Advanced Ecotechnology for Decentralized Wastewater Treatment and Reuse - Restorative Redevelopment of Devastated Ecocultural Landscapes - page 391

SOIL
FIGURE 19.8 Surface-flow wetland section schematic. Depth is typically 0.3 m or less. The
underwater plant thatch surface is large and biologically active. Both settling and predation
mechanisms for pathogen removal are strongly present.
Pathogen removal is well documented in surface-flow systems (Kadlec and
Knight 1996). Removal mechanisms are essentially the same as in lagoons. Based
on settling theory and known pathogen removal rates, a surface-flow wetland system
could meet WHO reuse standards as a final treatment stage.
Surface-flow wetlands are best suited to a final treatment stage characterized by
aerobic conditions. If loaded with raw wastewater, anaerobic conditions will become
established, creating an ideal mosquito (disease vector)-breeding habitat. A posi-
tive dissolved oxygen concentration favors predators of mosquito larvae. Suitable
pretreatment, such as a mechanical system or a subsurface-flow wetland, must be
part of the overall design for a surface-flow wetland to be part of a reuse system.
The most common type of wetland treatment system is a subsurface-flow wet-
land treating effluent from a septic tank (Figure 19.1) (Wallace and Knight 2006).
There are many advantages to this type of treatment. Septic tanks provide primary
treatment for raw wastewater. Wastewater flows through aggregate underneath the
surface of the wetland (Figure 19.9), preventing the creation of mosquito-breeding
habitat. The treatment area requirements are much smaller than those of surface
wetlands (Table 19.1). Treatment is passive, requiring no energy inputs. Most impor-
tantly, subsurface-flow wetlands are effective at removing Ascaris and other para-
sitic nematode eggs (Stott, May, and Mara 2003). Removal of bacteria pathogens to
WHO standards (1,000 colonies of fecal coliforms/100 ml effluent) by subsurface-
flow wetlands has been well documented (Kadlec and Knight 1996).
Wastewater lagoons and surface- and subsurface-flow wetlands can all be passive
technologies working on gravity alone. The advantage of a purely passive system
is simple and low-cost operation. A need for more advanced treatment or smaller
treatment areas may not allow a purely passive system (Table 19.1). Local topog-
raphy may not support a purely passive system and may require some pumping to