Water & Wastewater Treatment

www.wwtonline.co.uk | WWT | MARCH 2020 | 31 ported sludge and liquid wastes. Like many similar sites, it had been operating traditional lobe air blowers for the critical aeration processes in its Biologi- cal Air Flooded Filter (BAFF). In the process, a constant flow of low-pressure air is sup- plied by the blowers to BAFF media at the base of treat- ment tanks. This provides the oxygen required to sustain the organic bacteria that is treating the wastewater with each cell scoured and back- washed once per day. Blowers of the type in opera- tion at the Nigg plant have been a constant area of concern and attention for the site mainte- nance team. Graham Ellis, asset manager of Scottish Water Ser- vices, was concerned that the 19 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 run- ning 24/7, the BAFF process represents a significant share of the Nigg plant's total energy consumption. To remedy the situation Ellis and the management team un- dertook an asset-liability review to determine the equipment replacement options currently available to them: Starting off with seven blower providers, they went through an elimina- tion process, based on available technology. An energy study was undertaken to see who had the most energy-efficient and cost-effective solution to meet their needs. At this point, it was decided to switch from roots blowers to screw blowers. The plant's original 19 blow- ers 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 blow- er and wide turndown function to match process demands. 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 20 cells had to be taken offline individually, in sequence, with the backup blower utilised to minimise the risk to the technically sensi- tive process. Scottish Water was pleased that 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 (dis- solved oxygen) control process optimisation. This is made possible by the screw blow- ers' turndown capabilities and internal air compression. It has also enabled the Nigg plant to fully integrate all 17 blowers into its SCADA system via Modbus protocols. Such integration, along with built-in independent individual control systems, provides for full con- trol and monitoring via SCADA, as well as for an emergency protocol for when those signals are lost. This 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. Before this successful upgrade project work was carried out, 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 sup- plier service contract – a total responsibility plan that ensures maximum uptime. ZS blowers at the Nigg site.

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