Navigating the Pre-Selection of Membrane Bioreactor (MBR) Equipment in Nantucket, Massachusetts – Lessons Learned

Nantucket is an Island resort community located approximately 26 miles off the southern coast of Massachusetts Access to the Island is limited to boat or airplane, which makes delivery of building materials, chemicals, and other supplies difficult and costly to arrange. Interruptions in travel are frequently experienced during the winter due to rough seas, or fog. These conditions make it difficult and costly to implement heavy construction on the Island.The Surfside WWTF has historically provided chemically enhanced primary treatment using coagulants such as aluminum hydroxide and anionic polymer. Effluent is discharged into the Islands sole source aquifer using open sand rapid infiltration basins. In late 2002 the Town received an administrative consent order from the Massachusetts Department of Environmental Protection (DEP) to upgrade the level of treatment from primary to advanced treatment to achieve effluent total nitrogen concentrations of 10.0 mg/L or less on a year round basis, and to increase the capacity of the existing facility from 1.8 million gallons per day (mgd) up to 3.5 mgd‥ The Town began work on a sequencing batch reactor (SBR) to provide biological nutrient removal (BNR), and performed a value engineering (VE) study using the Preliminary (30 Percent) Design Report to verify selection of the BNR process. The VE team recommended that the Town consider the use of MBR technology for the upgrade and expansion for the following reasons:Sole source aquifer, possible requirements to implement extensive water reuse program.Potential for more stringent effluent discharge limits in the future due to the sole source aquifer.Sole source concrete supplier on the Island, important to minimize quantities of concrete used in the upgrade and expansion to control costs.Limited site availability due to extensive on-site area used for rapid infiltration basins, and close proximity to the Atlantic Ocean.Although the MBR process was estimated to have a slightly higher life-cycle cost compared to the SBR design. However, the Town elected to proceed with the MBR design based on these non-economic considerations.During the design the following types of MBR equipment were chosen for consideration based on each manufacturer list of installations, length of service, track record, and site visits to other operating MBR facilities:Immersed hollow fiber gravity or pumped flow.Immersed flat plate gravity or pumped flow.It was decided to allow all four system types in the specifications to maximize competitive bidding. The base contract drawings showed a immersed flat plate gravity flow arrangement in order to define the required tank volume, and process layout. The specifications outlined the process performance parameters, and equipment requirements, and evaluation criteria for the pre-selection.Manufacturers were required to arrange their system to fit within the confines of the structure designed. Tank volumes could be re-proportioned by the manufacturer, but the total volume of the MBR process tanks could not be reduced. Manufacturers were required to submit detailed layout and electrical drawings, pricing data on all components, energy consumption, cleaning frequency, and chemical usage information on detailed proposal forms one month prior to the general bid. Each manufacturer submittal was rated on the following:Life-cycle cost (capital and O&M)Number of similar size applications and track recordPretreatment requirementsCleaning requirements and frequencyAfter a two week review period the most favorable system was selected and the pre-selected vendor was published by addendum that included detailed arrangement and electrical drawings and guaranteed maximum purchase price as a bid allowance.This paper will outline the MBR design parameters, evaluation criteria, life cycle costs, and lessons learned during the complex pre-selection process for this large project.