Phosphorus (P) is an essential plant nutrient and a non-renewable resource of which the future
supply to agriculture is challenged by limited and geopolitical unevenly distributed mineral P
reserves. Recycling of P from waste is an important mean to minimise the dependence on the
limited mineral P reserves. Wastewater represents the largest urban flow of P in waste. Hence,
knowledge about plant P availability of products from the wastewater treatment system, and also
comparison to other waste P sources and mineral P is essential to obtain an efficient recycling
and to prioritise between different P recycling options.
The work of this PhD focused on the plant P availability of sewage sludge, a P-rich residue from
wastewater treatment which is commonly applied to agricultural soil in Denmark. The overall
objective of the PhD work was to evaluate the plant availability of P in sewage sludge and other
wastewater-derived products, and to relate this to the availability from other P-containing waste
products and mineral P fertiliser. This included aspects of development over time and soil
accumulation, as well as effects of soil pH and the spatial distribution in soil.
The P sources applied in this PhD work included anaerobically digested and dewatered sewage
sludges from six different wastewater treatment plants, thermally dried sewage sludge, four
sewage sludge incineration ashes, thermochemically treated sewage sludge ash, struvite, two
rejectwater evaporation products, composted household waste, cattle manure and mineral P
fertiliser. The applied methods consisted predominantly of pot studies and soil incubation studies
with focus on describing and evaluating the plant P availability of the P sources against each
other. In addition, rhizobox studies for root observations and characterisation of soil from a longterm
field experiment were included in combination with 33P isotope techniques.
In particular sewage sludges, but also sewage sludge incineration ashes, from different
wastewater treatment plants varied substantially in P availability. The variation between different
sludges could be partly related to the use of precipitation chemicals during the wastewater
treatment; thus, precipitation with Al (compared to Fe) and increased (Fe+Al)/P molar ratio were
indicated to have a negative impact on P availability. The sludge treatment options thermal
drying and in particular incineration was found to clearly reduce P availability compared to the
undried/un-incinerated sewage sludge. However, as opposed to an often stated view, the ashes
were shown to have some degree of P fertilisation value, with a tendency to increase in
availability over time. Soil pH was found to have a clear effect on sewage sludge P availability,
but not on ash P availability. Furthermore, it was demonstrated that products from alternative
recovery treatment, e.g. struvite and thermochemically treated ash, do not necessarily imply
higher P availability than sewage sludge.
Compared to mineral P fertiliser (TSP, triple super phosphate), sewage sludges generally showed
lower availability of P. Thus, average relative fertiliser efficiencies ranged from 51-77% when
added to a moderately acidic soil, and 20-37 % when added to moderately alkaline soil, for four
sludges derived from wastewater treatment plants using combined Fe-precipitation and
biological P removal (the common approach for wastewater P removal in Denmark). However,
the difference between TSP and sludges decreased over time (examined for up to 197 days),
indicating more stable and slow-release properties from the sewage sludge compared to TSP.
Compared to composted household waste, sewage sludge showed higher P availability in both a
short-term and a long-term perspective. Following long-term excess waste P applications to soil,
the availability of accumulated residual P was higher when applied with sewage sludge than with
composted household waste, but lower than when applied with cattle manure.
Localised application of sewage sludge to maize resulted in increased root proliferation in and
around the sludge patch (similar to localised TSP). However, P uptake from the sludge was not
enhanced compared to a situation with homogenous distribution; rather it was slightly reduced
(contrary to TSP). Localised application of sewage sludge ash did not imply any root responses
and the plants did not take up P from the localised ash. In contrast, homogenously distributed ash
resulted in a well-developed root system and a substantial uptake of P from the ash. Based on
these results, localised applications of sewage sludge and sewage sludge ashes cannot be
recommended.
Methodological considerations included an evaluation of the WEP (water extractable P) method
used in most of the experiments to describe P availability after application to soil, was evaluated.
This suggested that the simple WEP method can be a useful alternative to more expensive and
advanced methods in terms of ranking the P availability from different P sources.
Overall, the research conducted within the present PhD work contributes to a better
understanding of P availability from sewage sludge and related products, and it can potentially
serve as input for prioritisation of waste management strategies with focus on potential changes
in wastewater and sludge management to enhance efficient P recycling from wastewater. This
may be particularly relevant for wastewater treatment plants in areas where increased recycling
of wastewater P is targeted. In Denmark, this may apply to the Sjælland area (Region
Hovedstaden + Region Sjælland) where recycling of P from wastewater is low compared to other
parts of the country due to a high degree of incineration in the Copenhagen area. Finally, in this
context, tentative recommendations and specific suggestions for future research objectives are
provided.