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Jan Knappe

PhD Researcher
Dept. of Civil, Structural & Environmental Engineering (& Mathematics Applications Consortium for Science and Industry, University of Limerick)


Modelling of soil biomass for on-site wastewater treatment

Keywords: on-site domestic wastewater treatment system; septic tank; vadose zone; percolation; subsoil; nitrogen; phosphorus; groundwater protection; biomat; soil treatment.

With circa 500,000 septic tank systems currently installed, approximately one third of domestic wastewater generated in Ireland is treated in on-site domestic wastewater treatment systems (DWWTs). DWWTs are most commonly composed of a septic tank and a soil attenuation system for further treatment and effluent dispersal. In the soil, water is cleaned by microbial activity which depends on nutrient cycling but also controls it and, thus, determines whether excess nutrient levels contaminate groundwater or surface runoff and constitute a pollution risk.


In collaboration with the University of Limerick and the Limerick City and County Council, the project aims at understanding and modelling soil biomass growth in percolation trenches commonly used in Ireland to disperse on-site effluent.

Previous research has shown that the development of a microbial biomat which forms along the base and wetted sides of percolation trenches is key to the hydraulic loading and subsequent attenuation of pollutants in the unsaturated zone. The quality of the biomat, therefore, influences the overall pollutant loading to the groundwater. Irish studies on different subsoil permeabilities have found higher nitrogen loading to the groundwater under secondary treated effluent fed percolation areas compared to septic tank effluent fed areas due to significantly reduced biomat formation.


Based on experimental studies at field sites where water, nutrient, and pollution levels as well as weather conditions, effluent flow patterns, and microbial composition are monitored, effective mathematical models to describe the growth and interaction of soil biomass with nutrients are developed and calibrated. A variety of design alternatives (primary and secondary treated effluent, pumped vs. gravity-fed distribution) is being evaluated to predict the vertical and horizontal extent of wastewater contamination zones.



Project Supervisors: Prof. Laurence Gill & Dr. Andrew Fowler (UL)