What is sludge dewatering?

Sludge dewatering is a critical step of the wastewater treatment process that sets the stage for removing solids from the water. These solids are referred to as biosolids and are either used as fertilizer for local farms or are disposed via incineration or landfilling.

What is meant by sludge dewatering?

Sludge dewatering means the removal of liquid including water from the sludge to produce a sludge with lump-like consistency. As will be seen in this article, sludge dewatering is not the same as sludge thickening and there are several processes for dewatering sludge.    

What is the difference between sludge thickening and sludge dewatering?

Sludge thickening and sludge dewatering remove different quantities of water. Sludge thickening concentrates the sludge by removing part of the free water which means that at the end of the process, the sludge retains some of the liquid water to give it the free-flowing characteristics of a feed sludge. 

Sludge dewatering removes significantly more water from the sludge compared to the sludge thickening process. It removes free water and some interstitial (floc) water. The dewatered sludge is referred to as cake, is not free-flowing, and forms lumps.

The dry solids content resulting from the sludge thickening process increases to 4 to 6% whereas for sludge dewatering, the dry solids content is between 15 to 45%. 

What is the process of dewatering separation in sludge?

Sludge contains a mixture of complex organic chemicals that binds water tightly and for the dewatering process to be effective, the sludge needs to be conditioned prior to dewatering. Conditioning the sludge uses chemicals such as iron salts and lime or coagulants and flocculants. These release the interstitial and bound water from the sludge and this water is then removed from the sludge dewatering process.    

The sludge dewatering process is designed to remove as much water as possible with a high dry solids content. It includes pressing the sludge to remove the water, centrifuging to force more water out of the sludge, and drying to evaporate the sludge.

How do you remove water from the sludge?

The four most common methods for removing water from sludge are pressing, centrifugation, rotary pressing and sludge bed drying.

Pressing

In this process, the sludge is pressed to force the water out through a permeable medium such as a belt filter, a crew press, or a filter press.

Centrifugation

In this process, the sludge is rapidly rotated in a cylindrical vessel and this forces the denser solids towards the wall of the vessel while the water separates from the solids.   

Rotary press

In the rotary press, the sludge from the centrifugation process passes through a narrow, parallel flow channel that has porous walls. The water passes through the porous walls as part of the separation process. 

Sludge bed drying

Sludge bed drying can involve simply spreading the sludge as a thin layer of up to 300 mm over a porous bed of sand and gravel and drainage pipes underneath the bed. The water evaporates and some of the water also seeps into the bed and is then collected via the drainage pipes.

What equipment is used to dewater the sludge?

Filter press

A filter press usually has a series of 20 to 80 porous, rectangular plates arranged parallel to each other. The edge of the filter plate has a filter cloth with a pore size of less than 0.1 mm.  Filter presses can be used to dewater industrial and municipal wastewater sludges. 

In general, the sludge is pressed between these plates by applying high pressure and this forces the water out. Specifically, the spaces between the plates is slowly filled with the sludge. Then a pressure of 7 bars to 20 bars is applied over a period of one to two hours.  Due to the pressure, the filtrate that contains the water is forced out. 

The sludge is retained due to the porous filter cloth lining the edge of the filter plate that keeps the sludge in and drains the water out. Air is then flushed through the system to help the water drain out from the space between the plates. The plates are then separated to allow the cake sludge, which is about 25 to 40 mm thick, to fall out.  The whole process can take up to five hours, depending on the wastewater treatment system.  

Centrifuge

The solid bowl centrifuge is the most common type of centrifuge for dewatering sludge.  It is a horizontal centrifuge with a screw design interiorly. The sludge is fed in one direction and as the centrifuge spins, the solids travel in one direction and they move onto the wall of the centrifuge while the water collects at the bottom. The water is removed from one end and the dewatered sludge (i.e. the cake) is removed from the other end, all made possible because of the interior screw design of the centrifuge system. The use of centrifuges is useful for dewatering heavy oil sludge and municipal wastewater sludges. 

Advantages and disadvantages of using centrifuge for dewatering sludge

Besides requiring relatively small spaces, operators have lower exposure to contaminants including pathogens because the centrifuge is a closed piece of equipment. However, experience is required to operate the centrifuge to achieve optimal dewatering of the sludge. The internal parts could also be prone to abrasive wear depending on the interior design of the centrifuge and the sludge composition.  

Rotary press

The rotary press has a cylindrical vessel and inside this vessel, there are two circular screens. The sludge enters between these two screens and a pressure of 0.1 to 0.5 bars is applied as the sludge goes around in a circular path. The applied pressure and the shear forces created due to the circular motion allows the water to move out of the sludge resulting in the dewatered sludge.  

Other techniques in sludge dewatering

While the previously listed techniques are common and have been used for many years in the industry for sludge dewatering, newer technologies and options are making themselves known in the wastewater treatment market.

Electro-dewatering technology

Electro-dewatering does not require the need for chemical additive prior to dewatering the sludge. Used for oily sludges, this technique applies electric current with the ability to disintegrate the chemical bonds within the sludge. The electric field impels the water molecules towards the cathode, thus separating the water from the oily sludge. 

A scholarly study on the electro-dewatering technology showed that optimal dewatering was achieved at 40V with a final water content of 63.3% in the dewatered sludge. Another study mentioned that tightly bound extracellular polymeric substances in the sludge had a great influence on sludge dewatering and the use of an external electric field damaged the sludge cellular structure resulting in the dissolution of these substances and the release of bound water. 

Ultrasonic treatment

Ultrasound has the ability to disintegrate sludge flocs and release tightly bound water from the sludge structure. An ultrasonic energy density of 9.8W for 30 seconds was optimal for effective sludge dewatering. Higher amounts and duration of ultrasonic treatment may yield unfavorable outcome for dewatering effectiveness.  

What is the final water content of the sludge after dewatering?

The specific amount of the final water content in the dewatered sludge depends on the composition of the sludge, the processes, and the equipment used to dewater the sludge.  The amount of the final water content in the dewatered sludge could be as low as 47.8% and a value of 53.1% for sludge from a municipal wastewater treatment plant has also been achieved. 

Advantages of sludge dewatering

The advantages of sludge dewatering are:

Waste reduction

Because sludge dewatering removes free water and interstitial water from the sludge to form a cake, it also reduces the weight and volume of the cake to reduce the disposal cost. 

Wastewater recycling

The water removed from the sludge dewatering process is treated to remove microorganisms and other contaminants. In some cases, the treated water is reclaimed and used in different applications such as reclaimed water for steam powered plants.   

Landfill safety

In some places, the dewatered sludge is disposed of on landfills. Dewatered sludge prevents mudslides at landfills besides preventing the leaching of hazardous chemicals because the dewatered sludge contains very little water and no moisture.