Comparative Analysis of Parallel IFAS and ASP Reactors
Authors:
- Diego Rosso - University of California, Irvine
- Don Howard - Greensboro Water Resources
- Sarah E. Lothman, Alan L. Stone, W. James Gellner, Paul Pitt - Hazen and Sawyer
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Figure 1: Off-gas testing locations for the IFAS (Tank 12, top) and ASP (Tank 11, bottom) reactors.
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Figure 2: Oxygen uptake rate, air use, and oxygen transfer efficiency in process water for both processes.
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Figure 3: Breakdown of air use for mixing and oxygen transfer for ASP (left) and IFAS (right).
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Figure 1: Off-gas testing locations for the IFAS (Tank 12, top) and ASP (Tank 11, bottom) reactors. -
Figure 2: Oxygen uptake rate, air use, and oxygen transfer efficiency in process water for both processes. -
Figure 3: Breakdown of air use for mixing and oxygen transfer for ASP (left) and IFAS (right).
The T.Z. Osborne Water Reclamation Facility located in Greensboro, NC is a 40-mgd water reclamation plant operating BNR with surface water discharge. Several process alternatives were evaluated as strategies to meet stringent forthcoming regulatory nutrient limits. Integrated fixed film activated sludge (IFAS) is one of the alternatives being considered for implementation.
A full scale IFAS pilot with AnoxKaldnes media and coarse bubble aeration were installed and a year-long study carried out to quantify nitrification kinetics, aeration requirements, process performance, and identify potential operational issues. In order remove kinetic limitations associated with DO diffusional gradients through the fully aerobic biofilm, the IFAS system was operated at an elevated dissolved oxygen (DO) concentration. As part of the full-scale evaluation, an off-gas test was performed as described by the ASCE Protocol (1997). Previously, one off-gas test was performed by another IFAS manufacturer (Viswanathan, 2008). To our knowledge, our test is the first independent off-gas test on an IFAS system to date.
The IFAS process is characterized by elevated air flux and air use per unit load treated, due to elevated mixing requirements and high DO required to prevent oxygen diffusion limitations within the biofilm, with associated lower OTE and αSOTE. Note that for Tank 12 only the IFAS reactor cells (testing positions 1-6 in Fig. 1) contain coarse bubble diffusers. The remaining aerobic cells in Tank 12 (testing positions 7-9) contain fine bubble diffusers, as does the entire activated sludge process (ASP).
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 (Fig. 2). Throughout the coarse bubble aeration zones (positions 1-6), the IFAS has elevated air flow and therefore the relative air use is in the range of 1.3-3.0 times that of the ASP. Although the exact fraction of air flow used for mixing cannot be quantified, it is possible to calculate the fraction of air flux used for oxygen transfer from off-gas data, then calculate the fraction of air flux used for mixing by subtraction (Fig. 3).
Although relative patterns are similar, the IFAS has roughly double the air use for mixing when compared to the ASP.
Profiles for greenhouse gases and energy footprint calculations will also be discussed in the full-featured paper.
References
ASCE (1997) Standard Guidelines for In-Process Oxygen Transfer Testing, ASCE, NY, NY
Viswanathan, S., et al (2008) Evaluation of Oxygen Transfer Efficiency via Off-gas Testing at Full Scale Integrated Fixed film Activated Sludge Installation, Proc. WEFTEC 2008 Conference.
