The evolution of pH is intimately tied to the release and removed of organic matter in the biofilm surrounding the solid phase.A sensitive parameter of that organic matter evolution may be the COD concentration in the biofilm (De Guardia et al., 1998).The COD concentrations for G and GDEG strips first increased then decreased quickly until day 22.This decrease in COD may be due to the mineralization of the organic matter leading to the production of carbon dioxide and water.A slight increase in COD was observed after 25 days for GDEG and this may be due to the presence of non-biodegradable dissolved organic matter, which may cause a fall in microbiological metabolism.The gradual increase in COD value until day 22 for GBIO may be due to the availability of much organic matter during this period as compared to G and GDEG.After day 22, the COD concentration decreased, indicating stabilization of organic matter to biological activity.