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Mitigating exorbitant energy costs with membrane bioreactor efficiencies
Schofield Barracks in Honolulu is home to Hawaii’s largest U.S. Army installation. The base primarily provides troop and family housing, as well as several Army support buildings. The Schofield Barracks Wastewater Treatment Plant (SBWWTP) is an important part of base operations. The military wastewater treatment plant was constructed in the 1970s and was converted to membrane bioreactor (MBR) technology when Aqua Engineers purchased the plant in 2006.
When Aqua Engineers purchased and upgraded the plant, it became the largest privately-owned R-1 facility in Hawaii. An R-1 facility provides premium recycled water to irrigate lawns, golf courses, parks and other sites.
Most wastewater contains high concentrations of organic matter, nutrients such as nitrogen and phosphorus, and colloidal solids. The main objective of biological treatment is removal or reduction of pollutant concentrations in the wastewater before it is discharged or reused.
The microorganisms convert the colloidal and dissolved pollutants into biomass that can be removed by a subsequent separation process step (final clarification or membrane separation).
Suez’s LEAPmbr systems use ZeeWeed ultrafiltration membranes to retain the solids produced by microorganisms from pollutants. These membranes produce high-quality effluent that can be reused for various purposes. In addition to high-quality effluent, the MBR system has a small footprint and is energy efficient.
The upgrade to MBR technology at the SBWWTP positively impacts the nearly 28,000 military personnel and their families and civilians who work on base and in the surrounding area. U.S. Army Garrison, Hawaii, has credited the upgrades for enabling the military to conserve water, decrease pollution and contribute to sustainability goals. The MBR technology has reduced the plant’s output of nitrogen and phosphates, turning the former liability into an asset.
Energy costs in Hawaii are among the highest in the U.S. In 2016, the average energy cost in Hawaii was approximately 150% more than the U.S. national average. The negative economic impact of high energy costs took a toll on the Schofield plant, and it was looking for a more energy effi-cient alternative for its MBRs.
The decision to upgrade to the LEAPmbr system was driven by the promise of increased energy efficiency and cost savings. This upgrade was performed during a routine membrane replacement.
The plant influent is standard municipal wastewater. The plant consists of typical headworks with 2-mm screens, a grit chamber and equalizer basins. After the headworks, the influent passes through primary clarifiers and flows by gravity into an influent splitter box. The splitter box has two pipelines that distribute wastewater to a bioreactor distribution channel, where it is combined with recirculated mixed liquor and distributed evenly to the anoxic zones of the biological process trains.
Mixed liquor flows by gravity through an anoxic zone and spills over a weir into an aerobic zone. A recirculation pump transfers mixed liquor from the aerobic zone to the membrane tank. Mixed liquor from the membrane tank overflows into a mixed liquor recirculation channel, where it is conveyed by gravity back to the distribution channel at the head of the bioreactor. The plant effluent is sent to ultraviolet channels and is used for food crop and agriculture purposes.
There are four membrane trains, and prior to the system upgrade, Schofield utilized the ZeeWeed 500D 340-sq-ft membranes in all four trains with 10/10 cyclic aeration mode.
Because Schofield’s primary objective was to reduce energy consumption and operation and maintenance costs, Suez proposed an MBR upgrade that would accomplish this with minimal change to the plant configuration. Existing 340-sq-ft membranes were replaced by 370-sq-ft membranes as part of the regular membrane replacement process. The upgrade aimed to reduce energy consumption, and the proposed configuration:
• Increased membrane reliability due to design simplification (reduced aeration equipment and controls);
• Significantly reduced blower energy use and blower run time;
• Reduced requirements for compressed air; and
• Eliminated requirements for high-frequency cyclic valves, reducing operations and maintenance costs.
LEAPmbr aeration technology is a recent advancement for wastewater treatment that incorporates a simplified, more efficient membrane aeration system for savings in operating cost. Due to its aerator design, larger air bubbles are formed intermittently, which increases the shear along the membrane surface, resulting in effective cleaning of the membranes.
This aeration technology was retrofitted in each of the existing cassettes. Suez supplied all materials for the MBR aeration upgrade.
Air is delivered on a continuous basis, and it accumulates in the aerator at the base of the cassette. As accumulated air reaches the fixed target volume, it burps out to provide large-bubble scouring of the membrane. Since these large mushroom-cap bubbles are so effective, a lower volume of air is required to achieve the same solids removal performance.
This increase in efficiency eliminates the need for and maintenance of high-frequency cyclic aeration valves. For new plants, only one air header and one automated valve is required per train. However, to minimize changes to the existing trains, one cyclic valve is left in place in closed position and one air header with the cyclic valve on it is utilized.
The cyclic valve now operates as an actuated valve and only closes when the train is not being aerated (i.e., shutdown, specific cleaning steps or in standby), reducing wear and maintenance needs. This aeration technology operates at two airflows, LEAP-Lo and LEAP-Hi, based on the plant operating conditions. Under average daily flow (ADF) conditions, where most plants operate the majority of the time, the membrane cassettes will operate under the LEAP-Lo airflow condition. LEAP-Lo airflow rate is 50% of LEAP-Hi. Under flow conditions greater than ADF, where the membranes are operated at a higher flux, the airflow rate increases to the LEAP-Hi condition. The modulation between Lo and Hi airflow conditions is managed by the con-trol code through the blower variable-frequency drive according to the number of blowers in operation or controlling the number of blowers in operation, as applicable.
Retrofitted cassettes were installed in all four trains in a single stage. Suez completed the installation in approximately seven days while working closely with the customer.
The plant continued to run well at low transmembrane pressure (TMP) after installation of the system.
After the LEAPmbr upgrade, many changes in the mechanical elements of the plant were noted. The blowers became quieter and more efficient. Less run time per blower and less electric demand also were observed, and since the system eliminated the requirement for high-frequency cyclic valves, Schofield reduced maintenance costs of those valves. The reduction in the number of functioning pneumatic valves also increased compressor performance. The compressor run time decreased slightly due to this change.
The economic benefits of the upgrade were apparent almost immediately. Average savings of about 2,376 kilowatt-hours (kWh) per day were recorded, which amounts to approximately 867,240 kWh or $216,810 per year at 25 cents per kWh. These savings provided a full return on investment for the aerator upgrade in just over a year. Due to the gentle nature of the LEAPmbr aeration, it is expected that membrane life will exceed 10 years, providing further economic benefit to Schofield.
This system can be a solid investment for customers to reduce energy requirements and operation costs for MBR systems.