384 Organic waste reuse and recycling: technology and management
return from marketable agricultural crops, exchange of wastewater for irrigation
purposes in arid climates to achieve overall water conservation, and
development and preservation of open space and green belts can also be
obtained from land treatment process.
Land treatment systems are less energy intensive than conventional systems.
(Examples of conventional systems include activate sludge, trickling filters and
aerated lagoons, etc.) In land treatment, energy is needed for transportation and
application of wastewater to the land. But in conventional treatment systems
such as activated sludge, energy is needed for transportation of wastewater,
mixing and aeration of wastewater and sludge, return sludge, effluent
recirculation, and transportation of digested sludge.
Since less mechanical equipments are needed for land treatment process
when compared with other conventional treatment processes, the maintenance of
the land treatment system is easy and less expensive.
However, land area, soil condition and climate are the main limiting factors
of this treatment process. Large area of land is normally required for application
of wastewater; for some cities, these land treatment sites may be too expensive
or too far away from the wastewater sources. Soil condition is important for the
removal mechanisms of wastewater constituents, and a soil having too coarse or
too fine texture is not appropriate for land treatment. Climate conditions where
the magnitude of evaporation and evapotranspiration is greater than that of
precipitation are generally preferred for land treatment of wastewater and
sludge.
8.2 Wastewater renovation processes
Depending on the rate of water movement and the flow path within the process,
the treatment of wastewater by land is classified as:
- Slow rate (or irrigation) process (SR),
- Rapid infiltration process (RI), and
- Overland flow process (OF)
Selection of process depends on the required objectives as well as the soil
condition of the land. Figure 8.1 gives the relationship between loading rate, and
soil type for the above three processes. A description of soil textural classes is
given in Table 8.1.
A comparison of design features for alternative land treatment processes is
shown in Table 8.2 and the expected quality of treated wastewater from land
treatment processes are given in Table 8.3. The site characteristics for the land
treatment processes are comparatively shown in Table 8.4. Excellent effluent
quality is usually obtained with land treatment systems (Table 8.3) because the