WATER & WASTE TREATMENT INDUSTRY FOCUS BLOWING AWAY MAINTENANCE ISSUES
Scottish Water’s switch to Atlas Copco’s rotary screw blowers has reduced maintenance and cut costs
B
y replacing its traditional lobe air blowers with modern Atlas Copco VSD
rotary screw blowers, Scottish Water’s Nigg wastewater treatment works now achieves uninterrupted operation, with a reduced maintenance programme and savings of up to 25% in energy costs − plus the potential for further savings when the system is fully optimised. The Nigg wastewater treatment works,
located to the south-east of the City of Aberdeen, serves a population of 250,000 and is operated by Scottish Water Services (Grampian). In addition to treating wastewater, the company treats indigenous sludge and other liquid wastes. Like many similar sites, it had been operating traditional lobe air blowers for the critical aeration processes in its Biological Air Flooded Filter (BAFF). In the process, a constant flow of low-pressure air is supplied by the blowers to BAFF media at the base of treatment tanks. This provides the oxygen required to sustain the organic bacteria that is treating the wastewater with each cell air scoured and back-washed once per day. However, this was a constant area of concern and attention for the site maintenance team. Graham Ellis, asset manager of Scottish Water Services, explained: “Our nineteen
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 2272m3
/h dependent upon process
The 17x Atlas Copco ZS machines can run at considerably reduced loads whilst delivering the same or larger volumes of air as the previous 19 units did before
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 as most of the blowers were running 24/7. To remedy the situation, Ellis and the
management team undertook an asset- liability review to determine the equipment replacement options currently available to them, and eventually decided to switch from roots blowers to screw blowers. “This was followed up by a maintainability and totex model which came out strongly in favour of Atlas Copco’s offering,” said Ellis. The replacement units are Atlas Copco
ZS VCA VSD low-pressure, rotary screw blowers. Each comprises a complete, fully integrated package based on a simple internal principle: precision timing gears
PACKAGED PUMPING STATION REPLACED WITH A GREEN ALTERNATIVE
Willsbridge Mill, near Bristol, attracts tourists from far and wide. Set in an isolated area far from the mains drains however, dealing with wastewater poses a challenge. When the old packaged pumping station at the site failed, Willsbridge Mill turned to Rotamec, who installed a green, cost-effective treatment system that removed the need for a pump altogether. As it wasn’t connected to mains drains, the mill was relying on a packaged pumping station buried underground. As wastewater entered a chamber, the system intermittently pumped the sewage 80 metres uphill so it could enter the mains drain outlet. Carrying out continued maintenance on the pump was time consuming and costly due to repetitive blockages. This sometimes resulted in unpleasant smells, which at a tourist hot spot, is not ideal. To solve the issue, the local council approached Rotamec and ask them to propose a new treatment system. Paul Pearce, sales director at Rotamec, carried out a site inspection to determine the best course of
action: “After assessing the visitor numbers at the site and its geography, I was convinced that a sewage treatment plant that harnessed the anaerobic digestive process would be an optimal solution.” The system Pearce proposed removed the need for a pump altogether. Instead, sewage flows into an underground tank, where digestion by bacteria occurs, assisted by a low voltage air blower. Once digestion is completed after a period of weeks, the now treated water is then gravity fed back into the water table. Pearce said: “These sewage treatment systems feature minimal moving parts, which greatly enhances
reliability and reduces maintenance requirements. Typically, an annual inspection will suffice, compared to every few months with a pumping station. As the only powered aspect of the system is the low voltage air compressor, power consumption is also greatly reduced. These both provide substantial cost savings in operation. Furthermore, the end product is expelled water that is environmentally safe...” With everything approved, Rotamec teams removed the old pumping station and installed the new sewage treatment system, which has now eliminated the risk of unwanted smells. Rotamec
www.rotamec.com
demand. The plant’s original nineteen blowers of four sizes were reduced to seventeen Atlas Copco screw blowers of similar size. This comprised 11 x ZS30 machines and 6 x ZS160 units, all of which feature built-in inverter drives and Atlas Copco Mk5 control and regulation systems. The Atlas Copco ZS machines can run at
reduced loads while delivering the same or larger volumes of air as the previous nineteen units did before. The phased installation and
commissioning programme carried out by Design Air, an Atlas Copco distributor, 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 twenty cells had to be taken offline individually, in sequence and utilised the backup blower 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. All seventeen blowers are integrated into a SCADA system via Modbus protocols, utilising only two network cable runs and two Atlas Copco Mk5 Gateway units. The SCADA system, along with individual control systems, ensures every blower can operate independently and can always revert to a pre-programmed operating point in case of primary signal loss. Commenting on the upgrade project,
Ellis said: “Before this work, regular blower failures meant we were experiencing unacceptable process and compliance risks. Now, we can 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. Working with Atlas Copco and the installers on this project has gone exceptionally well, and we have put the blowers under an Atlas Copco total responsibility service plan to ensure maximum uptime.”
Atlas Copco Compressors
www.atlascopco.com
PROCESS & CONTROL | MARCH 2020 27
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