Abstract
Wastewater stabilization ponds (WSPs) are commonly used to treat municipal wastewater in the Canadian Arctic. Bacterial community structure and functionality remain mostly uncharacterized for arctic WSPs, yet are presumed important for treatment outcomes during the 3-month summer treatment season with open water in the WSPs. The objective of this study was to investigate treatment performance and related temporal and spatial changes in the structure and putative function of bacterial communities during treatment of municipal wastewater in the WSPs of Pond Inlet and Clyde River, Nunavut over two consecutive summer treatment seasons. Influent raw wastewater contained a high organic load and large bacterial communities (~9 log 16S rRNA copies/mL) belonging mainly to Proteobacteria. Although designed to be facultative ponds, both WSPs remained anaerobic with neutral pH values (7.5–7.8) throughout the summer treatment season. Water quality data showed that nutrients [measured as carbonaceous biological oxygen demand (CBOD5)], total suspended solids, and total ammonia nitrogen were progressively reduced during treatment in the ponds as the summer progressed. The pond bacterial population size and species richness depended on the pond temperature (2–18°C), with 8.5 log 16S rRNA copies/mL and the largest alpha diversities (Shannon-Wiener index of 4-4.5) observed mid-season (late July). While the phylogenetic beta diversity in raw wastewater from the two locations remained similar, pond bacterial communities underwent significant (p < 0.05) changes to dominance of Comamonadaceae, Geobacteracea, and Porphyromonadaceae. Multivariate distance based redundancy analysis and predicted gene functionalities in the microbiota agreed with water quality results that microbial removal of nutrients (e.g., CBOD5) peaked in the middle of the summer coinciding with the treatment period with the highest pond temperatures. Information from this study will be useful for further development of models to predict biological treatment outcomes, which could be used to size and assess the feasibility of WSPs in extreme climates. Higher pond temperatures resulted in optimal biological processes and nutrient removal in the middle of the summer. While it is challenging to control environmental factors in a passive wastewater treatment system there are some design considerations that could be used to optimize temperature regimes, such as the depth of the pond.
Highlights
Wastewater stabilization ponds (WSPs) are commonly used to treat municipal wastewater in remote arctic communities in Canada
Raw wastewater samples from both communities consistently contained an average of 9.0 log 16S rRNA copies/mL, a level which showed no change (p > 0.05) over the duration of the study
The bacterial population size followed the changes in pond temperatures with midseason peaks of 8–8.9 log 16S rRNA copies/mL followed by significant (p < 0.05) decrease to 6.5–7 log 16S rRNA copies/mL at the end
Summary
Wastewater stabilization ponds (WSPs) are commonly used to treat municipal wastewater in remote arctic communities in Canada. Engineered WSPs have typically been designed to be facultative (i.e., aerobic) and to hold the annual volume of wastewater generated within the community (Ragush et al, 2015) These WSPs are passive (i.e., non-mechanical) treatment systems that depend on physical settling and biological processes and are strongly affected by the climatic conditions including ambient temperature (Heinke et al, 1991). Due to the low water consumption per capita (∼100 L/person × day) in arctic regions such as Nunavut compared to Southern Canada (average of 274 L/ person × day), the deposited sewage tends to be more concentrated (Daley et al, 2014; Ragush et al, 2015) This can lead to high organic loading rates (OLRs) and anaerobic conditions in the facultative ponds which may affect the microbiological processes negatively (Ragush et al, 2017). Past studies have shown that primary treatment can be attained in arctic WSPs with removal of around 25–40% of nutrients, oxygen demanding material, suspended solids, and 99% of fecal indicator Escherichia coli (Marais, 1974; Heinke et al, 1991), even in WSPs with high OLRs and anaerobic conditions (Ragush et al, 2015; Huang et al, 2018)
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