Attentions in Selecting Flocculants for Screw Dehydrator

When screw dehydrator is used in sewage and sludge treatment, in order to achieve good treatment effect, flocculants are often needed. When selecting the appropriate flocculant, the following four aspects should be considered.

1. Sludge Properties

For sludge with high organic content, cationic flocculant is more effective. The higher the organic content is, the cationic flocculant with higher degree of polymerization should be selected. For sludge with high inorganic content, it is better to use the anionic flocculant. Different sludge properties directly affect the conditioning effect, scum and excess activated sludge are difficult to be dewatered.

Generally speaking, the more difficult to be dewatered the sludge is, the more flocculant should be added. If the solid particles in the sludge are very small, it will lead to the increase of flocculant consumption. High organic content and alkalinity will also lead to the increase of flocculant consumption. In addition, the solid content of the sludge also affects the dosage of flocculant. Generally, the higher the solid content is, the larger the dosage of flocculant is.

2. Sludge pH

The pH of sludge determines the form of the hydrolysate, when the same flocculant is used to treat the sludge with different pH, the treatment effect is very difficult. The hydrolysis reaction of aluminum salt flocculant is greatly affected by the pH value, the excellent pH value range of its condensation reaction is 5-7. Ferric salt flocculant is less affected by the pH value, the excellent pH value range is 6-11.

Therefore, when selecting inorganic salt flocculant, the pH value of sludge should be considered. If the pH value deviates from the excellent range of coagulation reaction, it is better to replace it with another one. Otherwise, it is necessary to add acid or alkali to adjust the pH of the sludge before dewatering.

3. Flocculant Concentration

The concentration of flocculant (especially the organic polymer flocculant) not only affects the conditioning effect, but also affects the consumption of agents and the yield of mud cake. Generally speaking, the lower the concentration of organic polymer flocculant, the less the consumption of the agent, the better the conditioning effect. However, too high or too low concentration of organic polymer flocculant will reduce the yield of mud cake.

But the conditioning effect of inorganic polymer flocculant is almost not affected by its concentration. The excellent concentration range of organic polymer flocculant is 0.05%-0.1%. The excellent concentration range of ferric salt flocculant is 10%. The excellent concentration range of aluminum salt flocculant is 4%-5%.

4. Temperature of Sludge

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℃, the flocculation effect of the screw dehydrator will be worse, the treatment effect can be improved by increasing the flocculation time appropriately. When the temperature is low in winter, we should pay attention to the heat preservation of the screw dehydrator, no less than 15℃, so as to minimize the heat loss in the process of sludge conveying.