The function of domestic sewage supplementation as co-substrate with composite vegetable based market waste was studied during the process of fermentative hydrogen (H(2)) production. Significant improvement in H(2) production and substrate degradation were noticed upon supplementing the waste with domestic sewage. Maximum H(2) production (cummulative) was observed at 5.2 kg COD/m(3) with pulp operation and 4.8 kg COD/m(3) with non-pulp operation accounting for improvement of 51 and 55% respectively after sewage upplementation. Substrate degradation was also found to improve with respect to both carbohydrates [8% (with pulp); 5% (non-pulp)] and chemical oxygen demand [COD, 12% (with pulp); 13% (non-pulp)] after adding domestic sewage. Specific H(2) yield improved especially at lower concentrations. Supplementation of waste with co-substrate helps to maintain good buffering microenvironment supports fermentation process and in addition provides micro-nutrients, organic matter and microbial biomass. Variation in the outlet pH was less in supplementation experiments compared to normal operation. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved