Slurry Density Instrumentation

ERDC TN-DOER-T7

October 2004

injecting the diluted polymer through an in-line mixer are economical for smaller systems.

However, the cost/pound of liquid polymer is usually higher than for dry polymers. Equipment

configuration and high flow rates sometimes dictate the use of flocculant mixing tanks in lieu of

in-line mixers. Polymers may be introduced at several points within the treatment train,

preceding the thickener for example, and again preceding the principal dewatering equipment.

Polymer dosage prior to a belt filter press may be as much as three times higher than the dosage

preceding the thickener, which is one of the reasons slurry is not generally fed directly to the

press (slurry volume is reduced at the thickener, thus reducing polymer consumption).

Various means of controlling the injection rate of mixed or blended polymer into the slurry are

commercially available. The degree of automation that is desired depends on a number of

factors, including the variability of the feed solids characteristics during the dredging process

and/or the change in the percent solids in the slurry. Because many factors affect the amount of

polymer that is required and because conditions can change so quickly, automatically adjusted

injection can sometimes be difficult to justify from a capital cost perspective.

Slurry Density Instrumentation. To achieve optimal system performance, including

maximum dewatering effectiveness and minimum polymer consumption, the percent solids in

the slurry feeding the dewatering equipment should be continuously monitored and adjusted to a

preset level. This is frequently accomplished with a densometer and dilution water from the

thickener/clarifier overflow. A controlled proportional valve can be used to introduce the

required water flow into the clarifier/thickener underflow.

Belt Filter Presses. Belt filter presses have historically been used in the paper industry and

for dewatering of sewage sludge, and are well-proven for dewatering dredged material. They

offer continuous operation, low operator labor, reasonable capital cost, ease and simplicity of

maintenance, high solids throughput, moderate footprint, high solids capture rate (i.e., low solids

in filtrate), low power consumption, reasonable polymer consumption rate, high reliability and

availability, and adaptability to changing process conditions. However, solids content of the

cake may be lower than that produced by a plate and frame press. Typically, a belt filter press

will produce a cake of approximately 40 to 50 percent solids by weight, as compared to 50 to 65

percent solids achievable with plate and frame presses. These values are a function of the

density of the solids in the material as well as the dryness and compression achieved by the

equipment. Solids content for very low-density materials may be more in the range of 30-40

percent solids. Solids content may be a pivotal factor where the final weight or volume of the

processed material factors significantly into management economics, as it does when transport or

off-site disposal costs must be considered.

A belt press has three dewatering zones: gravity, wedge, and high pressure. Free water drains

from the conditioned sludge in the gravity zone. The press is equipped with rakes to distribute

the material and encourage drainage (Figure 6). In the wedge zone, the sludge is captured

between two moving belts where additional water is squeezed out under low pressure as the belts

converge. The belts sandwich the now more-compressed sludge and pass over a series of rollers

of decreasing size and increasing pressure to further reduce the water content (Figure 7). The

number of rollers employed can be varied according to product specifications and requirements

of the material being processed.