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Apr 17, 2018



--Lifeng Environmental Equipment Co.,LTD


1.1. Origin of the sludge:

During the course of the water treatment, products coming from the pollution are extracted while the treated water is released in the environment.


Amongst these products coming from the pollution one can distinguish:

● Particles that decant naturally or that come from the physico-chemical treatment

● Excess micro-organisms coming from the dissolved organic matter treatment

● Mineral matter that is non biodegradable


All these products are suspended in more or less concentrated forms and the resulting liquid is called sludge.


1.2. The different types of sludge:

1.2.1. Primary sludge:

Primary sludge comes from the settling process. It is therefore made of easily decantable suspended particles: large and/or dense particles. It has a low level of Volatile Solids content (VS around 55% to 60%) and its dewatering ability is excellent. It is also very easy to concentrate this type of sludge with a static thickening step just before dewatering. The drawback is that this sludge ferments very easily.


1.2.2. Biological sludge:

 Biological sludge comes from the biological treatment of the wastewater. It is made of a mixture of microorganisms. These microorganisms, mainly bacteria, amalgamate in bacterial flocs through the synthesis of exo-polymers. A simple decantation in the clarifier will easily separate the bacterial flocs from the treated water. Only part of this settled sludge is sent to dewatering: the excess biological sludge; part of it is recirculated to maintain the bacterial population in the reactor.

SLUDGE DEWA TERING There are several types of sludge that have specific characteristics that will influence:

● The choice in conditioning chemical (cationic flocculant, ferric chloride, lime…)

● The choice in the dewatering equipment to be used (filtration, centrifuge…) These choices will also depend on the final use of the sludge (incineration, agricultural spreading…) sludge characterisation

To simplify, we will not differentiate between the different qualities of biological sludge (prolonged aeration, low charge, high charge…); their main properties are:

● A high Volatile Solids content: VS around 70% to 80%.

● A low dry solids content: 7 g/l to 10 g/l. It is often necessary to introduce a dynamic thickening step by flotation or gravity belt.

● The dewatering ability is medium. It depends partially on the VS. The higher the VS the harder it is to extract the water from the sludge. 1.2.3. Mixed sludge: Mixed sludge is a blend of primary and biological sludges. The blending ratio is often as follows:

 ● 35% to 45% of primary sludge.

● 65% to 55% of biological sludge. This blending will permit an easier dewatering as the intrinsic properties of this sludge are between the other two types.


1.2.3. Digested sludge:

Digested sludge comes from a biological stabilizing step in the process called digestion. This stabilization is performed on biological or mixed sludge. It can be done under different temperatures (mesophilic or thermophilic) and with or without the presence of oxygen (aerobic or anaerobic). Following this stabilization step the properties of the sludge are:

● A lower Volatile Solids content: VS around 50%. A mineralisation of the sludge occurs during digestion

● A dry solids content around 20 g/l to 40 g/l

 ● A good dewatering ability.


1.2.4. Physico-chemical sludge:

 This type of sludge is the result of a physico-chemical treatment of the wastewater (see brochure “Coagulation Flocculation”). It is composed of flocs produced by the chemical treatment (coagulants and/or flocculants). The characteristics of this sludge is the direct result of the chemicals used (mineral or organic coagulants) and of course of the pollutants in the water.


1.2.5. Mineral sludge

This name is given to sludge produced during mineral processes such as quarries or mining beneficiation processes. Their nature is essentially mineral particles of various sizes (including clays). They have a very good aptitude to settle by gravity and very high concentrations are frequently obtained.

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