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

Following installation, the monitoring systems initially generated about 40 alarm texts a day (mostly at night), but significant improvements have now been implemented. The Macclesfield works is a relatively large plant and wastewater is drawn from a network of pipes and drains that extend for hundreds of miles.

Historically, the industrial waste influent at the plant has been problematic. Illegal discharges from unknown sources have resulted in highly variable ammonia levels that – in April 2007 – caused the plant to exceed its discharge consent.

The works was chosen to host the trial of a Hach Lange monitoring system. The system is able to issue alarms by text messages to key UU staff. Barry Sherwood, Macclesfield’s process controller, said: “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 NH4D 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 at 3mg/l and the process probes at 25mg/l. Sherwood reported “about 40 to 50 text messages a night when the units were first installed”.

Quick access to reliable data has enabled Sherwood’s team to identify the sources of the peaks. He said: “It is now down to one or two text messages a day – which is great news for me, the company… and my family.”

Data from the monitors is available on the internet via a dial-up connection, soon to be upgraded to broadband. This means Sherwood and his team are able to study trends at any time, from anywhere. As a result, staff are able to check if an alarm was 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 is designed to provide continuous monitoring of ammonium levels. Included in the NH4D sc are:

  • A pHD (differential pH) electrode as the reference electrode, which provides stability
  • A temperature sensor to compensate for temperature effects
  • A potassium ion-selective electrode (ISE). The most significant potential interference in wastewater matrices is from potassium ions and the NH4D sc compensates for this by using the ISE to correct the ammonium value

The SC 1000 Controller is a modular system consisting of a display module and one or more probe modules. Each SC 1000 provides power to the system and can accept up to eight digital sensors. The probe modules can be networked together to accommodate more sensors.

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, which will also connect to the SC 1000.

Commenting on the monitoring equipment, Sherwood has nothing but praise: “Hach Lange staff came to commission the instruments and to train our own engineers, but the probes have been in the water for the past 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.”

Sherwood has also been pleased with the controllers. He explained: “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, a number of other United Utilities staff have visited Macclesfield to study the results of the trial. As a result, many of them have bought similar systems. Sherwood believes that in the near future almost all of UU’s works will benefit from the same probe/controller on inlet and 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. This has enabled them to avoid serious financial penalty. The number of illegal discharges and alarms has radically decreased – and Sherwood can now get some sleep.

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