Land treatment of wastewater 3978.2.3 Overland flow process (OF)
A schematic view of overland flow treatment including a pictorial view of a
typical OF sprinkler system is shown in Figure 8.4.
OF is a treatment method in which wastewater is applied over the upper
reaches of sloped terraces and allowed to flow across the vegetated surface to
runoff collection ditches. The objectives of OF process are: (1) to treat the
wastewater to a degree comparable to that of secondary or tertiary treatment and
(2) to produce forage grasses or preserving greenbelts and open space. OF is
subject to the same type of limitations as SR, but it can be applied to relatively
impermeable soil and a gently sloping terrain. The technique has considerable
potential for treatment of municipal wastewater. The effluent is of a quality
approaching that from tertiary treatment. In addition to a low construction cost,
the system produces little or no sludge, which is convenient for the system's
operation and maintenance.
Operating costs are considerably lower than the conventional and advanced
waste treatment because of the relative simplicity of operation. It has the
advantages of avoiding groundwater degradation, providing economic return
through the growth and sale of hay, and providing a high quality effluent
suitable for industrial or agricultural reuse. Sodium effects (discussed in section
8.3) on overland flow systems are less critical because the infiltration rates of
the soils are low.
Reliability
Overland flow systems at various places using wastewater have been monitored
to determining removal efficiencies. The expected ranges based on results at
these sites are BOD 5 and SS removal of 95 to 99%, nitrogen removal of 70 to
90%, and phosphorus removal of 50 to 60% (see Table 8.3). Biological
oxidation, sedimentation, and grass filtration are the primary removal
mechanisms for organics and suspended solids. Nitrogen removal mechanisms
include crop uptake, biological uptake, denitrification, and fixation in soil.
Phosphorus is removed by adsorption and precipitation.
Site selection
Soils with limited drainability, such as clay and clay loams are suitable for OF
system (Figure 8.1). The land may have a slope between 2 to 8% and a very
smooth surface so that the wastewater will flow in a sheet over the ground
surface. Grass is planted to provide a habitat for the bacteria to stabilize the
wastewater and for further use as animal feeds. Because groundwater will not
likely to be affected, it is of little concern in site selection.