In the process of sewage and sludge disposed by the screw dehydrator, it is necessary to add some flocculants to achieve good effect. In order to choose the appropriate flocculants, the following four factors should be taken into consideration.

1. Sludge properties

For sludge with high organic content, it is advisable to choose cationic flocculant with high polymerization degree. The anionic flocculant can be used for the sludge mainly composed of inorganic substances. The different properties of sludge will directly affect the treatment effect. For example, the scum and residual activated sludge are difficult to be dewatered, the dewatering performance of mixed sludge is between them.

Generally speaking, the more difficult the sludge to be dewatered, the more flocculants will be used. The smaller the sludge particles, the more flocculant consumption. The higher the organic content and alkalinity, the more flocculant consumption. In addition, the solid content of sludge will also affect the dosage of flocculants. Generally, the higher the solid content of sludge, the larger the dosage of flocculants.

2. Sludge pH

The pH of sludge determines the type of hydrolysates, and the treatment effect of the same flocculant on sludge with different pH is also different. The hydrolysis reaction of aluminum salt is greatly affected by the pH value, its excellent pH value range is 5-7. The effect of pH value on high iron salt conditioner is small, and its excellent pH value range is 6-11. The pH value range of 8-11 is suitable for ferrous salt.

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

3. Flocculant concentration

The concentration of flocculant not only affects the conditioning effect, but also affects the consumption of chemicals and the output rate of sludge cake.

Generally speaking, the lower the concentration of organic polymer flocculants, the less the agent consumption, the better the conditioning effect. However, no matter the concentration is too high or too low, the output rate of sludge cake will be reduced. But the conditioning effect of inorganic polymer flocculant is almost not affected by the concentration.

4. Sludge temperature

The temperature of sludge directly affects the hydrolysis of flocculants. When the temperature is low, the hydrolysis will slow down. If the temperature is lower than 10℃, the flocculation effect of the sludge dehydrator will become worse, so it is necessary to increase the flocculation time to improve the treatment effect. In winter, the temperature is low, heat preservation of the screw dehydrator is necessary. The temperature should be no lower than 15℃, so as to minimize the heat loss in the process of sludge conveying.