The Influence of Aeration Ratio on Energetic Aspects of Composting Process of Sewage Sludge With Agricultural Waste

Composting process of sewage sludge is one of the most common methods in the world for sludge management. Due to, inter alia, unfavorable ratio of carbon to nitrogen and the high hydration, sludge waste requires usually additional preparation before composting process. Especially in agricultural areas, the use of agricultural waste seems to be beneficial to improve the properties of the compost mixture. Such co-substrates will create favorable water and air conditions inside the pile or in-vessel. In addition to the composition of the mixture, another important aspect is the appropriate aerating of the compost. It depends on what time and how effective will be the composting process. The aim of the work was to present the influence of aeration ratio on energetic aspects of composting process of sewage sludge with agricultural biomass. In this case maize straw was the structural addition. COMPOSTING OF SEWAGE SLUDGE WITH AGRICULTURAL WASTE Reducing the number of livestock farming has contributed to the decline in the production of natural fertilizers, which resulted in the search for alternative, environmentally safe sources of organic matter, which can be composted sludge, processed into fertilizer in agriculture (Szwedziak 2006). Wastewater sludge is a significant biomass resource in the scale of the economy, which can be used in various ways. Analysis of Eurostat data shows that Poland is at the top of the sewage sludge production (above 568 000 tons/year, data from 2015 data) only from the urban wastewater treatment process (Eurostat 2017). Large volumes of such waste are only produced by Germany, United Kingdom, France Spain and Italy it is estimated that these five countries generate altogether almost 75% of the European wastewater sludge (Kacprzak 2017). Analyzing the directions of wastewater sludge management in Europe and Poland in the recent years, there is a strong trend towards reducing the amount of sediment deposited on landfills in favor of their agricultural use and composting (Duan 2017, Eurostat 2017). Properly managed composting process is one of the most common waste management methods, as far as sustainability issues are concerned to stabilize organic waste including wastewater sludge results in stable and harmless end-product, which may be used as a fertilizer or soil conditioner that does not pose an environmental hazard (Białobrzewski et al. 2015, Kosicka 2015 et al., Malamis et al. 2016). Composting is a viable, beneficial option in biosolids management and it is a proven method for pathogen reduction and results in a product that is easy to handle, store, and use (BauzaKaszewska 2010 et al., Starzyk and Czekała 2014, Mroczek-Krzyzelewska et al. 2017). Wastewater sludge composting technology is typically done at higher temperatures (Berggren 2004) which imitates an accelerated natural process that takes place on open 240 floor where the organic materials (leaf litter, animal wastes) are broken down. Resulting in an overall reduction of volume, or converted to more stable organic materials. CHARACTERISTICS OF WASTEWATER SLUDGE The composition of sewage sludge depends on the organic and mineral content of the wastewater and the degree of mineralization of the organic substance in the stabilization processes. Dewatered wastewater sludge (dry), depending on stabilization processes, contains on average 50–70% of organic matter, and 30–50% mineral components (including 1–4% of inorganic carbon), 3.4–4.0% N, 0.5–2.5% P and significant amounts of other nutrients, including micronutrients (Kacprzak 2017, Kosicka 2016 et al.). Due to inter alia unfavorable ratio of carbon to nitrogen and the high hydration, sludge waste requires usually additional preparation before composting process (Czekała et al. 2015, 2016). For these reasons, the use of wastewater sludge as an unconventional fertilizer contributes, according to Jezierska-Tyś et al. (2004) to improve the physicochemical properties of the soil, have a soil-forming effect, and also promote the accumulation of humus compounds. Especially in agricultural areas, the use of agricultural waste seems to be beneficial to improve the properties of the compost mixture (Ucaroglu and Alkan 2016, Grau et al. 2017). Such co-substrates will create favorable water and air conditions inside the pile or in-vessel. In addition to the composition of the mixture, another important aspect is the appropriate aerating of the compost. METHODOLOGY, SCOPE AND OBJECT OF RESEARCH The research was carried out at the Institute of Biosystems Engineering at the Poznań University of Life Sciences, in the Laboratory of Environmental Technology. The composting process was carried out in specially designed bioreactors (Fig. 1). The influence of external factors was eliminated by the use of thermal insulation. In order to ensure adequate structural properties of the composted feed, the ingredients were selected in appropriate proportions. The sewage sludge from the municipal wastewater treatment plant and the agricultural waste in the form of straw rape were used for the study. The dry weight was 18.61% for sludge and, for rapeseed straw, 87.71%. In the experiment 25 kg of sludge and 5.4 kg of rape straw were used in each compartment. Seven models of bioreactors were used for testing. Each of them was built in the shape of a cuboid with a side length of 0.5 m, height of 0.7 m and has a capacity of 165 dm3. The walls of the chambers are made of plastic and additionally reinforced by metal elements. The walls of the chambers were insulated thermally. Aeration of the bioreactors was carried out using an air-pumping pump (Fig. 1, item 1). The amount of air injected into the chambers was controlled and evenly distribution of the pressurized air allowed the metal grate, at the bottom of the tank.