Environmental Engineering Reference
In-Depth Information
named lagoons. Recommended solids content of the sludge is 2% to 5%. When full, they
are either covered or dredged. High disposal rates, ranging from 9100 to 28,400 m
3
/h, are
normal.
The land application of sludge in the case of beneficial use has the advantage of
providing plant nutrients and improving soil fertility, in addition to the main objective to
dispose of it. Macro-pollutants such as nitrogen, phosphates and potassium, and micro-
pollutants such as Cu, Fe and Zn, are undesirable characteristics in the case of effluent
discharges, but at the same time they are essential for plant growth. The organic matter
improves the soil structure, enhancing the water holding capacity, cation exchange
capacity and aeration. However, the land application of wastewater could be a potential
source of surface and groundwater pollution, phytotoxicity, salinity, and change in soil
structure and could be a potential health hazard if polluted surface or ground water is
used for drinking, washing or recreational purposes. In order to prevent such scenarios,
proper design, operation and maintenance of sludge disposal sites are essential. Metcalf
& Eddy (1991) provide the following steps in the development of sludge application
systems:
• Characterization of sludge quantity and quality;
• Review of pertinent federal, state and local regulations;
• Evaluation and selection of site and disposal option; and
• Determination of process design parameters - loading rates, land area, application
methods and scheduling.
3 REGULATING THE PRACTICE OF SLUDGE LAND DISPOSAL
3.1
Hydraulic loading rates
In cases when the sludge is applied to land in a wet form, one of the main design
parameters, which would determine the necessary size of the field required for sludge
disposal, would be the hydraulic rate of sludge application. The following factors would
affect the recommended hydraulic rates:
• The nature of soil - sandy, loamy overlaying gravel soils could have higher hydraulic
rates than fine clay soils.
• The nature of sludge effluent - the concentration and level of treatment of sludge would
affect hydraulic loads, with higher loads recommended for a higher level of treatment
of the sludge.
• Climatic conditions - in a dry and hot climate, hydraulic loads could be much higher
than in a wet and cold climate.
• Vegetation cover - hydraulic loads are higher for land with no crops, compared to cases
of a beneficial use of sludge for crop irrigation when restrictions are imposed with
respect to the water requirement of the specific crop grown. However, in cases of
pastures or indigenous vegetation, evapotranspiration rates are higher, compared to
bare lands, thus allowing for higher hydraulic loads.
