Abstract:
A growing population, urbanization and an increase in the number of
industries is causing an increase in sewage sludge (SS) that needs to be
either beneficially used or disposed of. Application of SS to agricultural lands
is a well-known practice but the plant available phosphate and phosphate
fertilizer value of SS has been of concern. This is especially due to the wide
variety of soils that SS is applied to and the different phosphate removal
processes that sewage waste water undergoes at the water care works and
the source used to produce SS.
Phosphate phyto-availability and phosphate fertilizer value of petrochemical
and municipal wastewater sludges (SS) were determined in four different soils
using an incubation study over 168 days, a pot trial over 42 days and a field
trial over one season. Phosphate phyto-availability was determined/calculated
by means of an incubation approach. Soils were incubated with sludge as well
as mono ammonium phosphate (MAP). The soils were then subjected to a
Bray-1 extraction after a certain time (42 days, 168 days). The relative
phosphate fertilizer value (RPFV) was then expressed as a percentage of the
Bray-1 extractability of the sludge-amended soil, relative to the MAP-amended
soil. The influence of soil properties, especially clay content, and sludge
properties namely phosphate extraction method (chemical and biological)
from waste water stream, was investigated to determine the effect on phytoavailability
and fertilizer value of phosphate in SS-amended soil. Soil
properties were the dominant factor determining plant available phosphate,
where plant available phosphate decreased with increasing clay content,
irrelevant of the type of treatment. There were, however, significant differences between the chemical and biologically removed SS, where the
biologically removed SS had higher plant available phosphate.
The RPFV % of the SS was comparable to MAP in terms of its plant
availability. Wetting and drying cycles in the pot trial influenced the plant
available phosphate from the SS, where the chemically treated sludge
showed in general lower plant availability. The RPFV % of the biologically
removed phosphate sludges was better than that of MAP and that of the
chemically phosphate removed sludge were lower.
Application of all the different sludge types resulted in a positive reaction on
plant available phosphate for all the soils. All the trials were conducted at pH
of about 5.5. However, it is expected that biologically P removed sludge will
perform better in acid soils. The reason being that ferric phosphate in the
chemically treated sludge is less soluble under pH conditions lower than 5.5
than above it.
