Alexander Szabo and Oscar Engle - Svenskt Vatten

Facultative Ponds
The main features of the facultative ponds are that they are horizontally divided into two layers,
with and without oxygen presence. In the top layer (down to 20 - 30 centimeters of depth)
photosynthesis makes micro-algae produce oxygen during daytime, the oxygen is then used by
aerobic bacteria to degrade organic substrate. Some of the oxygen in the top layer has its origin
from surface mass transfer, but it has been estimated that more than 80% of the oxygen is
produced by algae (Shilton et al., 2005). The depth of oxygen presence in the water is changing
and depends on many factors such as sunlight, water turbidity and organic loading (Shilton et al.,
2005). In the bottom of the pond, no dissolved oxygen is available, which gives anaerobic
bacteria opportunities to degrade substrate. The retention time of facultative ponds is relatively
large, up to several weeks. The long retention time and the shallow design, typically depth is 1.5
meters, make the ponds consume relatively large land areas (Mara, 2003).
Since the light needed for the algae arrives through the surface, facultative ponds are usually
designed by surface loading. In temperate climates, algae are more productive in terms of oxygen
production and the pond may therefore receive a higher loading of BOD. Eq. 2.1 (Mara, 2003) is
used to calculate the maximum possible BOD loading per area:
λ = 350∗(rtial-mix system (EPA, 2002). Since the aeration in a complete mix pond keeps large
quantities of suspended solids mixed in the pond, a following post-settling pond should be
installed. Partial-mix aeration may be installed into an existing facultative pond to improve the
overall treatment efficiency.
2.4 Degradation processes
Removal of pollutants in Waste water treatment ponds
The nature is capable to degrade human waste water when it is released into waterways. Problems
occur when the load on our natural environment is bigger than the self-cleansing mechanisms
available in our lakes and rivers. The more populated the world becomes, the higher the stress on
our environment. The waste water treatment ponds are cleaning the water using the same
processes as in nature. The task of the waste water treatment ponds is to optimize and isolate the
processes, so that the cleaning has been done before the water is released in our waterways. In
nature several processes are responsible for the treatment of polluted water, and these processes
should be used and optimized as much as possible. Some of nature’s processes will be discussed
in this section.
Aerobic degradation
When oxygen is present in water, aerobic microorganisms will use oxygen to oxidize organic
compounds and produce carbon dioxide (CO 2 ), water and biomass (sludge). Since oxygen is
consumed by the degradation, the process will be limited by the presence of oxygen. If large
quantities of organic material are to be degraded, large amounts of oxygen must be supported. In
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