468 Organic Waste Recycling: Technology and Management
- Minimum slope is largely left to the investigator’s discretion. Where
no surface waters are endangered slopes as high as 30 percent have
been approved. Generally, however, the maximum allowable slope
is 12 percent and, in cases where sensitive waters are nearby,
maximum slopes may be held at 7 percent or less.
In general, pasture and grassland receive sludge applications during the
winter months; agricultural land growing seasonal crops receives sludge during
the warmer months, before planting or after harvesting. When weather prevents
sludge application, the sludge is stored in lagoons at the treatment plant.
Sludge is usually applied by haul trucks. If the application soil is too wet or
otherwise unsuitable for direct truck access, then the sludge is sprayed on to the
application site.
Analyses of the sludge-amended soil showed virtually no change in soil
chemical and physical characteristics. Groundwater from wells on or within 150
m of sludge application sites was sampled and analyzed before and after
application. Results showed no significant changes in groundwater quality over
a period of 3 years and now only selected wells are sampled approximately
every 3 years. Crop tissue sampling and analysis during the initial years of the
sludge application showed no significant difference between crops grown in
sludge amended soils and control crops.
Case study II: Canada
The Halton region in Ontario Province operates seven municipal wastewater
treatment plants treating over 220,000 m^3 /day of wastewater. The primarily treated
sludge is stabilized in anaerobic digesters and the stabilized sludge, rich in N, P and
organic matter, is produced at about 220,000 m^3 /year.
The sludge is applied on farmland (growing field crops, e.g. wheat) by tractors
equipped with the injectors for direct incorporation of sludge into the ground.
Another mode of sludge application by pressure spread on the field surface is also
employed. At the Halton Region, sludge application rate is based on NH 3 and NO 3 -
N loading which should not exceed 135 kg/ha during the 5-year period; this
practice also results in about 220 kg/year of P applied on the farmland in 5 years, a
cost-saving for N and P for the farmers.
According to the Ontario Ministry of Agriculture and Food (1993), the
application of this sludge on the farmland follows the provincial guidelines for
sewage sludge utilization on agricultural lands, and has proven to be safe.
Sludge is routinely analyzed for N, P, and heavy metal contents, and strict
control is maintained to prevent heavy metal overloading to the soils.