Articles & Case Studies
Carlisle's solid sludge solution
Posted: Wednesday 21st November 2012
United Utilities wastewater treatment
works at Willowholme, Carlisle, treats
the equivalent of 100,000 people’s
waste, 24 hours a day. The resultant
solid waste is treated to make it
suitable for return to the environment.
The situation
In the final stage of the solid waste
treatment process the collected sludge is
dosed with lime in order to kill pathogens
and produce a sludge that is suitable to
be dried in centrifuges. The process is
required to meet targets in three areas,
namely the throughput, quality of centrate
and the quality of the cake produced.
The ability to meet the target qualities
is dependent upon a number of factors
which include the precise measurement
of the pH within the lime dosing tanks.
Historically, this has proved to be an
extremely diffi cult measurement to
perform, with previous efforts failing to
provide the accurate control of the lime
dosing required. The pH of the dosed
sludge has to reach a set level to ensure
full pathogen kill and to ensure the
correct consistency of sludge is sent to
the centrifuges. Get the pH wrong and
you risk low pathogen kill, increased
costs due to overuse of lime or decreased
effi ciency of the centrifuges. If the sludge
going through to the centrifuges is too
heavily dosed, a large degree of manual
intervention is required and the sludge
has to be sent back to the works inlet. If
the pH is too high the team are unable to
process it.
Due to the unreliable measurements
generated with the existing systems, the
team at Willowholme had never been able
to meet all three targets simultaneously. As
a result they have reverted to carrying out
manual checks rather than relying on the
installed pH equipment – which has led to
a reduced time window for processing the
sludge. The requirement was to start lime
dosing automatically in the early hours of
the morning so that the sludge was ready
to be processed when the operations staff
arrived on site. However, as the dosing
was being controlled using manual pH
checks, they were unable start processing
the batch until site was manned.
The solution
Endress+Hauser Ltd was approached by
the contractor on site to investigate a
solution to this problem. Having obtained
further details, the Topcal pH system was
proposed. Topcal is a fully automated
self-cleaning and self-calibrating pH
system. It is fully confi gurable to suit the
conditions and processes into which it
is being installed, allowing for different
cleaning times, variable calibration
intervals and the option to hold the sensor
out of the process if required. Specifi ed
in conjunction with the appropriate pH
sensor and a retractable holder, two
systems were supplied.
The system was installed by the contractor
and commissioned by Endress+Hauser
staff before being monitored and handed
over to United Utilities in October 2011.
Benefi ts to the customer
When initially installed, the controller was
set up to calibrate the sensor on a daily
basis. This interval was soon extended
to three times a week, with calibrations
taking place on Sunday, Tuesday and
Thursday nights, a couple of hours prior to
the batching and dosing of the sludge.
During the trial period, manual checks
were made of the pH against the Topcal
unit, to prove the reliability and accuracy
of the system and to provide the operators
confi dence in its ability. In addition, after
the systems have been handed over to
United Utilities, the site operators have
carried out additional sporadic manual
checks to further enhance their confi dence
in the Topcal system. Throughout their
time in operation, both units have proved
to be extremely reliable, and accurate
against the checks. According to United
Utilities, the real advantage has been
the saving in man-hours that has been
achieved at Carlisle because the team’s
confi dence in the system allows for
autonomous control of the process.
The result is that the entire lime dosing
process now operates automatically, with
the dosing controlled via the Topcal pH
measurement at a time to allow maximum
throughput. But what about those targets?
Since installation of the Topcal system,
all three targets, namely throughput,
centrate and cake quality, are being met
simultaneously and consistently.
Top marks for Topcal pH system
Topcal system automates wastewater treatment pH dosing
