Effect of processing temperature applied to mixtures of sewage sludge and plant waste on the content of macro- and microelements in the product and on the luminescence of Vibrio fischeri

The aim of the research was to determine the effect of temperature and addition of plant material to sewage sludge on the efficiency of thermal treatment of biomass. Another goal was to assess the content of selected macro- and microelements in the end product, taking into account the possibility of their agricultural use. The input material for the research consisted of anaerobically stabilized, dehydrated sewage sludge. The experimental design consisted of: sewage sludge + bark (SS + B), sewage sludge + wheat straw (SS + WS), sewage sludge + sawdust (SS + S). The mixtures were thermally treated in a chamber furnace, under anaerobic conditions. Two temperature settings were applied: 300°C and 600°C; the exposure time in both cases was 15 minutes. Thermal, anaerobic transformation of organic matter at a temperature up to 600°C causes even a twofold increase in the content of ash elements in the product. An increase in the nitrogen content can only be obtained under conditions of low-temperature transformation of a mixture of sewage sludge with organic materials. Using the temperature of 600°C for the transformation of organic materials caused an increase in the content of easily hydrolyzing nitrogen as well as forms of phosphorus, potassium and sodium extracted with water in comparison to mixtures transformed at the temperature of 300°C. Of the analyzed trace elements, only the content of water-soluble forms of zinc (apart from mixture of sewage sludge and straw) increased along with the increase in temperature of the transformation process, which indicates higher zinc bioavailability. In terms of fertilization, as a result of increasing the processing temperature applied to sewage sludge with the addition of different mass of plant origin, a lower value product is obtained due to nitrogen losses and a decrease in the availability of some elements for plants. The extracts from the mixtures were not toxic to bacteria. Stimulation of the luminescence of V. fischeri was observed in the research.