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Zero Liquid Discharge Solutions 499

13.4 SOLIDS SEPARATION EQUIPMENT

Although good in terms of water recovery and the elimination of effluent
discharge, the ZLD process does generate solids. This section discusses
aspects of solids separation inherent in all ZLD systems.
13.4.1 Centrifuge

Here the solid gets separated through the application of mechanical energy.
The important types of centrifuge to be considered for this process are
pusher centrifuges.

13.4.1.1 Pusher Centrifuge
This type of centrifuge provides continuous filtering used for solid-liquid
separation in the chemical and mineral industries. The capacity sizing of a
pusher centrifuge completely depends upon the type of solids or salts being
handled. There are mainly two types of inorganic salts we look at for pusher
centrifuge operation. These salts are:
• NaCl (sodium chloride)
• Na 2 SO 4 (sodium sulfate).
The operation of the pusher centrifuge is a function of many parameters such as
particle size, viscosity, solids concentration, cake quality, and particle attrition.

13.4.2 Dryer
In dryers, the separation of solids is accomplished through the application of
thermal energy. The major types of dryers are:
• Rotary drum dryer
• Spray dryers
• Agitated thin film dryer.

13.4.2.1 Agitated Thin Film Dryers
ATFDs are characterized by a mechanically agitated thin product layer inside
the dryer. The dryer consists of a vertical, cylindrical body with a heating
jacket and a rotor inside of a shell which is equipped with rows of blades
all over the length of the dryer. The hinged blades spread the wet feed
product in a thin film over the surface of the heated wall. The turbulence
increases as the product passes through the clearance before entering a calming
zone situated behind the blades. The volatile component evaporates contin-
uously. The product layer is typically less than a millimeter in thickness.
The following case studies discuss attempts to implement zero discharge
solutions.