- Metropolitan areas today must dispose large volumes of
sewage sludge produced during the wastewater treatment process.
This research was conducted to study the effect of large applications
of municipal sewage sludge on (a) the distribution of N and trace
elements (Cd, Cr, Cu, Mn, Ni, Zn) in the soil profile, (b) the
uptake of these elements by plants growing on the sludge-treated soil,
and (c) the potential for groundwater contamination.
Five sludge disposal sites in the Willamette Valley were
selected for the project, Milwaukie, Eugene, Hillsboro, Forest
Grove, and Woodburn, Oregon. The soils were sampled quarterly,
July, 1974, December, March, and June, 1975, as a function of depth.
Surface soil samples (8 to 10 cores) were collected in September,
1974, from each disposal location to determine the uniformity and
amount of sludge applied. Plant samples from the disposal area and
water samples from wells adjacent to each area were also collected. The soil, plant, water, and sludge samples were analyzed for
total N, NH₄-N, NO3-N, Cd, Cr, Cu, Mn, Ni, and Zn; the water,
plant, and sludges were also analyzed for P. The soil samples collected
during the winter, spring, and summer, 1975, were analyzed
for NH₄-N and NO₃-N.
An estimated 500, 290, 96, 72, and 20 dry m tons /ha of sludge
were applied to the Eugene, Hillsboro, Forest Grove, Woodburn, and
Milwaukie disposal areas, respectively. The total N and P content of
the five sludges ranged from 3.9 to 6. 3% and 0.5 to 2. 9%, respectively.
The inorganic N was primarily in the form of NH₄-N. The
trace element (Cd, Cr, Cu, Mn, Ni, Zn) content of the sewage sludge
fell within general ranges reported for municipal sludges, except for
the Cr content (17, 700 ppm) of the Milwaukie sludge.
The Cd, Cr, Cu, Ni, Zn, NH₄-N, NO₃-N, and total N content
increased in the surface soil of each sludge disposal area. The Cd,
Cr, Cu, Mn, Ni, Zn, NH₄-N and total N content of the treated soils
compared closely to the control soils below 50 cm in the soil profile
suggesting restricted movement of heavy metals.
The NO₃-N content in the soil profile increased with the sludge
application rate. The NO₃-N level of the Eugene and Hillsboro disposal
areas was as high as 120 and 20 ppm, respectively. During the
winter and spring, 1975, increased rainfall and cooler temperatures
combined to decrease the NO₃-N content in the surface soil and
increase NO₃-N levels in the lower soil horizons.
The pH of the surface soil at the Eugene and Hillsboro disposal
areas decreased from pH values of 6.4 to 4.6 and pH 4.8 to 4.4, respectively, a result of the nitrification reaction. The pH values of
the other disposal areas compared closely to the control soil.
The N and Zn content of the grass growing on the sludge-treated
areas increased at the high sludge application rate compared to the
grasses growing in the control area. The Cd, Cr, Cu, Mn, Ni, and
P concentration increased in the grass sampled from the Eugene disposal
area, while the Cd, Cr, Cu, Mn, Ni, and P content of grasses
from the other sludge - treated areas compared more closely to the
grasses from the control areas. The lower pH at the Eugene disposal
area and the high sludge application rate combined to enhance the
trace element uptake by plants.
The NO₃-N, NH₄-N, P, Cd, Cr, Cu, Mn, Ni, and Zn content
in the water samples from the Eugene, Milwaukie, Forest Grove, and
Woodburn disposal areas were below the Public Health Service drinking
The long-term disposal of municipal sewage sludges on
agricultural land appears to be a viable waste disposal method, providing
the sludge application rate and metal content are not excessively
high. In any land disposal program for sewage sludge, the
heavy metal accumulation in the soil surface and plants growing in
the sludge-treated soil should be monitored.