# Evaluation of Waste Blends with Sewage Sludge as a Potential Material Input for Pyrolysis
## 1 Introduction  
The large quantities of waste and waste packaging produced by consumer society are a risk factor for both human health and ecosystems. During waste production and its disposal, secondary substances may escape into the environment, the constituents of which may be contaminated. The substances contained in waste and packaging can enter the human body through the food chain, and thus negatively affect human health [1, 2]. In the Czech Republic (an EU country with 10 million inhabitants), the annual production of corporate waste has been around 22 million tons. In terms of the origin of this waste, a constant trend can be observed in the increasing production of waste particularly by companies engaged in the treatment, collection, and processing of industrial or municipal waste. This includes waste from municipal waste treatment facilities and wastewater treatment plants (so-called secondary waste). However, these materials are not perceived as waste only, but represent secondary raw materials due to their energy potential. At present, the key waste management trend is to move towards a circular economy [3], product reuse, recycling, and the conversion of waste to energy [4], instead of miningand the additional landfills that lead to the issue of land scarcity [4,5]. Beside the EU waste policies that restrict landfilling (99/31/EC on the landfill of waste) while promoting waste prevention, minimization, and recycling, legislation also focuses on the treatment of sewage sludge--91/271/EEC on urban waste water treatment, and 86/278/EEC on the use of sewage sludge in agriculture [6, 7, 8]. Sewage sludge (SS) is an unavoidable waste arising in connection with wastewater treatment. In the EU, the production of SS dry matter per capita is estimated to be 90 g per day [9], meaning that the current annual production of sewage sludge dry matter exceeds 12 million tons. It can be expected that with a growth in the population, and pressures on the construction of new wastewater treatment plants (WWTPs), the production of SS will soar. The development of SS production in tons of dry matter in the Czech Republic, and a comparison of the development in SS disposal in a percentage ratio, in the years 2009-2018 are shown in Figure 1 below. While a similar trend has been observed in several other EU countries due to stricter registrations or new regulations, Figure 1 implies that the total production of SS in the Czech Republic is growing. As a counter-measure, most SS has been applied directly, particularly in land reclamation projects. On the other hand, the quantities of compost SS are likely to continue to fall due to the stricter requirement for compost sanitization. Most recently, the \"thermal utilization\" of SS from WWTPs via incineration or pyrolysis has grown in importance, and this area is expanding due to the currently tightening legislation. Pyrolysis is considered advantageous for the decomposition of organic pollutants, the neutralization of pathogens, the reduction of waste, the application for a wide range of products, and significant energy recovery [10]. Referring back to the circular economy principle, and trends for reducing waste, it must be emphasized that the formation of SS cannot be prevented. Besides excess biomass from biological treatment, SS contains heavy metals and drug residues. Research is heading towards solutions regarding the choice of SS disposal technologies to reduce its quantity, while maximizing its energy potential [12]. However, this shows that SS is difficult to use  
Figure 1: Proportional comparison of sewage sludge disposal in the Czech Republic in the years 2009–2018, modified according to [11]. on its own as fuel, due to its high moisture content [13; 14; 15]. After dewatering (depending on the equipment used--filter presses, centrifuges, etc. ), the SS dry matter can range from 15 to 32 wt. %. The calorific value of such SS is very low. The calorific value of dried SS is comparable to brown coal or wood biomass in terms of energy content [16; 17; 18].