An investigation of potential relationships between septic tank microbial communities and system design and performance

Abstract

Septic tanks are utilized by many households across North America for wastewater treatment. Despite the economic and environmental importance of septic tanks, there has been limited innovation in septic tank design and research on the microbial communities responsible for wastewater treatment within these systems. InnerTube systems are septic tanks that employ a novel design to reduce solid accumulation in comparison to conventional septic tanks. For this project, 16S metabarcoding was employed to characterize conventional and InnerTube septic tank microbial communities and evaluate relationships between community composition, system design, and treatment efficacy. Wastewater was sampled along the length of InnerTubes to determine patterns of microbial succession and how they may impact InnerTube function. Wastewater was separated into liquid and solid fractions to identify differentially abundant taxa in each fraction. Populations of methylotrophic methanogens increased with distance from the InnerTube inlet. Solid communities were differentially more abundant in methanogens than liquid communities. Higher rates of solid degradation in InnerTubes may be due to longitudinal stratification of substrates and functionally distinct communities and the activity of methanogenic biomass. Septic tanks throughout Ontario were also surveyed to evaluate the effect of system design (conventional vs. InnerTube) and operational flow (single-pass vs. recirculation) on microbial community composition and to identify taxa correlated with chemical oxygen demand (COD) reduction. Single-pass InnerTube communities were more abundant in Pseudomonas which was attributed increased availability of long-chain fatty acid substrates. Recirculating conventional communities were more abundant in Arcobacter and Desulfomicrobium which was attributed to greater resistance to oxidative stress. Desulfovibrio and Brevundimonas were positively correlated with COD reduction. These putative hydrogen producers may facilitate greater COD reduction by forming syntrophic relationships with hydrogenotrophic methanogens. The findings of this project may be used to develop bioaugmentation inoculum, system designs, or operational strategies to optimize septic tank performance.