Flow and transport in a constructed infiltration system for wastewater treatment characterised by electrical resistivity and 2D numerical unsaturated zone modeling.

Disposal of domestic sewage effluents in soil has been used for several decades in Norway and more than 100000 constructed systems for wastewater purification have been built with capacities between 5 and 8000 pe (person equivalent). However, the infiltration of wastewater effluents into soils and the estimation of application rates for a given system design and environmental setting are extremely complex and often poorly understood and oversimplified (Siegrist, 2004). The infiltration system presented here consists of 26 horizontal distribution pipes separated by 1.25 m over an area of 1100m2. The distribution pipes are placed in a coarse gravely distribution layer at about 1m depth. A pump ensures regular injection of wastewater into the system. Below the distribution layer there is a 20 cm thick layer of local natural soil, which is a coarse sandy soil with gravel, followed by a 25 cm thick layer of light weight aggregates (LWA). Below the LWA layer there is natural soil and the water drains freely to the groundwater at about 5m depth. Hence the retention time and flow pattern are key factors determining whether phosphates are retained and organic components are degraded before water leaves the filter system or enters the phreatic level. In this study a combination of time lapse electrical resistivity (ER) measurements and numerical modeling of an unsaturated system have been performed in order to examine the wastewater distribution and its potential effect on flow and transport in a 2D unsaturated layered profile. Measurements were performed in June 2005. In addition to the ER measurements, an inactive tracer was applied and the breakthrough curve monitored at three depths below the constructed filter. Temporal changes in electrical resistivity revealed spatial patterns coinciding with the locations of the pipes, hence suggesting a heterogeneous distribution of waste water. The numerical simulations show the importance of the hydraulic conductivity of the distribution layer, and support the conclusion that the distribution may be non uniform over the area, hence causing preferential flow paths in the filter system. References Siegrist R.L. McCray J.E., Lowe K., 2004, Wastewater Infiltration into soil and the effects of infiltrative surface architecture, Small flows quarterly Vol. 5 No. 1, pp. 29-39.