Organic Waste Recycling

Land treatment of wastewater 385

BOD 5 loadings applied (Table 8.2) are much lower than the BOD 5 removal
capabilities. Design of a land treatment system is based generally on hydraulic
application rates and nitrogen loadings. However, BOD 5 loading to a land
treatment site needs to be carefully evaluated so that anaerobic conditions do not
occur. The occurrence of anaerobic conditions is usually harmful to the root
system of crops and affecting the availability of nutrients to the crop; it may also
enhance mobility of toxic materials previously precipitated there.

Table 8.1 Soil texture classes and general terminology used in soil descriptions (U.S.
EPA 1981)

General terms

Common names Texture

Basic soil textural class names

Sandy Soil Coarse

Moderately coarse

Sandy

Loamy sand

Sandy loam

Fine sandy loam

Loamy soils Medium

Moderately fine

Very fine sandy loam

Loam

Silt loam

Silt

Clay loam

Sandy clay loam

Silty clay loam

Clayey soils Fine Sandy clay

Silty clay

Clay

8.2.1 Slow rate process (SR)

SR is the controlled application of wastewater to a vegetated land at a rate of a
few centimetres of liquid per week. The flow path depends on infiltration, and
usually lateral flow within the treatment site. Treatment occurs by means of
physical, chemical and biological processes at the surface as the wastewater
flows through the plant-soil matrix. A portion of the flow may reach the
groundwater, some is used by the vegetation, but off-site runoff of the applied
wastewater is avoided by the proper system design. Typical hydraulic pathways for
SR treatment are shown in Figure 8.2 (a) in which surface vegetation responsible
for evapotranspiration and soil percolation is essential components in this treatment
process. The percolated water can be collected through underdrains placed under
the vegetation soil or from the recovery wells constructed within the vicinity.