466 Organic Waste Recycling: Technology and Management
9.3.3 Monitoring program
Since the design is normally based on limiting both N and Cd, monitoring for
these two parameters should not be necessary. Routine soil testing every 2 to 4
years for plant available P and K and to determine lime requirements for pH
maintenance at 6.5 is all that should be necessary.
The schedule for sludge applications will depend on the type of crop and on
the climate for the area. Sludge is not usually applied when the ground is
flooded or frozen to reduce risk of runoff contamination. Sludge can be applied
to the fields for row crops prior to planting and after harvest. Sludge application
to forage grasses is possible in all months of the year when the ground is not
flooded.
Periodic sludge analyses are needed to provide nutrient and heavy metal
concentrations so that rates of application can be determined to meet crop
nutrient needs, and total heavy metal loadings can be recorded from year to
year.
9.3.4 Case studies
The followings are two case studies of agricultural application of sludge in the
U.S.A and Canada.
Case study I: U.S.A
The city of Salem, Oregon, generates 121,120 m^3 /year of anaerobically digested
sludge containing 2.7 percent solids, and 90 – 95 percent of this sludge is
recycled to local farmland (U.S.EPA 1984). Typical sludge characteristics after
digestion are shown in Table 9.16. The sludge N levels are raised by the
addition of ammonia during the treatment processes because the raw sewage
contains a high percentage of food processing waste, which are deficient in
nutrients. The sludge is applied to about 1,200 ha of local agricultural land. At
virtually all application sites, the sludge is applied only once per year. Sludge
application rates are based on the N needs of the crop and the nutrient content of
the sludge. They vary from 2.2 to 6.3 dry tons/ha, averaging approximately 3.4
dry tons/ha.
The sludge is applied primarily to fields used to produce grains. Sludge-
amended sites are also used to produce seed crops, Christmas tree farms,
commercial nurseries, and filbert orchards. No sludge is applied to fruit and
vegetable crops. For poorly drained soils, sludge can be applied from April to
October. For well-drained soils, sludge can be applied anytime except during or
immediately after rainstorms. Schedules for application of sludge to soils with
intermediate drainage capacity fall between these two extremes. Cation