Solar degradation of effluents in board paper industries has been studied using different photo-catalysts: Fenton reagent and TiO(2). p-Toluenesulfonic acid was chosen as a model compound for sulfonated pollutants already present in the incoming waters. The abatement of a 0.005 M solution of this pollutant after 6 h was found to be 47% for photo-Fenton and 27% for TiO(2) (pseudo-first-order rate constants 0.002 and 0.001 min(-1) respectively). Eugenol and guaiacol were chosen as models for lignin degradation products. They were efficiently degraded by both photo-catalysts, and reaction rates were higher for eugenol (0.0024 min(-1)) than for guaiacol (0.0018 min(-1)). A solution of sodium acetate, sodium butyrate and D-glucose was chosen to study the effect of photo-catalysis towards volatile fatty acids and saccharides arising from starch degradation. In this case a clearly worse performance was observed: only 20% degradation was observed after 7 h of treatment. When the real wastewater was treated with photo-catalytic methods, the best performance was obtained in closed circuits, when the COD values were higher. This fact can be explained by taking into account that closure of the circuits results in an accumulation of reluctant phenolic pollutants, while starch derivatives are continuously degraded by microorganisms in the circuits; as phenolic compounds are more easily degraded by photo-catalytic means, these methods are suitable for closed circuits. Finally, changes in the BOD(st) were determined by means of active sludges respirometry. A noticeable BOD(st) increase (30-50%) was observed in all cases, attributable to chemical oxidation of biodegradable species. (c) 2005 Elsevier Ltd. All rights reserved.