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Membrane aeration is one of the biggest energy requirements for a membrane bioreactor (MBR) wastewater treatment system, and it can consume a significant portion of a plant's annual operating budget. Membrane fibers benefit greatly from coarse bubble aeration, and despite the energy cost, the aeration process is extremely effective at scouring debris from the surface of the membrane fibers and at increasing the time intervals between backpulse and maintenance cleanings.
In the long run, membrane aeration reduces the rate of membrane fouling, saves money spent on cleaning chemicals and helps to maximize plant up-time. A new aeration strategy can reduce membrane aeration costs by up to 75% over continuous aeration. This Intelligent Aeration process was developed by Zenon Membrane Solutions, a part of GE Water & Process Technologies, and is the product of more than a decade of operating experience at more than 250 MBR plants worldwide. The process combines 10/10 sequential aeration, membrane relaxation and new 10/30 ecoaeration to optimize membrane performance at the lowest possible energy cost.
The key drivers
The city of Pooler, near Savannah, Ga., has been using the system for more than a year at its 3-million gal per day (mgd) MBR plant, which serves this city of more than 10,000 and the neighboring city of Bloomingdale. Rapid growth, increasingly stringent discharge standards and the lure of additional revenue from a beneficial water reuse program were the key drivers behind the city of Pooler's move from an oxidation ditch treatment plant to a state-of-the-art MBR plant.
The new ZeeWeed MBR enabled the city to increase capacity from 1 to 3 mgd in approximately one-tenth of the space that the treatment lagoons occupied. In addition to the dramatic reduction of the plant's space requirements, the city of Pooler has also saved over 200,000 kWh during 2006 in blower power consumption through the use of the system.
The system works by sharing a common aeration blower between two trains and then alternating the aeration of the membrane cassettes within each train. At the Pooler wastewater treatment plant, four membrane trains each contain three cassettes of ZeeWeed membranes. Each train has two aeration headers that supply the membrane aeration grids. The PLC system selects the appropriate aeration pattern depending on incoming wastewater flow and the permeability of the membranes. In 10/10 aeration, air is delivered to half of the cassette for 10 seconds and then diverted to the other half of the cassette for 10 seconds. Typically, this process operates during times of peak flow, or when fouling reduces permeability.
10/30 eco-aeration works in a similar fashion, except it is applied when the plant is operating below average daily flows and when membrane fouling is relatively low. Air cycling occurs among four aeration headers, so air delivery is on for 10 seconds in each header and then off for 30 seconds. Permeation is continuous during the entire cycle, except when the train is in a relaxation cycle. The results show that Intelligent Aeration Controls can dramatically reduce energy requirements, while still maintaining optimum performance of the Zee-Weed membranes.