Municipal water and wastewater plants, pumps, and lift stations can consume more than 40% of a city’s energy in a year to distribute, disinfect, treat, and pump water for their customers. The City of Mesa realized this and took action to make improvements to their systems.
The City of Mesa Southeast Water Reclamation Plant (SEWRP) aeration improvements project represents a unique application of different technologies brought together to achieve two main goals; reduce energy use and optimize aeration and process control in the activated sludge process. The different technologies installed at the Mesa SEWRP include fine bubble aeration, high speed turbo blowers, and real time analyzers for nutrients (ammonia and nitrate) and dissolved oxygen. The blend of these technologies as well as the cutting edge high speed turbo blower technology is a unique application in the State of Arizona.
Collectively, the turbo blowers and the fine bubble aeration saved the city more than 2 million kilowatt-hours annually which equates to more than 60% savings in energy use, or approximate $166,000. Check out City of Mesa video and learn about their SRP rebates.
The heart of this unique aeration system is the high speed turbo blower. The new high speed turbo blowers (200hp) provide additional 30% energy efficiency over the existing centrifugal blower (400hp) technology due to several key benefits of this cutting edge technology. The benefits include:
- Lower installed motor horse power (HP) due to a more efficient blower design
- Efficient blower design components include the direct drive impeller to motor configuration coupled with an air bearing design and the more efficient permanent magnetic synchronous motor
- Up to 40% turndown capability due to variable frequency drive
Like many WRPs, the Mesa SEWRP experiences diurnal swings in influent flow and loading. A conventional aeration system consisting of constant speed centrifugal blowers, dissolved oxygen analyzers and modulating or manually operated airflow control valves is typically utilized and was the original method installed at the facility. The conventional system has two main shortcomings, limited ability for airflow turndown on centrifugal blower, and thus limited ability to optimize energy consumption and process control. Furthermore it lacked the partial optimization of energy use and process control that can be realized with dissolved oxygen only control.
Due to the shortcomings of the existing aeration system, the Mesa staff at the SEWRP found the existing centrifugal blowers to be limited in their turndown capabilities, resulting in high devolved oxygen (DO) concentrations during low flows and forcing staff to blow off air excess to maintain DO concentrations. The generally higher DO concentrations consequently also increase the DO concentration in the anoxic zone due to internal mixed liquor return flows, resulting in inefficient nitrogen removal. Mesa operations staff had devised an interim control strategy for inlet valve modulation which achieved some turndown, but it could not provide significant energy savings.
The combination of fine bubble aeration, high speed turbo blowers, and both dissolved oxygen and nutrient analyzers offers a combined solution that optimized energy use while at the same time improved process control. The fine bubble aeration alone has resulted in approximately 30% greater oxygen transfer efficiency over the existing coarse bubble diffusers. The fine bubblers were designed with a tapered heads allowing for minimal transfer of oxygen from final aerated zone back to the anoxic zone.
Although fine bubble aeration is not a new technology, coupling it with nutrient/DO controls provided for a unique application with this project. The fine bubble aeration control valves at each of the aeration basin zones will now modulate and control both the DO set point and an overriding ammonia set point. Thus, even when the optimum DO requirement is met and ammonia set point indicates treatment to be complete, the control strategy will further decrease the DO set point to further optimize energy usage.
In addition to the energy, control and technology benefits, the high speed turbo blowers required less overall maintenance than conventional blowers and operate at a significantly reduced noise level. The operational noise is a real added tangible benefit for plant staff that has to work in close proximity with the system on a daily basis.
Bringing together new technologies and blending it with various applications has provided the City of Mesa with an innovative solution to accomplish major financial and operational goals. The techniques that have now been applied provide a significant energy savings which reduces overall operational costs and CO2 emissions. This project has been a great example of how technology and the use of variable control techniques in the application of that technology can solve many of our greatest wastewater challenges.