Comparison of two techniques for measuring biodegradable dissolved organic carbon in water

Abstract

This article compared two methods for measuring biodegradable dissolved organic carbon (BDOC) concentrations in water. These techniques consist of following the reduction of dissolved organic carbon (DOC) in water samples incubated either for 30 days with suspended bacteria or for 5–7 days with bacteria attached to sand particles. BDOC concentration is the difference between initial and minimum DOC values. The aim of this work was to assess the effect of operational conditions on the final BDOC result. The role of inoculum size, incubation period and aeration were tested. Finally, BDOC results obtained from these two methods were compared. For BDOC determination using attached bacteria, the results showed that a sandrwater ratio of 100 g:300 ml allowed a rapid decrease in DOC concentration and an optimal biodegradation of the organic matter. Results of BDOC measurements increased with sand:water ratio, showing biosorption of a low fraction of DOC (0.10–0.20 mg DOC per 100 g of sand). Moreover, aeration accelerated the biodegradation process and increased the BDOC results (linear relationship: BDOC_{non aerated} = 0.72 BDOC_aerated + 0.10; r = 0.90; n = 47). For the BDOC determination using suspended bacteria, aeration had no effect on BDOC values. However, for most of water samples, biodegradation was incomplete after a 28 day incubation period (depending on the water type). BDOC concentrations determined after 28 days increased by 0–125% when incubation was extended to 85–120 days. The estimation of BDOC concentration depended on the applied method. BDOC values were higher when using attached bacteria (4 1 h^‐1 aeration) (linear relationship: BDOC_susp. = 0.61 BDOC_fix. ‐0.05, r = 0.93; n=30), probably resulting from a greater bacterial diversity in biological sand than in river water inoculum, a higher biodégradation ability for fixed bacteria than for suspended bacteria and a low DOC adsorption on sand biofilms.