The effects of acetate and propionate on the performance of a recently proposed and characterized photosynthetic biological sulfide removal system have been investigated with a view to predicting this concept's suitability for removing sulfide from wastewater undergoing or having undergone anaerobic treatment. The concept relies on substratum-irradiated biofilms dominated by green sulfur bacteria (GSB), which are supplied with radiant energy in the band 720 - 780 nm. A model reactor was fed for 7 months with a synthetic wastewater free of volatile fatty acids (VFAs), after which time intermittent dosing of the wastewater with acetate or propionate was begun. Such dosing suppressed the areal net sulfide removal rate by similar to50%, and caused the principal net product of sulfide removal to switch from sulfate to elemental-S. Similarly suppressed values of this rate were observed when the wastewater was dosed continuously with acetate, and this rate was not significantly affected by changes in the concentration of ammonia-N in the feed. The main net product of sulfide removal was again elemental-S, which was scarcely released into the liquid, however. Sulfate reduction and sulfur reduction were observed when the light supply was interrupted and were inferred to be occurring within the irradiated biofilm. A preexisting conceptual model of the biofilm was augmented with both of these reductive processes, and this augmented model was shown to account for most of the observed effects of VFA dosing. The implications of these findings for the practicality of the technology are considered. (C) 2004 Wiley Periodicals, Inc.