What is sludge cake?
Sludge cake is a resource produced by dewatering sludge from wastewater treatment plants. The term “filter cake” usually refers to the solids that remain after the water is removed from the sludge.
How Is sludge cake formed?
Wastewater treatment produces sludge as suspended and dissolved contaminants are separated from water.
Fresh sludge normally contains a large amount of water. Before it can be handled efficiently, part of this water is removed.
A simplified process is:
Wastewater → Sludge → Thickened Sludge → Conditioned Sludge → Dewatered Sludge Cake
During dewatering, water is separated from the sludge while solids are retained. As more free water is removed, the sludge changes from a pumpable slurry into a semi-solid material known as sludge cake.
The word “cake” refers to this concentrated physical form. It does not mean the material is completely dry.
The cirth of sludge cake
Sludge dewatering can take many different forms and use different methods. One of the most effective methods is dewatering using a belt filter press. This type of dewatering uses mechanical equipment to “squeeze” or squeeze out the liquid and requires polymer chemicals to be added to the process at one or more points. The polymer helps to regulate the formation of the solids and allows the machine to do its job. After dewatering, it is subjected to a drying process. The sludge cake is then hauled to an appropriate disposal site.
There is no long-term future for placing raw or lime treated sludge cake directly on agricultural land, and landfilling of sludge cake is costly and a lost opportunity. Therefore, the safe use of sludge cake is a major driving force for wastewater treatment companies.
Dewatered sludge cake typically contains 18 – 35% dry solids and is a non-free flowing material with mechanical handling properties that vary depending on the process, location and/or season.
Sludge cake processing methods
Sludge for Advanced Anaerobic Digestion (AAD)
This is a method of treating sludge cake to generate additional anaerobic activity and facilitate energy recovery in anaerobically digested AD plants.
Energy-from-Waste Sludge (EfW)
Sludge cake is widely used in EfW plants and cement plants as a way to process material for beneficial energy when the dry solids content is high. Sludge cake can be burned when mixed with other waste fuels or burned alone. Using waste heat to dry sludge cake can make sludge a valuable fuel.
Phosphate recovery of sludge
The main method of recovering phosphate is from the ashes of sludge mono-combustion units, so there is an incentive to move away from sludge blending and build new mono-combustion units.