Land treatment of sludge 467exchange capacity is used to limit cumulative metal loadings added by sludge
application. However, if soil pH is less than 6.5, as it is in most of the Salem
area, then cumulative Cd addition is limited to 4 kg/ha, regardless of soil cation
exchange capacity. As the sludge generated by Salem is very low in metals,
application sites generally have a life well over 25 years.
Table 9.16 Typical characteristics of digested sludge at Salem, Oregon (U.S.EPA 1984)a
Characteristics Concentration
pH
Total solids (%)
Total Nitrogen (%)
Ammonium Nitrogen (%)
Phosphorus (%)
Potassium (%)
Zinc (%)
Copper (mg/kg)
Nickel (mg/kg)
Cadmium (mg/kg)
Lead (mg/kg)
Chromium (mg/kg)
Magnesium (mg/kg)
Calcium (mg/kg)
Sodium (mg/kg)7.3
2.5
10.3
5.9
2.0
0.96
980
470
43
7
230
60
200
12,200
3,000a All constituents except pH are reported on a dry-weight.
Each sludge site is investigated by the Oregon Department of Environmental
Quality, which makes recommendations on a case-by-case basis. General
guidelines for sludge application sites are as follows:
- Minimum distance to domestic wells = 61 m
- Minimum distance to surface water = 15 m
- Minimum rooting depth (effective depth of soils) = 0.61 m
- Minimum depth to groundwater at time that sludge is applied = 1.22
m - Minimum distance of sludge application to public access areas
varies with the method of sludge application:
a. If the sludge is incorporated into soil = 0
b. If the sludge is not incorporated into soil = 30.5 m
c. If the sludge is pressure-sprayed (‘big gun’-type sprayer)
over the soil = 91 – 152 m - Sludge application is not approved close to residential
developments, schools, parks, and similar areas.