Differences in the isotopic signature of activated sludge in four types of advanced treatment processes at a municipal wastewater treatment plant

Abstract

The natural abundance of stable isotopes is a powerful tool for evaluating biological reactions and process conditions. However, there are few stable isotope studies on the wastewater treatment process. This study carried out the first investigation on variations in natural abundance of carbon and nitrogen stable isotope ratios (δ¹³C and δ¹⁵N) of activated sludge in four types of advanced treatment process (extended aeration activated sludge (EAAS), aerobic-anoxic-aerobic (A²O), recycled nitrification-denitrification (RND), and modified Bardenpho (MB)) at a municipal wastewater treatment plant. The δ¹³C and δ¹⁵N values of influent suspended solids settled in the primary sedimentation tank (i.e., primary sludge) ranged from -25.4‰ to -24.6‰ and 0.5‰-2.9‰, respectively, during monitoring periods. The δ¹³C values of the activated sludge were -24.6‰ to -23.6‰ (EAAS), -25.4‰ to -24.3‰ (A²O), -25.7‰ to -24.9‰ (RND), and -25.7‰ to -24.3‰ (MB). The δ¹³C values of the activated sludge were similar to those of influent suspended solids. However, the δ¹³C values of activated sludge in EAAS was significantly higher than in A²O, RND, and MB. Meanwhile, the δ¹⁵N values of activated sludge were obviously higher than influent suspended solids; 5.8‰-7.5‰ (EAAS), 6.6‰-8.1‰ (A²O), 5.5‰-7.5‰ (RND), and 5.3‰-7.6‰ (MB). Changes in δ¹³C and δ¹⁵N values of the activated sludge within the treatment system were also found. These findings indicate that changes in δ¹³C and δ¹⁵N values of the activated sludge rely on important function for biological wastewater treatment such as nitrification, denitrification, and methane oxidation through wastewater treatment over time.

Keywords: Activated sludge; Advanced treatment system; Carbon stable isotope ratio (δ(13)C); Nitrogen stable isotope ratio (δ(15)N); Nutrient removal.