Water & Wastewater Treatment Magazine
Issue link: https://read.utilityweek.co.uk/i/481381
T reatment processes for sewage sludge (also called biosolids) are currently evolving at a very fast pace in response to current pressures, that can be ultimately summarized as the need to look for opportunities for enhanced resource recovery. Sewage sludge has a high content of organic carbon compounds (20-40% of total solids) and nutrients (nitrogen 1.5-4% of TS and phosphorus 0.5-2.8% of TS) but also heavy metals and trace pollutants, containing resources that can be recovered as energy (biogas, hydrogen syngas), nutrients, bio- diesel, bio-plastics, bio-sorbents, construction materials, heavy metals, proteins, enzymes, and so on. Traditional methods for resource recovery from sludge are focused on energy production through biogas, using anaerobic digestion processes. The main goal of anaerobic digestion is to stabilize the sludge, i.e. reduce pathogen counts and reduce the volume for re-use as soil fertilizer or conditioner. Anaerobic digestion is able to capture the energy content from sludge as a biogas, which is usually 40% CO2 and 60% methane, but only 20-30% of the organic matter is actually mineralized, against the total estimated energy stored in sludge, i.e., 23,000-29,000 kJ/kg TS in primary sludge and 19,000-23,000 kJ/ kg TS in activated sludge. The use of pre-treatment processes that promote fast hydrolysis is becoming more widely spread to enhance breakdown of the complex compounds into simpler molecules, resulting in higher methane production yields. Some examples include sludge disintegration through mechanical shear, ultrasound (e.g. Ultrasonic), a combination of temperature and pressure (eg: Cambi and BioThelys) or enzymes. New research has also demonstrated the benefits of enriching anaerobic digesters with external carbon dioxide. A recent study completed at Cranfield University has established an increased methane production of 96-138% during the first 24 hours of digestion a'er adding CO2 Dr AnA SoAreS, Lecturer in BioLogicaL engineering, cranfieLd university new roads to recovery an increased emphasis on resource recovery is driving innovation and development in sludge treatment processes In the know Technically speaking: sludge treatment www.wwtonline.co.uk | WWt | aPriL 2015 | 39