Characterisation of Microbial Biomat Development in Soil Treatment Units Receiving Domestic Effluent

Abstract

Soil treatment units are an essential part on on-site wastewater treatment system. In Ireland there are approximately 500,000 on-site wastewater treatment systems with 87% configured as a septic tank followed by a soil treatment unit. The treatment of domestic wastewater which is a high in organic and nutrients involves a complex series of biogeochemical reactions. These chemical transformations are mediated by microbial activity who utilise organic and nutrients as an energy source. These organisms can often coexist in soil environments and produce extracellular polysaccharide to form microbial mats `biomats?, which have been shown to enhance the functionality of soil treatment units. With accelerated development of molecular techniques microbial ecology has become an essential component in environmental engineering. The identification of carbon and nitrogen cycling species has allowed for the optimisation of microbial activity in engineering processes. However, much of the application of microbial ecology in wastewater treatment is focused primarily on centralised wastewater treatment plant processes, particularly activated sludge treatment systems.

The aim of this research is to develop a better understanding of the microbial ecologies present within soil treatment units and the ecological profile of the microbial mats. Three on-site wastewater treatment system were surveyed for this study. The sites were built to EPA standards with the sites utilising both a standard septic system and packaged secondary treatment unit with the standard 4 percolation trenches assigned influent from either source to assess the effect of domestic wastewater pre-treatment. These sites were purpose built and instrumented with lysimeters and a three-dimensional soil sensor network to enable characterisation of the clogging of the biomat over time and chemical composition of the porewater below the systems. Additionally, four subsoil mesocosms were constructed in the laboratory, designed to mimic domestic wastewater dosing on a soil treatment unit and to analyse the effects of pre-treatment and porosity on the development of the biomat. Chemical and physical monitoring was combined with regular biomat and subsoil genomic sampling campaigns for both the field studies and the mesocosm experiments. Environmental DNA was extracted from the subsoil and was genetically sequenced to profile the biomat communities.

The results show that the pre-treatment of effluent has a significant effect on the microbial community structure of organisms present within the on-site wastewater treatment units and the laboratory mesocosms. The application of high organics and nutrients provides a selective pressure on communities with the community structure being determined by species sorting. Over time communities exposed to high organic loading exhibit increases in their diversity as the availability of substrate aided by the clogging within the soil, this promotes the transformation of nutrients and the creation of microbial hotspots. However, over time many bacteria within these communities will be out-competed by copiotrophic bacteria capable of dominating the high nutrient environment. The functionality of the bacteria is wholly linked to the reduced hydraulic conductivity delivered by a robust soil biomat. In comparison, in soils dosed with secondary treated effluent the presence of a muted biomat, due to a low concentration of organics in the effluent feed appears to delay these shifts in community structure in a pattern that is distinct to biomats dosed with primary treated effluent.