**1. Introduction**

Depletion of the world's natural resources and an increase in the environmental footprint [1] has stimulated the increased utilization of renewable energy sources, resource efficiency, better waste management, circular economy and sustainability. Among the solutions is greater utilization of waste materials, which could partially address the challenges of resource depletion and ecosystem health. Waste materials are widely available and often mismanaged [2]. Developed and developing countries produce large amounts of waste per capita, with a significant increase in recent decades, owing to a higher level of consumption [3]. Waste management continues to improve in several countries; however, significant amounts of potential secondary raw materials are lost, such as metals, wood, paper and other waste streams [4]. Waste could also be a potential source of several value-added products, such as enzymes, fuels, fertilizers, pesticides, polymers, and plastics [5].

The use of several waste types is limited, despite their vast availability. Waste materials such as sewage sludge and organic waste contain significant amounts of elements such as carbon, nitrogen, phosphorus that could be efficiently used by thermal or biological processes [6]. The EU's Landfill Directive [7] requires that waste should be pretreated by physical, thermal, chemical or biological processes to reduce its negative impact on the environment and to help increase the scope of waste recycling and recovery. However, there are several limitations to their use, such as their complex structure, the non-homogeneity of waste and the presence of hazardous waste. On the other hand, advances in microbiology, biotechnology and genetic engineering are leading to new concepts for converting these materials into valuable products [8].

Several solid waste products and waste from municipal sources have higher shares of lignocelluloses [9]. Such examples are agricultural residues, forest woody residues, industrial waste, microalgae and municipal solid waste [10]. Lignocellulosic materials are also widely abundant renewable materials, and are composed mainly of cellulose, hemicellulose, and lignin [11]. Pretreatment of waste materials facilitates further hydrolysis and fermentation [11] for the production of fuels, chemicals and other materials [12]. Pretreatment can partially remove lignin and hemicellulose, and often also cellulose (such as by cellulose-degrading enzymes [13], rumen fluid [14], white rot fungi [15], etc.). Ideally, pretreatment should be simple, with a low environmental footprint [16] and should be economically efficient [17], while it should produce pretreated substrate that is easily hydrolyzed/fermented, and should avoid the loss of the desirable fraction of the material and the formation of inhibitory compounds [18].

Pretreatment of waste materials could help in reducing the amount of waste, in stabilizing waste, overcoming the recalcitrance of lignocellulosic waste, and in more efficient utilization of waste materials as fuels and/or chemicals. Various pretreatment methods exist, which can be classified into chemical, physical, physico-chemical, biological and combined or multiple pretreatment methods. However, each pretreatment method acts differently on lignocellulosic structures [12]. Each method has only limited applications, as no pretreatment technique suits all types of waste material. Commonly used pretreatment techniques still do not meet sustainable industrial production requirements despite being studied for a number of years [17]. A combination of more than one pretreatment technique and/or novel techniques has the potential to significantly improve the efficiency of the process [17]. In order to better understand and improve specific pretreatment process(es), it is important to analyze the changes in the properties of waste materials during pretreatment [19].

The literature review has shown that there has been limited research on analyzing changes in the various parameters during pretreatment that are typical in related fields, such as wastewater characterization, anaerobic digestion and composting. Further, to the best of the authors knowledge, no studies have been performed on pretreatment of sewage sludge, grass and rumen fluid. The aim of this work was to examine the change of parameters during thermal pretreatment of sewage sludge and grass *Typha latifolia* and to investigate the impact of cattle rumen fluid (microbial consortium) presence on pretreatment.

In this work, various parameters have been measured during thermal and biological pretreatment of two waste materials, sewage sludge and riverbank grass (*Typha latifolia*), and their combination (1:1 ratio on a dry basis). Thermal pretreatment was studied at both an elevated temperature of 80 ◦C, which lasted for 5 days, and at milder conditions of 38.6 ◦C, which lasted for 8 days. Biological pretreatment was also studied at 38.6 ◦C by adding an enzyme mixture (cattle rumen fluid) to the waste materials. An analysis of various parameters in the liquid phase was conducted before and after pretreatment and in the gas phase after pretreatment. In the liquid phase, the following parameters were measured: nitrogen, phosphorus and potassium (NPK) content, total organic carbon (TOC), chemical oxygen demand (COD), pH and conductivity, and in the gas phase concentrations of CH4, CO2 and H2S were measured.
