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 (delta C-13 and delta N-15) 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 delta C-13 and delta N-15 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 delta C-13 and delta N-15 of suspended solids (SS), retained sludge, and macrofauna (oligochaetes and fly larvae) in the system. The delta C-13 and delta N-15 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 delta N-15 of the retained sludge in the DHS reactor increased drastically from the inlet towards to the outlet, from -0.7 parts per thousand to 10.3 parts per thousand. This phenomenon may be attributed to nitrogen conversion processes (i.e. nitrification and denitrification). The delta N-15 of oligochaetes also increased from the inlet to the outlet, which corresponded well to that of the retained sludge. Thus, the delta N-15 of the oligochaetes might simply mirror the delta N-15 of the retained sludge. On the other hand, the delta C-13 and delta N-15 of sympatric fly larvae differed from those of the oligochaetes sampled, indicating dietary differences between the taxa. Therefore delta C-13 and delta N-15 reflected both treatment and dietary characteristics. We concluded that delta C-13 and delta N-15 values are potentially useful as alternative indicators for investigating microbial ecosystems and treatment characteristics of biological wastewater treatment systems. (C) 2014 Elsevier Ltd. All rights reserved.