Prior to understanding the various types of sewage sludge treatment plants that are in common use, it is essential to get an idea of the material that is handled by these plants. Sludge is the material that remains after sewage has been treated in a sewage sludge treatment plant. It consists of the byproduct of the treatment of wastewater. It ty[ically consists of solid, semisolid, or slurry material. Sludge can be produced through a number of processes. There are two types. Primary sludge, which is produced through the action of chemical precipitation, sedimentation or other processes. Biological treatment of wastewater results in the production of activated waste biomass. It is not unusual for primary and secondary sludge to be combined prior to further treatment and eventual disposal.
The goal of a sewage sludge treatment plant and the treatment of sludge prior to final disposal are:
A) To reduce sludge volume.
B) To stabilize organic materials.
A sewage treatment plant has a number of functions: https://kingtigergroup.com/continuous-carbonization-furnace/.
The first of these is to thicken the sludge. The primary reason for this is that it is simply not feasible to handle slurry suspended in water. This treatment usually takes place in a gravity thickening tank. This can reduce the volume of sludge by up to 50%. There is an alternative where bubbles are fed through the sludge, forcing the solid waste to the surface of the tank resulting in a layer of thickened sludge. This is known as ‘dissolved-air flotation’.
A second treatment is known as digestion. During this biological process, organic compounds are decomposed – the result being a stable substance. The process reduces the amount of solids present. The process also has the benefit of destroying pathogens – and makes the process of drying the sludge easier. The most popular method of achieving this is a two-stage digestion system that uses bacteria in an environment where oxygen is not present. The bacteria (in a heated environment) convert proteins and fats into smaller water-soluble components. The process is an almost closed cycle with a second tank allowing the product of the first tank to be digested by bacteria into biogas – which is then used to heat the first tank. In some instances, this carbon dioxide and methane can also be used to supply power to the sewage sludge treatment plant. There are other refinements to the process such as thermal hydrolysis in which the sludge is placed under pressure and heated – then released into a tank, where the sudden drop in pressure bursts the cell walls of the organic matter.
The final stage is dewatering. In this process, the sludge is spread out on a bed of sand to dry naturally and drain through the sand bed. After this stage is complete the sludge has changed in character and may be handled and disposed of as a solid. That disposal can be at ba landfill or it can be used as fertilizer.