Abstract
Municipal wastewater treatment plants have produced about 160,000 t/y to 210,000 t /y (Eurostat, 2014) of raw sludge (expressed as dry ratio) in the Czech Republic. Considering the calorific value of the dry sludge is about 15 GJ/t, it represents the caloric content in the raw sludge of about 3,000 GJ/y. In many cases, the processing of the separated raw sludge includes the process of anaerobic fermentation, which generates biogas for energy utilization. In the Czech Republic, the produced sludge, mainly digested and dewatered, is used for land reclamation or compost production or it is landfilled. However, the incineration of sludge is a way that is able to significantly eliminate the risks associated with levels of undesirable minor components contained in sludge. Incineration of sludge is a modern technology which is considered as a modern basis in dealing with sludge issues.
The calorific value of the raw sludge or product of anaerobic fermentation is significantly reduced to values from 1 to 3 GJ/t due to water content of about 65 to 70 % after mechanical dewatering (Elsäßer, 2009) and late (Elsäßer, 2012) and problematic of mechanical dewatering is summarised by Ning (2013). The aim of this paper is to determine under which conditions the sludge incineration process is energy self-sufficient. The paper presents results of a series of balance calculations to determine the optimum rate of drying of raw or anaerobically pre-treated sludge so that the caloric content of the combusted material is sufficient to meet the energy needs of its own technology for energy production intended for export. It is shown that energy production can be significantly affected if the partial drying of the material after mechanical dewatering by waste heat from other operations becomes available. This article builds on the contribution of Fryba (2013) and discusses the conditions of sewage sludge incineration in detail.
