Ecological approach to mitigate toxic shocks in activated sludge systems.

摘要

In this study, an interdisciplinary approach was used to provide a potential solution to the problem of accidental and intentional discharge of toxic chemicals to activated sludge sewage treatment systems. As an overarching framework, this study was guided by two hypotheses. The first hypothesis states that the extent of biodiversity of microbial communities is related to the operating conditions of activated sludge systems, in particular solids retention time (SRT). The second hypothesis states that activated sludge systems with a greater extent of biodiversity demonstrate enhanced functional stability (resistance to perturbation) to toxic shock loads. The first hypothesis was tested theoretically using an ecology-based competition model and experimentally by running lab-scale bioreactors and using a molecular fingerprinting tool, terminal restriction fragment length polymorphism (T-RFLP), to assess bacterial diversity. The results of the model and T-RFLP show that bioreactors operated at an SRT of 2 days have significantly (P 0.05) more biodiversity than bioreactors operated at an SRT of 8 days. Combined, the model and T-RFLP results showed that engineering decisions could be optimized to enhance biodiversity in activated sludge systems.; To objectively test the second hypothesis, a novel use of respirometry, a widely used bioassay for assessing the impact of toxins on the oxygen uptake rate (OUR) of microorganisms, was used to measure resistance of nine municipal activated sludge sewage treatment systems in Cincinnati, Ohio to toxic shock loads of copper (II) (Cu2+), 3,5-dichlorophenol (3,5-DCP), and 4-nitrophenol (4-NP). Respirometric batch experiments were designed to determine the resistance of activated sludge to the three toxicants using two different approaches. In the first approach, resistance was measured as IC50 or the concentration of toxicant that resulted in 50% inhibition in oxygen utilization rate (OUR) compared to a referential state represented by a control receiving no toxicant, where higher IC50 values implies higher resistance. In the second approach resistance was measured as the % decrease in active competent biomass concentration in response to toxicants compared to a referential state represented by a control receiving no toxicant, where lower % reduction implies higher resistance. Biodiversity in activated sludge was assessed using two PCR-based molecular fingerprinting techniques, namely T-RFLP and denaturing gradient gel electrophoresis (DGGE). Statistical analysis of the respirometric and molecular fingerprinting data suggested that there was a significant correlation (P 0.05) between biodiversity and functional stability for the three toxicants, where resistance increased as biodiversity increased. These results show that activated sludge sewage treatment plants with higher biodiversity are functionally more stable to toxic shock loads.; This fundamental new knowledge obtained about the role of biodiversity in the functional stability of activated sludge system will make a vital contribution to the field of biological wastewater treatment systems. At the same time it will positively affect human health and the environment, because wastewater treatment systems will be better able to remove toxins before they reach the environment.