Alexander Szabo and Oscar Engle - Svenskt Vatten

2.2 Pretreatment
Screening
The purpose of a screening device, which is located before the treatment pond, is to remove
coarse materials like pieces of plastic and woods etc. The material caught and removed from the
screen is often of inorganic characteristics and hard to break down biologically in the treatment
pond. The coarse material can also damage pumps and aerators used in the subsequent treatment
stages.
It is recommended that the water velocity through the screen should be less than 1 m/s so coarse
material will be able to stick between the screens (Mara, 2003). The space between the bars
should be between 15-25 mm (Mara, 2003). If the flow is above 1000 m 3 /day mechanical screens
which automatically remove the captured objects should be used instead of screens that have to
be maintained manually. This is because mechanical screens can be cleaned more frequently. It is
although wise to have a screen that could be cleaned by hand as a back-up for the mechanical
screen in case the mechanical screen would stop working. The waste products collected from the
screen can be taken to the nearest landfill or be incinerated.
Grit removal
Grit is made up of heavier and smaller particles (often of inorganic characteristics) like glass,
sand eggshells etc. Grit can damage pumps and aerators in the subsequent treatment steps and
should be removed before the treatment pond if the waste water contains large quantities.
Before water enters the screen and the treatment pond, the waste water may flow in long grit
channels where the water passes through at a desired velocity. The heavier grit particles will settle
in the channel as the waste water flows through. The grit channels are made up of two parallel
channels so one of the channels can be closed for grit removal. A possible problem with a grit
channel is that it is difficult to keep the velocity at a constant level (Mara, 2003).
2.3 Pond Types
Anaerobic ponds
Anaerobic ponds operate without the presence of algae or oxygen, and have an advantage over
the facultative pond since they can deal with higher organic loading. They can reduce the organic
load by 40 to 70% with a retention time of only a few days (Shilton et al., 2005).
Methane (CH 4 ) and sulphide (H 2 S) are gasses that are produced under anaerobic conditions.
Methane gas may be considered as a fuel resource if collected, but if not collected as a
greenhouse gas, contributing to the climate change as it escapes into the atmosphere from the
pond. If sulphide is produced and released, it is not appreciated since it is releases a bad odor. If
the anaerobic pond is not properly design, or when overloaded, the release of sulphide may be
problematic for people living in the surroundings (Mara, 2003). This is confirmed by a study at
Lee Summit WSP, Missouri, where odor nuisance occurred after the load was increased above the
pond capacity (McKinney, 1968).
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