Animal protein processor JBS was discharging anaerobically-treated wastewater from its packing plant in Grand Island, Neb., directly into the City of Grand Island wastewater treatment plant (WWTP).
Two original lined and covered anaerobic treatment basins operated in series and provided seven-day flow equalization. A study revealed that the most cost-effective alternative for JBS was to design and construct its own biological pretreatment system and pay the city only a flow charge rather than a surcharge.
Biological Treatment Reborn
A new aerobic treatment system featuring a Modified Ludzack Ettinger (MLE) process, which accomplishes nitrification and denitrification of the anaerobic effluent prior to discharging to the City of Grand Island WWTP was selected. The MLE system consists of two process trains operating in parallel; each having an anoxic basin and aeration basin. The effluent from the two aeration basins flows by gravity to a final clarifier prior to being discharged to the city’s WWTP.
The design flow, BOD5 and volatile suspended solids loadings to the anoxic basins are 3.45 mil- lion gal per day, 610 mg/L and 265 mg/L, respectively. The recycle rate from the aeration basins to the anoxic basins is three times the plant influent flow. The BOD5 loadings to the aeration basin from the anoxic basin drop to 250 mg/L. This BOD5 reduction is achieved when the bacteria uses the oxygen from the nitrates formed in the aeration basin to oxidize the BOD5.
The basic design philosophy of the aerobic treatment system was to separate the mixing requirement from the oxygen requirement in the aeration basins. The oxygen transfer system could be controlled solely by oxygen demand, knowing that the mixing requirement was being met by a separate system.
The other design philosophy was to minimize the operating costs by reducing the energy requirements for the mixing system and limiting the amount of air supplied to the aeration basins to 2 mg/L while nitrifying. A comprehensive SCADA system monitors and controls the air supplied to the aeration basins and minimizes the number of operators needed because the critical parameters are monitored remotely.
Design Phase
During the project’s design phase, two alternative approaches were evaluated when considering the mixing and aeration systems to be utilized in the aeration basins: the first was a pumped flow with induced air aeration and mixing system and the second was a floor-mounted medium-bubble-diffused aeration system used in conjunction with submersible mixers.
Gonzalez Cos. LLC presented the options to JBS. The company selected the submerged medium-bubble-diffused aeration system with slow-speed, large-blade submersible mixers. This was the most cost-effective alternative based on higher oxygen transfer efficiency and lower overall horsepower requirements.
When determining the alpha factor to be used for the design of the medium-bubble sub- merged aeration system, the design engineer relied on a technical paper titled “Alpha Factors in Full Scale Wastewater Aeration Systems,” presented at the 2006 Water Environment Federation Technical Exposition and Conference and authored by Diego Rosso and Michael K. Stenstrom of the University of California – Los Angeles Civil and Environmental Engineering department. The paper presented the following alpha factors: 0.490 for conventional activated sludge, 0.650 for nitrification and 0.715 for nitrification and denitrification.
The authors stated that the higher alpha factor for nitrification and denitrification was due to “... the prompt uptake of low molecular weight surfactant by the selectors in the denitrification zone.” An alpha factor of 0.65 was used for design, although the expectation was that the aeration system likely would achieve an alpha factor of 0.715.
Energy-Wise Mixing
Gonzalez Cos. LLC originally considered the medium-bubble-diffused aeration system with high-speed submersible mixers. KSB Inc. however, approached the design engineer with the concept of mixing the basins with their high-efficiency, slow-speed, large-blade mixers. The engineer recommended the latter option to JBS due to the significant energy savings it would yield, and JBS accepted this approach.
The submerged mixing system consisted of 12 submersible mixers with a total of 36 hp. Two Amaprop slow-speed mixers (each 2.2 hp, 29 rpm) were installed in each of the anoxic zones, and four Amaprop slow-speed mixers (each 3.4 hp, 34 rpm) were installed in each of the aeration basins. The result was a mixing power reduction of more than 200 hp to mix the basins as compared with using high-speed, small-blade mixers. Startup took place in September 2011.
JBS has stated that the low-energy mixing system was exactly what it was seeking, as it provides both effective mixing and high oxygen transfer efficiency at minimal energy demand. Because mixing of the mixed liquor in the aeration basin does not require air from the diffusers, the new mixers in the aeration basin also allow the aeration system to cycle on/off to obtain a higher rate of denitrification. This leads to the added benefit of alkalinity recovery.
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