Chemical sludge conditioning

Posted on 18 June 2020

Chemical conditioning refers to the chemical pre-treatment of sludge in advance of thickening and dewatering processes.

Sludge conditioning chemicals and their action

Sludge conditioning is carried out to aid the removal of the water from the solids and is most often achieved through dosing with chemicals. Conditioning coalesces (or agglomerates) the smaller sludge particles (1−10 µm) to produce larger ones and reduces the sludge colloidal (the sub-0.1 µm particle/macromolecular size) content. This increases both particle settling and the product cake permeability, consequently increasing the efficiency of the downstream thickening and dewatering processes.

Chemical conditioning is achieved through the dosing of sludge with:

inorganic (or mineral) coagulants (such as iron or aluminium), and/or
organic flocculants (or polymers).

Mineral reagents are better suited to processes such as mechanical dewatering (such as a belt or filter press) where a strong floc is preferred to a large, compressible one. Mineral coagulants include:

ferric or aluminium chloride or sulphate (FeCl₃, Fe₂(SO₄)₃, AlCl₃, Al₂(SO₄)₃)
ferric chlorosulphate (FeClSO₄)
polyaluminium or polyferric salts
lime (slaked lime, Ca(OH)₂) or quick lime, CaO), used in alkaline stabilisation.

Ferric salts, and ferric chloride in particular, are often preferred both on economic grounds and on the basis of their effectiveness. Coagulants work by reducing the repulsive sludge particle surface charge, allowing them to agglomerate through this destabilisation effect. Coagulant dosing may be supplemented by lime dosing to increase the cake filterability and retain more of the toxic metals with the solids. Lime dosing additionally stabilises the sludge, reducing its putrescence, odour and pathogenic micro-organism content.

Sludge conditioning chemical dose

The coagulant or polymer dose required to condition the sludge depends on both the sludge characteristics and the dewatering technology being applied downstream of conditioning. Laboratory tests are always required to determine the polymer chemical type and dose, but doses are generally in the range:

40−120 kg/teDS FeCl₃
50−300 kg/teDS CaO (quicklime)
2−11 kg/teDS polymer.

Table 1. Mean coagulant and polyelectrolyte doses for dewatering operations (Andreoli et al, 2007)
Sludge origin	Filter press FeCl3, kg/teDS	Belt filter Polymer, kg/teDS	Centrifuge Polymer, kg/teDS
*Waste activated sludge
Primary	40−60	2−3	4−5
Mixed primary/WAS*	−	3−5	6−9
Mixed digested	40−100	4−5	6−9
WAS*	70−100	4−6	7−11

A high fibre or dense granular solids content of the sludge tends to reduce the required coagulant chemical dose, whereas a high protein content increases it. Adding inorganic reagents increases the sludge inert solids concentration; ~60−70% of the added FeCl₃, and 80−90% of the dosed lime, ends up in the dewatered solids (or cake fraction). Organic polymers, on the other hand, can be thermally destroyed along with the other organic content and so do not negatively impact on the sludge calorific value in downstream anaerobic digestion or thermochemical treatment processes.

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Sludge conditioning chemical dosing method

Inorganic chemical conditioning of sludge may involve flocculation in successive mixed tanks, the first for metal salts (usually alum or ferric chloride), and the second for lime. High agitation rates (150−300 W/m³) with short durations (5−10 minutes) are used, followed by a longer period under slow-mixing conditions to allow the agglomerated particles (or flocs) to grow.

The flocs formed by conditioning must be preserved by maintaining low shears when conveying the sludge, for example by using screw or progressive cavity pumps. Storing the conditioned sludge for extended periods may also adversely impact on its filterability, since natural bio-degradation of the organic matter can generate colloidal matter.

The conditioning chemicals can be provided either in solid or liquid form, with liquid polymers supplied as an emulsion. If supplied as a powder, then the polymer has to be stored dry and rapidly pre-mixed with water to form a 2−5 g/L solution ready for use, then allowed to rest (20−30 minutes of maturation to allow the polymer to hydrolyse) before application. Solutions must be prepared regularly since the effectiveness of the dissolved polymer slowly decreases over the course of a few days.

Dosing with polymer can be through simple in-line mixing in the case of centrifugal or gravity drum thickening since the rotary action within the process provides sufficient effective mixing for promoting agglomeration. For other thickening/dewatering technologies, dedicated premixing for 20−60 seconds upstream is normally required.

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Sludge conditioning testing

Selection of the most appropriate chemical(s) for sludge conditioning is critically important. The effectiveness of agglomeration – i.e. the formation of large aggregates from the particles – can be assessed using standard jar testing equipment. However, other properties, such as the mechanical strength and deformability of the aggregates formed, can also be important in determining the efficiency of the thickening or dewatering operation.

A number of other tests have been developed to assess conditioning performance. These include:

drainability: the measurement of the rate at which water drains under gravity through a standard filter paper
filterability: the measurement of the rate at which water passes through a standard filter paper under the action of a constant positive pressure on the feed side or a negative pressure (vacuum) on the filtrate side

settling velocity: the measurement of the rate at which either:
- individual particles settle, or
- bulk particles settle, as represented by the rate at which the boundary between the settled material and the supernatant moves in a settling vessel