Liquid purification or separation – Processes – Making an insoluble substance or accreting suspended...
Patent
1997-04-16
1998-12-08
Hruskoci, Peter A.
Liquid purification or separation
Processes
Making an insoluble substance or accreting suspended...
210727, 210734, 210735, 210928, C02F 1114
Patent
active
058464336
DESCRIPTION:
BRIEF SUMMARY
This application is a 371 of PCT GB95 01333, filed Jun. 6, 1995.
This invention relates to processes in which a suspension (for instance sewage sludge) is flocculated to form a flocculated suspension which is then dewatered to form a thickened sludge or cake and a separated liquor (filtrate or centrate or supernatant).
It is usually adequate to flocculate the feed suspension by use of a polymeric flocculant alone, prior to the dewatering. However it is also known to treat the suspension with a coagulant, which can be an inorganic coagulant such as alum, or a polymeric coagulant. Polymeric coagulants have a lower molecular weight than flocculants and, usually, a higher charge. Processes in which polymeric coagulant and flocculant are used are relatively uncommon but are known, for instance in U.S. Pat. No. 4,588,508 and EP-A-479616.
It is well known that it is necessary to select the optimum dose of flocculant in order to obtain optimum dewatering, but what constitutes optimum dewatering in any particular process depends upon the objective of that process. For example, in some processes the primary objective is to provide a cake having the highest possible solids content whilst in other processes the objective may be to provide separated liquor having the highest clarity and lowest turbidity. There is usually a conflict between optimising cake solids and optimising clarity, probably because the very fine hydrophilic suspended solids that cause inferior clarity may tend to hold water in the cake if they are trapped in the cake rather than left as turbidity in the separated liquor. Accordingly, the operator has to select an optimum which may be based on optimising cake solids or clarity or a compromise. In many commercial processes some or all of the separated liquor is recycled to an earlier stage in the process, thereby allowing optimisation of cake solids provided the recycled turbidity does not cause an unacceptable gradual deterioration in the process.
The suspension that is being dewatered is likely to be of variable composition and so it is desirable, in theory, to vary the dose of flocculant according to variations in the composition of the suspension in order to hold performance at the chosen optimum. Such variations can be made as a result of observation of the performance of dewatering, for instance changes in the solids content of the cake. However this is a rather slow, inconvenient and inefficient way of controlling the dewatering process. Accordingly various ways are known for observing a dewatering parameter of the feed suspension, the flocculated suspension or the separated liquor and utilising the observed parameter to control the flocculant dosage.
For instance the zeta potential of the suspension is utilised in JP-A-02052100. Minimisation of zeta potential is discussed in Pulp and Paper, August 1977, Volume 51, 9, pages 112 to 115 and controlling a process in response to zeta potential measurement is also discussed in U.S. Pat. No. 3,594,313. Optimising cake solids by adjusting the addition of polymer in response to measurement of charge by streaming current is described in JP-A-04083600. Reduction in the amount of polyvalent metal salt coagulant required for treating water is achieved by monitoring charge in the water in J Inst Water Environment Management 1992, 6(1), pages 1 to 9. Other ways of controlling polyvalent metal coagulant in the treatment of tap water in response to measurement of, for instance, acidity in the tap water are described in JP-A-03284305.
These methods are indirect in that they determine a physical property, but it is also known to control the process by monitoring the actual dewatering performance of the suspension. Control by monitoring the filterability of the suspension is described in WO94/17895 and control by monitoring the turbidity of the separated liquor is described in WO94/17394. Control of the amount of alum in a process which also uses high molecular weight anionic polymer for the treatment of water is made by measurement of the ortho-phosphorous conc
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Field John Rodney
Sorensen Jens
AB CDM
Allied Colloids Limited
Hruskoci Peter A.
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