CA2558038A1 - Sewage sludge dewatering process - Google Patents

Abstract

A process of dewatering aqueous suspension employing a flocculating system comprising treating the suspension with a flocculating amount of a first flocculant and a dewatering amount of a second flocculant, and subjecting the suspension to mechanical dewatering to form a cake, wherein the first flocculant brings about flocculation and assists thickening of the suspension and the second flocculant is a water-soluble or water swellable polymer that is mixed into the suspension is a water-soluble or water swellable particulate polymer having a particle diameter of at least 20 microns.

Description

Dewatering Process The present invention concerns the flocculation and dewatering of aqueous suspensions to form a dewatered cake.
It is well known to apply flocculants to aqueous suspensions in order to separate solids from the suspension. For instance it is common practice to flocculate and then dewater suspensions containing either suspended solid organic material or mineral solids. For instance it is common practice to flocculate sledges such as sewage sludge, waste waters, textile industry effluents, red mud from the Bayer Alumina process and suspensions of coal taifrngs etc. Flocculation is usually achieved by mixing into the suspension the tlocculant, allowing fihe suspended parkicles to flocculate and then dewatering the flocculated suspension fio form a dewatered cake.
?5 In the dewatering of suspensions it is known to add a high molecular weight, water soluble polymer as a flocculant to the suspension in order to remove the liquid from the suspension and greatly increase the dry solids of the suspension.
High molecular~weight flocculants may be cationic, anionic, nonionic or amphoteric in nature. The choice of polymeric flocculant will largely depend upon the susbstrate, which is being treated. For instance it is common practice to use high molecular weight cationic flocculants to treat aqueous suspensions comprising suspended organic material, for insfiance sewage sludge. In paper-making it is known to use either cationic, nonionic, anionic or amphoteric flocculants. Flocculation of mineral suspensions is frequently effected by use of anionic flocculants.
It is standard practice to apply polymers as aqueous compositions to flocculate suspensions containing suspended organic material. Generally the compositions of polymers are relatively dilute, for instance no more than 1 %
and usually no more than 0.5%, and can be as low as 0.2% by weight or lower.

Various alternative methods of introducing a flocculant into a suspension have been proposed. WO A 02/079099 describes in a method in which at least one flocculant emulsion and is added directly to a solids liquid separation process and inverted in situ such that flocculant is released directly into the application.
The emulsion is added specifically at the solids liquid separation process and subjected to an efFective amount of high shear for sufficient time and pressure to ensure rapid inversion of the emulsion and complete release of the flocculant info the bulk suspension before any initial separation.
WO A-98/31749 and WO-A-98/31748 and of both concerned with preparing dispersions of high intrinsic viscosity cationic polymers in an aqueous and medium containing dissolved low intrinsic viscosity cationic polymer. The product thus formed is an aqueous dispersion of undissolved high intrinsic viscosity cationic polymer which is a convenient way of providing high molecular weight flocculants. The dispersion polymer can be dissolved in water to a conventional concentration or can be added directly to a suspension.
It is also known to use two different polymeric flocculants in the same process.
In commercial practice the dewatering of sewage sludge may involve the addition of two polymeric flocculants which have the same charge (co-ionic).
In other processes it is known to apply two polymers of opposifie charge.
(counter ionic). Where two polymeric flocculants are applied to an aqueous suspension they may be added simultaneously or more usually sequentially.
W09950195 describes a process of dewatering an aqueous suspension of suspended organic solids by adding to the suspension an aqueous composition comprising a first water-soluble ionic polymeric flocculant and a second water-soluble ionic polymeric flocculant. The first flocculant is in excess over the second flocculant and both flocculants are counterionic. It is a requirement that the first filocculant and second f(occulant form particles of counterionic precipifiate. The counter ionic precipifiafie is designed to rupture in order to release the second flocculant and contained within the counter ionic precipitate.
US 6063291 discloses flocculation of suspensions using the addition of a mixture of counter ionic flocculants in which one of the flocculants is particulate.
US 5112500 discloses separate addition of dry cafiionic polymer particles and dry anionic polymer particles to a suspension fio flocculate it.
Such counter ionic floccuiant systems can bring about deleterious effects when attempting to dewater certain sledges, especially sledges thafi contain significanfi , amounts of organic components, such as sewage sledges.
WO-A 01/05712 reveals a process of dewatering a suspension by adding.fio the suspension concentrated a dilute solution of polymeric filocculants substanfiially simultaneously. Both the concentrated and diluted solufiions of polymer and are added at conventional concentrations of no more than 1 % and usually much less than this.
WO-A 02/72482 describes a process of flocculating and dewatering an aqueous suspension of suspended solids in which a polymer composition comprising 40% and 60% by weight polymer and a polymer composition comprising between 0.05 and 0.2% by weight polymer are introduced simultaneously.
Although fihe process. brings aboufi some improvements in filtration and free drainage, it would be desirable to improve upon the cake solids obtained in dewatering suspensions, especially for sewage sledges.
International application PCTJEP03/09381, unpublished at the priority dafie of present applicafiion, describes a method of dewatering a suspension employing a composition comprising a cationic polymer flocculant and a coagulanfi in which the coagulant is encapsulated. After free drainage of the suspension the coagulanfi is released into the suspension for example by rupturing of the capsules which enclose the coagulant or by migration from a mafirix in which the coagulant is entrapped. Although significant improvements in cake solids can be obtained in it would be desirable to provide equivalent or improved cake solids using flocculant products that can be more easily manufactured andlor applied.
However, achieving high cake solids can sometimes be difFcult, particularly in the dewatering of sewage sludges. It is also known to add a flocculant or coagulant to assist the initial dewatering of a suspension followed by further addition of flocculant or coagulant and then further dewatering to achieve high cake solids. Such processes are described in JP-A-10-249398, JP-A-61-257300, JP-A-06-343999, JP-A 06-344000 and EP-A-1035077. However, these processes have the disadvantage that they require two stages of dewatering involving two separate treatments with flocculant.
It would be desirable to provide an improved process that results in dewatering of suspensions to provide increased cake solids. In particular it would be desirable to provide such a process that involves treatment agents that can be more easily and conveniently manufactured and applied. A further objective of the present invention is to provide a process that avoids the necessity of employing flocculant additions in two separate steps.
According to the present invention we provide a process of dewatering aqueous suspension employing a flocculating system comprising treating the suspension with a flocculating amount of a first flocculant and a dewatering amount of a second flocculant, and subjecting the suspension to mechanical dewatering to form a cake, wherein the fiirst flocculant brings about flocculation and assists thickening of the suspension and the second flocculant further dewaters the suspension, characterised in that the second floccuiant is a water-soluble or wafer swellable polymer that is mixed into the suspension in the form of a water-soluble or water swellable particulate polymer having a particle diameter of at leasfi 20 microns, wherein the first and second flocculants are not counter ionic.
It is important that first and second flocculants do not form a counterionic 5 precipitate. For instance, fihe firsfi flocculant may be nonionic whilsfi the second flocculant can be anionic but preferably is cationic. Alternafiively, fihe first flocculant may eifiher be cationic or anionic and the second flocculanfi would be nonionic. Ifi is especially preferred that both the first and second flocculants are co-ionic so thafi both flocculants are either anionic buff most preferably are .. cationic.
The invention is applicable to any suitable suspensions in which ifi is desirable concentrate the suspended solids. This includes wasfie waters, and textile indusfiry effluents mineral suspensions such as red mud from the Bayer Alumina process or coal tailings, in paper mill wastes such as cellulosic sludges. The process is particularly applicable to the dewatering of sewage sludge.
In the dewatering processes the suspension is first thickened following the addition of fihe first flocculant. This stage involves fihe inifiial flocculation and release of free water to produce the thickened suspension. Generally the release of free wafer may be achieved by free drainage or filtration and it is common to employ mechanical means such as a belt thickener, belfi press rotary drum thickener or centrifuge. The flocculant should be applied in sufficient quanfiity fio bring about inifiial flocculafiion and partial dewatering of the suspension. Preferably the suspension is thickened to produce a semi solid sludge paste. In general this first flocculant will be a polymer added at a conventional concentration, for instance 0.1 % to 1 % by weight, especially 0.2%
to 0.5%.
Typically addifiion of the first floccufant and second flocculant would be infio the initial bulk suspension.

The dewatering process involves the action of the second flocculant on the thickened suspension in which the second flocculant is in the form of polymer particles having a particle diameter of at least 20 microns. ThE particulate second ffocculant may be partially hydrated although it is preferred that it is substantially dry. We find that the particulate sECOnd flocculant does not substantially mix into the bulk suspension prior to thickening but it does integrate into the thickened suspension during mixing and/or mechanical dewatering and brings about further release of water to produce a dewatered cake. The polymeric particles can be easily mixed into the thickened suspension and distributed throughout using conventional mixing equipment.
Suitable mixing equipment includes far instance ribbon type mixers on or kneading mixers. Ribbon type mixers consist of helical or spiral mixing blades that sweep across nearly the entire surtace of the mixing vessel. Kneading mixers consist of two kneading arms that intermesh as well as farm a close tolerance to the mixer wall. Alternatively the second flocculant can be distributed throughout the thickened sludge during mechanical dewatering_ Typically, this mechanical dewatering will normally involve compression and can for instance be any of belt press, filter press, screw press or centrifuge.
When this treated thicl~ened suspension is subjected to mechanical dewatering unExpectedly high cake solids are achieved.
Usually the second flocculant will be a particulate polymer having a particle diameter of at least 50 microns. The polymeric particlES may have a particle diameter as high as 2000 or 3000 microns or higher or can be as low as 10 or 20 microns oC lower, although usually will not be below 50 microns. Generally the particle diameter will be in the range of 50 microns to 2000 microns.
Preferably, the particles will have an dierneter between above '100 and 800 microns, for instance ~ 20 or 150 to 800 microns. More preferably the particles will range between 250 and 750 microns. The particles may also be defined by weight average particle diameter generally this wilt be between 50 and 1000 microns, preferably 100 to 800 microns and more preferably between 300 and 700 microns.
The first and second flocculants may be any suitable natural or synthetic polymeric flocculant and typically will be high molecular weight. Natural polymers include for instance cationic starch, anionic sfiarch and chitosan etc.
Synthetic polymers include linear, branched and cross-linked polymers of ethylenically unsaturated monomers. The first flocculant may be the same as the second flocculant or alternatively the two filocculants may be different.
Usually the polymer of the first flocculant and second flocculant will be of molecular weight in excess of 500,000, usually at least one million and normally 5 million up to 30 million.
The first and second flocculants of the present invention may be cationic, anionic, nonionic in nature. The choice of polymeric flocculant will largely depend upon the substrate, which is being treated. For instance it is common practice to use high molecular weight cationic flocculants to treat, aqueous suspensions comprising suspended organic material, for instance sewage sludge. In treating paper-mill waste it is known to use either cationic, nonionic, anionic or amphoteric flocculants. Flocculation of mineral suspensions is frequently effected by use of anionic flocculants. As indicated previously either the first and second flocculants should be co ionic or that at least.one of them may be non-ionic. Hence, the first and second flocculants should not be counter-ionic.
The polymer may be prepared by polymerisation of a water soluble monomer or water soluble monomer blend. By water soluble we mean that the water soluble monomer or water soluble monomer blend has a solubility in wafer of'at least 5g in 100 ml of water. The polymer may be prepared convenienfily by any suitable polymerisation process.

When the water soluble polymer is nonionic the polymer may be formed from one or more water soluble ethylenically unsaturated nonionic monomers, for instance acrylamide, methacrylamide, hydroxyethyl acrylafie, N-vinylpyrrolidone.
Preferably the polymer is formed from acrylamide.
When the wafer soluble polymer is anionic the polymer is formed from one or more ethylenically unsaturated anionic monomers or a blend of one or more anionic monomers with one or more of the nonionic monomers referred to previously. The anionic monomers are for instance acrylic acid, methacrylic acid, malefic acid, crotonic acid, itaconic acid, vinylsulphonic acid, allyl sulphonic acid, 2-acrylamido-2-methylpropane sulphonic acid and salts thereof. A
preferred polymer is the copolymer of sodium acrylate with acrylamide.
Preferably the water soluble polymer is cationic and is formed from one or more ethylenically unsaturated cationic monomers optionally with one or more of the nonionic monomers referred to herein. The cationic polymer may also be amphoteric provided that there are predominantly more cationic groups than anionic groups. The cationic monomers include dialkylamino alkyl (meth) acrylates, dialkylamino alkyl (meth) acrylamides, including acid addition and quaternary ammonium salts thereof, diallyl dimethyi ammonium chloride.
Preferred cationic monomers include the methyl chloride quaternary ammonium salts of dimethylamino ethyl acrylate and dimethyl aminoethyl methacrylate. A
particularly preferred polymer includes the copolymer of acrylamide with the methyl chloride quaternary ammonium salts of dimethylamino ethyl acrylate.
The polymers may be linear in that they have been prepared~substantially in the absence of branching or cross-linking agent. Alternatively the polymers can be branched or cross-linked, for example as in EP-A-202780.
Desirably the polymer may be prepared by reverse phase emulsion polymerisation, optionally followed by azeotropic dehydration to form a dispersion of polymer particles in oil. Alternatively the polymer may be provided in the form of beads by reverse phase suspension polymerisation, or as a powder by aqueous solution polymerisation followed by comminution, drying and then grinding. The polymers may be produced as beads by suspension polymerisation or as a water-in-oil emulsion or dispersion by water in-oil emulsion polymerisafion, for example according to a process defined by EP-A-150933, EP-A-102760 or EP-A-126528. .
It is particularly preferred that the second flocculant is formed from at least 30%
by weight cationic monomer or monomers. Even more preferred are polymers comprising at least 40 or 50% by weight cafiionic monomer units. It may be desirable to employ cationic polymers having very high cationicities, for instance up to 80 or even 100% cationic monomer units. It is especially preferred when the cationic second flocculant polymer is selected from the group consisting of cafiionic polyacrylamides, polymers of dialkyl diallyl ammonium chloride, dialkyl amino alkyl (meth) -acrylates (or salts thereof) and dialkyl amino alkyl (meth)-acrylamides (or salts thereof).
As stated previously the second flocculant is desirably of relatively high molecular weight. Normally the second flocculant wilt be a polymer that exhibits an intrinsic viscosity of at least 0.5 dl/g. Typically the intrinsic viscosity will be the least 3 dllg, and often ifi can be as high as 20 or 30 dl/g but preferably will be between 4 and 10 dllg.
Intrinsic viscosity of polymers may be determined by preparing an aqueous solution of the polymer (0.5-1 % wlw) based on the active content of the polymer. 2 g of this 0.5-1 % polymer solufiion is diluted to 100 m! in a volumetric flask with 50 ml of 2M sodium chloride solution that is buffered to pH 7.0 (using 1.56 g sodium dihydrogen phosphate and 32.26 g disodium hydrogen phosphate per litre of deionised water) and the whole is diluted to the 100 ml mark with deionised water. The intrinsic viscosity of the polymers are measured using a Number 1 suspended level viscometer at 25°C in 1 M buffered salt solution.
One particularly useful cationic polymer type for use as the second flocculant 5 includes 50 to 100% by weight methyl chloride quaternary ammonium salt of dimethyl amino ethyl (meth) acryiate and 0 to 50 % by weight acryiamide of intrinsic viscosity between 4 and 10 dl/g. Preferably the cationic polymer comprises at least 80% methyl chloride quaternary ammonium salt of dimethyi amino ethyl (meth) acrylate.
Other suitable polymeric second ftocculants include polyvinyl amidine and polyvinyl amines of intrinsic viscosity greater than 1 dl/g, preferably greater than 2 dl/g.
~ Another particularly suitable category of second flocculants are Mannish addition polyacrylamides. Ideally such polymers will exhibit an intrinsic viscosity greater Than 1 dl/g and quite often can be at least 4 dl/g, for instance at least 7 or 8 dl/g. Such polymers may be made by reacting formaldehyde / amine adducts with polyacrylamide. The amine may for instance be dimethylamine or other secandary amines. Preferably the Mannish addition po(yacrylamides are quaternised salts and these could be prepared by reacting the~free base Mannish with a suitable quaternising agent such as methyl chloride or dimethyl sulfate.
Another suitable polymer as the second flocculant includes poly dimefihyl dialiyl ammonium chloride of intrinsic viscosity greater than 0.5 dl/g, preferably.at least 1 dl/g.
Effective dewatering of suspensions can be achieved when these polymers are , used as the second floccuiant.

The dose of aqueous composition depends on the substrate and usually fihis will be a conventional amounfi. Typically for sewage sludge treatment the dose of fihe aqueous composition (second flocculant) found to be an effective dewatering amount is often at least 50 mg active polymer per litre of suspension. Usually fihe amount would be higher for insfiance up to 400 mg per titre. Preferred doses are between 60 and 300~mg per litre. The quanfiity of first flocculanfi used will usually be at least 50 mg active polymer per lifire of suspension and can be as high as 500 or 600 mg per litre. Preferred doses would be between 100 and 400 mg per litre.
Various polymers may be used as the'first flocculant in order to obtain a suitably thickened suspension for treafimenfi with fihe second flocculanfi. Preferably fihe first floccutant is a cafiionic organic polymer. This is particularly firue when the suspension is a sewage sludge. Preferred cationic polymers include polymers selected from the group consisfiing of acry(amide polymers, polyvinyl amidine, polyvinyl amine, poly dimethyl diallyl ammonium chloride, poly amines;
polyefihyleneimines, mannich polyacrylamides and quaternised mannich polyacrylamides.
The first and second flocculanfis may be added sequentially and in which case .
usually the second flocculanfi is added to the suspension firsfi although the reverse order may be employed. Normally the first and second flocculants are added in close proximity and preferably they are added substantially simultaneously. When the two flocculants are added in this way fihey can desirably be added separately although in some situations the firsfi.flocculant and the second flocculant are combined infio a single camposifion with beneficial results. In one preferred aspect the single composifiion is a particulate polymer product in which the first flocculant comprises particles having a diamefier below 10 microns and the second flocculant comprises particles having a diamefier above 20 microns, preferably above 50 microns. The parfiicie size of the second flocculant may be as defined previously. We have found the first flocculant tends to act substantially immediately on the suspension and brings about flocculation and thickening and then the larger parfiicle size second flocculant can distribute easily throughout the thickened suspension to bring about further dewatering.
Particularly effecfiive results can be achieved when the first and second flocculants are added to the sludge simultaneously but separately. Without being limited to theory it is believed that the first flocculant results in flocculation of the sludge and the undispersed polymer of the second flocculant becomes trapped within the flocculated structure but does not bring about any significant dewatering anti! the flocculated sludge is thickened and then by mixing the polymer of the second flocculant is allowed to distribute throughout and integrate with fibs sludge and achieve further dewatering.
In a further embodiment the second flocculant comprises polymeric particles having a coating applied to the surFace. The coating delays the dissolution of the second flocculant particles so that on addition to the suspension the first flocculant acts on the suspension to bring about flocculation and produce a thickened suspension and the coated second flocculant particles are distributed throughout the thickened suspension and bring about further dewatering. The coating may for instance be a silicone compound or alternatively it may be a water-soluble wax. The water-soluble wax and can for instance be a polyethyleneglycol or a polypropylene glycol. A suitable water-soluble wax is for instance polyethyleneglycol with a molecular weight of 600 (PEG600) or above.
It may be desirable to combine first and second flocculants into a single composition in which the second flocculant comprises coated particles.
In a still further embodiment of the present invention, the second flocculant may be introduced in the suspension in the form of a slurry of second flocculant polymer particles in a liquid. The liquid may be a suitable liquid that does not adversely interact with either the particles of the second tlocculant or the suspension. Suitably the liquid can be a polyethyleneglycol.
The following example is an illustration of the invention.

Exam le Dewaterina of aqueous suspensions via a one-staae addition of a conventional solution and drv particles of organic nolymer flocculants Polymers Polymer A is a linear, high molecular weight, high cationic acrylamide based polymer of intrinsic viscosity 12 dl/g in the form of a dehydrated emulsion (liquid dispersion product). Polymer B is a linear, low -medium molecular weight, cationic homQpolymer of quaternised dimethyl amino ethyl methacrylate of intrinsic viscosity 5 dl/g based polymer in bead form.
Unless otherwise stated intrinsic viscosity is measured using a Number 1 suspended level viscometer, in 1 M sodium chloride buffered to pH 7 in accordance with the information given in the description.
Test substrate Dewatering tests were conducted on a sample of a digested, mixed priinary/activated sludge, The sample had a dry solids content of 3.28%.
Experimental procedure ,~,A1 One-staae addition of pol my.--er(s_1 i) Polymer A was first dissolved in deionised water to give a homogeneous~.9 % w/v solution and further diluted to 0.25%w/v prior to use. Polymer B was dissolved in deionised water to give a 1 %w/v solution. The 1 %w/v solution was further diluted with deionised water to 0.25%w/v prior to use.

ii) 250 ml of a digested, mixed primary/acfiivafied sludge was placed in a 1 litre plasfic beaker (120cm diameter by 120cm tall). A standard laboratory stirrer was secured over the beaker with fihe stirrer shaft located through a hole in the centre of the beaker lid. The stirrer is a four bladed, flat 5 crosshead type (each paddle is 25cm width by 1.lcm).
iii) An appropriate volume of a 0.25% solution of Polymer A, and a 0.25%
solution or substantially dry particles (250-500 micron) of Polymer B, were added simultaneously to the sludge and the lid secured to the 10 beaker. The sludge was flocculated by stirring at 1500rpm for 15s. The flocculated sludge was poured into a filtration cell, which had ~a filter membrane, comprising an 8cm diameter belt-press filter cloth and the filfirate collecfied in a measuring cylinder.
'l5 iv) After 30s drainage the thickened sludge retained on fihe filter cloth was subjected to a 'furrowing' technique, whereby a spatula was slowly drawn across the sludge in several directions fio encourage release of more water. Furrowing was carried out for 30s. The volume of filtrate was noted.
v) The thickened sludge was transferred to a 250m1 beaker and stirred by hand for 45s with a spatula, using a slow, circular folding action.
vi) The thickened sludge was then transferred to a piston-press device and subjected to a compression dewatering stage. Dewatering was commenced using a pressure of 20psi for 2 minutes, followed by increases of l0psi, at one minute intervals, for a further 3 minutes to a maximum of 60psi. Pressure was maintained at 60psi for a further 5 minutes, giving a total compression dewatering fiime of 10 minutes. The weft cake was removed and fihe cake solids content was determined by heating at 110°C overnight.

(B ) One-stage addition of polymer(s) excluding mixing of the thickened sludgie.
The procedure was exactly the same as that described in Section (A) except:
Section v) was omitted ,~C) Control -Addition of dry polymer particles in a two-stag~e'process The procedure was as that given in Section (A) except:
Section iii) - the appropriate volume of a 0.25% solution of Polymer A
was added to the sludge using a syringe and the lid secured to the beaker. The sludge was flocculated by stirring at 1500rpm for 10s. The floccutated sludge was poured into a filtration cell comprising an 8cm diameter belt-press filter cloth and the fiiltrate collected in a measuring cylinder.
Section v) - the thickened sludge was transferred to a 250m1 beaker. The appropriate weight of substantially dry particles of Polymer B (250-500 micron) was added into the thickened sludge. To mix in the polymer the treated thickened sludge was stirred by hand for 45s wifih a spatula, using,a slow, circular folding action.
Results The results are given in Table 1 Table 1 Data Test Polymer FiltratePolymer Cake A B

Set Procedure Dose Volume Dose(mg/l)Solution solids(%) {mg/I) (ml) strength(w/v) 200 182 100 0.25% 15.85 1 A 200 182 150 0.25% 16.65 200 184 200 0.25% 17.05 200 - 100 0.25! 17.37 2 B 200 - 150 0.25% 17.99 200 - 200 0.25% 17.36 200 160 75 Dry particles*18,50 3 A 200 - 100 Dry particles*19.81 200 . 150 Dry particles*21.15 200 164 75 Dry particles*18.27 4~ C 200 - 100 Dry particles*19.80 200 170 150 Dry particles*21.4?

*250-500 micron range Data set 1 and 2 represent conventional addition of dilute polymer solutions to sewage sludge. The results show that, with conventional treatment, additional mixing of the thickened sludge (Set1 ) has an adverse efFect on cake solids compared to no additional mixing (Set 2).
Data Sets 3 and 4 show that improved cake solids can be achieved by adding substantially dry polymer at the first stage (Set 3) and that this is just as .
effective as adding substantially dry polymer at the second stage (Set 4).

Claims (19)

1. A process of dewatering aqueous suspension employing a flocculating system comprising treating the suspension with a flocculating amount of a first flocculant and a dewatering amount of a second flocculant, and subjecting the suspension to mechanical dewatering to form a cake, wherein the first flocculant brings about flocculation and assists thickening of the suspension and the second flocculant further dewaters the suspension, characterised in that the second flocculant is a water-soluble or water swellable polymer that is mixed into the suspension in the form of a water-soluble or water swellable particulate polymer having a particle diameter of at least 20 microns, wherein the first and second flocculants are not counterionic.

2. A process according to claim 1 in which the aqueous suspension is sewage sludge.

3. A process according to claim 1 or claim 2 in which the mechanical dewatering employs an apparatus selected from the group consisting of belt press, filter press, screw press and centrifuge.

4. A process according to any of claims 1 to 3 in which the second flocculant is a particulate polymer having a particle diameter of at least 50 microns, preferably 100 to 2000 microns.

5. A process according to any of claims 1 to 4 in which the second flocculant is a cationic polymer.

6. A process according to any of claims 1 to 5 in which the second flocculant is formed from at least 50% by weight cationic monomer or monomers.

7. A process according to any of claims 1 to 6 in which the second flocculant is selected from the group consisting of cationic polyacrylamides, polymers of dialkyl diallyl ammonium chloride, dialkyl amino alkyl (meth) -acrylates (or salts thereof) and dialkyl amino alkyl (meth)-acrylamides (or salts thereof).

8. A process according to any of claims 1 to 7 in which the second flocculant has an intrinsic viscosity of at least 0.5 dl/g, preferably 4 to 10 dl/g.

9. A process according to any of claims 1 to 8 in which the second flocculant is selected from the group consisting of, i) a polymer formed from 80 to 100% by weight methyl chloride quaternary ammonium salt of dimethyl amino ethyl (meth) acrylate and 0 to 20%
by weight acrylamide of intrinsic viscosity between 4 and 10 dl/g, ii) polyvinyl amidine and polyvinyl amines of intrinsic viscosity greater than 1 dl/g, iii) quaternised salts of Mannich addition polyacrylamides of intrinsic viscosity greater than 9 dl/g, and iv) poly dimethyl diallyl ammonium chloride of intrinsic viscosity greater than 0.5 dl/g.

10. A process according to any of claims 1 to 9 in which the first flocculant is a cationic organic polymer.

11. A process according to claim 10 in which the polymer is selected from the group consisting of acrylamide polymers, polyvinyl amidine, polyvinyl amine, poly dimethyl diallyl ammonium chloride, poly amines, polyethyleneimines, mannich polyacrylamides and quaternised mannich polyacrylamides.

12. A process according to any of claims 1 to 11 in which the first flocculant and second flocculant are added substantially simultaneously.

13. A process according to any of claims 1 to 12 in which the first flocculant and second flocculant are combined into a single composition.

14. A process according to claim 13 in which the single composition is a particulate polymer product in which the first flocculant comprises particles having a diameter below 90 microns and the second flocculant comprises particles having a diameter above 20 microns, preferably above 50 microns.

15. A process according to any of claims 1 to 14 in which the second flocculant comprises polymeric particles having a coating applied to the surface.

16. A process according to claim 15 in which the coating is a silicone.

17. A process according to claim 15 in which the coating is a water-soluble wax.

18. A process according to any of claims 1 to 17 in which the second flocculant is introduced into the suspension in form of a slurry in a liquid.

19. A process according to claim 18 in which the liquid is polyethylene glycol.