Environmental Engineering Reference
In-Depth Information
systems, which resemble closely the biological processes in a natural water environment.
The reactions involved are complex and mutually interrelated, and for this reason the
most commonly applied design techniques are based on empirical data. The design
parameters of highest importance are the organic loading rate, the water depth in the pond
and the hydraulic retention time, which would allow for the treatment processes to take
place in an appropriate environment.
The main processes taking place in an AP are the sedimentation of suspended material
and the anaerobic digestion of accumulated sludge, together with partial bio-degradation
of the suspended organic material in the wastewater. In general, these are the reactions
involved in a septic tank, thus AP could be considered as large open septic tanks. In order
to provide anaerobic conditions within the whole volume of the pond, AP are designed
for high organic loading rates and as relatively deep structures, with a recommended
depth between 2 and 5 m. The organic loading is given as a volumetric loading rate, e.g.
kg BOD 5 /m 3 , which would depend on the average annual air temperatures. The required
retention time is 3-5 days (Mara & Cairncross 1989). A well functioning anaerobic pond
would be characterized by a vigorous release of biogas, as the end product of the
anaerobic process. The biogas bubbles, formed at the bottom of the pond, would rise and
will be released in the atmosphere. They would contribute to the mixing of the pond and
would seed the upper layers with active microorganisms, which would contribute to the
anaerobic degradation of the wastewater with corresponding partial reduction of BOD 5 .
In facultative ponds, both aerobic and anaerobic biological processes take place in
addition to the sedimentation of decaying material. The major process of importance
during the wastewater treatment is the biodegradation of organic material (reduction of
BOD 5 ). The vast majority is decomposed by aerobic microorganisms, present in the
facultative layer, with the active involvement of algae, as shown in Figure 2.4 (Chapter
2). Therefore, the presence of DO and sunlight are crucial for a successful treatment
process together with adequate temperature and pH conditions. DO is supplied by natural
means through reaeration and through the algae respiration. Based on this, the pond is
usually designed to have a relatively low depth (between 1 and 2 m) and a large area, to
ensure a good reaeration rate due to a large contact area between the water and the air
surface. The low depth also allows the sunlight to penetrate through a large portion of the
reactors' volume. Organic loading rates are expressed as surface loading rates, e.g. kg
BOD 5 /m 2 , which would depend on the average annual air temperatures. The biological
degradation processes show a distinctive diurnal variation due to the photosynthetic
activity of algae. During the day, the upper 50 cm layer of the pond is rich in DO due to
the considerable contribution of oxygen as a product of the algae respiration. The oxygen
rich conditions have their peak concentrations during midday, and this would be the time
when the rate of the biological oxidation would be at its maximum. Due to the high
intensity of the rate of photosynthesis, and the utilization of carbon dioxide by algae, the
pH in the pond could rise to values of 8 to 9. During the night, algae switch from
photosynthesis to respiration and start utilizing the available DO, thus reducing the
reaction rate of the aerobic biological process. In the same pond, the sludge accumulated
at the bottom would undergo anaerobic degradation, leading to the mineralization of the
organic fractions. During the night, the anaerobic portion of the ponds' volume would
increase. This type of fluctuations would be more pronounced in PF, which usually are
subject to considerably higher organic loads, compared to SF. In general, due to the
Search WWH ::




Custom Search