Case study: a sense of security

DS Smith produces around 2.8 million tonnes of corrugated case materials and specialist industrial products annually.

In order to protect the environment and ensure compliance with regulations, DS Smith’s Kemsley Mill in Kent has to closely monitor the wastewater that it discharges into the Swale estuary.

It takes me a few minutes to change the probes on site and the job is done. It saves an awful lot of time.

Ian Castle, DS Smith's electrical control support engineer

This includes measuring the pH value and dissolved oxygen (DO) content of the water to ensure the levels are kept within strict parameters. In the past, maintaining the analytical instruments delivering these crucial measurements has been a time consuming and laborious process.

The DO measurement takes place in a lagoon where waste effluent is re-oxygenated before being transferred to settlement tanks and discharged to the river.

For health and safety reasons, any work at the lagoon has to be carried out by two people, making the on-site calibration of sensors a labour-intensive task.

“It was a waste of resources,” says Ian Castle, DS Smith Kemsley’s instrumentation and electrical control support engineer.

“Both of you could be out there for up to two-and-a-half hours per instrument. We wanted to see if we could do all that work in the lab instead and cut the amount of time spent at the lagoon.”

The same thought process was applied to the company’s pH sensors, as calibrating in the field was inconvenient and time consuming.

“You’d be sitting out in the middle of nowhere trying to stick probes in buffer solutions,” says Castle.

“You just had to hope the probe would stay in the beaker and that the calibration slope was ok, otherwise you’d have to put a new probe in and start all over again.”

When the company sought a better solution, it looked at Memosens digital sensors from Endress+Hauser, which could store all operation and calibration data in the head of the sensor. This meant they could be pre-calibrated and replaced in the field.

Because each sensor holds its calibration data, the sensors can also be rotated periodically, keeping the freshest sensor in the process while the other is cleaned, calibrated and allowed to rejuvenate in the lab or workshop.

Because Memosens technology offers noncontact transmission of the measured value from the sensor to the transmitter, the problems of corrosion, leaks and measured value distortion from moisture could also be avoided, says Castle.

DS Smith replaced all of its pH sensors used in the effluent treatment process with Memosens sensors and is now trialling the introduction of Memosens sensors for dissolved oxygen. The main benefit has been the efficiency gains, says Castle.

“With the other manufacturer’s pH probes it took perhaps an hour to clean and calibrate them,” he says.

“Now I can do this in the lab, in a nice clean location, using the Memobase Plus software.

“It takes me a few minutes to change the probes on site and the job is done. It saves an awful lot of time.”

The accuracy of the measurement has also improved, proven by the readings given by the two pH transmitters required for regulatory purposes.

“The two meters correlate very closely to one another, within about 0.02 pH,” says Castle.

“Our measurement didn’t used to be as accurate as that. It’s probably because we’re now able to calibrate in a much more controlled environment.”

Operating costs have also been reduced, says Castle, because a more common, cheaper buffer solution can be used and the sensors last longer.

The Memobase Plus software used to calibrate the sensors also helps with compliance, as all calibration records are stored digitally.

“That was another big plus,” says Castle. “I’ve got a record of all my calibrations and we know that it’s there should the Environment Agency want to take a look.”