There are several sludge characteristics that have a great influence on the costs of sludge treatment. The water content is very important as it determines the sludge volume and therefore feasibility and costs of transportation and disposal. The solids and water content of sludge depends on the type of sludge (e.g. primary sludge, waste activated sludge, chemical sludge) and the type and quality of its treatment (e.g. sludge digestion) and on the method of sludge thickening and dewatering.
The chemical composition of the sludge also depends on wastewater characteristics and the wastewater treatment method. Of particular importance is the content of heavy metals as their concentration is a limiting factor for land application of biosolids. Despite the fact that land application within the limits set by European biosolids regulation is not only a safe and beneficial reuse of valuable nutrients, there are politically motivated concerns endangering the long-term continuation of land application. The existence of endocrinic substances in biosolids is also an additional concern though these pharmaceutical substances are unlikely to be taken up and incorporated into plants.
European regulations specify that only waste with a volatile solids (organic) content of below 5% may be landfilled. This means the end of the sludge landfilling only the ash from sludge incineration plants may be landfilled. The combination of sludge drying and incineration is about energy neutral. Dried sludge has approximately the same thermal value as brown coal. If dried sludge is incinerated, the generated heat is sufficient for sludge drying.
HUBER offers a virtually complete chain of sludge treatment processes.