Proceedings

Biosolids (treated sewage sludge) provide a valuable source of plant available nutrients and
organic matter that can be of benefit to soil quality and fertility. In 2004, c.62% of UK sludge
production was recycled to farmland. Development of thermal drying technologies has
produced new enhanced treated biosolids products that have different nutrient supply
properties compared with conventional digested liquid and sludge cake products.

This paper summarises results from the first year of a field study where the fertiliser nitrogen
(N) replacement value of two different types of enhanced treated biosolids (thermally dried
and plate-pressed vacuum dried) was measured, using winter wheat as the test crop (harvest
year 2006). The biosolids products, produced by Severn Trent Water, were applied at two
rates (c.125 and 250 kg/ha total N) and two timings (autumn ploughed down and spring
topdressed). Following the autumn 2005 application of thermally dried biosolids, nitrate
leaching losses were measured over winter 2005/06.

The autumn application of thermally dried biosolids increased nitrate leaching losses by a
small amount (c.10 kg N/ha) compared with the untreated control (equivalent to 4% of the
total N applied), although the differences could not be confirmed statistically (P > 0.05). The
thermally dried (autumn and spring applications) and plate-pressed (spring application)
biosolids had N utilisation efficiencies in the range 10-18% of total N applied compared with
inorganic fertiliser N. There were no differences (P > 0.05) in N utilisation efficiency between
the two thermally dried biosolids application timings (i.e. autumn vs. spring) or between the
two biosolids types.

This work forms part of a longer term study that is evaluating the soil quality benefits of
repeated annual applications of enhanced treated biosolids.

KEYWORDS
biosolids, nitrate leaching, nitrogen, thermally dried, vacuum dried