Drying and gas or vapor contact with solids – Process – With contacting of material treated with solid or liquid agent
Reexamination Certificate
1999-06-29
2001-05-29
Wilson, Pamela (Department: 3749)
Drying and gas or vapor contact with solids
Process
With contacting of material treated with solid or liquid agent
C034S379000, C034S386000, C034S389000, C210S609000, C210S770000, C073S427000, C073S428000, C071S064080
Reexamination Certificate
active
06237246
ABSTRACT:
This invention relates to treating sewage sludge or similar organic sludge, such as that obtained from industrial sites such as waste water treatment plants.
EP 0 283 153 B1 discloses a method of treating wastewater sludge to provide a fertiliser for agricultural lands which can be applied directly to the land. The method involves mixing the sludge with alkaline material of specified fineness to raise the pH of the mixture to at least 12 for at least a day and to effect pasteurisation, and then drying the mixture. Drying is done either a) by aeration and maintaining the pH above 12 for at least seven days until the solids levels reach and maintain a minimum of 65% bulk solids, or b) by aeration and heating to a temperature of at least 50° C. and so that the solids levels reach and maintain a minimum of 50% solids. In b) the temperature should not be so high as to destroy all non-pathogenic organisms. The elevation of the pH and the drying are effected so as to reduce odour and undesirable viruses, bacteria, parasites and vector (e.g. flies) attraction to the sludge and prevent significant pathogen regrowth while not eliminating beneficial non-pathogenic microorganisms. At least some of the temperature rise is due to the exothermic reaction with the alkali. The product is allowed to air cure for about 10 days after achieving the desired solids content. Drying and curing may be effected by windrowing, turn-over or other forced air methods, and curing or aeration time is dependent on the aeration procedure and other factors and clearly need to be determined, for any particular circumstances, experimentally to see that the required end point has been reached. This involves taking samples to measure the solids content, testing for:
animal viruses—less than one plaque forming unit per 100 ml
salmonella bacteria—less than three colony forming units per 100 ml
parasites—less than one viable egg per 100 ml
significant regrowth of the pathogenic microorganisms—there should be none
and assessing the reduction of odour to a level tolerable in a closed room and that this reduction is maintained indefinitely under any climatic conditions, and whether flies are less attracted to the product. At the same time, the presence of at least some beneficial non-pathogenic microorganisms must be established.
If method b) is used, the minimum recommended time for maintaining the temperature at at least 50° C. (but not so high as to destroy all non-pathogenic microorganisms) is at least 12 hours and the heat treated alkaline stabilised dewatered sludge cake is then air dried (while the pH remains above 12 for at least 3 days) through intermittent turning of windrows until a minimum of 50% solids content is achieved.
GB 2 276 876 A, which refers to EP 0 283 153, describes treatment of sewage sludge having a solids content of at least 15% by weight with an alkaline material containing free lime, and storing and/or drying and/or composting the resultant mixture, adding sufficient lime to the mixture to achieve a pH of at least 10. Higher pH values, even higher than 12, are not ruled out, but it is suggested that excellent reduction in pathogens can be achieved at pH levels below 12 with reduced evolution of ammonia, which renders the treated sludge and its surroundings less unpleasant and means that the treated material retains more nitrogen increasing its value as a fertiliser.
The method of GB 2 276 876 involves dewatering the sludge, e.g. in a press, and measuring it using a weigh hopper into a mixer into which is delivered alkaline material in the appropriate amount, the mixed material being delivered to a skip to deliver it to a windrowing area where it is turned periodically, normally for a period up to seven days. However, it is suggested also that provided the amount of calcium oxide that has been introduced is sufficient to raise the pH to a value greater than 12, and sufficient, indeed, to ensure that it stays above 12 for at least 2 hours, then it is possible to store the mix for not less than two hours, but then apply it directly to the land.
It is apparent that the methods described in these two specifications are labour intensive and to a large extent dependent upon a subjective judgement (the assessment of odour in a closed room, for example) or upon results of biological tests which themselves could take days or weeks before you have a result.
It is necessary, for effective treatment, to mix the alkali with dewatered sludge, but sludge, nonetheless that comprises a substantial quantity of water, as it is only in the presence of water that the desired exothermic chemical reaction takes place. The end product—as is clear from the specification discussed—is required to have less water content than is appropriate during the heat-development stage. It is an essential part of the process, then, that the product be eventually dried, and this is the reason for treating it by windrowing. Windrowing, especially aeration by turning over windrows, is not only labour intensive and floor space intensive, it is a slow method of drying.
The present invention provides processes and apparatus for treating sewage sludge or similar organic sludge which are more efficient, less labour and space intensive, and more controllable (so as not to depend upon the results of bio-technological testing and measurement procedures) than prior art processes to date.
The invention comprises a process for treating sewage sludge or similar organic sludge in which dewatered sludge cake and an alkaline admixture are mixed and then dried under pasteurisation conditions utilising heat from the exothermic reaction with the alkali, in which drying is effected by extraction of moisture evaporated from the mixture by the exothermic heat.
The drying may be effected in a first in first out (FIFO) hopper.
Drying may be effected in a cycling operation until a predetermined moisture content is detected, then the dried material is discharged.
The drying may be effected to dry the product to between 50 and 65%, preferably between 55 and 60% dry solids content. The drying product may pass two or more times through a drying hopper. Drying may be assisted by an air flow, which may be fan generated. Gases given off during drying may be passed through an ammonia scrubber.
The dried material may be tested for moisture (or dry solids content) by a sensor the output of which determines whether the product is recycled or delivered to another location, e.g. a stockpile. The dried mixture may be tested by an infra-red moisture measuring transducer.
At the beginning of the process, the dewatered sludge may be measured for water content and the measure used to control the alkaline admixture—the dewatered sludge may pass beneath an infra-red moisture measuring transducer.
The dewatered sludge cake may be fed to a storage bin from where it is delivered to a mixing arrangement with the alkaline admixture. The storage bin may have a level sensor controlling the supply of sludge cake to the storage bin and/or delivery of sludge cake to the mixing arrangement. The delivery of sludge cake to the mixing arrangement may be measured and the measurement used to control the supply of alkaline admixture to the mixing arrangement. Delivery of sludge cake to the mixing arrangement may be effected by a weigh belt.
The moisture content of the mixed dewatered sludge cake and alkaline admixture may be measured and the measurement may be used to control the addition of said admixture to said sludge cake.
The moisture content of the mixed dewatered sludge cake and alkaline admixture may be measured before drying and the measurement used to control the drying process.
Dewatered sludge cake may be fed directly from a dewatering arrangement such for example comprising a belt press arrangement.
The belt press arrangement may comprise primary and secondary high pressure belt press arrangements.
The output from the infra-red moisture measuring transducer may be used to control sludge cake throughout via adjustment of the operation of the belt process arrangement.
The process may
Gunter, Jr. Charles D.
R3 Management Limited
Wilson Pamela
LandOfFree
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