Sleepless in Macclesfield

A Cheshire wastewater treatment works was picked to trial a Hach Lange ammonia monitoring system following a history of illegal industrial discharges, which had caused problematic influent. But, at first, the system caused a few sleepless nights.

New ammonia probes with sophisticated controllers have enabled staff at United Utilities’ (UU) Macclesfield wastewater treatment works (WwTW) to ensure that the plant does not exceed its discharge consent.

Following installation, the monitoring systems initially generated around 40 alarm texts a day – mostly at night – but significant improvements have now been implemented.

The Macclesfield WwTW is relatively large (PE 87,000) and treats industrial and domestic waste from a large area drawn from a network of pipes and drains that extend for hundreds of miles. Historically, the industrial waste influent at the plant has been most problematic, particularly as a result of illegal discharges from unknown sources that have resulted in highly variable ammonia levels. And, in April 2007, these caused the plant to exceed its discharge consent.

Environmental improvement is fundamentally important to UU – the company recognises that its investment in wastewater treatment is a major contributor to enhancements in river and coastal water quality.

So, it is clear that reliable control of discharge quality is extremely important at Macclesfield. And, for this reason, the site was chosen to trial a Hach Lange monitoring system that can issue instantaneous alarms by SMS text message to key UU staff.

Barry Sherwood, Macclesfield’s process controller, says: “This site was chosen because of its strategic importance and because we were anxious to gain a higher level of understanding of the ammonia levels throughout the plant.”

Initially, an ammonium probe, the NH4 sc, was installed about 50m downstream from the inlet with a further four probes strategically placed around the site. Each probe was connected to a Hach Lange SC1000 Controller and each controller was fitted with a SIM card so that key staff would receive text messages should an alarm condition arise.

The inlet alarm was set at 45mg/l, the outlet set at 3mg/l and the process probes were set at 25mg/l. Sherwood reports “around 40 to 50 text messages per night” when the units were first installed. However, quick and easy access to reliable data has enabled him to identify the sources of the peaks. He says: “It is now down to one or two text messages per day, which is great news for me, the company and my family.”

Studying trends

Data from the monitors is available on the internet via a dial-up connection, which is soon to be upgraded to broadband. This means that Sherwood and his team can study trends at any time from anywhere, even via a BlackBerry. As a result, staff can check if an alarm was the result of a temporary peak or something more serious.

In addition to the increased ability to identify illegal discharges, the alarms also help to run the plant efficiently and maintain discharge quality below consent levels. For example, inlet alarm conditions can be dealt with in a number of ways including flow management, dilution, chemical dosing and dormant tanks.

The NH4D sc sensor is designed to provide continuous trending of ammonium levels to control the nitrification process in the aeration tanks and lanes of wastewater treatment plants with less than 30% industrial waste. Included in the NH4D sc sensor is a differential pH electrode as the reference electrode, which provides stability, a temperature sensor to compensate for temperature effects, and a potassium ion-selective electrode (ISE). The most significant potential interference in wastewater matrices is from potassium ions (K+) and the NH4D sc sensor compensates for this by using the potassium ISE to correct the ammonium value.

The sensor detects ammonium ions (NH4+) directly in the process as ammonium nitrogen (NH4-N). Potential interferences are further reduced using Cartrical technology that calibrates each electrode individually and also calibrates the three electrodes to each other.

The SC 1000 Multi-parameter Universal Controller is a modular system consisting of a Display Module and one or more Probe Modules.

Each SC 1000 Probe Module provides power to the system and can accept up to eight digital sensors. However, the Probe Modules can be networked together to accommodate many more sensors attached to the same network. The SC 1000 Display Module controller has a large colour touch-screen display. Hach Lange says this is an intuitive, easy-to-use interface that can be used for any number of parameters. One Display Module controls either a single Probe Module or a number of Probe Modules connected by a digital network. The Display Module is portable and can be disconnected and moved anywhere within the network.

The Controllers at Macclesfield are only connected to the ammonium probe but have the potential to accept inputs from other sensors. For example, Sherwood is about to take delivery of six optical dissolved oxygen probes, the LDO, which will also connect to the SC 1000.

Lab results

Commenting on the monitoring equipment, Sherwood says: “Hach Lange staff came to commission the instruments and to train our own engineers, but the probes have been in the water for the last six months and we have not had to touch them. We have checked them against lab results and found them to be accurate, so there has literally been nothing further to do.”

He has also been pleased with the controllers: “I have set the controllers to display the previous 16 hours of data so that I can make a quick check every morning and look for trends.”

Since the ammonia monitoring systems have been installed, several other UU staff have visited Macclesfield to study the trial results. Many of have since purchased similar systems and Sherwood believes that in the near future almost all of UU’s WwTWs will benefit from the same probe/controller on both the inlet and the outlet.

The ability to receive instantaneous alarms from a reliable sensor with web access to the data has provided staff at Macclesfield with a high level of process control, which has enabled them to avoid serious financial penalty.

The number of illegal discharges and alarms has substantially decreased, and Sherwood says he is sleeping much better now.

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