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Chair of Sanitary Engineering and Water Recycling
Optimisation of inlet structures in final
sedimentation tanks through scumboards with
variable height
In conventional final sedimentation tanks (FST), water
and sludge mixture is fed in a constant height. The
selection of the inlet height is very important, since
high structures cause a stringer density flow and thus a
hydraulic overloading of the tank, where low structures
lead to a stir in the bottom sludge. Ideal case would
be feeding the sludge directly into the water/sludge
separation line, yet the line can move depending on
the hydraulic state. This movement can be covered
only by a construction with variable height. With this
idea on mind, the project aims at investigations by
means of a scumboard with variable height.
Financing Institution:
Ministerium für Umwelt, Klima und Energiewirtschaft
Baden-Württemberg
Contact:
Prof. Dr.-Ing. H. Steinmetz,
Dipl.-Ing. RBM C. Meyer
Dr.-Ing. J. Gasse, D. Antakyalı, M.Sc.
Project partner:
Abwasserzweckverband Ermstal, Metzingen;
iat - Ingenieurberatung für Abwassertechnik GmbH,
Stuttgart;
Fa. Gröpler GmbH, Metzingen;
hydrograv GmbH, Dresden
Duration:
12/2009 - 04/2012
WWTP Metzingen was selected as pilot plant for this
project. There are two identical FST in the plant, and
one of them was equipped with a variable scumboard.
The other one remained without any change, to
enable a comparison. Hydraulic load for both tanks
was equal.
Within the past investigation period, a measurement
program mainly consisting of sludge index and sludge
level in the FSTs, as well as total suspended solids in the
effluent was conducted. Sludge level measurements
presented a clear difference between two tanks, the
one having the variable scumboard (FST 2) indicating
lower sludge levels. Also the sight depth indicated a
clear improvement in the FST 2. Suspended solids
measurements did not produce meaningful results.
This can be explained by very low suspended solid
levels in the effluent, in fact close to the detection
limit.
Scumboard with variable height, installed in final sedimentation
tank
A further way to determine the impact of the scumboard
was the comparison of tanks in case of an
overload. Through the past operational values, the
absolute maximum hydraulic load for the tanks was
known. During an overloading by 20% regarding the
absolute maximum, the variable scumboard prevented
the sludge outflow into the effluent for 90 minutes.
Hence the experiment produced very clear and visible
evidence on the positive impact of the variable scumboard.
More comparable experiments are planned to
strengthen the results.
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