Water Research 38 (2004) 1653–1662 Mesophilic and thermophilic temperature co-phase anaerobic digestion compared with single-stage mesophilic- and thermophilic digestion of sewage sludge Young-Chae Song*, Sang-Jo Kwon, Jung-Hui Woo Division of Civil and Environmental System Engineering, Korea Maritime University, 1, Dongsam-Dong, Yeongdo-Gu, Busan 606-791, South Korea Abstract The performance of thermophilic and mesophilic temperature co-phase anaerobic digestions for sewage sludge, using the exchange process of the digesting sludge between spatially separated mesophilic and thermophilic digesters, was examined, and compared to single-stage mesophilic and thermophilic anaerobic digestions. The reduction of volatile solids from the temperature co-phase anaerobic digestion system was dependent on the sludge exchange rate, but was 50.7–58.8%, which was much higher than 46.8% of single-stage thermophilic digestion, as well as 43.5% of the mesophilic digestion. The specific methane yield was 424–468 mL CH 4 per gram volatile solids removed, which was as good as that of single-stage mesophilic anaerobic digestion. The process stability and the effluent quality in terms of volatile fatty acids and soluble chemical oxygen demand of the temperature co-phase anaerobic digestion system were considerably better than those of the single-stage mesophilic anaerobic processes. The destruction of total coliform in the temperature co-phase system was 98.5–99.6%, which was similar to the single-stage thermophilic digestion. The higher performances on the volatile solid and pathogen reduction, and stable operation of the temperature co-phase anaerobic system might be attributable to the well-functioned thermophilic digester, sharing nutrients and intermediates for anaerobic microorganisms, and selection of higher substrate affinity anaerobic microorganisms in the co-phase system, as a result of the sludge exchange between the mesophilic and thermophilic digesters. r 2004 Elsevier Ltd. All rights reserved. Keywords: Anaerobic digestion; Thermophilic; Mesophilic; Co-phase; Sewage sludge 1. Introduction Single-stage mesophilic completely mixed anaerobic digestion has been widely used for the reduction in volume of organic sludge from wastewater treatment processes, and for obtaining energy in the form of methane gas. Here, the mesophilic digestion usually requires over a 20-day retention time, but is not so efficient in the reduction of volatile solids and the deactivation of pathogenic organisms. To overcome these limitations, interest in thermophilic digestion, using the higher metabolic rate of thermophilic micro- organisms, has increased [1–4]. Although better perfor- mance in the reduction of volatile solids and deactivation of pathogenic organisms can be obtained from thermophilic digestion, the effluent quality and ability to dewater the residual sludge are poor, and require additional energy to heat the digester [2,3,5]. Especially, thermophilic digestion is a little more sensitive to operational conditions, such as temperature, and the organic loading rate, as well as to the characteristics of the influent sludge [5,6]. Generally, anaerobic processes can be characterized from the digestion environments, microorganisms and process configuration, and each process has its unique advan- tages. According to previous studies [7–10], two-phase or two-stage anaerobic processes showed good perfor- mance in the effluent quality, methane yield, volatile ARTICLE IN PRESS *Corresponding author. Tel.: +82-51-410-4417; fax: +82- 51-410-4415. E-mail address: soyc@mail.hhu.ac.kr (Y.-C. Song). 0043-1354/$-see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2003.12.019