge stream. 
     
     
       3. The system of claim 1 wherein said gases undergoing the combustion process include air at ambient temperature and said first portion of gaseous discharge, said gases being combined with the products of the combustion process to provide said hot gaseous effluent which is characterized by a temperature that is below the combustion point of the feedstock and in sufficient volume and low moisture content to permit absorption in the drier of moisture from the feedstock to obtain a predetermined moisture level in the dehydrated particulate sludge. 
     
     
       4. The system of claim 1 wherein a part of said fuel is solid fuel burned on a furnace apparatus selected from the group of furnaces including: a moving grid furnace,   a fluid-bed furnace,   a shelf furnace, or   an underfeed stoker.   
     
     
       5. The system of claim 4 wherein said fuel is selected from the group of solid fuels including: wood,   coal,   refuse derived fuel, or   dehydrated particulate sludge.   
     
     
       6. The system of claim 1 wherein said fuel is in liquid or gaseous form. 
     
     
       7. The system of claim 1 wherein the dehydrated particulate matter is dehydrated particulate sludge previously produced by the rotary drier. 
     
     
       8. The system of claim 1 wherein said fan is disposed between the separator and the gas flow proportioning valve. 
     
     
       9. The system of claim 1 wherein said separator includes a plurality of cyclonic separators arranged in a bank connected in series. 
     
     
       10. In the system of claim 1, said afterburner being vented to atmosphere through a stack. 
     
     
       11. The system of claim 10 wherein said stack includes a heat recovery apparatus for transferring heat from the gaseous discharge after passing through the afterburner. 
     
     
       12. A sewage sludge stream treatment system wherein a concentrated stream of sewage is dried in a rotary drier by a hot gaseous effluent from a combustion chamber, the gaseous discharge from the rotary drier having entrained particulate matter and a high moisture content, the gaseous discharge is further processed sequentially by mechanically separating particulate materials, scrubbing in a wet gas scrubber and heating in an afterburner prior to being vented to the atmosphere wherein the improvement comprises: a proportional valve means downstream from the rotary drier for directing a portion of the gaseous discharge to an inlet port in the combustion chamber whereby energy requirements of the system are reduced by the use of preheated gases in the combustion chamber and by reducing the volumetric requirements of the afterburner.   
     
     
       13. The sewage sludge stream treatment system of claim 12 wherein the proportional valve means directs said portion of the gaseous discharge prior to scrubbing in the wet gas scrubber. 
     
     
       14. The sewage sludge stream treatment system of claim 12 wherein the proportional valve means directs said portion of the gaseous discharge after scrubbing in the wet gas scrubber. 
     
     
       15. The sewage sludge stream treatment system of claim 12 wherein said proportional valve means directs said portion of the gaseous discharge prior to scrubbing in the wet gas scrubber, and a second proportional valve means is provided after scrubbing in the wet gas scrubber for directing a third portion of the gaseous discharge to the inlet port in the combustion chamber. 
     
     
       16. The sewage sludge stream treatment system of claim 15 wherein the temperature and humidity of the hot gaseous effluent is adjusted by changing the volume of said portion of the gaseous discharge prior to scrubbing in the wet gas scrubber and by changing the volume of said second portion of the gaseous discharge to obtain a predetermined moisture level in the dehydrated particulate sludge thereby compensating for variation in the composition of the sewage sludge stream. .Iadd. 
     
     
       17.  A process for drying sewage sludge comprising the steps of: introducing a stream comprised of sewage sludge to a rotary drum dryer whereby the sludge comes in contact with a stream of heated gas passing therethrough to produce a stream of dried sludge and heated gas;   separating the stream of dried sludge and heated gas exiting from the dryer into at least one stream comprised substantially of heated gas and at least one stream comprised substantially of dried sludge;   dividing the stream of heated gas into at last two substreams;   directing at least one substream of heated gas to a position prior to the entrance of the dryer; and   subjecting at least one other substream of heated gas to a contaminant removal process. .Iaddend. .Iadd.   
     
     
       18.  The process of claim 17 wherein the stream comprised of sewage sludge further comprises dried sludge. .Iaddend. .Iadd.19. The process of claim 17 wherein the step of separating the exiting streams of heated gas and sludge comprises passing them through means for mechanically separating particulate matter. .Iaddend. .Iadd.20. The process of claim 19 wherein the means for mechanically separating particulate matter are comprised of one or more cyclonic separators. .Iaddend. .Iadd.21. The process of claim 17 wherein the step of dividing the stream of heated gas into at least two substreams comprises passing the stream through at least one proportional valve means. .Iaddend. .Iadd.22. The process of claim 17 wherein the substream of heated gas directed to a position prior to the entrance of the dryer is introduced into a combustion chamber which directly supplies its exhaust gases to the dryer. .Iaddend. .Iadd.23. The process of claim 22 which produces a stream of heated gas exiting from the rotary dryer having an increased water vapor concentration relative to the water vapor concentration of said exhaust gas from said combustion chamber. .Iaddend. .Iadd.24. The process of claim 22 further comprising the step of subjecting some portion of the substream of heated gas to a contaminate removal process prior to its introduction into the combustion chamber. .Iaddend. .Iadd.25. The process of claim 24 wherein the contaminate removal process is comprised of a wet gas scrubber. .Iaddend. .Iadd.26. The process of claim 17 wherein the step of subjecting at least one other substream of heated gas to a contaminate removal process comprises passing it through an afterburner capable of heating the gas to a temperature sufficient to cause combustion of noxious contaminants. .Iaddend. .Iadd.27. The process of claim 26 further comprising the step of passing the substream of heated gas through a wet gas scrubber prior to its passage through the afterburner. .Iaddend. .Iadd.28. The process of claim 17 further comprising the step of passing at least one substream of heated gas through a heat recovery apparatus. .Iaddend. .Iadd.29. The process of claim 28 wherein the substream of heated gas is passed through an afterburner prior to its passage through the heat recovery apparatus. .Iaddend. .Iadd.30. The process of claim 28 wherein the substream of heated gas is introduced to a combustion chamber after its passage through the heat recovery apparatus. .Iaddend. .Iadd.31. A sewage sludge treatment system comprising: a combustion chamber producing a hot gaseous effluent;   a rotary drier which receives a material to be dried and the hot gaseous effluent and produces a gaseous discharge having entrained particulate matter, noxious contaminants, and a high moisture content;   an afterburner in which at least a portion of the gaseous discharge is heated to a temperature sufficient to cause combustion of noxious contaminants; and   means for directing another portion of the gaseous discharge to the combustion chamber where such means are downstream from the rotary drier and result in a reduction in the amount of gaseous discharge heated by the   
     
     
        afterburner. .Iaddend. .Iadd.32.  The sewage sludge treatment system of claim 31 further comprising means for reducing the moisture content of the gaseous discharge. .Iaddend. .Iadd.33. The sewage sludge treatment system of claim 32 wherein the means for directing directs said portion of the gaseous discharge after said portion has passed through said means for reducing the moisture content of the gaseous discharge. .Iaddend. .Iadd.34. The sewage sludge treatment system of claim 32 wherein the means for directing directs said portion of the gaseous discharge before said portion has passed through the means for reducing the moisture content of the gaseous discharge. .Iaddend. .Iadd.35. The sewage sludge treatment system of claim 34 further comprising a second means for directing positioned after the means for reducing the moisture content of the gaseous discharge wherein said second means for directing directs another portion of the gaseous discharge to the combustion chamber. .Iaddend. .Iadd.36. The sewage sludge treatment system of claim 31 further comprising a heat recovery apparatus through which at least one substream of heated gas is passed. .Iaddend. .Iadd.37. The sewage sludge treatment system of claim 36 wherein the substream of heated gas is passed through the afterburner prior to its passage through the heat recovery apparatus. .Iaddend. .Iadd.38. The sewage sludge treatment system of claim 36 wherein the substream of heated gas is introduced to the combustion chamber after 
     
     
        its passage through the heat recovery apparatus. .Iaddend. .Iadd.39.  A process for drying sewage sludge comprising the steps of: introducing a stream comprised of sewage sludge to a rotary drum dryer whereby the sludge comes in contact with a stream of heated gas passing therethrough to produce a stream of dried sludge and heated gas;   separating the stream of dried sludge and heated gas exiting from the dryer into at least one stream of heated gas and at least one stream of dried sludge;   dividing the stream of heated gas into at least two substreams;   directing at least one substream of heated gas to a position prior to the entrance of the dryer; and   passing at least one other substream of heated gas through an afterburner capable of heating the gas to a temperature sufficient to cause combustion of noxious contaminants. .Iaddend. .Iadd.40. The process of claim 39 further comprising the step of passing the at least one other substream of heated gas through a wet gas scrubber prior to its passage through the   
     
     
        afterburner. .Iaddend. .Iadd.41.  A process for drying sewage sludge comprising the steps of: introducing a stream comprised of sewage sludge to a rotary drum dryer whereby the sludge comes in contact with a stream of heated gas passing therethrough to produce a stream of dried sludge and heated gas;   separating the stream of dried sludge and heated gas exiting from the dryer into at least one stream of heated gas and at least one stream of dried sludge;   dividing the stream of heated gas into at least two substreams;   passing at least one substream of heated gas through a heat recovery apparatus;   directing said at least one substream of heated gas to a position prior to the entrance of the dryer; and   subjecting at lest one other substream of heated gas to a contaminant   
     
     
        removal process. .Iaddend. .Iadd.42.  The process of claim 41 wherein said at least one substream of heated gas passed through the heat recovery apparatus is first passed through an afterburner. .Iaddend. .Iadd.43. The process of claim 41 wherein the substream of heated gas passed through the heat recovery apparatus is subsequently introduced into a combustion 
     
     
        chamber. .Iaddend. .Iadd.44.  A process for drying sewage sludge comprising the steps of: introducing a stream comprised of sewage sludge to a rotary drum dryer whereby the sludge comes in contact with a stream of heated gas passing therethrough to produce a stream of dried sludge and heated gas;   separating the stream of dried sludge and heated gas exiting from the dryer into separate streams of heated gas and dried sludge;   passing the stream of heated gas through at least one proportional valve means to form at least two substreams of heated gas;   directing at least one substream of heated gas to a position prior to the entrance of the dryer; and   subjecting at least one other substream of heated gas to a contaminant removal process. .Iaddend.