A VFA MEASURING BIOSENSOR BASED ON NITRATE REDUCTION

A modified version of a biosensor (DENICON) used to measure nitrates i n wastewaters is presented which allows for the determination of VFA c oncentration as readily biodegradable COD (rbCOD). A sample of the dig ested effluent is mixed to denitrifying microorganisms in excess of ni trates. As a consequence, a fraction of NO3, proportional to the biode gradable organic carbon, is reduced and may be measured by titration. The concentration of rbCOD is then derived from the consumption of nit rates as Biodegrable Nitrate Demand (END). Taking into account that th e prevailing fraction of rbCOD in digested anaerobic liquors is made o f VFA, this paper investigates the possibility of measuring VFA concen tration with the proposed biosensor. Extensive tests have been carried out on a laboratory instrument, using acetate, butyrate and propionat e as model compounds. A satisfactory correlation (R-2 > 0.95) between organic carbon and reduced nitrate has been found for the three substr ates. Mixtures of acids, especially acetic + propionic have also been tested because this association is the one often found when unstable c onditions arise ina digester. It has been observed that the reaction r ate increases with respect to acetic acid only when a solution of acet ic + propionic acids is denitrified. This effect might allow for a qua litative evaluation of incoming instability. The instrument can be eas ily automated and might allow a VFA determination every 20-40 minutes, which is sufficient for most anaerobic applications. (C) 1997 IAWQ. P ublished by Elsevier Science Ltd.