Liquid purification or separation – Processes – Chemical treatment
Reexamination Certificate
2001-03-23
2002-05-14
Hoey, Betsey Morrison (Department: 1724)
Liquid purification or separation
Processes
Chemical treatment
C210S766000, C210S768000, C210S770000, C210S790000, C210S198100, C210S252000, C210S259000
Reexamination Certificate
active
06387281
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to processes and apparatus for treating sewage sludge or similar organic sludge, such as that obtained from industrial sites such as waste water treatment plants.
2. Description of the Prior Art
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, such as ambient temperature and humidity, 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 least 50° C. (but not so high as to destroy all non-pathogenic microorganisms) is at least 12 hours and the heat treated alkaline stabilized 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 belt 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 tat 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, but is a slow method of drying, and exposes personnel to excessive levels of ammonia and volatile organic compounds.
There are further problems and disadvantages with prior art methods such as those disclosed in EP 0 283 153 and GB 2 276 876. For example, a relatively large quantity of alkaline material is required: so much so that the alkaline material constitutes a significant proportion of the end product produced by the treatment. This adds to costs in three significant ways. Firstly, there are costs associated with providing large quantities of alkaline material. Secondly, there are considerable costs associated with materials, handling of increased mixed product volume through the heat pulse and windrow formation and turning. Thirdly, there are transportation costs associated with the removal of the end product from the site. A further consequence is that, because a significant proportion of the end product is alkaline material, the relative proportions of nutrients such as nitrogen and phosphorus in the end product are not only reduced but also are fixed due to the high pH of the product. This reduces the possible value of the end product as an agricultural fertiliser. Furthermore, since such large quantities of alkaline materials are added, a pH greater than 12 is maintained for extended periods and may result in the product being classified as a special waste by regulatory bodies. Special waste requires special land fill disposal.
It is difficult, if not impossible, to evenly control, culture and measure the temperature of a windrowed product. Furthermore, it has been found that, in practise, considerable problems with product odour, in particular ammonia, can be encountered when the method of EP 0 283 153 is employed, both during production and in the finished product.
It should be noted that, under United States of America regulations for the treatment of sewage sludge (EPA Rule 503 Regulations), it is necessary not only to effect pasteuisation (Class A pathogen reduction) but also to effect stabilisation of the product. The latter is termed the Vector Attraction Reduction Requiremnent, and is described at Rule 503.33(b). For the avoidance of doubt, the terms “stabilisers” and “stabilisation” as used herein refer to a process which satisfies the Vector Reduction Requirement as set forth in US EPA Rule 503.33(b).
SUMMARY OF THE INVENTION
The present invention overcomes the aforesaid problems and disadvantages and provides a new paradigm in the treatment of sewage sludge. The present invention provides processes and apparatus for treating sewage sludge which are more efficient, economic, less labour and space intensive, and more controllable, (so as not to depend upon the results of bio-technological testing, inaccurate manual sampling, and measurement procedures such as dry solids content, temperature and pH) than prior art processes to
Kurtz Frederick H.
Millard Robin
Bracewell & Patterson LLP
Hoey Betsey Morrison
R3 Management Limited
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