The significance of pre-treatment on microbial communities within soil treatment units for on-site wastewater systems

Abstract

Domestic on-site wastewater treatment systems (OWTSs) are configured with a variety of treatment technologies, resulting in varying degrees of effluent pre-treatment before the effluent is distributed into the soil for percolation via the soil treatment unit (STU), more usually known as a percolation area. This septic-STU configuration accounts for the vast majority of OWTS systems in Ireland (treating 29.5% of the nation’s wastewater) as well as being equally prevalent as a form of OWTS more globally. The subsoil’s texture and the level of effluent pre-treatment directly influence the development of dense microbial mats, or ‘biomats,’ along the interface where the effluent percolates into the underlying soil. This study aims to evaluate the effective role of effluent quality and soil permeability on community assembly patterns and the treatment efficiency of the STU biomat, and whether effluent quality will result in conformational changes in the microbial community assembly pattern. This was achieved by monitoring four separate soil columns for 510 days as a comparative mesocosm study. The columns were packed with subsoil of two levels of porosity (High: 26%; Low: 21%), with two columns dosed with primary treated effluent (PE) from a conventional septic system and the other two columns dosed with secondary treated effluent (SE) from a rotating biodisc contactor (RBC) at the same household, which resulted in approximately 90% Total Organic Carbon (TOC) removal from the PE. The columns were monitored for effluent pollutant and faecal indicator bacteria removal efficiencies for over 200 days. Changes in volumetric water content (VWC) were also monitored throughout the whole trial period using an automated 2-dimensional network of sensors which helped to determine the establishment of the biomat. The samples from varying depths of four columns, influents and effluents were characterised by 16 S amplicon gene sequencing analysis. Our findings demonstrate that effluent pre-treatment has a stronger effect on biomat community assemblage patterns than subsoil texture. We observed that greater pre-treatment led to more immigration at 3.8%, while columns dosed with PE exhibited greater levels of species sorting at 88.1% and 3.4% immigration. These insights will significantly impact research and development efforts focused on soil treatment systems, including managed aquifer recharge projects.