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Four Techniques for Selecting Flocculant in Sludge Dewaterer Operation


When sewage and sludge are treated by sludge dewatering machine, some flocculants are often needed to achieve better treatment effect. When choosing suitable flocculants, we usually start from the following four aspects.

I. Sludge Characters

For sludge with high organic content, the more effective flocculant is cationic. The higher the organic content, the better the cationic flocculant with higher degree of polymerization. For inorganic sludge, anions can be considered. Different sludge properties directly affect the conditioning effect. Sludge and excess activated sludge are difficult to dehydrate, while the dewatering performance of mixed sludge is between the two.

Generally speaking, the more difficult it is to dehydrate the sludge, the larger the flocculation dosage and the smaller the sludge particles will lead to the increase of the consumption of flocculant, the higher the organic matter content and alkalinity in the sludge, and the increase of the flocculant dosage. In addition, the solid content of sludge also affects the dosage of flocculant. Generally, the higher the solid content of sludge, the larger the dosage of flocculant when using sludge dewaterer.

II. Sludge acidity and alkalinity

The hydrolysate form of sludge with different acidity and alkalinity is different from that of sludge with the same flocculant. The hydrolysis of aluminium salts is greatly affected by the acidity and basicity. The optimum acidity and basicity range for the coagulation reaction is 5-7. The effect of acidity and alkalinity on the high iron salt conditioner is small, and the optimum acidity and alkalinity range is 6-11. Ferrite hydrolysates with high solubility can be oxidized into flocs with low solubility in sludge with acidity and alkalinity of 8-10.

Therefore, when choosing inorganic salt flocculants, we should first consider the specific acidity and alkalinity of dewatered sludge. If the acidity and alkalinity deviate from the optimum range of its coagulation reaction, it is better to replace another one. Otherwise, we should consider adding acid or alkali to adjust the acidity and alkalinity of sludge before the sludge is regulated.

III. Flocculant Concentration

The concentration of flocculant not only affects the conditioning effect, but also affects the consumption of reagents and the yield of sludge cake from sludge dewaterer, especially the organic macromolecule flocculant. Generally speaking, the lower the concentration of organic macromolecule flocculant, the less the consumption of reagent, the better the conditioning effect.

But too high or too low preparation concentration will reduce the yield of mud cake. The conditioning effect of inorganic macromolecule flocculant is almost not affected by the concentration of preparation. The optimum concentration of organic macromolecule conditioner is between O.05% and O.1%. The optimum concentration of ferric chloride is 10%. The optimum concentration of aluminium salt is 4%-5%.


The temperature of sludge directly affects the hydrolysis of flocculant. When the temperature is low, the hydrolysis will slow down. If the temperature is lower than 10 C, the flocculation effect of sludge dewaterer will become worse, and the conditioning effect can be improved by prolonging the flocculation time properly. When the temperature is low in winter, attention should be paid to the thermal insulation of sludge dewaterer, not less than 15 C, so as to minimize the heat loss during sludge transportation.


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