Comparison of Oxygen Transfer and Uptake Between an Integrated Fixed-Film Activated Sludge (IFAS) Process and a Conventional Activated Sludge Process (ASP)

Abstract

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

The use of nonlinear mono kinetics in modeling a mixing aeration tank with a biofilm on additional loading

“…In particular, it is essential to consider the effect produced on the formation of the active part of the biofilm in time by such important processes as growth, accumulation, decay and separation on the surface. It is also important to ensure the processes of purification (oxidation) by the required amount of oxygen, which has already been partially considered in works [25][26][27].…”

The Changing and Distribution Laws of Oxygen Transfer Efficiency in the Full-Scale IFAS Process

“…Thus, the specific surface area of bubbles increased. In addition, the blocking effect of the carriers could increase the path of the bubbles [32] and prolong the residence time of bubbles [33]. In addition to the high turbulence intensity, the oxygen mass transfer rate at the gas-liquid interface was improved.…”