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Abstract
The disposal of excess sludge from wastewater treatment plants is a serious problem that needs to be addressed. Temperature-phased anaerobic digestion (TPAD) which combines thermophilic and mesophilic processes in one, brings together the advantages of both systems. The aim of the present work was to develop a simple kinetic model to describe the TPAD of sewage sludge in batch completely stirred tank reactors (CSTRs) and to determine the kinetic parameters of both thermophilic and mesophilic stages. A zero-order kinetic equation described the thermophilic step after 2, 4 and 6 days of digestion time (experiment 1, 2 and 3, respectively), yet a first-order equation was found to be adequate to correlate the methane gas accumulated with time in the mesophilic step, the kinetic constant being 0.21 days−1. The methane yield coefficient obtained was found to be almost proportional to the digestion time used in the thermophilic step with values of 0.067, 0.132 and 0.193 L CH4 STP/g VSadded for experiments 1, 2 and 3, respectively. By contrast, the kinetic constant of the mesophilic stage was not influenced by the digestion time used in the thermophilic phase.
Acknowledgments
The authors wish to express their gratitude to the Spanish Ministry for the Environment, Rural Affairs and Marine Policy (Project 148/PC08/3-04.3) for providing financial support, and to the contract JAE-Doc from “Junta para la Ampliación de Estudios del CSIC” co-financed by European Social Funds.