soil products including composts, biosolids and digestates, plastic contamination
within the feedstocks or introduced via the processing has the potential to distribute
plastics into the environment, particularly when ‘beneficially applied’ to soils
utilised for the production of food. For example, analysis of a range of organic
products used for agriculture identified up to 24 microplastic particles (i.e. particles
under 5 mm) per kilogram, and as high as 895 microplastic particles per kilogram in
biowaste digestate (Weithmann et al. 2018). Although there is increasing research on
plastic and emerging contaminants, particularly those entering agroecosystems,
considerable uncertainty remains given the limited number of studies to date.
There has also been limited research to document and collect evidence relating to
the effects of these contaminants and what suitable loading concentrations may be
appropriate.
8.8 Biosolids Use
Raw sewage includes a supply of water and a solids component which is rich in
essential nutrients such as nitrogen, phosphorus and organic matter; and these are in
a form that is highly suitable for assimilation by plants (New South Wales 2000). As
such biosolids producers have been working with government regulators and end
users (agricultural sector) to reuse both the water and solids (biosolids) components
of sewage in a manner that is cost-effective, environmentally sustainable and safe
from a public health perspective. Sewage treatment plants are the main source of
biosolids in Queensland with production estimated anywhere between 30 and 50 kg
dry solids per equivalent person per day (equivalent to 150 kg of dry cake per
annum). The biosolids are produced as either a thickened slurry or a dewater cake
and, more recently, pellets; and contain useful quantities of organic matter and
nutrients such as nitrogen (N), phosphorus (P) and potassium (K) and lead to
improvements in soil characteristics such as improved microbial activities and
oxygen consumption. Beneficial use is now a prerequisite for disposal of biosolids
in many developed countries, which has necessitated formulation of guidelines to
prevent environmental contamination with heavy metals and pesticides, and infection of human and animal populations with pathogenic organisms, resulting in
different classifications (such as Grades A–E) to control the end uses for the material
(New South Wales Government 2000; Queensland Government 2016a). Whilst it is
commonly accepted that the utilisation of nutrients in biosolids at or below agronomic loading rates are highly beneficial, there is increasing concern related to
contamination of biosolids from both known and new sources and how the concentrations or leaching potential of these new contaminants will be impacted through
further pretreatment (such as pelletisation to improve handling and reduce the
amount of water being transported) or co-composting (to value-add to lower nutrient
organic waste streams such as green waste).
Over 85% of the total biosolids in Queensland are produced within 50 km of
Brisbane (Queensland Government 2017b) with 77,392 tonnes of biosolids (dry
8 Sustaining Queensland’s Agricultural Sector: Challenges and. . .
129
within the feedstocks or introduced via the processing has the potential to distribute
plastics into the environment, particularly when ‘beneficially applied’ to soils
utilised for the production of food. For example, analysis of a range of organic
products used for agriculture identified up to 24 microplastic particles (i.e. particles
under 5 mm) per kilogram, and as high as 895 microplastic particles per kilogram in
biowaste digestate (Weithmann et al. 2018). Although there is increasing research on
plastic and emerging contaminants, particularly those entering agroecosystems,
considerable uncertainty remains given the limited number of studies to date.
There has also been limited research to document and collect evidence relating to
the effects of these contaminants and what suitable loading concentrations may be
appropriate.
8.8 Biosolids Use
Raw sewage includes a supply of water and a solids component which is rich in
essential nutrients such as nitrogen, phosphorus and organic matter; and these are in
a form that is highly suitable for assimilation by plants (New South Wales 2000). As
such biosolids producers have been working with government regulators and end
users (agricultural sector) to reuse both the water and solids (biosolids) components
of sewage in a manner that is cost-effective, environmentally sustainable and safe
from a public health perspective. Sewage treatment plants are the main source of
biosolids in Queensland with production estimated anywhere between 30 and 50 kg
dry solids per equivalent person per day (equivalent to 150 kg of dry cake per
annum). The biosolids are produced as either a thickened slurry or a dewater cake
and, more recently, pellets; and contain useful quantities of organic matter and
nutrients such as nitrogen (N), phosphorus (P) and potassium (K) and lead to
improvements in soil characteristics such as improved microbial activities and
oxygen consumption. Beneficial use is now a prerequisite for disposal of biosolids
in many developed countries, which has necessitated formulation of guidelines to
prevent environmental contamination with heavy metals and pesticides, and infection of human and animal populations with pathogenic organisms, resulting in
different classifications (such as Grades A–E) to control the end uses for the material
(New South Wales Government 2000; Queensland Government 2016a). Whilst it is
commonly accepted that the utilisation of nutrients in biosolids at or below agronomic loading rates are highly beneficial, there is increasing concern related to
contamination of biosolids from both known and new sources and how the concentrations or leaching potential of these new contaminants will be impacted through
further pretreatment (such as pelletisation to improve handling and reduce the
amount of water being transported) or co-composting (to value-add to lower nutrient
organic waste streams such as green waste).
Over 85% of the total biosolids in Queensland are produced within 50 km of
Brisbane (Queensland Government 2017b) with 77,392 tonnes of biosolids (dry
8 Sustaining Queensland’s Agricultural Sector: Challenges and. . .
129