Preferential flowpath land treatment system

New Zealand produces around a billion litres of municipal wastewater per day. (inf-facts-issues-Sep09.pdf, 2009) Currently very little of this effluent is used for farm irrigation, despite declining water resources in many areas. A key obstacle to uptake of treated wastewater is perception. Thus whilst technology is sufficiently advanced to ensure that public health risks are acceptable in a scientific sense, there is a knowledge gap in achieving cultural acceptability.

The research proposes the use of constructed trenches set on a sloping hillside in low permeability soils (clay loam/clay). Trenches are excavated and filled with a local soil of higher permeability (silts and sands) with a reused topsoil covering. The effluent can move only slowly through the surrounding soil (low permeability) and so chooses an easier flowpath through the filtration soil. Whilst some effluent will be lost to ground, the vast majority will follow this preferential flowpath down the trench, receiving treatment through a number of mechanisms along the way. The system uses bio-mimicry to replicate the natural processes through which water is purified by the environment in the upper soils layers, and is used as a polishing treatment.

A trial trench was constructed and municipal wastewater passed through the trench over a three-week period. The system was also dose loaded with manufactured MS2 phage to investigate virus removal efficacy. Sampling at entry, intermediate points, and exit of trench indicated a four-log removal of e-coli and MS2 phage, high Biochemical Oxygen Demand removal (82%-98%), and 98% Phosphorus removal.