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In December 2000, the Jacksonville Electric Authority (JEA) assumed ownership of the Yulee Wastewater Treatment Plant (WWTP) in Nassau County, Fla. By 2003, it became evident that growth in the region and pending permit requirements would require a much larger facility capable of producing reuse-quality effluent.
After evaluating available technologies and suppliers, JEA selected the design-build team of Wharton-Smith, Inc. and Boyle Engineering Corp. to supply an Enviroquip, Inc. membrane bioreactor (MBR) system. Construction began in 2004, and the plant went online in February 2006.
Flexible & robust
At the direction of JEA, Enviroquip worked with the design-build team to provide a flexible and robust system capable of handling diurnal and seasonal fluctuations in loading without influent equalization. Moreover, the system needed to have sufficient turndown capabilities to reliably meet stringent 5/5/3/1 (BOD5/TSS/TN/P) limits at current loading conditions and at projected 2010 numbers.
Designed using the principles of biohydraulics, Enviroquip provided what is called a UNR process, incorporating several state-of-the-art technologies and design features as part of the Yulee MBR system. Some of the system features provided by Enviroquip are shown in Table 1.
Installed in more than 2,000 MBR systems around the world, the Kubota Flat-Plate Submerged Membrane Unit (SMU) has a reputation for being durable, reliable and easy to operate. At the Yulee WWTP, Enviroquip provided 56 Kubota double-deck EK-400 SMUs for installation in four separate MBRs (Figure 1), each capable of treating average daily flows of up to 0.67 mgd and peak flows of up to 2 mgd.
With only two MBRs in service at startup and virtually no surge capacity in the system, high diurnal flows routinely pushed membrane filtration rates (flux) up to ~48 gfd, even as mixed liquor suspended solids concentration reached ~18,000 mg/L.
However, to illustrate the capacity of the plant to reliably handle sustained peaking events, two performance tests were conducted with impressive results (Table 2).
The baseline flux for both tests was 14 gfd at design flow (Q) to substantiate reliability claims. Six-hour trial peak flow tests were first performed, and sustainable 24-hour fluxes were extrapolated. In Test 2, five 24-hour trials were run for a combined test time of 124 hours. Test 2 culminated in a four-hour peak flow test at roughly 42 gfd. Similar to the results of Test 1, the transmembrane pressure (TMP) never increased by more than 0.7 psi during any part of the test. Most importantly, the average permeability during peak flux testing (~42 gfd) showed no statistical signs of declining with an average value of 30 gfd/psi and a mean of 28 gfd/psi.
In addition to hydraulic performance testing, Enviroquip helped to fine tune the biological process using proprietary biomonitoring technology.
Meeting the objectives
The flow schematic for the Nassau County project involves a pre-anoxic zone for denitrification, a pre-aeration zone operated at low dissolved oxygen concentration using the patented SymBio process control to promote simultaneous nitrification and denitrification (SNdN), followed by a post-anoxic zone for further nitrate depletion, and finally the MBR for completion of nitrification and solid separation (Figure 2).
The SymBio process monitors the coenzyme-nicotinamide adenine dinucleotide (NADH) level in the biomass along with the dissolved oxygen level in the water to precisely predict the changes in the biological oxygen demand. Based on the results, the aeration is controlled to maintain low dissolved oxygen (< 1 ppm) for SNdN in the same basin.
Low flows at startup, combined with high MLSS operation, resulted in operating SRT to be outside of Enviroquip’s biohydraulics guidelines (i.e., 12 to 50+ days). Enviroquip and United Water (operations company) worked closely and adjusted aeration controls (i.e., operating DO levels) and MLSS concentrations to successfully adapt to this situation. One of the most beneficial features of the UNR system is its ability to handle a wide range of MLSS (8,000 to 18,000 mg/L), which helped in absorbing excessive DO in the recycle stream during low-flow scenarios. The system was fully optimized in May 2006 to meet the treatment objectives. Data collected from May to July confirms the system’s ability to meet and exceed future BOD, TSS and nutrient limits.
Following process optimization and performance testing, all four MBRs were put in service and Energy Pro control enabled. With a properly designed system, this proven control strategy can reduce power requirements by ~50% and, in the case of the Yulee WWTP, increase the turndown ratio of the plant to about 6:1.