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

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