Installing modern VSD rotary screw blowers, Scottish Water's Nigg wastewater treatment works now achieves uninterrupted operation, with a reduced maintenance programme reducing energy costs by up to a quarter...
Nigg wastewater treatment works, located to the south-east of the City of Aberdeen on the shore of Nigg Bay, serves a population of 250,000. It is operated by Scottish Water Services (Grampian), as part of a larger contract with Scottish Water.
Like many similar sites, it had been operating traditional lobe air blowers for the critical aeration processes in its Biological Air Flooded Filter (BAFF).
Graham Ellis, Asset Manager of Scottish Water Services, was concerned that the 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. With many of the blowers running 24/7, the BAFF process represents a significant share of the Nigg plant's total energy consumption.
Fitting for purpose
The management team undertook an asset-liability review to determine the equipment replacement options: A study was undertaken to see which blower providers had the most energy-efficient and cost-effective solution. At this point, it was decided to switch from roots blowers to screw blowers.
It installed VSD, low-pressure, rotary screw blowers. Each comprises a complete, fully integrated package based on a simple internal 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 at a pressure band between 700-800 mbar is delivered at an output rate of up to 2272 m3h dependent upon process demand.
The plant’s original nineteen blowers of four sizes were reduced to 17 screw blowers of similar size. This comprised 11 x 30kW machines and 6 x 160kW units, all of which feature built-in inverter drives and control and regulation systems to maximise performance under a variety of conditions. The reduction in the number of blower units has been enabled by the ability to use the extra capacity per blower and wide turndown function to match process demands.
Vindicated by results
The phased installation and commissioning programme took place over six months. When taken offline, a wastewater treatment cell needs from 12-18 hours to restore its operation. As a result, each of the twenty cells had to be taken offline individually, in sequence, with the backup blower utilised to minimise the risk to the technically sensitive process. The planned process interruption was limited to just a half-day per cell. As of now, the site is reporting around 25 per cent energy savings in aeration, with potential for further savings through DO (dissolved oxygen) control process optimisation. This is made possible by the screw blowers’ turndown capabilities and superior internal air compression.
It also enabled the Nigg plant to fully integrate all 17 blowers into its SCADA system via Modbus protocols. This provides for full control and monitoring as well as an emergency protocol. It ensures that every single blower can operate completely independently and could always revert to a pre-programmed operating point in case of primary signal loss.
Previously, regular blower failures meant the Nigg plant was experiencing unacceptable process and compliance risks. Now, it can operate reliably at the plant's design capacity while also using less energy. Furthermore, it has gone from having to spend an average of 14 hours a week maintaining the blowers to just carrying out daily checks within its supplier service contract – a total responsibility plan that ensures maximum uptime.