How to choose a sludge conditioner?

2021-04-01

(1) Variety characteristics of conditioners. As far as the commonly used inorganic conditioners of aluminum and iron salts are concerned, the dosage of the medicament when using aluminum salt is larger, the density of flocs formed is smaller, and the conditioning effect is poor. The filter cloth will be blocked during the dehydration process.

Therefore, when using a centrifugal dewatering machine and a belt filter press to dewater the sludge, organic conditioning agents such as polyacrylamide can be considered. When it is difficult to achieve the desired conditioning effect with one of the inorganic or organic conditioning agents, you can consider combining the inorganic and organic conditioning agents, which can sometimes achieve better conditioning effects. For example, the combined use of lime and ferric chloride can not only adjust the pH value, but also the calcium carbonate particle structure formed by the lime and the heavy calcium carbonate in the sewage can increase the porosity of the sludge and promote the separation of mud and water.

Compared with inorganic conditioners, the dosage of organic conditioners is smaller, and the formed flocs are coarser, but the flocs have lower strength and are more easily broken than the flocs formed by inorganic conditioners. Moreover, once the flocs are destroyed, it is not easy to restore to the original state no matter if an inorganic or organic conditioner is used.

Therefore, when using a centrifugal dewatering machine and a belt filter press to dewater the sludge, organic conditioners can be considered. When it is difficult to achieve the desired conditioning effect with one of the inorganic or organic conditioning agents, it is possible to consider the combination of inorganic and organic conditioning agents, which can sometimes achieve better conditioning effects. For example, the combined use of lime and ferric chloride can not only adjust the pH value, but also the calcium carbonate particle structure formed by the lime and the bicarbonate in the sewage can increase the porosity of the sludge and promote the separation of mud and water.

(2) The properties of sludge. For sludge of different nature, the type and dosage of the conditioning agent used are also very different. For sludge with high organic content, a more effective conditioner is a cationic organic polymer conditioner, such as cationic polyacrylamide. And the higher the organic content, the more suitable the cationic organic polymer conditioning agent with the higher the degree of polymerization.

For the sludge that is mainly inorganic, an anionic organic polymer conditioner can be considered. The different properties of the sludge directly affect the conditioning effect: the sludge in the primary settling tank is easier to dewater, while the scum and the remaining activated sludge are more difficult to dewater, and the dewatering performance of the mixed sludge is somewhere in between.

In order to achieve a certain conditioning effect, there are significant differences in the number of conditioning agents required. Generally speaking, the more difficult to dehydrate sludge, the greater the dosage of conditioning agents, the finer sludge particles, which will increase the consumption of conditioning agents, and the high organic content and alkalinity in the sludge will also increase the dosage of conditioning agents. Big. In addition, the sludge solid content also affects the dosage of the conditioning agent. Generally, the higher the sludge solid content, the greater the dosage of the conditioning agent.

(3) Temperature. The temperature of the sludge directly affects the hydrolysis of inorganic salt conditioners. When the temperature is low, the hydrolysis will slow down. If the temperature is lower than 10oC, the conditioning effect will be significantly worse. The conditioning effect can be improved by appropriately extending the conditioning time.

When using an organic polymer conditioner, if the temperature of the mother liquor or tap water or the temperature of the sludge is too low, the dynamic viscosity of the water and the viscosity of the polymer conditioner solution will increase and it is not conducive to dilution. Uniformity and conditioning are mixed uniformly, which in turn affects the sludge conditioning effect and dewatering effect.

Therefore, when the temperature is low in winter, attention should be paid to the thermal insulation of the sludge conveying system (the temperature of the sludge discharged from the sewage treatment system is generally not lower than 15°C) to minimize the heat loss during the sludge conveying process. If necessary, it is possible to improve the dissolution conditions by heating the organic polymer conditioning agent dilution tank or by appropriately extending the mixing and dissolving time and increasing the stirring intensity.

(4) pH value. The pH value of the sludge determines the form of the hydrolysate of the inorganic salt conditioner, and the same conditioner has very different conditioning effects on sludge with different pH values. The hydrolysis reaction of aluminum salt is greatly affected by the pH value, and the optimal pH value range for the condensation reaction is 5-7. When the pH value is greater than 8 or less than 4, it is difficult to form flocs, which means that the conditioning effect is lost.

The ferric salt conditioner is less affected by the pH value. No matter whether the sludge is acidic or alkaline, it can form hydrolysate Fe(OH)3 flocs, and the optimal pH range is 6-11. In the sludge with a pH value of 8-10, the hydrolyzed product of ferrous salt with higher solubility can be oxidized into Fe(OH)3 flocs with lower solubility.

Therefore, when selecting an inorganic salt conditioner, the specific pH value of the dewatered sludge must first be considered. If the pH value deviates from the optimal range of its coagulation reaction, it is best to switch to another conditioner. Otherwise, it is necessary to consider adding acid or alkali to adjust the pH value of the sludge before conditioning the sludge. Under normal circumstances, this measure is not taken.

The pH value also has an impact on the conditioning effect of polyelectrolytes. The pH value of the sludge affects the ionization, charge status and molecular shape of the conditioning agent molecules.

The ionization degree of cationic polyelectrolyte in acid sludge with low pH value is larger, and the molecular shape tends to stretch; while in alkaline sludge with high pH value, the ionization degree is smaller, and the molecular shape tends to curl. Contrary to the nature of cationic polyelectrolytes, the ionization degree of anionic polyelectrolytes in acidic sludge with low pH value is small, and the molecular shape tends to curl; while in alkaline sludge with high pH value, the ionization degree is larger and molecular shape Tend to stretch.

The situation of anionic and cationic polyelectrolytes is slightly different. At the isoelectric point, the entire molecule is neutral, and the positive and negative charges attract each other, so the molecules are tightly curled into clusters. On both sides of the isoelectric point, there will be a kind of excess charge on the molecules, which tends to stretch due to mutual repulsion.

(5) Preparation concentration. The preparation concentration of the conditioning agent not only affects the conditioning effect, but also affects the consumption of the medicament and the yield of the mud cake, and the effect of the organic polymer conditioning agent is more significant. Generally speaking, the lower the concentration of the organic polymer conditioning agent, the less the consumption of the agent and the better the conditioning effect.

This is because the lower the concentration of the organic polymer conditioning agent, the easier it is to mix uniformly, the better the molecular chain stretches, the better the bridging and cohesion, and the better the conditioning effect. However, if the preparation concentration is too high or too low, the yield of mud cake will be reduced.

The conditioning effect of the inorganic polymer conditioning agent is hardly affected by the formulation concentration. Experience and related studies have shown that the concentration of organic polymer conditioning agent is between 0.05% and 0.1%, the concentration of ferric chloride is 10%, and the concentration of aluminum salt is between 4% and 5%.

(6) Dosing order. When more than one conditioning agent is used, the order in which the conditioning agent is added will also affect the conditioning effect. When iron salt and lime are used as the conditioning agent, the iron salt is generally added first, and then the lime is added, so that the flocs formed are easier to separate from the water, and the total consumption of the conditioning agent is also less.

When an inorganic conditioner and an organic polymer conditioner are used to jointly condition the sludge, the inorganic conditioner is added first, and then the organic polymer conditioner is added. Generally, a better conditioning effect can be achieved.

(7) Mixing reaction conditions. In order to achieve the best conditioning effect, it is very necessary to achieve complete and sufficient mixing of sludge and conditioning agent. However, it is worth noting that after the sludge and the conditioner are mixed and reacted to form flocs, they must not be destroyed again, because once the flocs are damaged, it is difficult to restore the original state.

Experience has shown that for a certain sludge, when a certain conditioner is used, only when the intensity and time of the mixing reaction are within a certain range can a better conditioning effect be achieved, and the conditioning effect will decrease as the residence time increases.

That is to say, after the time and intensity of conditioning are determined through experiments, they must be strictly followed in actual operation. On the one hand, the mixing reaction time cannot be prolonged or shortened at will, on the other hand, the conditioned sludge must enter the dehydrator as quickly as possible.


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