Effects of Wastewater Treatment Plant Effluents on the Microbial Biomass and Community Structure of the Big Walnut Creek

Abstract

Excessive organic matter input and nutrient loading are considered a major ongoing threat to water quality. Communal wastewater treatment is a process whereby sewage water is treated to remove contaminants before being discharged back to the environment. As most Wastewater Treatment Plants (WWTPs) do not remove all contaminants from sewage waters, we examined the impact of WWTPs effluents on microbial biomass and community structure of the Big Walnut Creek, Greencastle, IN. We took triplicate sediment samples along a 6,000 meter transect at five different locations, two upstream and three downstream of the outfall in June, September, and November of 2019. We quantified microbial community structure as community level physiological profiling (CLPP) using Biolog EcoPlates as well as microbial biomass using phospholipid phosphate analysis. The samples in June right above and below the outfall had comparably lower carbon to nitrogen (C/N) ratios (39.63 and 32.44), and therefore more available nutrients for microbial uptake. The samples further downstream from the outfall (1,452 m and 4,677 m) had high C/N ratios of 65.8 and 88.09 respectively, while samples farther upstream from the outfall (1,600 m) had a C/N ratio of 93.53. Microbial biomass increased from June to November for all five sites due to seasonal allochthonous material inputs. We noticed that the C/N ratio correlated to the increase in microbial biomass observed at the outfall for the June samples. Interestingly, the further away from the outfall, the lower the biomass measured and the higher the C/N ratios. We did not observe any significant difference in microbial biomass across sites during the months of September and November. Increased inputs of allochthonous materials during these months (falling leaves and algae) might have obscured the effects of the WWTP effluent. Principal component analysis (PCA) of the CLPP showed two distinct microbial communities for samples collected at the outfall across seasons and separately grouped together microbial communities from non‐outfall samples. PC 1 explained 28.04% variation observed in microbial community and the most dominant microbial groups on the positive component of PC 1 includes Corynebacteriaceae, Acetobacteraceae, and Lactobacillaceae. Overall, this study showed that WWTP effluent impacts the immediate surroundings of its input, but can be obscured by allochthonous materials or biological transformation of nitrogen downstream. Further research should look into antibiotic resistant genes and resistant bacteria at these separate sites to see if WWTP effluents might have further health impacts on the stream ecology of the Big Walnut Creek.Support or Funding InformationSupport from the Biology Department and the Science Research Fellows program at DePauw University.