Liquid purification or separation – Processes – Making an insoluble substance or accreting suspended...
Reexamination Certificate
2001-05-31
2004-10-26
Hruskoci, Peter A. (Department: 1724)
Liquid purification or separation
Processes
Making an insoluble substance or accreting suspended...
C071S013000, C071S022000, C210S721000, C210S725000, C210S737000, C210S738000, C210S756000, C210S764000
Reexamination Certificate
active
06808636
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for treatment of sewage sludge and in particular to a method for reducing the viscosity of dewatered or concentrated sewage sludges having a solids concentration of at least 10% (w/w).
BACKGROUND OF THE INVENTION
Sewage sludges typically have concentrations of 0.5 -3.0% (w/w) solids. The sludges may be thickened, with removal of water, by a variety of methods, including gravitational thickening, flotation thickening, centrifugation and screw and belt pressing. Surfactants may be added to sludge to facilitate particle flocculation as part of the steps to thicken or concentrate the sludge.
Sludges of about 3-5% (w/w) solids are sufficiently low in viscosity that they are still capable of being pumped, which facilitates transfer of the liquid sludges to and from storage and transporting tanks. In addition, the ability to pump sludges facilitates transfer of the sludges to land application equipment and also application of the liquid sludges to the land by surface spraying or subsurface injection.
There is substantial interest in concentrating liquid sludges, having 0.5-5% (w/w) solids, to reduce the volume of material required for transport, storage and/or further handling. However, the high viscosity of more concentrated sludges makes them less suitable for pumping, spraying and injection. It is also recognized that polymeric and other kinds of flocculating agents and conditioners, used to produce concentrated sludges, may also contribute to the increased viscosity of these sludges. Hence, there is a great interest in finding cost effective methods for reducing the viscosity of more concentrated sludges so that they are more easily pumped and/or sprayed and/or injected. and/or further processed.
U.S. Pat. No. 5,188,741 describes a process to treat sewage sludge to increase sludge solids content and to decrease viscosity, which includes a step of heating to about 500° F. (about 260° C.) at a pressure of 750 psig under a blanket of nitrogen. U.S. Pat. No. 5,783,073 describes a process and apparatus to liquefy sludges containing up to 10% solids at a pH of greater than 12. U.S. Pat. No. 5,618,442 describes a process for destroying harmful pathogens in sludge under alkaline conditions in a pasteurization chamber. U.S. Pat. No. 4,471,916 describes a process for reducing sludge viscosity involving mechanical treatment and addition of an alkali metal silicate. U.S. Pat. No. 5,356,540 describes a pumpable aqueous sewage composition that contains a non-ionic water-soluble alkoxylated alkyl phenol, and a process for preparing aqueous sewage sludge having a solids content of 30-65% by weight. U.S. Pat. No. 5,681,481 describes treatment of sewage sludge with a solids content of less than 10%, at a pH of 12.0 or higher for a period of time e.g. at least 50° C. for at least about 12.0 hours.
Methods described to date for producing low viscosity sewage sludge concentrates are either limited to the sludges having <10% solids content or are disadvantageous in that extreme conditions of temperature and/or pressure are required to reduce viscosity to facilitate pumping and/or land spraying and/or injection of the sludges.
SUMMARY OF THE INVENTION
An improved method has been developed for production of concentrated sludges having low viscosities.
Accordingly, one aspect of the present invention provides a method for reducing sludge viscosity of a sewage sludge having a solids concentration of at least 10% (w/w), comprising the steps of:
(a) increasing the pH of the sludge to the range of 9.5-12.5;
(b) selecting at least one step from
(i) maintaining the sludge at the pH of (a) and at a temperature of 10-37° C. for a period of at least one day, and (ii) adding one or more inorganic or organic chemicals to the sludge, such chemicals contributing to viscosity reduction;
(c) incubating the sludge by maintaining the resultant sludge at a temperature in the range of 40-100° C. for a period of time of at least one hour;
(d) subjecting the sludge to physical shearing or disintegration; and
(e) subsequently discharging the sludge.
In embodiments of the process of the present invention, step (b)(i) is selected, step (b)(ii) is selected or both step (b)(i) and step (b)(ii) are selected.
In the embodiment in which step (ii) is selected, at least one of a sodium or potassium salt is added, especially at least one of sodium or potassium chloride, optionally with addition of an oxidizing agent is added in step (c) e.g. sodium hypochlorite or hydrogen peroxide.
In another aspect of the present invention, a low viscosity sewage sludge is obtained, which may be further processed by physical, chemical and/or biological methods, may be applied to land by spraying, injection or other methods, or disposed by any method.
A further aspect of the present invention provides an apparatus for reducing the sludge viscosity of a sewage sludge, comprising:
(a) a device for concentrating and/or dewatering the sludge;
(b) a device in which the viscosity of the sludge obtained from the device of (a) is reduced;
(c) a device to subject the sludge obtained from the device of (b) to shearing; and
(d) means to control the flow and temperature of sludge, said means including means to subject the sludge to holding steps.
DETAILED DESCRIPTION OF THE INVENTION
The term “sewage sludge” as used herein may be broadly defined as “water-carried wastes”, particularly from municipal sanitary sewage lines, containing one or more of body wastes, community waste such as street washings, etc., and industrial wastes and sludges. The total solids content of the sewage in municipal lines is usually about 0.5% w/w or more. The solids in sewage are mostly animal or vegetable substances, “organic matter”, i.e., fats, carbohydrates, and proteins. Some mineral or inorganic constituents present include sand and clay, as well as the common mineral salts found in the water supply.
Sewage sludge contains raw sewage and may contain a number of pathogens known to be health hazards to humans. Almost any type of microorganism may be found in sewage, including bacteria, protozoans, viruses and fungi. It is understood that raw sewage would normally have been passed through screens for the removal of large pieces of stone, wood, metal, and other trash which would clog channels or damage pumps. Coarse heavy inorganic materials, i.e. gravel, cinders, and sand are allowed to settle out. A sewage in the form of an aqueous suspension of sewage sludge and liquids is obtained. Concentration of the sewage sludge may be accomplished by any suitable way for separating solids and liquids, e.g. gravity settlings, filtration, centrifugation, hydrocyclone, or a combination thereof. Preferred methods of concentration include use of a screw press, belt press or centrifuge, or use of pressure, vacuum, heat or evaporation. It is the concentrated material that is preferably subjected to the processes of the present invention. The sludge that is treated according to the process of the present invention has a solids concentration of at least 10% (w/w).
In the concentration of the sewage sludge, a flocculating agent may be added to facilitate the concentration of the sewage sludge. A wide variety of flocculants may be used. The flocculants may be anionic or cationic polyelectrolytes, and in the form of polymers, copolymers and polymeric mixtures. Examples of the flocculants include polymers containing acrylamides, acrylate, quaternary acrylates; poly-N-vinylformamides, polyethyleneimine, chitosan, carboxymethyl-celluloses or -starches, cationized celluloses or starches and sodium alginate. For instance, the flocculating agent may be a cationic polyacrylamide copolymer, e.g. Percol™ 757 cationic polyacrylamide copolymer. The flocculating agent, if used, is preferably used in concentrations of 0.01-0.05% w/v.
The pH of the concentrated sludge is adjusted by the addition of an alkaline material. The alkaline material may be any of a number of caustic compounds including mono- and di- valent hydroxides, especially alkali metal
Burd Henry
Ward Owen P.
Anthony Asquith & Co.
Hruskoci Peter A.
Lystek International Inc.
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