Oily Sludge Dewatering Equipment and Technology (2)

3. Dewatering of Oily Sludge

Oily sludge needs to be pretreated before mechanical dewatering. The sludge dewatering process is actually the relative movement of the suspended particles and water in the sludge. The pretreatment of sludge is to adjust the properties and arrangement of the solid particles through certain means to make them suitable for different dewatering conditions.

3.1 Pretreatment of Oily Sludge

The main purpose of sludge pretreatment is to improve the dewatering performance of sludge and increase the production capacity of dewatering equipment.

The pretreatment methods of sludge include chemical conditioning, elutriation conditioning, etc. Chemical conditioning is mainly used in China.

The chemical conditioning method is to add coagulants to the sludge, so to condense the suspended particles in the sludge and improve the dewatering efficiency. However, for oily sludge, in order to achieve the three-phase separation of oil-water-solid, it is also necessary to add surfactants, demulsifiers, pH regulators, etc. Only in this way can the adsorbed oil with high viscosity be desorbed and demulsified.

The chemical conditioning method of oily sludge must be carried out on the basis of testing the properties of oily sludge.

For oil content not less than 10%, it is appropriate to use hydrophilic surfactants. After separation, water and solid are in the lower layer, and oil is in the upper layer.

For oil content not more than 4%, it is appropriate to use lipophilic surfactants. After separation, oil and solid are in the lower layer, and water is in the upper layer.

The water layer contains soluble oil and micro-emulsified oil.

3.2 Dewatering of Oily Sludge

At present, the following are the main methods of oily sludge dewatering.

3.2.1 Vacuum Filtration

Vacuum filtration refers to the use of porous filter media to intercept the solid particles in the suspension and form filter cake on the surface of the medium. Although the vacuum filter has the advantages of simple design, wide application range and continuous production, its vacuum driving force is low, the filtration rate is not high, and the moisture content of the filter residue is high. It cannot be used normally for the scum produced by the flotation process of oily wastewater, and the filter medium will be easily contaminated by oil.

The main equipment for vacuum filtration includes rotary drum vacuum filter, horizontal belt vacuum filter, etc.

3.2.2 Pressure Filtration

Pressure filtration refers to the process of pressurizing the filter medium on both sides to reduce the moisture content. The filter press has the advantages of simple structure, strong filtration driving force, good dewatering effect. The moisture content of the filter cake is very low (about 50%), but the equipment is bulky, the operation labor intensity is high. It cannot operate continuously, the efficiency is low. The filter medium will also be easily contaminated by oil.

The main equipment for pressure filtration includes plate and frame filter press and chamber filter press.

3.2.3 Rotary Filtration

The main equipment for rotary filtration is belt filter press. The belt filter press refers to the process of applying pressure to the filter belt and using the pressure or tension of the filter belt to dewater the sludge.

When dewatering the sludge, the sludge and flocculant are sent to the mixing drum for full mixing to promote flocculation, and then be sent to the gravity dewatering section. The free water in the sludge will be removed by gravity, so that the sludge loses fluidity and is easy to squeeze (extending the gravity dewatering section can improve the efficiency of gravity dewatering).

After gravity dewatering, it will be sent to the "wedge-shaped" pressing section. Under the pressure of filter belt, the filtrate will be discharged to the liquid outlet. The filter cake will be peeled off by the scraper, and the sludge stuck on the filter belt will be flushed off by flushing water and then be discharged.

The belt filter press cannot treat the scum produced by the flotation process, and the filter belt will also be easily contaminated by oil.

3.2.4 Centrifugal Filtration

Sludge centrifugal dewatering does not require filter medium. It relies on the centrifugal force to make solid particles settle quickly and achieve solid-liquid separation. The main equipment for centrifugal filtration includes the following:

(1) Three-legged sedimentation unloading centrifuge with skimmer and scraper. This equipment can treat relatively sticky sludge and can achieve better results without adding chemical agents. The moisture content of the filter cake is related to the sedimentation time. The clarified liquid and sediment are discharged separately by the skimmer. However, its processing capacity is small and the operation is intermittent, which is not suitable for processing large amounts of sludge. For the treatment of oily sludge, especially those containing asphaltene and paraffin, the asphaltene and paraffin are easy to adhere to the skimmer and block it.

(2) Horizontal screw sedimentation unloading centrifuge. This equipment has the advantages of large processing capacity, good processing effect (the moisture content of the filter cake is generally 75% to 85%), simple operation, high degree of automation, good adaptability to the original liquid, sanitary working environment, low infrastructure cost, and small land occupation. It has become a new type of efficient sludge dewatering equipment.