The influence of a growing microbial biomat on water retention and pollutant attenuation in soil treatment units receiving domestic effluent

Soil treatment units (STUs) are an integral component of most domestic on-site wastewater treatment systems. Understanding their long-term environmental impacts and performance will help to improve design guidelines and operational procedures. In this study, the first systematic tracing of primary and secondary treated effluent percolating into the unsaturated subsoil below shallow gravel trenches in STUs was undertaken using a three-dimensional network of soil sensors continuously recording volumetric water content, soil temperature, and electric conductivity in the pore water. Three full-scale on-site systems were constructed to treat the domestic effluent of individual households (2 to 4 PE) at field sites in rural Ireland. Following a conventional two-chamber septic tank, the partially treated effluent was, then, split equally into two streams for (i) direct discharge of primary effluent into one half of the STU, and (ii) further treatment in a secondary treatment unit before final discharge to soil in the other half of the STU. Data obtained from the sensor network and chemical analysis of effluent as it percolated through the vadose zone serve as proxy for determining the existence and extent of zones of effective percolation and biomat spread at the base of percolation trenches. From results of the field measurements and numerical modeling using HYDRUS 2D, it can be inferred that soil moisture retention within a biological clogging zone at the infiltrative layer – also known as a microbial biomat – expressed distinct responses to hydraulic and organic loading patterns, and environmental factors such as periods of extended drought or temporarily saturated conditions following heavy rainfall events. The results of this study, in turn, will contribute to a more insightful assessment of existing guidelines and regulations governing the design and operation of on-site treatment systems in Ireland and internationally