Northumbrian Water opts for energy efficient Atlas Copco screw blower technology

The company, which has been awarded the ‘Queens Award for Enterprise: Sustainable Development’ and ‘Water Company of the Year’ previously, needs a constant, reliable supply of air in the region of 480mbar for aerobic treatment of waste water at its site in Newton Aycliffe. This process uses bacteria to break up waste particles and requires large amounts of air to be blown into the aeration tanks to maintain the desired level of dissolved oxygen within the wastewater containment system - a biological process that typically utilises up to 70% of the energy consumed on site. There is a changing perspective within the water industry that the widely-used lobe blower design principle no longer meets the needs of today’s low carbon economy and that operators can benefit substantially from energy savings through the replacement of conventional lobe equipment with innovative screw technology. Such was the experience of Northumbrian Water, whose aeration air supply was originally delivered to the site by four lobe type blowers. However, when one of these units started to develop reliability issues, the opportunity was taken to investigate the possibility of installing one of the new generation of high efficiency screw type blowers which, compared to standard lobe type machines, offer potential savings of up to 30% in energy consumption. Northumbrian Water decided to replace one lobe blower with the Atlas Copco ZS screw blower technology. On completion of the installation of the new ZS blower, Northumbrian Water carried out a comparison performance test between the existing lobe blowers and Atlas Copco’s ZS technology. Data from air flow meters and power monitors was put into the site’s PLC/SCADA system to calculate the specific power of all four air blowers. The calculated values of air production were averaged over a 12 hour period. In order to compare like for like, the ZS unit was compared with the performance of the other blowers at the same time on the following or preceding days during similar weather, flow and biological load conditions. It revealed that the existing lobe blower used approx. 19% more power for the same output. Based on these test results and continuous reliability issues with the lobe blowers it was decided to replace a second lobe blower with an Atlas Copco ZS90-VSD, low-pressure, oil-free rotary screw blower. The blower unit comprises a complete 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 specially designed seals prevent 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 pressures of approx. 480 mbar is delivered at an output rate of more than 4000m 3/hr dependent upon process demand. Energy savings are derived from the efficiency of the internal compression screw design and the direct drive, integrated gearbox that ensures fewer losses compared to the traditional belt and pulley system. For example, to deliver a flow of 1600 m³/hr (942 cfm) at a pressure of 0.8 bar (e) (11.6 psig), a tri-lobe blower consumes 61 kW (82 hp) on average. By comparison, the screw blower consumes only 43 kW (58 hp) to match this performance. At the same time reliability has been improved and maintenance costs reduced. Northumbrian Water has now placed a third order with Atlas Copco to replace the remaining two lobe units installed at Newton Aycliffe Waste Water Treatment Plant with a further two ZS 90 screw blowers. Alan Harle, the Northumbrian Water engineer who conducted the comparison tests, commented: “From an operational perspective, Atlas Copco’s ZS blowers have performed well, especially when compared with the incumbent blowers, and we are very pleased with the 20% improvement we have already seen. I would say that the proof is in the pudding and given that NWL have chosen to replace all blowers with the Atlas Copco units speaks volumes.” Written by Joanna Canton Long