During the first moments of a sludge filtration cycle, sludge particles are retained by a layer of precoat material and/or filter media with the liquid fraction of the sludge passing through the media and out of the filter plate in the form of filtrate.  From this point on, the rate of filtration, hence the effectiveness of dewatering is determined by the porosity of permeability of the formed sludge layer by the pressure differential available to cause flow across that layer and by the viscosity of the sludge.

In some cases, the characteristics of the sludge to be dewatered are such that its natural permeability is sufficient to allow dewatering to proceed satisfactorily without doing anything further to it than pumping it into a suitable dewatering device.  Generally speaking, sludges falling into this category will have one (1) or two (2) characteristics: they will contain predominant amounts of fibrous material or they will contain predominant amount of crystalline or granular material.  Examples of the first type would be most pulp mill primary and some paper mill primary sludges.  Examples of the second would be most lime softening sludges and the sludge produced in coal washing operations.

When the permeability of a sludge is not adequate for the dewatering process to proceed on an effective basis, it becomes necessary to alter this permeability and sometimes to reduce the viscosity of the sludge.  This is known as conditioning.

There are three (3) basic approaches which can be taken to conditioning a sludge to improve its permeability: chemical, mechanical and heat.

Chemical Conditioning:

As the name implies, chemical conditioning of a sludge involves the addition of chemicals to enhance dewaterability.  The effect of such additions can take any one and generally takes a combination of the following forms:

  1. The pH of the sludge is altered to a level at which the electrical charges on the particles cause the particles to reorient themselves.
  2. The reoriented structure resulting from (1) above is strengthened.
  3. Small particles are coalesced into larger, heavier particles.
  4. Reaction products are formed which are interposed between the sludge particles, holding them apart.
  5. Particles of the chemical itself are interposed between the sludge particles, holding them apart.


Charge Reversal:

Most sludge which have resulted from a coagulation process, either of the natural type as that resulting from biological treatment of an organic waste or the type induced by the addition of coagulants as that resulting from clarification of water, share two (2) properties: the sludge particles are in the shape of platelets and the electrical charges on these particles are concentrated on the larger or plane surfaces of the particles. Consequently, these particles will tend to orient themselves plane surface to plane surfaces, much in the fashion of a deck of cards lying on a table and there is little area between particles through which flow can occur.  There are generally two (2) pH levels of which this condition will change.  One in the area of 10-12 and the other in the area of 1-2.  If the pH is changed to one or the other of these levels, the electrical charges will tend to leave the plane surfaces of the particles and, instead, will concentrate at the thin edges of the platelet shaped particles.  When this occurs, the particles will tend to orient themselves like a house of cards, opening up large flow areas between particles, thereby increasing the permeability, hence, dewaterability of the sludge.

Coalescence or Coagulation:

To the extent that sludge particles are relatively compact, that is, that they have a reasonably high ratio of mass to surface area, the larger is the particle, the greater is the permeability of the sludge.  An analogy would be comparative flows through course gravel and fine sand.  The larger the gravel, the faster is the flow, and the finer is the sand, the slower is the flow.

The effective particle size of many sludges can be increased by the addition of a suitable coagulant, the action of such coagulant being to draw together many fine particles into the form of larger, heavier particles.  It must be emphasized at this point that the desired end result is relatively compact particles.  The large floc that can be produced with many polymeric type coagulants with very high surface areas and very low mass can be highly detrimental to the dewatering process.  Such floc will tend to flatten out in the dewatering process, much like a sheet of plastic, and can be completely blind off the permeability of the sludge.

Generally speaking, iron salts such as ferric chloride have tended to produce a tough floc and have tended to strengthen the structure produced by charge reversal better than have the aluminum salts as alum, hence their greater application for this service.

Reaction Products:

When an alkali such as lime is added to sludge, the effect is to soften and reduce the alkalinity of that sludge, precipitating calcium carbonate and magnesium hydroxide in the process.  Because of its granular nature, calcium carbonate is very desirable from the standpoint of improving permeability of a sludge.  Generally speaking, because of its gelatinous nature, magnesium hydroxide is not a particularity desirable material, however in the case of a water clarification plant in the southwestern part of the U.S., use of dolomitic lime to introduce some magnetism hydroxide into the sludge gave superior results to those achieved with conventional lime.

Interposition of Chemical Particles:

When lime is used to raise the pH of a sludge to achieve charge reversal, the quantity of lime required to obtain the desired sludge permeability will generally be found to exceed that required to produce the required pH and frequently will exceed the solubility of lime in the conditioned sludge.  In such cases, the excess lime particles become mixed with the sludge particles and, by their interposition between the sludges particles, become a dewatering aid in themselves.  The use of waste dust from kilns used in the cement making process, because of its similar properties to lime from a pH adjustment standpoint and its better properties from the standpoint of particle structure, has been highly effective in the conditioning of steel mill wastes in a mill in the south and another in the Midwestern area of the U.S., the quantity required being lower than that of lime.

Because of the above, sodium hydroxide or caustic soda will rarely be used for elevating the pH of a sludge.  It is highly soluble material and no benefit can be derived from a particle interposition standpoint.

A detailed summary of the chemical reactions involved, method of prediction of reaction products, characteristics of the chemicals most normally used for conditioning purposed, etc., will follow.

Mechanical Conditioning:

Mechanical conditioning involves the feed of a suitably sized, inert material to the sludge to be dewatered.  This material becomes interposed between the sludge particles, holding them apart to open up flow paths, hence, increasing the permeability of the sludge.

Principal characteristics affecting the suitability of a material for use as a mechanical conditioning aid are the density, size and shape of the particles of that material.

Effect of Density:

Inasmuch as the use of a material to physically establish flow path through the sludge is a volumetric proposition, requiring a certain size conditioning particle to hold apart sludge particles, the lower the particle density of the conditioning material, the less pounds will be required to achieve the desired end result.

In the case of conditioning materials having extremely high particle densities, some problem can also be experienced in maintaining these in a homogeneous suspension with the sludge during the conditioning process and pumping of the conditioned sludge into the filter.  As is obvious, failure to maintain a homogeneous suspension of sludge and conditioning material will result in uneven permeability in different areas of a forming cake and will quickly lead to process failure.

Effect of Size:

The size of particles used as a mechanical conditioning aid is highly critical to the success of this approach.  If the particles are too small, they cannot hold the sludge particles apart sufficiently to achieve the desired permeability.  If they are too large, there will not be a sufficient number of particles per unit of weight fed to accomplish the required dispersion between sludge particles and it will be necessary to use an excessive amount of conditioning material to achieve the desired end result.  Generally speaking, conditioning material particles in the 10 micron range will be too small and in the 200 micron range too large to be effective for conditioning the sludge.  The optimum material used for this purpose will normally consist of particles predominantly sized in the 40 to 100 micron range with 60-80 microns being ideal.

Effect of shape:

The ideal shape of a material used as a mechanical conditioner is that of a piece of popcorn that is highly irregular and jagged.  Spherical, bead-like material will not generally work too well in this service and will require relatively high dosages to achieve the desired result.

Chemicals used for sludge conditioning

The following is a summary of the characteristics of chemicals most normally used for the conditioning of waste sludges.  It is by no means a complete list, as such a list cannot be prepared.  If complete understanding exists of the requirements of conditioning and the manner in which a given chemical can satisfy those requirements, this list will be continually expanding to satisfy the needs of particular type sludge.  The above is especially important in the disposal of sludge generated in industrial plants.  It frequently is possible to achieve the ideal goal of treating one waste with another waste in such plants or of drastically reducing the operating cost of a dewatering system by using a low cost, locally available conditioning material.








Because of its ready availability, its relatively low cost and its effectiveness both as a chemical and mechanical conditioning agent, the alkali most commonly used for sludge dewatering applications is lime.  This material can be procured in either anhydrous form as quick lime or in hydrated form, the determining factor generally being local availability and the quantity to be used.  Larger plants invariably can justify the additional cost of slaking equipment required for anhydrous or quick lime because of the lower continuing cost of purchasing quick lime in bulk.

Quick Lime:

Other Common Names: Chemical Lime, burnt lime, calcium oxide

Chemical Formula: CaO

Molecular Weight: 56.1

Equivalent Weight: 28.0

Normal Purity: 90% CaO

Grades: Lump, pebble, ground, pulverized

Solubility: Slakes with water to form calcium hydroxide (Ca(OH)2 .  For solubility of calcium hydroxide, see hydrated or slaked lime.

Slaking Reaction: CaO + H2 =  Ca(OH)2

        1 lb. CaO will produce 1.32 lbs. CaO2

        1 lb. CaO requires approximately ½ gallon slaking water

Bulk Density and Storage Requirements:

Grade        Bulk Density#/Cu. Ft.             Storage Requirement Cu. Ft. /Ton

Lump                     50-65                                       31-40

Pebble                   60-65                                       31-34

Ground                  50-70                                       29-40

Pulverized             39-71                                       28-52


Normal shipping container: bag, barrel, bulk

Miscellaneous:       Slaking reaction is exothermic


Hydrated Lime:

Other common names: Slaked lime, calcium hydroxide

Chemical Formula: Ca(OH)2

Molecular Weight: 74.1

Equivalent Weight: 37.1

Specific Gravity: 2.24

Normal Purity: 93%

Grades: Powder

Solubility in water:

Temperature, Deg. F                           Solubility, #/1000 gal.

32                                                        15.0

50                                                        14.2

68                                                        13.3

86                                                        12.5

Bulk density: 25 to 50 #/cu. ft.

Storage requirements: 40 to 80 cu. ft. /ton

Normal shipping container: bag, barrel, bulk

Desirable feed strength: 7.5%





Feed characteristics:

Conc. %                      Density #/gal.              Feed #/ Ca(OH)          2Gal.*

5.0                               8.567                           0.428                                                                           7.5                         8.692                           0.652                                                                           10.0                       8.819                           0.882                                                                           *# of lime containing 93% Ca(OH)2


Hydrated Lime @ 93%    100% Ca(OH)      2      100% CaO      Quick Lime @ 90%

1 Lb.          =                      0.93 Lb.                =     0.70 Lb.     =   0.78 Lb.

Hydrated Dolomitic Lime:

Chemical Formula: Ca(OH)2MgO

Molecular Weight: Combined function of Ca(OH)2 and MgO content which is variable (normally approx. 62% Ca(OH)2.32% MgO).

Grades: Powder

Bulk density: 28 to 52 #/cu. ft.

Storage requirements: 39 to 72 cu. ft. /ton

Normal shipping container: bag, barrel, bulk

Desirable feed strength: 7.5%










Coagulants (Other than Polymers)

The primary function of a coagulant, when used for sludge conditioning, is to draw together fine sludge particles into larger, heavier particle having better settling and dewatering properties.  Historically, this has most frequently been accomplished through the trivalent forms of iron and aluminum salts.  In recent years, large steps have been taken in the development of long chain molecule, polymeric or polyelectrolyte type coagulants for clarification and, to a lesser degree, dewatering application (discussed elsewhere), and it is expected that these will see greater use in the future.  However, for the present time, it is expected that the work-horse type coagulant in dewatering applications will continue to be the metallic salt type.

Generally speaking, iron based coagulants have been more effective that aluminum based materials, producing tough, heavier floc with better dewatering characteristics.

It has been reported that good results have been obtained in Europe with some sludges using aluminum chloride. Work with this material in the U.S. has been very limited, and, for the most part, has not been successful.

In the U.S., the coagulant most frequently used for dewatering applications has been ferric chloride.  In Europe, because of limited availability and high cost of ferric chloride, ferric sulfate and ferrous sulfate have been used more commonly.  It should be noted, at this point, that use of ferrous sulfate requires the prior oxidation of this material to the ferric state.  This oxidation does not occur readily, and a considerable amount of equipment is required if ferrous sulfate is to be used in a commercial type installation.

It should be noted that iron salts, both of the sulfate and chloride types, are produced by passing sulfuric and hydrochloric acid, respectively, over iron.  In a given area, this material may be available at very low cost as the waste product of acid pickling operations in metal working industrial plants.

Because of its high chloride content and acidic nature, selection of materials for use in equipment, piping, tankage, etc., handling ferric chloride is very limited and plastic, fiberglass, rubber or rubber lined, etc., materials should be used whenever possible.  Ferrous or ferric sulfate can, on the other hand, be successfully handled in stainless steel.






Ferric Chloride (Hydrate)

Chemical Formula: FeCl3.6H2O

Molecular Weight: 270.2

Grades: Powder

Equivalent Weight: 90.1

Purchased form: Liquid (generally at 40-42 deb. Baume)

Ferric Chloride (Hydrate) (continued)

Concentration and Density:

Deg. Baume    Concentration %         Specific Gravity          Density #/ Gal.

32.6                 30                                1.2910                         10.77

37.6                 35                                1.3530                         11.29

42.0                 40                                1.4175                         11.82

47.4                 45                                1.4850                         12.39

Reaction with Lime: 2FeCl3 + 3Ca (OH)2        2Fe(OH)3 + 3CaCl3

Lime Required for Reaction: 1 lb. FeCl3 = 0.41 lb. Ca (OH)2

Ferric Hydroxide Produced: 1 lb. FeCl3 = 0.40 lb. Fe(OH)3

General: Highly corrosive cannot be handled in stainless steel.  Preferred materials-plastic, fiberglass, rubber.


Ferric Sulfate

Other Common Names: Ferrisul, Ferrifloc

Chemical Formula: Fe2(SO4)3

Molecular Weight: 399.8

Equivalent Weight: 66.6

Normal Purity: Variable, depending on source, ranging from 70% to 90%

Solubility in water: Highly soluble.  If in cold water, use 2 parts water to 1 part ferric sulfate.

Bulk density: 60 to 70 lbs/cu. ft.

Storage requirements: 29 to 34 cu. ft. /ton

Normal shipping container: bag, keg, barrel, bulk

Reaction with Lime: Fe2(SO4)3 + 3Ca (OH)2  2Fe (OH)3 + 3CaSO4.2H2O

Lime Required for Reaction: 1 lb. Fe2(SO4)3 = 0.56 lb. CA (OH)2

Reaction Products: 1 lb. Fe2(SO4)3 0.53 lb. Fe (OH)3 +

1 lb. CaSO4.2H2O (gypsum)

Equivalent: 1 lb. Fe2(SO4)3 = 1.36 FeCl3


Ferrous Sulfate

Other Common Name: Copperas

Chemical Formula: Fe2(SO4)3.7H2O

Molecular Weight: 277.9

Equivalent Weight: 139.0

Solubility in Water:

Temperature, Deg. F                                 Solubility, #/ gal.

32                                                        2.392

50                                                        3.125

68                                                        4.042

86                                                        5.017

Bulk density: 63-66 lb. /cu. ft.

Storage requirements: 30-32 cu. ft. /ton

Normal shipping container: bag, box, barrel, bulk

Equivalent: 1 lb. FeSO4.7H2O=0.48 lb. Fe2(SO4)3

Filter Alum

Other Common Names: Aluminum Sulfate, Alumina Sulfate

Chemical Formula: Al2(SO4)3.18H2O

Molecular Weight: 666.1

Equivalent Weight: 111.1

Normal Purity: 14.5 to 17.5% Al2O3

Grades: Slab, lump, ground, powdered

Temperature, Deg. F                                 Solubility, #/ gal.

32                                                        5.067

50                                                        5.442

68                                                        5.917

86                                                        6.567

Bulk density: Slab, lump, ground-57-67 lb. / cu. ft.

Powdered-38-45 lb. /cu. ft.

Storage requirements: Slab, lump, ground-30-35 cu. ft. /ton

Powdered-45-53 cu. ft. /ton

Normal shipping container: bag, keg, barrel, bulk

Reaction with Lime: 2FeCl3 + 3Ca (OH)2        2Fe(OH)3 + 3CaCl3

Lime Required for Reaction: 1 lb. FeCl3 = 0.41 lb. Ca (OH)2

Ferric Hydroxide Produced: 1 lb. FeCl3 = 0.40 lb. Fe(OH)3

Reaction with Lime: Al2(SO4)3 + 3Ca (OH)2  2Al(OH)3 + 3CaSO4

Lime Required for Reaction: 1 lb. Al2(SO4)3.18H2O = 0.33 lb. Ca(OH)2

Reaction Products: 1 lb. Al2(SO4)3 0.23 lb. Al(OH)3 + 0.78 lb. CaSO4.2H2O




As indicated previously, many advances have been made in recent years in the development of polymers or polyelectrolytes for coagulation applications, and such materials, are more and more, taking over the water clarification field.  Their potential advantage as sludge conditioners lies in the fact that they are normally fed in relatively low dosages and, therefore, result in an insignificant addition to the amount of sludge requiring dewatering.

Polymers, as applied to clarification and sludge conditioning applications, fall into one of three categories.  They are cationic, anionic, and non-ionic.  This nomenclature refers to the fact that some of these materials produce a floc that carries an electrical charge (cationic and anionic).  A positively charged floc will attract negatively charge sludge particles.  A negatively charged floc will attract positively charged particles.  The cationic and anionic categories are further broken down into strongly cationic, weakly cationic, strongly anionic and weakly anionic subcategories.  Several of the manufacturers of polymers use the designation “C”, “A” and “N” as part of their product identification to assist in their recognition.  Selection of one type polymer vs another is strictly empirical, being possibly only following extensive pilot plant testing under the range of conditions at which a dewatering system is expected to operate.

Although there are a few dewatering plants operating successfully with polymers, more extensive use of these materials has been restricted by the following:

  1. Many of these polymers are highly specific in their performance insofar as conditions required for their successful usage are concerned. In some cases, this specifically relates to the pH at which they will operate, with relatively small deviations from the optimum causing drastic deterioration in performance.  Insomuch as conditions in waste sludge tend to be variable, particularly in industrial applications, the complications involved in trying to maintain conditions at the optimum have outweighed the advantages of using the polymers.

In other cases, the specificity has related to dosage rates which in turn have related to sludge concentration.  Generally speaking, increasing the dosage of a polymer will improve performance to a point.  Beyond that point increasing dosages will cause performance to deteriorate.  During the start-up of a municipal sewage plant in the southeast, it was found that the permeability of the sludge, as measured by specific filtration resistance, improved on virtually a linear basis with increased dosage of polymer for a given sludge concentration and then got worse, also on virtually a linear basis.  At the maximum sludge concentration achieved during the early stages of operation of this plant, these two lines met at a point.  This point representing only marginally satisfactorily performance of the plant.  At all concentrations below the maximum, satisfactory performance could not be achieved.  Fortunately, in this particular case, later operation of the plant gave increased sludge concentration, opening up the range of dosages at which satisfactory operation could be achieved to the point where these dosages could be practically controlled.

  1. Relatively minor changes in the quality of a polymer, as delivered at different times, can seriously affect operation. At a municipal sewage plant in the Midwest, tests performed on the full scale operating plant using a polymer provided in small quantities by the manufacturer (in conjunction with incinerator ash) for conditioning over a 30 day period showed excellent results.  When this polymer was ordered in bulk as required for continuing operation, however, the material received varied sufficiently from the material tested that satisfactory could not be achieved and the use of polymer had to be discontinued.
  2. Many of the polymers produce a large, relatively light floc which tends to flatten out like a sheet of plastic in a filter completely binding off flow.
  3. There has not been sufficient operating experience with polymers in full scale operation to insure that detrimental side effects will not result from their operation. It has been reported by a municipal sewage plant in the north central area that the use of a polymer for conditioning of their sludge apparently lowered the fusion temperature of ash in their multiple hearth sludge incinerator to the point that severe slagging and clinkering problems were experienced.  These made it necessary to discontinue the use of polymers.  There has also been some unconfirmed suspicion that use of polymers has caused blinding of filter media over a period of time, increasing the frequency of required washing.


Regardless of the above, it is felt that the potential advantages to the use of polymer type sludge conditions are sufficiently great that families of such materials will be developed in the future that will be capable of operation under the varied conditions experienced in a waste treatment plant and that these materials will gradually take over dominant usage in the service.