Maximum power at Minworth

With no less than 16 sludge digesters at Severn Trent Water’s WwTW in Minworth near Birmingham, there is no shortage of methane from which to power the site and sell on to the National Grid.

Currently there are five engines producing typically in excess of 8mW from this desirable source of renewable energy. But for any site that has more than one sludge digester, even if it is only two and not 16, there has until now been no accurate, reliable online system for determining gas production from each digester. Carrying out manual checks on two digesters is obviously not going to require as much legwork as Minworth’s 16 but it is still

laborious and can lead to inconclusive results.

However, Severn Trent is believed to be the first water company in Europe to introduce a fully automated sludge digester gas monitoring system. Designed and installed by Coventry-based Gas Data, all of the gas performance criteria can be seen at a glance on a dedicated page of Minworth’s Scada system.

The geographic overview shows the amount of gas being produced by each dige-ster, giving an on-screen guide showing the percentages of:

• methane – the prime energy source,

• carbon dioxide – the main biogas,

• oxygen – ideally a nil reading,

• hydrogen sulphide (ppm) – always present but large concentrations are very harmful to the plant’s engines.

“The main advantage of using Gas Data’s system is that we can monitor the gas quality, which is critical to the efficiency of the CHP engines, said Severn Trent’s senior engineer Martin Dent.

“The LMS system quickly highlights any deterioration in gas quality attributed to a

specific digester, which enables the root causes to be identified and measures implemented to restore gas quality as soon as possible.”

Serving each block of four digesters, a fixed position Gas Data LMS analyser (self-contained in its own cabinet with additional equipment) samples gas in turn from each digester at 15-minute intervals – though at any time, Severn Trent has the option to

manually operate.

The analyser and all sample pipes are maintained with dry gas between samples, with the actual sample time taking only 30-60 seconds.

Chris Dakin, director of Gas Data, which is celebrating ten years in business in 2003 and has a proven track record in analysers for environmental gases such as those found on landfills and areas of contaminated land, explained: “This technique has been highly

successful in similar environmental monitoring applications where conventional online analysers have failed due to onerous maintenance requirements. This system

is designed to be virtually self-maintaining.”

On the roof of the massive digester block at Minworth, each of the 16 Gas Data sampling points demonstrates another advantage of the new monitoring system. The hot, steamy gas (from sludge temperatures of around 37°C) entering the sample point on the left hand side of the sampling tube is visible in its saturated state as it cools in the outside air, but whereas previously sample lines would often become blocked, the new equipment allows condensation to be released,

keeping the pipes dry and blockage-free.

This is demonstrated by the crystal clear appearance of the sampling tube on the right hand side of the sampling point. “We have worked very closely with the engineering team at Minworth to develop the most effective, yet easiest possible monitoring system to operate”, said Dakin.

“When reading the geographic overview on the performance of the digesters, we have had to ensure Severn Trent can do this with the utmost confidence.

“This is why we have created a system whereby they can be safe in the knowledge every reading has been taken within the last 15 minutes and that updates to the online screen are achieved in seconds.” To give some idea of the scale of the Minworth operation, Gas Data also completed its installation with the minimum of cabling to keep the four independent analysers as local as possible. Maximising the methane to power

the seven engines at the site involves a wide variety

of sludge intakes.

One of the sets of four digesters handles household or domestic (green route) sludge, while the other three sets of digesters process industrial sludge, including one block of the four digesters utilising sludge that is imported to the site. However, and particularly in the case of the industrial intake, there is never any certainty as to the exact quality of the sludge and its subsequent potential to produce high quantities of methane

for energy use.

“In conjunction with our Scada-based control, Gas Data’s system removes any uncertainty over the quality, content and from which digester the varying levels of gas are being produced”, added Dent. “Now, when

gas levels change, we can

see immediately which of

the 16 digesters is not

producing sufficient gas.

“We can also keep an eye on the hydrogen sulphide levels so any unusually high levels containing harmful acids can be prevented from reducing the performance and lifetime of our engines.”

If any of the LMS analysers need servicing, they can be removed in a matter of seconds. The system has also been designed with a GSM modem so that from its offices, Gas Data can dial into the monitoring equipment, run diagnostic checks, make adjustments and notify Severn Trent Water accordingly.

Data could also be downloaded via the Modems to provide data trending and analysis for sites without their own Scada control systems.