Ultrasonic Disintegration of Sewage Sludge to Increase Biogas Generation

Renewable energy will play a key role in the future in limitation of greenhouse gas emissions, with particular focus on carbon dioxide. Biomass and waste are considered among the dominant renewable energy sources. Due to their availability on the energy resources market, it has potential to ensure permanent energy generation.1–3 According to the International Energy Agency (IEA), fossil fuels accounted for 81 % of primary energy supply in 2007, whereas renewable energy sources were merely 13 %. Although particular interest is paid to technological and economic aspects of development and implementation of renewable energy sources, fossil fuels remain a dominant energy carrier all over the world, with its 77 % share in the world energy balance in 2007–2030.4, 5 The very fast increase in sludge production, mainly due to the implementation of the European Council Directive 91/271/EEC concerning urban wastewater treatment, requires new strategies in sludge management to cope with this critical issue in an economically and environmentally fully acceptable manner.6 Anaerobic stabilization, which is the biochemical process of organic matter conversion, is widely applied in wastewater treatment. The origins of the industrial use of fermentation date back to the first half of the 20th century. Methane fermentation is a chemical conversion of organic waste into inorganic compounds that occurs in the microbial cells present in water environment, which means that biomass sources that contain high levels of water can be converted without using initial physical processing.7,8 The use of methane fermentation as a method for processing sludge, the waste product of wastewater treatment, is a highly beneficial solution. It limits the direct negative effect of sewage sludge on the environment and provides a good source of biogas. Biogas, which contains 65 % CH4, 35 % CO2 and trace amounts of H2S, H2 i N2, is an effective and environmentally friendly energy source. Its combustion is associated with low-level emissions of harmful pollutants.9 Disintegration of sewage sludge involves destruction of the sludge structure using external forces that affect their physical and chemical parameters and change the character of the structure. The use of an ultrasonic field on a medium would cause mechanical and sonochemical effects.10–13 During the first phase of ultrasonic field conditioning, according to Tiehm et al.14,15 depending on the amount of supplied energy and the duration of sonification, floccular structures undergo disintegration. At this stage of conditioning, no destruction of microorganism cell walls was observed. The destruction of the cell walls of activated sludge microUltrasonic Disintegration of Sewage Sludge to Increase Biogas Generation