Environmental Engineering Reference
In-Depth Information
recommended depth for facultative ponds is between 1 m and 1.2 m, in order to allow for
a relatively wide aerobic zone, where the major organic reduction occurs.
Algae play a significant role in the purification process, as shown in Figure 2.5, by
acting in symbiosis with aerobic bacteria, consuming the end products of their activity
and releasing oxygen in addition to the one supplied by reaeration from the atmosphere.
The design of facultative ponds is based on empirical values for recommended organic
surface loading rates, expressed as kg BOD or COD/ha.day, which are strongly
dependent on the average temperature conditions (Feachem et.al. 1977). The usual
retention time of the water to be treated in the pond varies from 7 to 30 days, which
explains the large area requirements.
Maturation ponds serve as a polishing step in the treatment process and provide
mainly for pathogen removal. Empirical data from existing pond systems show a
considerable pathogen removal efficiency, the second higher after disinfection. The
process mechanism is not well studied but many authors mention the action of sunlight as
the main disinfection agent. For this reason the recommended depth of maturation ponds
is between 0.75 m and 1.2 m. Maturation ponds are usually located as a last stage of the
pond system, after the facultative ponds, and it is strongly recommended that the ponds
configuration should allow for several maturation ponds to be located in series (Horan
1990).
In general, stabilization ponds are robust treatment facilities, which require low
operation cost and skills, and have a considerable buffer capacity under variable load
conditions, which makes them suitable for runoff treatment. Their capacity to reduce
nutrients concentrations is relatively low-up to 50%, and they retain heavy metals
pollution, mainly through sedimentation.
3.3.3
Wetlands
Wetlands are shallow ponds, covered with intense vegetation. Natural wetlands are
considered to be receiving water bodies and are subject to regulatory instruments to
control their quality and maintain a healthy environment. If polluted runoff is to be
discharged and treated into a natural wetland, it should comply with these regulations.
Typical characteristics of natural wetlands include:
• The depth of water may vary throughout the year or be absent for some time.
• The saturation with water is the dominant factor determining the nature of soil
development and the types of plant and animal communities living in it.
• Often they might be found in the boundary between terrestrial and truly aquatic systems
(rivers or lakes).
Man-made wetlands are specifically constructed treatment structures, similar to the
natural ones. However, selected hydraulic enhancements are provided in order to control
the inflow and the hydraulic conditions, and in addition, to provide for certain retention
time for biological reaction processes to take place. They have the same advantages as
ponds, but have even higher land requirement, as their depth is lower. One specific
advantage of wetlands, compared to ponds is their significant capacity of nutrients
removal due to vegetative nutrients uptake. Therefore, their application for runoff
treatment is strongly recommendable in cases of nutrients rich runoff.
