The goal of the project was to improve the utilization of sludge in a cost-efficient manner. Thermophilic anaerobic digestion (TAD) is an option to improve the energy efficiency of sewage sludge due to potentially higher biogas yield. TAD also offers other advantages over mesophilic process such as reduction in sludge volume and better hygienic properties of the treated sludge and thus better potential for agricultural utilization. The experimental part of the project focused on the modification of anaerobic digestion process from mesophilic to thermophilic. The aim was to investigate and compare the biogas yield and quality, process loadability, digestate and reject water properties and digestor material durability in the two processes. Basic data on anaerobic digestion was gathered in comparative pilot scale experiments. Loading tests were carried out to determine the maximum loading rate of the digestion processes. In the first set of loading tests, the TAD process produced more biogas than the MAD process. The average total biogas production and unit production in TAD were as high as 1.3 times those of MAD. The biogas unit production started to decrease when the retention time was shorter than 17 (TAD) or 15 (MAD) days. In the second set of loading tests, thickened sludge was let to hydrolyse for ca. two days and the formation of gas bubbles caused flotation. This procedure more than tripled the biogas unit production compared to previous test set, but now both TAD and MAD processes produced similar amounts of methane. Biogas methane content was similar in both reactors throughout the pilot runs. However, the elevated temperature caused increased siloxane concentrations in the gas phase. Also the sulphur concentration in TAD was higher than in MAD. Overall, the sulphur content of the biogas was low in both processes. TAD process hygienized the sludge when the retention time was > 15 days. According to CST measurements (Capillary Suction Time), dewatering properties of thermophilic sludge were inferior and the reject water contained more NH4+-N and CODCr compared to mesophilic sludge. Thermophilic sludge was times more odorous and more volatile organic compounds were released from it. Reactor material tests indicated that thermophilic process caused slightly more corrosion in stainless steel. However, no significant differences between the two processes were observed.
|Title of host publication||The 12th Nordic/NORDIWA Wastewater conference, Conference book|
|Place of Publication||Helsinki|
|Publication status||Published - 2011|
|MoE publication type||A4 Article in a conference publication|
|Event||12th Nordic Wastewater Conference - Helsinki, Finland|
Duration: 14 Nov 2011 → 16 Nov 2011
Conference number: 12
|Conference||12th Nordic Wastewater Conference|
|Period||14/11/11 → 16/11/11|
- trace compounds
Merta, E., Kangas, A., Arnold, M., Lund, C., & Saarinen, R. (2011). Jätevesilietteen energiatehokas käsittely termofiilisesti mädättämällä. In The 12th Nordic/NORDIWA Wastewater conference, Conference book (pp. 299-308).