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Biocolloids, or particles less than 4.2 μm which are made up primarily of protein and polysaccharides, created a significant amount of the polymer demand for digested biosolids. A good linear correlation was found between this biocolloid concentration and the optimum polymer dose of the samples. Digestion processes that used higher temperatures, such as thermophilic, ATAD, and TPAD had significantly greater amounts of biocolloids and polymer demand than mesophilically digested sludges.

For the 34 different polymers examined, the polymer charge density is a key factor in explaining differences in polymer demand for different polymers. The different cationic polymers studied have different charge densities and characteristics such as linear or branched configurations, and hydrophobic moieties. In addition, a second set of polymers with similar varying properties including a wide range of molecular weights (MW) was examined. Lower MW polymer had a significantly greater polymer demand than higher MW polymers (greater than about 1 million).

Shear during dewatering is an important factor in affecting the optimum polymer dose, with greater shear requiring more polymer for conditioning. Lower MW polymers were not as resistant to shear as higher MW polymers. In addition, branched polymers appear to have better resistance to shear than linear polymers.

As the temperature of digestion increased, the sludges became less resistant to shear, and were more susceptible to floc breakup. This means for increasing amounts of shear, the polymer demand increased more for thermophilic sludges compared to mesophilic sludges. Undigested sludges were also very susceptible to shear.

The results demonstrate that both charge neutralization and polymer bridging are key mechanism for conditioning with polymer.

The shear associated with several full-scale dewatering devices was measured and quantified using the unitless Gt term. The shear of two high solids centrifuges was measured in the range of 75,000–120,000, medium solids centrifuges 25,000–35,000, and a belt filter press was 10,000. This means, for the same sludge type, a greater polymer demand would be encountered for high shear devices such as the high solids centrifuge as compared to a relatively low shear belt filter press.
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Document Type: Research Article

Publication date: 2005-01-01

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