River oxygen uptake and respiratory decay of sewage fungus biofilms

Abstract

Benthic oxygen uptake rates (BUR) were measured for sewage fungus biofilms in rivers using a chamber in situ. The accuracy of the chamber measurements of BUR were tested by determining a river oxygen mass balance. This demonstrated that the chamber BUR values were within the range calculated when operated at the river boundary velocity (0.25 m s −1, measured 0.05 m above bed). The maximum BUR value recorded was 70.2 g m −2 d −1 at 7.0 g m −3 dissolved oxygen (DO) concentration with a median value of 18 g m −2 d −1. The large BUR response to flow velocity, increasing 1.9-fold for a doubling of flow velocity (0.125-0.25 m s −1), suggested a turbulent mass transfer process predominate for nutrient exchange between river biofilms and the overlying flow. Biofilm respiratory decay coefficients for homogenised biofilms were k = 0.375 – 0.100 day −1 (base e) giving a respiratory half-life of 1.85–5.54 days. These measurements suggest that intermittant effluent loading to rivers with biofilms established is unlikely to result in substantial respiratory decline unless the with-holding period is of the order of days.