Spatial changes in carbon and nitrogen stable isotope ratios of sludge and associated organisms in a biological sewage treatment system

Carbon and nitrogen stable isotope ratios (δ13C and δ15N) have been utilized as powerful tools for tracing energy or material flows within food webs in a range of environmental studies. However, the techniques have rarely been applied to the study of biological wastewater treatment technologies. We report on the spatial changes in δ13C and δ15N in sludge and its associated biotic community in a wastewater treatment system. This system consisted of an upflow anaerobic sludge blanket (UASB) and a down-flow hanging sponge (DHS) which is a novel type of trickling filter. The results showed clear spatial changes in the δ13C and δ15N of suspended solids (SS), retained sludge, and macrofauna (oligochaetes and fly larvae) in the system. The δ13C and δ15N was used as a natural tracer to determine the SS dynamic throughout the system. The results imply that SS in the DHS effluent was mainly eluted from the retained sludge in the lower section of the DHS reactor. The δ15N of the retained sludge in the DHS reactor increased drastically from the inlet towards to the outlet, from −0.7‰ to 10.3‰. This phenomenon may be attributed to nitrogen conversion processes (i.e. nitrification and denitrification). The δ15N of oligochaetes also increased from the inlet to the outlet, which corresponded well to that of the retained sludge. Thus, the δ15N of the oligochaetes might simply mirror the δ15N of the retained sludge. On the other hand, the δ13C and δ15N of sympatric fly larvae differed from those of the oligochaetes sampled, indicating dietary differences between the taxa. Therefore δ13C and δ15N reflected both treatment and dietary characteristics. We concluded that δ13C and δ15N values are potentially useful as alternative indicators for investigating microbial ecosystems and treatment characteristics of biological wastewater treatment systems.