Water Research 39 (2005) 3819–3826 Increased biological hydrogen production with reduced organic loading Steven W. Van Ginkel, Bruce Logan à Department of Civil and Environmental Engineering, The Pennsylvania State University, 212 Sackett Building, University Park, PA 16801, USA Received 23 December 2004; received in revised form 8 June 2005; accepted 8 July 2005 Available online 29 August 2005 Abstract An experimental matrix consisting of reactor hydraulic retention time (HRT) and glucose loading rate was tested to understand the effect of organic loading on H 2 production in chemostat reactors. In order to vary the glucose loading rate over a range of 0.5–18.9g/h, the glucose concentration in the feed was varied from 2.5 to 10gCOD/L under conditionswheretheHRTvariedfrom1to10h(30 1C,pH ¼ 5.5). Decreasing the glucose loading rate over this range increased the hydrogen yield from 1.7 to 2.8mol-H 2 /mol-glucose. High yields of hydrogen were consistent with a high molar acetate:butyrate ratio of 1.08:1 as more hydrogen is produced with acetate as a product (4mol-H 2 /mol-acetate) thanwithbutyrate(2mol-H 2 /mol-butyrate).Itwasthoughtthatthedecreaseinyieldwithorganicloadingrateresulted fromanoveralldecreasedrateofhydrogenproduction.Astherateofgasproductionisreduced,H 2 supersaturation in the liquid phase is likely reduced, relieving inhibition due to H 2 . Flocculation was also an important factor in the performanceofthereactor.Atthe5–10gCOD/Linfluentglucoseconcentrations,substantialflocculationwasobserved particularly as the feeding rate was increased due to a reduction in the HRT from 10 to 2.5h. At the HRT of 2.5h, biomass concentrations reached as much as 25g/L. The flocculant nature of the biomass allowed reactor operation at low HRTs with steady H 2 production and 490% glucose removal. However, when the HRT was reduced to 1h at a glucose feed concentration of 2.5gCOD/L, there was little flocculation evident resulting in wash-out of the culture. These results suggest that hydrogen yields will be optimized for more dilute feeds and lower organic loading rates than have typically been used in biohydrogen reactor studies. r 2005 Elsevier Ltd. All rights reserved. Keywords: Biohydrogen; Chemostat; Fermentation; Hydrogen; Inhibition; Yield 1. Introduction Hydrogen production from the bacterial fermentation of sugars has been examined in a variety of reactor systems, with overall yields generally larger in contin- uous flow systems than in batch reactors. Hydrogen yields in continuous culture typically can range any- wherefrom0.7to4.0mol-H 2 /mol-glucose depending on thebacterialcommunity(typeofinoculum,suchaspure or mixed culture), temperature, pH, retention time, and other factors (Nandi and Sengupta, 1998; Schro¨der etal.,1994).Inbatchtestsinthemesophilictemperature range, it has been observed that the overall hydrogen yield is a function of the organic loading with the highest yields obtained at the lowest organic loading (Van Ginkel et al., 2001). In continuous culture tests, ARTICLE IN PRESS www.elsevier.com/locate/watres 0043-1354/$-see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2005.07.021 à Corresponding author. Tel.: +8148637908; fax: +8148637304. E-mail address: blogan@psu.edu (B. Logan).