Water Research 38 (2004) 1685–1694 Formaldehyde degradation in an anaerobic packed-bed bioreactor S.V.W.B. Oliveira, E.M. Moraes, M.A.T. Adorno, M.B.A. Varesche, E. Foresti, M. Zaiat* Laborat ! orio de Processos Biol ! ogicos, Departamento de Hidr ! aulica e Saneamento, Escola de Engenharia de S * ao Carlos (EESC), Universidade de S * ao Paulo (USP), Av. Trabalhador S * ao-carlense 400, S * ao Carlos, SP 13566-590, Brazil Received 12 February 2003; received in revised form 12 November 2003; accepted 20 January 2004 Abstract The development of appropriate technologies for the treatment of formaldehyde discharged into the environment is important to minimize its impact. Aerobic systems have been employed, although alternative anaerobic treatments have also been widely studied, mainly due to their low energy consumption and sludge production. However, toxic substances can lead to disturbances in anaerobic reactors. Some research has already been developed on formaldehyde anaerobic biological treatment, but no consensus has yet been reached about its behavior nor has the most efficient system been identified. Aiming at finding supporting evidence for this issue, therefore, this study investigated the degradation and toxicity of formaldehyde in a Horizontal-Flow Anaerobic Immobilized Sludge Reactor. Formaldehyde concentrations of 26.2–1158.6mgHCHO/L were applied in the reactor, resulting in formaldehyde and chemical oxygen demand removal efficiencies of 99.7% and 92%, respectively. Volatile fatty acids with up to five carbons, found during the degradation of formaldehyde, are believed to indicate that the degradation followed routes unlike those suggested in the literature, which reports the formation of intermediates such as methanol and formic acid. The Monod kinetic model adhered to the experimental data well, with apparent kinetic parameters estimated as r app max ¼ 2:79 10 3 mgHCHO/mgSSVh and K app s ¼ 242:8mgHCHO/L. r 2004 Elsevier Ltd. All rights reserved. Keywords: Formaldehyde; Wastewater; Anaerobic treatment; Toxicity; Packed-bed reactor 1. Introduction Formaldehyde is used industrially as a component of some resins and glues in chemical and petrochemical plants, textile processing, paper manufacturing and wood processing, or as an active ingredient in disin- fectants and preservatives. These industries can generate wastewaters containing up to 10 g/L of formaldehyde [1]. According to the ranking of environmental impacts generated by 45 chemical products proposed by Edwards et al. [2], formaldehyde ranks in first place. Many studies have demonstrated the toxicity and the carcinogenicity of this substance [3–5]. As a disinfectant, a 0.5% formaldehyde solution (B5.4g/L) destroys all species of microorganisms in a period of 6–12h [6]. The literature contains little information about the anaerobic degradation and toxicity of formaldehyde. The pathway of anaerobic degradation of formaldehyde and the microorganisms involved in this process are not yet well defined. According to Gonz ! alez-Gil [7], it is not clear if formaldehyde is converted directly into methane or if intermediate products act as substrates for methanogenic microorganisms. Similarly, there is no consensus about the concentra- tion of formaldehyde that causes inhibition of the anaerobic biomass activity or about the most appropriate ARTICLE IN PRESS *Corresponding author. Tel.: +55-16-2739546; fax: +55-16- 273-9550. E-mail address: zaiat@sc.usp.br (M. Zaiat). 0043-1354/$-see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2004.01.013