• Lund, Charlotta
• Arnold, Mona
• Saarinen, Risto
• Kangas, Ari
• Merta, Elina

Abstract

The goal of the project was to improve the utilization ofsludge in a cost-efficient manner. Thermophilic anaerobicdigestion (TAD) is an option to improve the energyefficiency of sewage sludge due to potentially higherbiogas yield. TAD also offers other advantages overmesophilic process such as reduction in sludge volume andbetter hygienic properties of the treated sludge and thusbetter potential for agricultural utilization.The experimental part of the project focused on themodification of anaerobic digestion process frommesophilic to thermophilic. The aim was to investigateand compare the biogas yield and quality, processloadability, digestate and reject water properties anddigestor material durability in the two processes. Basicdata on anaerobic digestion was gathered in comparativepilot scale experiments.Loading tests were carried out to determine the maximumloading rate of the digestion processes. In the first setof loading tests, the TAD process produced more biogasthan the MAD process. The average total biogas productionand unit production in TAD were as high as 1.3 timesthose of MAD. The biogas unit production started todecrease when the retention time was shorter than 17(TAD) or 15 (MAD) days. In the second set of loadingtests, thickened sludge was let to hydrolyse for ca. twodays and the formation of gas bubbles caused flotation.This procedure more than tripled the biogas unitproduction compared to previous test set, but now bothTAD and MAD processes produced similar amounts ofmethane.Biogas methane content was similar in both reactorsthroughout the pilot runs. However, the elevatedtemperature caused increased siloxane concentrations inthe gas phase. Also the sulphur concentration in TAD washigher than in MAD. Overall, the sulphur content of thebiogas was low in both processes.TAD process hygienized the sludge when the retention timewas > 15 days. According to CST measurements (CapillarySuction Time), dewatering properties of thermophilicsludge were inferior and the reject water contained moreNH4+-N and CODCr compared to mesophilic sludge.Thermophilic sludge was times more odorous and morevolatile organic compounds were released from it. Reactormaterial tests indicated that thermophilic process causedslightly more corrosion in stainless steel. However, nosignificant differences between the two processes wereobserved.

Topics

• impedance spectroscopy
• compound
• stainless steel
• corrosion
• experiment
• organic compound
• durability
• gas phase
• Sulphur