Using a water treatment residual and compost co-amendment as a sustainable soil improvement technology to enhance flood holding capacity

Abstract

The recycling of clean wastes, such as those from the treatment of drinking water,
has gained importance on the environmental agenda due to rising costs of landfill
disposal and movement towards a ‘zero’ waste economy. More than one third of
the globe’s soils are degraded and as such policies towards determining soil health
parameters and reversing destruction of the globe’s most valuable non-renewable
source are at the forefront of environmental debate. This thesis questions the
opportunity for water treatment residual (WTR) to be used as a beneficial material
for the co-amendment of soil with compost to improve the soil’s flood holding
capacity (Kerr et al., 2016), which includes functions such as the water holding
capacity, hydraulic conductivity, soil structure and shear strength. Currently, water
treatment residual is typically sent to landfill for disposal, but this research shows
that the reuse of WTR as a co-amendment is able to improve the flood holding
capacity of soils. This research crosses the boundary between geotechnical and
geoenvironmental and provides a holistic approach to quantifying a soil from both
perspectives.

Iron based water treatment residual from Northumbrian Water Ltd was used in
both laboratory and field trials to establish the effect of single WTR and a compost
and WTR co-amendment on the water holding capacity (the gravimetric water
content, volumetric water content, volume change of samples i.e. swelling and
shrinkage), and the effect of amendment on the erosional resistance, hydraulic
conductivity and shear strength compared to a control soil. A series of four trials
were conducted to develop and establish a novel method to determine the water
holding capacity, supplemented by standard geotechnical methods to determine
the flood holding capacity. The use of x-ray computed tomography has provided
accompanying information on the morphology of dried WTR and changes in the
internal characteristics of amended soil between a dry and wet state. The
amendment application rate ranges from 10 – 50%.

Experiments have shown that the single amendment of WTR, compared to a
control soil, yields significant increases in the hydraulic conductivity (by up to a
factor of 28), increases the shear strength of soils at low testing pressure (25 kPa)
by 129%, increases the maximum gravimetric water content by up to 13.7%, and
improves swelling by up to 12% (but only at the highest amendment rate, 30%),
increases the maximum void ratio when saturated by 11%, and reduces shrinkage
by maintaining porosity by 14%. However the application of WTR as a single
amendment has implications for the chemical health of the soil as it is highly
effective at immobilising phosphorous as and such cannot not effectively be used
as a soil amendment. The single application of compost yielded significant
improvement in the water holding capacity (improving gravimetric water content
by up to 34.7%, increasing the sample volume by up to 83.3%, and increased the
void ratio by 8.2%), however this application reduces the hydraulic conductivity
by up to 84.5% and the shear strength by 3% compared to the control soil.

Co-amendment using compost and WTR (in two forms, air dried 80% solids and
wet at 20% solids, as produced from water treatment works) improved the flood
holding capacity of soils by retaining the structural improvements of amendment
using WTR and the water holding capacity improvements of compost. Compared to
the control soil, for co-amended soils the gravimetric water content was improved
by up to 25%, the volume increased by up to 51.7%, experienced 13% less
shrinkage and an 11.5% increase in maximum void ratio. The hydraulic
conductivity was also improved by up to 475%, and shear strength was increased
at both low and high testing pressures by to 53.8%.

Taking into account these effects of co-amendment on essential soil functions that
determines a soil’s flood holding capacity (maximum gravimetric water content,
volume change, resistance against shrinkage, void ratio (porosity), hydraulic
conductivity and shear strength), the economical and environmental sustainability
issues, the co-amendment of soil using compost and WTR may provide a solution
to both recycling clean waste product and improving the quality of soil.