Tony Kobilnyk describes how substantial reductions in operational energy consumption at wastewater treatment works (WWTW) using membrane bioreactor (MBR) technology can be achieved. Energy savings through careful plant design, changes to operational procedures, and the use of intelligent aeration control are explained and are illustrated by implementation of these measures at two WWTW.
MBR technology is recognised worldwide for reliably producing high quality effluent, offering distinct advantages over conventional activated sludge (CAS). MBR performance more robust than CAS, producing final effluent lower in solids and chemical oxygen demand. MBR systems are space efficient, occupying as little as 10% of CAS-clarifier systems, and flexible configurations make MBR ideal for both new-build and retro-fit plant upgrade applications.
As with alternative wastewater treatment options, MBR systems incur ongoing energy costs. Power is required for aeration, sludge recirculation, pumping effluent, chemical cleaning systems and air scour to keep the membranes clean. Membrane aeration is one of the major energy requirements. Membrane aeration and scouring is beneficial in maintaining treatment performance and reducing plant downtime. Measures to reduce power consumption, hence operating costs, can be accomplished through advancements in MBR technology.
Figure 1
ZeeWeed membrane cassettes immersed directly in the aeration tank on an MBR wastewater treatment application. Membrane surfaces are kept free of fouling by air scouring with supplemental back-pulsing and periodic manual cleaning.
Substantial power savings were achieved at the design stage at Fowler Water Reclamation Facility (Cumming, Georgia, USA). Fowler WWTW produces high quality effluent suited to urban water re-use or surface water discharge at 2.5 mgd (9500 m3/d) treatment capacity. GE MBR technology was the chosen technology, offering high quality treatment and a small plant footprint with low visual impact. The design team modified the conventional MBR configuration, eliminating 30 items of equipment and saving 470 kW. Modifications included:
• consolidated piping for aeration tanks and anaerobic digesters.
• installation of a common blower header to scour multiple membrane cassettes, utilising controlled air scour distribution.
• combination of recirculation piping with side-stream screening flow and waste sludge.
• side-stream screenings discharged to digesters, removing screening compactors and conveyors; these screens also act as sludge thickeners.
• the use of in-pipe UV disinfection allowing the permeate pump to boost flow to storage tanks.
Extra meters and fittings required in the improved plant design approximately matched capital savings from the plant eliminated. Additional power savings (15%) at Fowler WWTW were achieved through procedural changes in the first 12 months of operation, and the management team has subsequently improved energy efficiency, in combination saving an estimated 40% energy. Improvements included:
• Up to 30% reductions in energy consumption from the sludge recirculation pumps are achieved by decreasing recirculation at times of low influent load, typically overnight. Denitrification of lower nitrate loads in the anoxic zone is maintained, since less oxygen is produced through denitrification at these times.
• Up to 30% of the energy for process air blowers is saved by shutting them off for periods of 1-3 hours. These blowers, initially in continuous operation at Fowler, exceed the mixed liquor aeration requirements for much of the day; a cascade weir now entrains up to a third of the required oxygen.
• Cyclic air scouring of membranes saves up to 50% of related power costs by scouring each of four membranes sequentially from a combined air header with suitable controls.
Figure 2
Pooler MBR WWTW. 10/30 eco-aeration has resulted in significant savings (up to 50%) in energy costs from PLC controlled cyclic patterns for aeration scouring of ZeeWeed membrane cassettes.
In 2000 Zenon advanced MBR technology by sequentially scouring of one half of each membrane at a time thus halving aeration requirements, benefiting over 250 MBR plants worldwide. In 2006, the company further advanced this approach developing Intelligent Aeration Control.
Membranes require cyclic aeration scouring to prevent fouling and maintain performance, but during periods of low influent flow extending the aeration-off time period is possible without compromising performance. Under normal 10/10 cyclic aeration, each membrane in a train is scoured for 10 seconds before scouring is switched to a different cassette for 10 seconds before scouring resumes at the original membrane. Alternatively, each half of a membrane is sequentially scoured. Intelligent Aeration Control works by using a pattern of membrane scouring, controlled via a programmable logic control (PLC), that is responsive to the current wastewater treatment loading. Extension of the intervals between scouring to 30 seconds (10/30 eco-aeration) is possible under non-peak influent loads with concomitant energy savings.
Intelligent aeration control has been adopted at Fowler WWTW and at Pooler WWTW (near Savannah, Georgia), a 3.0 mgd (11400 m3/d) MBR municipal plant. Pooler MBR WWTW consists of four trains each with four GE ZeeWeed membrane cassettes; each train has two aeration headers. The PLC selects the appropriate aeration pattern, 10/10 during peak flow or when fouling reduces permeability, and 10/30 at average daily flows or lower, saving 200,000 kWh (50% of the power requirement).
Useful Links
GE water and process technologies: www.ge.com/water
About the Author
Tony Kobilnyk is Global Trade Media Relations Manager at GE Water & Process Technologies. For further information contact via the website.