Integrated fixed-film activated sludge (IFAS) processes are a combination of biofilm reactors and activated sludge processes, achieved by introducing and retaining biofilm carrier media in activated sludge processes. We tested a full-scale IFAS process equipped with AnoxKaldnes media and coarse-bubble aeration. This process operated independently in parallel with an existing full-scale activated sludge process. Both processes achieved the same percent removal of COD and ammonia, despite the double hydraulic load and double oxygen demand on the IFAS process. In order to prevent kinetic limitations associated with dissolved oxygen (DO) diffusional gradients through the IFAS biofilm and to avoid media coalescence on the reactor surface and promote biofilm contact with the substrate, high DO and high mixing requirements are specified for the IFAS system. These require an elevated air flux in the IFAS process, which was much higher than that of the parallel activated sludge process. Even though the air used per unit load removed should be the same for both processes, the IFAS reactors were characterized by higher air flux and air use per unit load treated due to the high DO and mixing requirements. This directly affected the energy footprint for aeration, which in this case was much higher for the IFAS system than activated sludge. KEY WORDS -activated sludge; integrated fixed-film activated sludge (IFAS); oxygen transfer; nutrient removal; energy footprint; aeration 368 WEFTEC 2011 NOMENCLATURE AFR = air flow rate (m 3 s -1 ) A TANK = tank bottom area BHP = power drawn by the blower (kW) c = molecular weight of air (kg kmol -1 ) COD = chemical oxygen demand (mg O2 l -1 ) DWP = dynamic wet pressure (Pa) h L = head loss in the air distribution line (Pa) J air = Air flux (m s -1 ) MCRT = mean cell retention time (d) MLSS = mixed liquor suspended solids (mg TSS l -1 ) MLVSS = mixed liquor suspended solids (mg VSS l -1 ) OD = oxygen demand (kg O2 s -1 ) OTE = oxygen transfer efficiency (%) OTR = oxygen transfer efficiency (kg O2 s -1 ) OUR = oxygen uptake rate (mg O2 l -1 h -1 ) P d = discharge pressure (Pa) P i = inlet pressure (Pa) Q = influent flow rate (m 3 d -1 ) R = universal gas constant (8.314 J mol -1 K -1 ) SOTE = standard OTE in clean water (%) SOTR = standard oxygen transfer efficiency (kg O2 s -1 ) T = ambient temperature (K) W air = ponderal air flow (kg s -1 ) Z = hydrostatic pressure corresponding to diffuser submergence (Pa) greek letters α = alpha factor = αSOTE / SOTE (-) αSOTE = Standard OTE in process water (%) ε = energy footprint per unit oxygen demand oxidised = (kWh kg O2 -1 ) γ = ratio of specific heats at constant pressure and volume (0.283 for air) η = combined motor and blower efficiencies (-) ρ air = air density (kg air m -3 air ) 369 WEFTEC 2011
A full scale IFAS pilot with AnoxKaldnes media and coarse bubble aeration (IFAS tanks only) was installed at the T.Z. Osborne Water Reclamation Facility located in Greensboro, NC, and a year-long study carried out to quantify nitrification kinetics, aeration requirements, process performance, and identify potential operational issues. As part of the full-scale evaluation, an off-gas test(s) was performed as described by the ASCE Protocol. In order to remove kinetic limitations associated with DO diffusional gradients through the biofilm, the IFAS system was operated at an elevated dissolved oxygen concentration. The IFAS process is characterized by elevated air flux and air use per unit load treated, due to elevated mixing requirements and the high DO required to prevent oxygen diffusion limitations within the biofilm, with associated lower OTE and αSOTE. In theory, when OTE is the same, the air used per pound of COD removed is expected to be the same. Nevertheless, the mixing requirements specified by the IFAS manufacturers affect air use. Throughout the coarse bubble aeration zones, the IFAS has consistently higher air flow and therefore the relative air use compared to ASP. Also, the IFAS has roughly double the air use for mixing when compared to the ASP. Nitrous oxide in the off-gas was measured in the Winter test, and at this time the data is still insufficient to support any conclusion on the two processes.
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