Liquid purification or separation – Processes – Making an insoluble substance or accreting suspended...
Reexamination Certificate
1999-10-27
2002-09-10
Hruskoci, Peter A. (Department: 1724)
Liquid purification or separation
Processes
Making an insoluble substance or accreting suspended...
C210S712000, C210S710000, C210S734000, C210S195100, C210S196000
Reexamination Certificate
active
06447687
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to processes and apparatus for dewatering sewage sludge so as to produce a cake and a reject liquor which can be a supernatant, filtrate or centrate.
BACKGROUND TO THE INVENTION
A typical sewage treatment plant takes in raw sewage and produces solids and clarified water. Typically the raw sewage is treated in a primary sedimentation stage to form a primary sludge and supernatant, the supernatant is subjected to biological treatment and then a secondary sedimentation stage to form a secondary sludge and clarified liquor, which is often subjected to further treatment before discharge.
The sludges are usually combined to form a mixed sewage sludge which is then dewatered to form a cake and a reject liquor. The reject liquor is usually recycled to the head of the plant and the start of the process, i.e., fed back to the primary sedimentation stage or a preceding stage in the plant. Any water which is required in the plant, for instance for dissolving polymeric flocculant, is usually either potable water (from the local drinking water supply) or is clarified water from the secondary sedimentation stage, optionally after any subsequent treatment procedures.
It is standard practice to dewater the sludge by mixing a dose of polymeric flocculant into that sludge at a dosing point, and then substantially immediately subjecting the sludge to the dewatering process and thereby forming a cake (typically having a solids content of 15 to 35%) and a reject liquor. The dewatering process may be centrifugation or may be by processes such as filter pressing or belt pressing.
The process is generally conducted so as to obtain the highest possible cake solids, preferably accompanied by the highest possible reject liquor clarity. It is well established that, for any particular performance parameter (such as cake solids) there is an optimum dosage of flocculant and that increasing the dosage above this optimum does not result in an improvement but instead tends to result in overdosing and in deterioration of performance. Accordingly, it can be assumed that polymer applied at dosages of up to the optimum is substantially all adsorbed on to the sludge so as to participate in bridging flocculation, whereas overdosing of polymer is liable to introduce additional polymer into the sludge with the result that excess polymer may remain in the reject liquor and the presence of this excess can interfere with the bridging flocculation performance.
There is a maximum solids loading rate (kg dry matter per hour) and a maximum hydraulic loading rate (liters sludge per hour) which can be handled in any particular dewatering apparatus. It has conventionally been regarded to be desirable normally to operate as close as reasonably practicable to the maximum solids loading rate, and in order that this can be achieved without exceeding the hydraulic loading rate it is necessary that the sludge which is dewatered should have a solids content within an appropriate range. If the sludge would normally have a solids content below this, it is conventional to subject it to a thickening stage (often promoted by the use of polymeric flocculant) before dosing the polymeric flocculant into the resultant sewage sludge substantially immediately prior to the final dewatering intended to form the cake and the reject liquor.
When setting up a dewatering process, it is therefore generally necessary to select the optimum dose and the optimum solids loading rate and hydraulic loading rate.
Thus the optimum combination of solids and hydraulic loading rates is selected to give the most efficient utilisation of the dewatering apparatus having regard to the sludge that is being dewatered. Thereafter, in most cases, it is generally undesirable to dilute the sludge since that increases the hydraulic loading rate without increasing dewatering efficiency.
In other processes, the sludge which is to be dewatered may have a solids content which is too high for optimum results, for instance because the viscosity of the sludge is such that it is difficult to achieve efficient mixing of the flocculant into the sludge and/or to achieve efficient distribution of the flocculated sludge in the dewatering apparatus. Under these circumstances, it is necessary to dilute the sludge prior to dosing the polymeric flocculant into it. The water which is used for dilution is usually the same water as is used for other purposes in the overall plant, namely either water from the drinking water supply or clarified water from the plant.
Various proposals have been made in the literature for recycling various streams to various points in a sewage treatment plant (additional to the normal recycling of the final reject liquor to the head of the plant, as discussed above).
For instance, in U.S. Pat. Nos. 5,075,012 and 5,248,416 a stream of unfiltered liquor which is approaching the dewatering apparatus and which contains flocculated solids is recycled to an earlier point in the feed line so as to promote flocculation of the feed.
When, as mentioned above, it is necessary to thicken the sludge by sedimentation or filtration prior to the final dewatering which forms a cake, the filtrate (or supernatant) from the thickener is sometimes not as clear as is desired. Instead of recycling all of this thickener filtrate or supernatant to the head of the plant, it is known to reuse the thickener supernatant or filtrate for diluting the polymeric flocculant which is added either to promote the thickening or to promote the subsequent dewatering.
In JP-A-57150480 it is proposed to hold effluent in a store tank and subject it to sedimentation in a sedimentation tank into which polymeric flocculant is added. Sludge is taken from the base of this tank and some of the supernatant from the tank is recycled to the store tank so as to dilute the liquor which is being passed to the sedimentation tank. It is alleged that the amount of flocculant which is needed in the sedimentation tank is reduced because of the increased dilution of the solids in that tank, but a disadvantage of this process is that the hydraulic load in the tank is increased.
In JP-A-58146498 sewage sludge, after aeration, is subjected to sedimentation in a sludge concentration tank in which sludge precipitates under gravity. The precipitated sludge is taken to a centrifugal thickener and the supernatant from the centrifugal thickener is combined with the supernatant from the sedimentation tank. The sludge from the centrifugal thickener is dosed with polymeric flocculant and is then subjected to dewatering to provide a cake and a filtrate. It seems that the amount of flocculant which is added is more than would normally be regarded as optimum as it is alleged that, due to the high dosage, large amounts of flocculant are lost with the filtrate and eutrophication is a significant problem in the resultant effluent.
Filtrate from the dewatering apparatus is recycled to the sludge entering the centrifugal thickener and/or to the sludge entering the sedimentation concentration tank and this is alleged to improve the sedimentation and/or thickening. However it should be noted that the only point at which flocculant is added is immediately prior to the dewatering apparatus and that the sedimentation concentration and the centrifugal thickening are both conducted without the deliberate addition of flocculant. Since these processes were being conducted without flocculant and since the filtrate is said to contain wasted flocculant, it is not surprising that adding some of this filtrate to the sludge which is to be subjected to sedimentation and/or centrifugal thickening in the absence of deliberately added flocculant would lead to an improvement in the sedimentation or thickening.
We are concerned with something entirely different, namely achieving a performance in a dewatering process which produces a cake and a reject liquor which is better than a performance which, prior to the invention, would have been regarded as already being optimum in that particular process. Thus, we are concerne
Hunter Dewey W
Winn Edward H
Ciba Specialty Chemcials Water Treatments Ltd.
Crichton David R.
Hruskoci Peter A.
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