PuK - Process Technology & Components 2024

Pumps and Systems
Peristaltic pumps
One of the central treatment steps
from raw water to drinking water is
flocculation to eliminate finely dispersed,
difficult to remove colloidal
particles and humic substances that
could cause turbidity, as well as microorganisms.
By adding flocculants,
the electrostatic repulsion of the particles
and dissolved substances can
be overcome. They then bind together
to form larger flocs that are easier
to remove. This means that organic
and mineral particles can be safely
filtered out.
“We use the flocculant ferric chloride
(FeCl 3
) as an aqueous solution with
a concentration of 40 %. It is dosed
into the supply lines to our four 300 m³
mixing and reaction basins,” explains
the water technician at Thüringer
Fernwasser. “Static mixers ensure the
necessary turbulence in the water directly
at the metering point, and flocculation
then takes place in the basins.
The flocs are then retained in a total
of 14 open multi-layer filters. Anthracite,
quartz sand and gravel are used
as filter material. Flocculant additives
and activated carbon can be added if
required. The retained flocs are processed
and used as dewatered sludge
in biogas production.”
Fig. 2: Flocculation is one of the central steps in the purification process of turning raw water
into drinking water and allows the filtration colloidal particles and humic substances.
Metering pumps for efficient
flocculation
The flocculant metering system is at
the heart of the flocculation process.
The metering pumps convey the ferric
chloride from the siphon vessels
of the storage tanks to the mixing and
reaction tanks. One metering pump is
in continuous 24/7 operation for each
mixing and reaction tank. For optimum
flocculation and filtration, the
metering quantity is adjusted to the
respective flocculant requirement; as
a rule, the pumps each dose at a rate
of around ten litres per hour, which
corresponds to a daily flocculant consumption
of around 1,000 litres of
ferric chloride solution.
Frequent maintenance of
diaphragms necessary
Diaphragm metering pumps, which
are standard in many metering stations,
were initially used in the new
Fig. 3: Check the dosing concentration at the outlet into the mixing and reaction basins
metering system installed in 2017.
However, after just a few months, it became
apparent that this type of pump
could not offer the reliability and longevity
required for continuous use. “After
just a few months, numerous, often
lengthy repairs became necessary.
The diaphragms had to be replaced approximately
every three months,” reports
the water technician. “While this
maintenance could at least be carried
In order to ensure uninterrupted operation
of the flocculation in all four
mixing and reaction basins, in addition
to frequent maintenance and repair
work, further measures became
necessary. “We had two more diaphragm
pumps than planned in stock
as spare pumps and at some point,
even had to bring in other pump
technologies as an additional solution,”
she recalls.
out in-house, we often had to hire an Increasingly dissatisfied with
external company for frequent repairs.
The manufacturer of the diaphragm
pumps advised us to reduce the stroke
length - unfortunately, this did not lead
to any improvement either and this
the unreliability of the diaphragm
pumps and the associated high
operating costs, and with no prospect
of fundamentally improving the
situation with the existing equipment,
measure was also associated with a reduction
the Thüringer Fernwasser
in the maximum possible metering
quantity,” adds the engineer.
team joined forces with an engineering
firm to look for a better alterna-
34 PROCESS TECHNOLOGY & COMPONENTS 2024