Abstract:
The research described in this thesis is part of a wider EU-LIFE project, the
WaterRenew project. The WaterRenew concept can be described as the recovery of
nutrients from wastewater which can lead to eutrophication of surface waters, by
irrigation of short rotation coppice in order to fertilise them. Such systems have been
proven to function properly as nutrient removal systems when studied for N removal
and have already been successfully and commercially implemented in different
countries. However, the factors potentially preventing them from operating sustainably
have not been identified nor their upper limits quantified with confidence. A WaterRenew
system can indeed be looked at as a unit composed with three main compartments;
soil, soil water and plant. Therefore, the sustainability of such a system will be
compromised if at least one of these compartments is changed irreversibly. The limits
can be hydrological with constant runoff or drainage being induced. They can also be
chemical with inadequate amounts of nutrients removed from the effluent applied or
irreversible accumulation of nutrients in soil. Finally, these limits can be physiological
with the trees’ health being irreversibly compromised. Moreover, the relevance and
effectiveness of such a system under UK conditions has not been established yet.
In this context, a field trial was set up at Cranfield University sewage treatment works
where the secondary treated effluent was irrigated on to Salix viminalis, Populus
trichocarpa and Eucalyptus gunnii trees planted at a density of 13,060 trees.ha-1, on a
chalky clayey soil, in order to maintain soil water content at field capacity. To tackle
more specifically P fate processes understanding, an independent P leaching soil
column experiment was also set up.
With the latter settings, it was possible to apply high volumes of effluent (3625 mm for
willow, 2895 mm for poplar and 3345 mm for eucalyptus for the 2 years of irrigation)
and high amounts of nutrients (1023 kg-N.ha-1 and 134 kg-P.ha-1 for willow, 834 kg-
N.ha-1 and 108 kg-P.ha-1 for poplar and 946 kg-N.ha-1 and 127 kg-P.ha-1for eucalyptus
for the 2 years of irrigation).
It was found that irrigation with effluent increased significantly tree yields so that they
were within the range reported in the literature for willow and eucalyptus but slightly
lower for poplar. The trees uptook between 20 % and 50 % of the total amounts of N
and P applied with eucalyptus uptaking more nutrients than willow, which in turn took
up more than poplar. Then, it was found that irrigation did not have any significant effect on N and P in soil and the amounts applied remained very low compared to the
existing nutrients soil pools. However, irrigation did have a significant effect on
increasing K and Na in soil. Na increased enough to induce a significant increase in
soil SAR but soil remained neither saline nor alkali. The trees had a smaller impact on
soil chemistry. Finally, it was found also that irrigation did not have any significant
effect on N and P in soil water with no P detectable in any of the soil water and
groundwater samples during the whole experiment. Irrigation did, however, increase
significantly K and Na concentrations in soil water and for K also in groundwater. From
the point of view of nutrients removal, although a tree effect was measurable, it was
not as important as the functions of the soil. Thus, when a WaterRenew system is
maintained under a hydrological constraint, with the soil moisture kept at field capacity,
it was still possible to apply high volumes of effluent, even on a clayey soil. In addition,
the consequent high amounts of nutrients applied were efficiently retained between
tree uptakes and mainly soil organic and inorganic nutrients’ pools. Indeed, the
amounts of nutrients lost by drainage remained low (<10 % of the total amounts
applied) for N and P and groundwater was efficiently protected from pollution on this
site.
On P dynamic processes understanding, it was found that P leaching patterns depend
highly on soil moisture and to a lesser extent on the amount of P applied. When soil is
saturated, P will start leaching even when applied at a very low concentration.
A model, the WR_MODEL, was developed which integrates the observations,
measurements and understanding of Cranfield University sewage treatment work field
trial and soil column experiment into a model. The purpose of WR_model is to help the
design and implementation of a WaterRenew system in any location as long as
climatic and soil data are available. The model default values are for England and
Wales climatic and soil data.