Classifications

• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F1/00—Treatment of water, waste water, or sewage
  • C02F1/52—Treatment of water, waste water, or sewage by flocculation or precipitation of suspended impurities
  • C02F1/54—Treatment of water, waste water, or sewage by flocculation or precipitation of suspended impurities using organic material
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F1/00—Treatment of water, waste water, or sewage
  • C02F1/52—Treatment of water, waste water, or sewage by flocculation or precipitation of suspended impurities

Definitions

• This invention relates to a process for separating suspended solids from an aqueous suspension and which includes a solids-liquids separation stage that is conducted by sedimentation, and is of particular value in the sedimentation of sewage.
• flocculant material it is well known to add a flocculant material to a suspension so as to cause the suspended material to flocculate before solids-liquids separation.
• Suitable flocculants for this purpose include alum and other polyvalent metal salts, bentonite, and various natural or synthetic polymers that can be non-ionic, anionic or cationic.
• flocculation we use the term "flocculation" as a generic term to embrace processes that might more accurately be referred to as coagulation and processes that are conventionally referred to as flocculation and which probably involve bridging flocculation.
• coagulant In the coagulation processes, charge neutralisation is a major effect and if the coagulant (flocculant) is a polymeric material then the polymer is generally of relatively low molecular weight and high cationic charge density. In the conventional flocculation processes (referred to herein as "bridging flocculation") the polymeric flocculant probably acts primarily by a bridging mechanism and so is generally of relatively high molecular weight.
• bridging flocculation the polymeric flocculant probably acts primarily by a bridging mechanism and so is generally of relatively high molecular weight.
• flocculant and flocculate are used in a similar generic sense.
• a process according to the invention for separating suspended solids from an aqueous suspension comprises flocculating the suspended material by mixing a cationic polymeric flocculant into the suspension, aggregating the flocculated material by mixing an anionic colloidal material into the flocculated suspension, and separating the aggregated material from the suspension by sedimentation.
• the invention can be applied to a wide variety of sedimentation processes where the flocculated particles are allowed to sediment under gravity either in a static tank or while flowing through a tank provided with a stack of plates or other surfaces to promote sedimentation.
• a preferred plate settler apparatus is described in EP-A- 354744.
• the process can be used for, for instance, treating industrial effluents such as paper mill effluent or pigment effluent
• the invention is preferably used for suspensions of organic material and is of particular value for sewage sedimentation stages in which raw sewage or a sewage sludge or biologically treated sewage is allowed to undergo sedimentation to separate solids from a clarified liquor.
• a conventional process for treating raw sewage includes the following treatment stages:
• effluent handling for instance by being discharged without prior treatment or after treatment chemically and further separation by a tertiary treatment stage.
• the invention is preferably applied to the first sedimentation stage, but can be applied to later stages in the process.
• the result of this process is the formation of a primary sludge and a clarified liquor that can then be subjected to biological treatment to convert soluble and colloidal materials in it to biomass which can then be separated from the liquor by a secondary sedimentation stage to form a secondary sludge and a secondary liquor that can either be discharged or subjected to a tertiary treatment.
• the treatment of the invention can, if desired, be applied to the second and/or third sedimentation stages as well as to the first.
• the cationic polymer that is used in the invention can be a natural cationic polymer such as chitosan or a modified natural cationic polymer such as cationic starch.
• the polymer is an organic synthetic polymer that is substantially water soluble and that is formed by polymerising one or more ethylenically unsaturated monomers, in generally acrylic monomers, that consist of or include cationic monomer.
• Suitable cationic monomers are dialkylaminoalkyl (meth) acrylates and dialkylaminoalkyl (meth) acrylamides, either as acid salts or preferably as quaternary ammonium salts.
• the alkyl groups may each contain 1 to 4 carbon atoms and the aminoalkyl group may contain 1 to 8 carbon atoms.
• dialkylaminoethyl (meth) acrylates dialkylaminoethyl (meth) acrylamides and dialkylaminopropyl (meth) acrylamides.
• These cationic monomers are preferably copolymerised with a non-ionic monomer, preferably acrylamide.
• Cationic amphoteric polymers (including a minor amount of anionic groups) can also be used.
• Preferred polymers are particulate as in EP 202780.
• cationic polymers that may be used include polyethylene imines, dicyandiamide polymers, polyamine epichlorhydrin polymers and polymers of diallyl monomers such as diallyl methyl ammonium chloride (DADMAC) , either as homopoly er or copolymer with acrylamide or other comonomer.
• DMDMAC diallyl methyl ammonium chloride
• the cationic polymer that is added to the suspension before the anionic colloid should have a sufficiently low molecular weight that the flocculation process can more properly be described as coagulation.
• the molecular weight of the polymer is such that IV is not above 3dl/g, e.g., 0.2 to 2dl/g, or molecular weight 50,000 to 2 million.
• Suitable low molecular weight polymers of this type are low molecular weight versions of polymers discussed above and in particular preferred polymers for use in the invention include polyamines, polyethylene imines, polymers of diallyl dimethyl ammonium chloride (for instance as homopolymer or copolymer containing a minor amount of acrylamide) and polymers of dialkylaminoethyl aerylate or methacrylate acid addition or quaternary salt, generally as homopolymers or copolymers with acrylamide.
• the cationic polymer generally has a relatively high charge density, for instance above 0.2 and preferably 0.4 to 2.5 equivalents of catonic nitrogen per kilogram of polymer.
• the cationic polymer has IV below about 3dl/g the cationic content is preferably relatively high.
• the polymer may be a substantial homopolymer or formed from a monomer blend at least 50% and generally at least 80%, by weight cationic monomer, any remaining monomer generally being acrylamide or other non-ionic monomer.
• DADMAC polymers especially homopolymers or, less preferably, copolymers with up to 20% or even 50% acrylamide, is especially preferred.
• the low molecular weight polymers can be of the type that are available commercially as solutions, but it is particularly preferred that they should have IV above ldl/g and that they should be of the type that is commercially available as beads since it seems better results are obtained using these bead polymers. They may be dissolved before addition to the sewage or they may be added as beads, for instance as published in WO92/00247.
• a relatively low molecular weight cationic polymer for instance a relatively highly charged cationic polymer having molecular weight 50,000 to 2 million
• a higher molecular weight cationic polymer for instance to act as a bridging flocculant, followed by the anionic colloidal material.
• a bridging flocculant alone.
• the bridging flocculant is preferably a polymer made from ethylenically unsaturated monomers as described above and having an IV of at least 4dl/g. Often the amount of cationic monomer is 8 to 40, often around 10 to 20, mole percent with the balance being acrylamide.
• Particularly preferred processes of the invention involve flocculation with a relatively low molecular weight cationic polymer (i.e., coagulation) followed by the addition of the anionic colloidal material, followed by the addition of further polymeric flocculant before the sedimentation.
• This further polymeric flocculant is preferably a bridging flocculant and so is preferably of high molecular weight, for instance IV above 4dl/g. It can be a cationic flocculant in which event it generally has been formed from 3 to 25, preferably 8 to 15, mole percent cationic monomer with the balance acrylamide.
• non-ionic flocculant or an anionic flocculant usually formed from 3 to 25 mole percent anionic monomer, for instance sodium acrylate or other carboxylic or sulphonic ethylenically unsaturated monomer, copolymerised with acrylamide or other suitable non-ionic monomer.
• the process can be conducted with relatively low degrees of mixing, with the result that there is little or no degradation of the initial floes before the anionic colloidal material is added, it is often preferred to stir sufficiently to break down any large particles that may have aggregated before adding the anionic colloidal material. In some instances it may be desirable to achieve an overall reduction in floe size by applying sufficient stirring or other agitation to the flocculated suspension.
• cationic polymer is made at some position ahead of the clarifier or other sedimentation equipment and anionic colloidal material is added after the polymer addition but before the sedimentation stage, and preferably bridging flocculant is added after the anionic colloid but before sedimentation.
• anionic colloidal material is added after the polymer addition but before the sedimentation stage, and preferably bridging flocculant is added after the anionic colloid but before sedimentation.
• the anionic colloidal material can be an emulsion of water-insoluble anionic organic polymer, or it can be an anionic inorganic polymer such as colloidal silicic acid, but preferably it is an anionic swelling clay.
• Such clays are known to swell to a very large extent when the dry clay 10 is contacted with water. They are generally smectites and are frequently mont orillonites. Suitable materials are referred to as bentonites such as Wyoming bentonite, or Fullers Earth.
• the bentonite or other clay may have been chemically modified, e.g., by alkali treatment to convert 15 calcium bentonite to alkali metal bentonite.
• the bentonite or other clay preferably swells by at least 10 or 20 times its own volume and the dry clay is contacted with water and the surface area of the anionic colloid before swelling is preferably at least 30m /g and the surface 20 after swelling is preferably at least 400, e.g, up to 800m /g.
• the bentonite or other clay may be provided as a powder that is mixed with water to form a slurry for convenient addition to the aqueous suspension, or may 25 initially be provided as a slurry, and this in turn may be provided as a concentrated slurry that contains low molecular weight sodium polyacrylate or other dispersing agent or other material that will render the concentrate sufficiently fluid for ease of handling.
• the amounts of cationic polymeric flocculant and anionic colloidal material used in the invention will depend upon the suspension that is being treated and the degree of agitation that is applied and the nature of the suspension and of the sedimentation process. The optimum 35 can be determined by routine screening. If the flocculated suspension is to be agitated significantly prior to adding the anionic colloidal material, it is desirable to include that are present after the agitation all carry a relatively heavy cationic charge due to the flocculant polymer.
• the amount of polymer that is included in the suspension is generally at least lppm or 2ppm (based on the volume of suspension) and is generally at least 5 or lOppm. It can be up to, for instance, 500ppm but the amount is generally below lOOppm and is frequently below 50ppm. Values of 2 to 50, often around 2 to 30, mg/1 are often preferred. These are all based on the total weight of suspension. Based on the solids content of the suspension, the amounts typically are at least 0.1% and often at least 0.5%, but generally below 5% and often below 2%, by weight of the solids content of the dispersion. If too much polymer is used, the disadvantages of overdosing will occur.
• the amount needed for optimum results may be reduced if the suspension already contains cationic polymer from a previous stage, for instance coagulant polymer may already be in the suspension in which event the amount of low molecular weight polymer that should be added can be reduced.
• the amount of anionic colloidal material is generally at least lOppm and usually at least 30ppm and preferably at least 50ppm, based on the volume of the suspension that is to be sedimented.
• the amount is generally below 500ppm and frequently it is below 250ppm. Amounts around 20 to 200mg/l with best results often in the range 30 to 100mg/l are often preferred.
• the amounts typically are above 0.1% and generally above 0.3% and often above 1%, but the amount is generally below 10%, and preferably below 3% by weight.
• the solids content of the liquor that is treated is normally typical for primary sedimentation liquors, for instance 100 to 400ppm, but if the liquor is a sludge then the solids content will be higher, tyically 2,000 to 8,000ppm.
• the test as a whole typically takes around 10 minutes.
• A bead polymer of diallyl dimethyl ammonium chloride having an IV of 1-2.5, in the form of a 1% solution or added as beads direct to the sewage.
• D low anionic high molecular weight polyacrylamide, in the form of a 0.05% solution.
• each run is given a relative order of effect of magnitude 1-4 as compared to the other runs in that example, 1 being the best and 4 being the worst.
• the order of effect in each example is independent of that in every other example.
• Example 1 This example investigates the combined effect of coagulant and bentonite as compared to their separate effects. The results are shown in Table l.
• Run 2 having an order of effect of 4, was the worst run in this example and illustrates that a dosage of 40ppm A overdosed the system.
• D the flocculant
• the presence of D had minimal effect when subsequently added to the A floe but in contrast worked relatively well on the A + bentonite flocc.
• This example investigated the effect of varying the concentration of bentonite used in the process of the invention.

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• Life Sciences & Earth Sciences (AREA)
• Hydrology & Water Resources (AREA)
• Engineering & Computer Science (AREA)
• Environmental & Geological Engineering (AREA)
• Water Supply & Treatment (AREA)
• Chemical & Material Sciences (AREA)
• Organic Chemistry (AREA)
• Separation Of Suspended Particles By Flocculating Agents (AREA)

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Cited By (4)

Citations (2)

• 1991
  • 1991-08-02 GB GB919116699A patent/GB9116699D0/en active Pending
• 1992
  • 1992-08-03 ZA ZA925818A patent/ZA925818B/xx unknown
  • 1992-08-03 MX MX9204513A patent/MX9204513A/es unknown
  • 1992-08-03 AU AU23801/92A patent/AU2380192A/en not_active Abandoned
  • 1992-08-03 WO PCT/GB1992/001431 patent/WO1993002968A1/en active Application Filing

Patent Citations (2)

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