Environmental Engineering Reference
In-Depth Information
The sludge effluent has to be applied intermittently since lands require some rest at
intervals. Also, for the conditions of the South African region, hydraulic loads would
differ during the wet and dry season. Then the size of the land to be dedicated for this
purpose would be determined by the recommended loads for the wet season, considering
the additional load from rainfall and the corresponding evapotranspiration rates. It should
be noted that the determination of the size of the area dedicated, should also consider the
need for intermittent irrigation by applying the rotational principle. It consists of dividing
the area in plots and irrigating a prescribed number of plots only, allowing the rest of the
plots to absorb the pollutants, infiltrate the moisture and recover until the next portion of
sludge is disposed on the same plot.
Hydraulic loads are an important design parameter to be considered during the design
stage and controlled during the operation of sludge land disposal sites, but they should
always be applied in conjunction with the pollutant loading rates. The final loading rate,
which would be used for the design of the field, should be based on the limiting factor for
each specific case. If the pollutant absorption rate is lower than the rate of application,
which has been calculated based on the hydraulic rate, then it would be the limiting factor
and the hydraulic application rate should be reduced.
In South Africa, the level of treatment of sewage sludge, which would influence the
hydraulic loads, determine four types of sludge: A, B, C and D in a decreasing order of
potential to cause odor nuisance and fly-breeding as well as the potential to transmit
pathogenic organisms to man and the environment (Lotter & Pitman 1997).
• Type “A” Sludge: Unstable with a high odor and fly nuisance potential; high content of
pathogenic organisms.
• Type “B” Sludge: Stable with low odor and fly nuisance potential, reduced content of
pathogenic organisms.
• Type “C” Sludge: Stable with insignificant odor and fly nuisance potential, containing
insignificant numbers of pathogenic organisms.
• Type “D” Sludge: It is similar in hygienic quality as type C, but is applied for an
unrestricted use on land at a maximum application rate of 8 dry tones per hectare per
year. The metal and inorganic content are limited to acceptable levels.
Type “A” sludge is not allowed to be applied on pastures with grazing animals, types “B”
and “C” sludge could be applied but should be mixed or covered with soil, whenever
possible.
3.2
Pollutants loading rates
Pollutants loading rates reflect the mass of pollutant constituents, which will be directed
to the designated site during a given time interval. They are recommended, based on the
absorption capacity of the soils and the plant uptake of the vegetation cover or crops, in
the case of beneficial applications. The pollution loading for each specific case is
determined by the mass (volume) of the sludge and the specific constituent concentration.
Usually, the limiting constituents are toxic metals, which could be accumulated in the soil
during the exploitation period of the site. The determination of the necessary land
requirement for any disposal site is based on the selection of the pollution load of a
critical parameter, which would lead to the maximum land requirement. In cases of
