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Municipal and industrial wastewater undergoes various treatment steps in WWTPs before it is released back into rivers and other waterways.
In secondary clarifiers most sludge is collected and recycled back to the aeration basin to generate a stable and productive activated sludge for nutrient removal. However, as more and more sludge enters the plant over time and a certain sludge age has to be maintained, some sludge has to be removed from the process. Removed sludge is also known as waste activated sludge (WAS). WAS might contain toxic components so that it cannot just be released into the environment.
Depending on the size of the facility, location and sludge volume different solid/ liquid waste management options are available.
Smaller plants in rural areas often compost sludge and/or use sludge drying beds for dewatering. The remaining sludge “cake” is either hauled of and disposed in landfills or in case the sludge doesn’t contain toxic components its used as a fertilizer.
Sludge drying beds are a low-cost solution for sludge dewatering however they require a lot of space and cause odors. Therefore, in areas with a denser population, sludge is mechanically dewatered for example with centrifuges or sludge presses. More people equal more sludge volume. As sludge hauling is expensive and a lot of sludge is available it makes economically sense to treat the sludge chemically to remove toxic components and to use most of it as a fertilizer.
Finally, in metropolitan areas with limited space and very high sludge volumes energy production from WAS is often economically beneficial. In anaerobic digesters microorganisms break down the organic matter in the sludge and convert parts of it to methane and natural gas. The gas is then burned either to generate electricity or to produce heat. Additionally, mechanical dewatering might be used to produce fertilizers or to prepare the sludge cake for incineration where its generating more electricity.
In the US about 35% of WAS is used as fertilizers for agriculture, 25% is disposed in landfills and each 15% is composted or incinerated. The remaining 10% is treated in anaerobic digesters or processed in different ways.