The Nigg Wastewater Treatment Works, located to the south east of Aberdeen, Scotland, serves a population of 250,000. It is operated by Scottish Water Services (Grampian), who runs four wastewater treatment plants serving the north east of Scotland.
Like many similar sites, the wastewater facility used traditional lobe air blowers in the critical filtering process. In this process, a constant flow of low-pressure air provides the oxygen required to sustain the organic bacteria that is treating the waste water.
This traditional type of blower was a constant area of concern for the maintenance team. As Graham Ellis, Asset Manager of Scottish Water Services explains: “Our nineteen roots-type lobe blowers required intense levels of maintenance and were frequently unable to operate at the upper end of their performance range. There was also a high incidence of air and oil leakage to deal with. Overall, the process was running too close to the wire.”
There was also an energy cost consideration to take into account. With the majority of the blowers running 24/7, the process represents a significant share of the Nigg plant’s total energy consumption.
Going for rotary screw blowers
To remedy the situation, Graham Ellis and the management team started investigating the options available: “We started off with seven blower providers, went through an elimination process, based on available technology.
We then did an energy study to see who had the most energy-efficient and cost-effective solution, at which point we decided to switch from roots blowers to screw blowers. This was followed up by a maintainability and total expenditure model which came out strongly in favor of Atlas Copco’s offering.”
The plant’s original 19 blowers of four sizes were reduced to 17 Atlas Copco ZS VSD screw blowers of similar size, thanks to the ability to use the extra capacity per blower and wide turndown function to match the demand. The Atlas Copco machines can run at considerably reduced loads whilst delivering the same or larger volumes of air as the previous nineteen units did before.
Each Atlas Copco blower comprises a fully integrated package based on a simple principle: precision timing gears maintain minute clearances between two intermeshing dry screw elements that never touch.
No lubrication is required in the compression space and special seals stop any rotor bearing oil from entering the compression chamber. Intake air is compressed between the rotors and their housing and oil-free, pulsation-free air is delivered at an output rate of up to 2272 m3h dependent upon process demand.
The Nigg Wastewater Treatment Works located to the south east of Aberdeen, Scotland.
Flexible installation ensured maximum uptime
The installation and commissioning took place over a period of six months. When taken off line, a wastewater treatment cell needs from 12-18 hours to restore its operation. As a result, each of twenty cells had to be taken offline individually, in sequence, and utilized the backup blower in order to minimize the risk to the technically sensitive process. Scottish Water was pleased that the planned process interruption was limited to just a half day per cell.
The site is now reporting around 25 per cent energy savings in aeration, with potential for further savings when the system is fully optimized. Commenting on the success of the upgrade project, Graham Ellis had this to say: "Before this work, regular blower failures meant we were experiencing unacceptable process and compliance risks. Now, we are able to operate reliably at the plant’s design capacity while also using less energy. What’s more, we have gone from having to spend an average of 14 hours a week maintaining the blowers to just carrying out daily checks within our service contract."
