Influence of Plant Biomass Added to Sewage Sludge on the Product Energy Potential

The authors of this study aimed at evaluating the influence of plant biomass added to sewage sludge on the product energy potential. In order to improve its physical properties, sewage sludge was mixed with wheat straw, sawdust, and bark of conifers. Materials with a natural water content were mixed at 1:1 weight ratio on a dry matter basis. The scheme of the experiment was as follows: sewage sludge without any additions (SS), sewage sludge + wheat straw (SS + WS), sewage sludge + sawdust (SS + S), sewage sludge + bark (SS + B). The moisture of materials, the contents of volatile matter, combined carbon, heat of combustion and calorific value were determined in the study. The addition of plant biomass to sewage sludge reduced the moisture content and increased the volatile matter content compared to sewage sludge without such addition, in which case the variation was relatively low. For the studied mixtures, larger variations were observed in relation to the ash and combined carbon contents. The highest contents of ash and combined carbon were found in the mixture of sewage sludge and bark. The highest value of heat of combustion was determined in the mixture of sewage sludge and sawdust (14,000 J g⁻¹). Calorific values of the mixture of sewage sludge and wheat straw and sewage sludge and bark were 13,640 J g⁻¹ and 11,540 J g⁻¹, respectively, and were higher by more than 40% on the average compared to the calorific value of sewage sludge without any additions.