Alexander Szabo and Oscar Engle - Svenskt Vatten

the case of waste water treatment ponds, the algae are the main producers of oxygen (Shilton et
al., 2005). Since the algae are only present in the upper part of a pond, it is difficult to achieve
aerobic decomposition through an entire pond. There are however more advanced pond systems
where algae are mixed through the water to oxygenate larger volumes. In mechanically aerated
ponds, oxygen is pumped or mixed into the water which will achieve high dissolved oxygen
levels. The disadvantage of mechanical oxygen support is that it consumes large amounts of
energy.
Aerobic degradation in waste water treatment ponds requires larger volumes and a longer
retention time then the conventional activated sludge-systems, because the concentrations of
active biomass, containing the microorganisms, are much lower.
Anaerobic degradation
When no oxygen is available, anaerobic degradation may occur by anaerobic microorganisms.
The benefit of anaerobic digestion is that it can deal with highly concentrated waste water and can
achieve good purification results within short retention times. The process is temperature
dependent and will, if properly designed, reduce the BOD concentrations by 40% at 10°C, 60% at
20°C and 70% at 25°C (Shilton et al., 2005). The anaerobic pond should be installed as the first
treatment step, when the load of waste water is the highest. The high load of organic matter
would inhibit the presence of algae. A typical anaerobic pond receives more than 3000 kg
BOD/ha/day, whereas facultative ponds should not receive more than 400 kg BOD 5 /ha/day in
order to sustain a healthy algal population (Mara, 2003).
Sedimentation
Large amounts of settleable and flocculated colloidal materials that enter the pond will settle to
the bottom of the pond and create a sludge layer as the water speed declines. Independent of
whether the particles settle in an anaerobic or facultative pond, the environment, within the
formed sludge layer, will be anaerobic. Even maturation ponds, which are at the last step of the
treatment, may produce a thin anaerobic sludge layer at the bottom. The sludge layer is broken
down anaerobically and thickens over time. Gases (methane and carbon dioxide) that result from
the degradation arises up though the water and escapes into the atmosphere. It is estimated that up
to 30% of the BOD load disappears as gas (Shilton et al., 2005).
Disinfection
There are several factors within a WSP that together contribute to the removal of illness-causing
bacteria, viruses, worms and protozoan parasites. It is however difficult to exactly evaluate the
contribution to the disinfection effect from a single factor, since the processes within a WSP are
both complex and dynamic. Factors that are known to contribute to disinfection are sunlight (UV)
exposure, pH, temperature, algal toxins, sedimentation and hydraulic retention time (Shilton et
al., 2005). The WSP:s are generally considered very good at removal of pathogens (Maynard et
al., 1999). Research about disinfection shows treatment results from 26 WSP:s around the world
(see Table H.1, Appendix H). The reduction performance of pathogens in WSP systems is
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