Molasses as C source for heterotrophic bacteria production on solid fish waste Oliver Schneider ⁎ , Vasiliki Sereti, Ep H. Eding, Johan A.J. Verreth Aquaculture and Fisheries Group, Wageningen University, P.O. Box 338, 6700 AH Wageningen, The Netherlands Received 25 January 2006; received in revised form 18 August 2006; accepted 23 August 2006 Abstract The drumfilter effluent from a recirculation aquaculture system (RAS) can be used as substrate for heterotrophic bacteria production. These bacteria can be reused as aquatic feed. In RAS drumfilter effluents are organic carbon deficient for bacteria production. This is due to nitrogen accumulation in the system water. In the present experiment, a bacteria growth reactor (3.5l) was connected to the drumfilter (filter mesh size 60 μm) outlet of a recirculation system. To counteract carbon deficiency, different supplementation levels of molasses (organic carbon) were tested (carbon fluxes of 0.0, 3.2, 5.8, 7.8, 9.7 gC molasses/l/d; C:N ratios: 3.4, 6.4, 9.4, 13.0 and 16.5). The hydraulic retention time in the bacteria reactor was 6 h. For the maximum flux, the VSS and crude protein productions were about 168 gVSS and 95 g crude protein per kg feed. The maximum conversion of nitrate and ortho-phosphate was 24 g NO 3- N and 4 gP/kg feed. This equals a conversion of 90% of the inorganic nitrogenous waste and 98% of the ortho-phosphate–P. Furthermore the maximum substrate removal rate and the K s for molasses were determined (1.62 gC/l/h and 0.097 gC/l respectively). The maximum specific removal rate was 0.31 gC/gVSS/h and the related half saturation constant was 0.008 gC/l. The observed growth rate reached a maximum for C fluxes higher than 8 g/l/d. The present integration of heterotrophic bacteria production in RAS represents, therefore, an innovative option to reduce waste discharge by conversion. If the bacteria are reused as feed, system's ecological sustainability increases. © 2006 Elsevier B.V. All rights reserved. Keywords: Bacteria; Fish waste; Conversion; Nutrients; Molasses 1. Introduction The drumfilter effluent from a recirculation aquacul- ture system (RAS) can be used as substrate for heterotrophic bacteria production. This biomass can be reused as aquatic feed. Such reuse of converted and otherwise waste nutrients increases the overall sustain- ability of RAS. Conversion of fish waste into heterotro- phic bacteria biomass depends on carbon (C):nitrogen (N) ratios. Optimal C:N ratios for heterotrophic bacteria production are about 12–15 g:1 g (Lechevallier et al., 1991; Henze et al., 1996; Avnimelech, 1999). In that respect, fish, which are receiving high protein diets, are producing carbon deficient waste due to high levels of excreted nitrogen. For example, the faecal loss of African catfish has a C:N ratio of approximately 12–13:1 (g/g) under commercial conditions. As consequence of nitro- gen excretion (non-faecal loss) and nitrate accumulation in the system water, this ratio drops to only 3:1 in the drumfilter effluent. For other fish species that are less effective in their nitrogen retention than African catfish, this ratio can be even lower. In earlier experiments a RAS effluent stream was supplemented with sodium acetate to Aquaculture 261 (2006) 1239 – 1248 www.elsevier.com/locate/aqua-online ⁎ Corresponding author. Tel.: +31 0031 317 485147; fax: +31 0031 317 483937. E-mail address: Oliver.schneider@wur.nl (O. Schneider). 0044-8486/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.aquaculture.2006.08.053