Dewatering...

Dewatering is a physical unit operation used to reduce the water content of sludge for the following reasons : (1) Lower cost for trucking sludge to the ultimate disposal site. (2) Easier handling and transporting. (3) Increasing energy content for incineration by lowering water content. (4) Lower cost for bulking agents for composting. (5) Rendering the sludge odor and putrescibility. (6) Reducing leachate production for landfilling.

Sludge conditioning...

Vacuum Filter : Advantages ;Skilled personnel not required, Maintenance requirements are low for continuously operating equipment. Disadvantages ;Highest energy consumer per unit of sludge dewatered, Continuous operator attention required, Vacuum pumps are noisy, Filtrate may have high suspended solids.

Solid Bowl Centrifuge : Advantages ;Clean appearance, minimal odor problems, fast startup and shutdown capabilities, Easy to install, Produces relatively dry sludge cake, Low capital cost-to-capacity ratio. Disadvantages ;Scroll wear potentially a high-maintenance problem, Requires grit removal and possibly a sludge grinder in the feed system, Skilled maintenance personnel required, Moderately high suspended solids content in centrate.

Belt Filter Press : Advantages ;Low energy requirements, Relatively low capital and operating costs, Less complex mechanically and easier to maintain, Very dry cake production. Disadvantages ; Hydraulically limited in throughput, Requires sludge grinder in feed system, Very sensitive to incoming sludge feed characteristics, Short media life, Automatic operation generally not advised.

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Plate Filter Press : Advantages ;Highest cake solids concentration, Low suspended solids in filtrate. Disadvantages ; Batch operation, High equipment and labor costs, Special support structure requirements, Large floor area required for equipment, Skilled maintenance personnel required, High dosage of chemical.

Performance Data for Sludge - Processing Facilities...

"Solids Capture"... "Solid Concentration"... "Recycle Flow Characteristics"...

Remote Control of Sludge Conditioner (Polyelectrolyde) Dosing...

In many cases the sludge is first chemically conditioned in order to accelerate dewatering and to raise efficiency of the centrifuges. Here, organic substances such as polyelectrolydes (polymeres) are used. Goal is now on the one hand to obtain more information from the process by employing analysing instruments. On the otherhand to build up a control loop. Taking into account that the dewatering result is mostly effected by the kind and amount of flocculent, by the dry mass (flow multiplied with suspended solid) and especially by the nature of the sludge itself, the importance of continuous measuring becomes obvious. The efficency of the flocculant dosage is changing with the different incredients of the sludge.

Control Methods...

Constant Dry Mass : A first step towards a controlled operation is the control of the arriving dry mass. Here, the multiplication resulting from sludge flow measurement and suspended solid measurment is kept on a constant level. Such a method controls the dry mass, however not the effect of chemicals on the changing sludge nature. This represents a feed-forward control without any feedback from the dewatering result. As a consequence, this strategy is commonly used in cases where no flocculents are added.

Measuring of Turbidity in the Centrate : Basis of this strategy is a turbidity meter in the centrate. For the measuring it has to be taken into account that the measuring probe is installed in a deareation vessel which has the task to eliminate foam and air-bubbles. The flocculent is dosed according to turbidity in the centrate.

In order to take care of sudden changing dry mass, the dry mass signal is switched as feed forward. However, this does not represent an optimisation yet. It means in general that with an increasing dosage of flocculents the turbidity in the centrate decreases. In case of over-dosing the turbidity increases again. Furthermore, it has to be considered that with a beginning dosage of flocculent the turbidity decreases relatively much. This proportion reverses with increasing dosage, since relatively higher dosing only effects a lower decrease of turbidity. This has the result that the optimum for a dosage of flocculent is not necessarly equal to the lowes turbitity. Often it is useful to determine a economic optimum. It specifies a relation of turbidity decrease and flocculent dosage. That means the control parameter is not turbidity of the centrate, but the slope of the curve which represents the relation between turbidity decrease and flocculent dosage.

"Remote Control System for Sludge Conditioning"...

Sludge lagooning...