Citric acid is an effective remediation agent for soils contaminated by heavy metals. The large-scale field use of this organic acid requires the development of efficient production techniques using low cost substrates such as sugar rich wastes. To develop such techniques, the objective of the project was to simultaneously optimize the initial glucose, nitrogen (N), phosphorus (P) and NaCl levels of a nutrient solution used to wet peat moss (PM) simulating a sugar rich waste on which Aspergillus niger NRRL 567 was grown to produce citric acid. Seventeen different combinations of nutrients were tested to grow A. niger at 30 degrees C for 48 and 72 h, and to measure the resulting citric acid production. With the central composite design method (CCD), the results were used to produce a second order equation defining citric acid production as a function of initial glucose, N, P and NaCl levels. Initial glucose and N levels were found to have a significant positive and negative effect on citric acid production, while P and NaCl were found to have a limited positive and an insignificant effect, respectively. A peak citric acid production of 82 g kg(-1) dry peat moss (DPM) was reached after 72 h with the following optimized nutrient solution, in terms of a kg(-1) DPM: 967.9 glucose, 15.4 (NH4)(2)SO4, 43.9 KH2PO4 and 4.0 NaCl. This represented a 50% increase in production, over a shorter fermentation period, compared to a basal nutrient solution optimize solely for initial glucose level. (c) 2005 Elsevier Ltd. All fights reserved.