Alkalinity and high total solids affecting H 2 production from organic solid waste by anaerobic consortia Idania Valdez-Vazquez, He ´ctor M. Poggi-Varaldo* CINVESTAV-IPN, Environmental Biotechnology and Renewable Energies R & D Group, P.O. Box 14-740, Me ´xico DF 07000, Mexico article info Article history: Received 18 February 2008 Received in revised form 9 February 2009 Accepted 11 February 2009 Available online 31 March 2009 Keywords: Alkalinity Hydrogen Response surface methodology Solid waste Total solids abstract The optimization of total solids in the feed (%TS) and alkalinity ratio (g) for H 2 production from organic solid wastes under thermophilic regime was carried out using response surface methodology based on a central composite design. The total solids levels were 20.9, 23.0, 28.0, 33.0 and 35.1% whereas the levels of alkalinity ratio (defined as g phosphate alkalinity/g dry substrate) were 0.15, 0.20, 0.30, 0.41 and 0.45. High levels of TS and g affected in a negative way the H 2 productivity and yield; both response variables signifi- cantly increased upon decreasing the TS content and alkalinity ratio. The highest H 2 productivity and yield were 463.7 N mL/kg-d and 54.8 N mL/g VS rem , respectively, predicted at 20.9% TS and alkalinity ratio 0.25 (0.11 g CaCO 3 /g dry substrate). The alkalinity require- ments for hydrogenogenic processes were lower than those reported for methanogenic processes (0.11 vs. 0.30 g CaCO 3 /g COD). Adequate alkalinity ratio was necessary to main- tain optimal biological activity for hydrogen production; however, excessive alkalinity negatively affected process performance probably due to an increase of osmotic pressure. Interestingly, reactor pH depended only on the alkalinity ratio, thus the buffer capacity was able to maintain a constant pH independently of TS levels. At g ¼ 0.15–0.30 the pHs were in the range 5.56–5.95, which corresponded to the highest hydrogen productivities and yields. Finally, the highest metabolite accumulation corresponded with the highest removal effi- ciencies but not with high H 2 productivities and yields. Therefore, it seems that organic matter removal was channeled toward solvent generation instead of hydrogen production at high TS and g levels. This is the first study that shows the requirements of alkalinity in solid substrate fermentation conditions for H 2 production processes and their interaction with the content of total solids in the feed. ª 2009 Published by Elsevier Ltd on behalf of International Association for Hydrogen Energy. 1. Introduction Hydrogen is a promising alternative to fossil fuels. As a clean fuel it can be directly used to produce electricity in fuel cells and/or burnt with minimal pollutants emissions [1–3]. Recently, the interest in fermentative biohydrogen production has increased because a wide range of organic compounds can be used as substrates, and also it can be coupled to the ‘‘bio- refinery’’ concept for further reclaiming of fermentation by- products [4,5]. There are reports where the organic fraction of municipal solid waste has been used as substrate for hydrogen production [6–10]. Yet, the total solids (TS) content * Corresponding author. CINVESTAV-IPN, Dept. Biotechnology and Bioengineering, Environmental Biotechnology and Renewable Energies R & D Group, Me ´ xico D.F., 07000, Me ´ xico. Tel.: þ52 55 5061 3800x4324; fax: þ52 55 5061x3313. E-mail addresses: valdez_idania@yahoo.com (I. Valdez-Vazquez), hectorpoggi2001@gmail.com (H.M. Poggi-Varaldo). Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he 0360-3199/$ – see front matter ª 2009 Published by Elsevier Ltd on behalf of International Association for Hydrogen Energy. doi:10.1016/j.ijhydene.2009.02.039 international journal of hydrogen energy 34 (2009) 3639–3646