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Sludge Dewatering

In wastewater treatment the pollutants are agglomerated and precipitated in form of flocs, called ‘sludge’. Typically, sludge from a final liquid solids separation unit may contain from 1 to 5% total suspended solids.

Because of the costs saving associated with handling smaller volumes of sludge, there is an economic incentive to remove additional water, cut the disposal costs, while complying with environmental, health and safety rules.

Inefficient sludge treatment leads to low-quality effluent, reduces sludge solids content, increases disposal costs, and necessitates sludge rework.

Conventional equipments (centrifuge, vacuum, pressure, decanter, belt press, filter press, screw press) can increase the drainage rate and the amount of water released from sludge, but the nature of the sludge, these equipments have limits in increasing the dry solid rate of the sludge.

Conventional equipment can extract the free water relatively easily and efficiently, by applying high pressure and using large amounts of coagulation and flocculation polymers. However, the dewatered sludge still contain a relatively high percentage of water (75-85%), consisting mainly of bonded water. Bonded water is physically and chemically bound to the solid molecules in the sludge and cannot be extracted using conventional mechanical dewatering equipments.

Until the advent of electro-osmosis dewatering equipment, only thermal processes, which are costly in terms of energy and capital, would allow achieving dryness (level of solids in total mass) of more than 15-35% in the sludge.

Composition of sludge

The Structure of Water in sludge

All sludges, organic and inorganic consist of a combination of solid phase with certain quantity of liquid. Behaviors of this liquid is different from the ordinary water. There are different physical forms of water in sludge and these different forms play an important role in determining the ease or difficulty of phase separation.

Liquid sludge contains a large proportion of water (95 to 99%). This water is mainly in the form of free water, external water, bonded water and internal water.

Free water is the water moves freely in sludge and therefore it is easily separately by flocculation. It represents the most relevant content in percentage in sludge and it is most easily to remove in pre-thickening process.

External water is the water remained at the surface of floc and  is removed by the pressure of press

Internal water is the water exists both in the suspended solid and generated floc and is removed by thermal process.

Bonded water is the water bonded with suspended solid. It can be classified as ‘colloidal water’ and ‘capillary water’. The removal of bonded water is done by drying process after dewatering. In any case this  water can not be removed completely.

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