Effect of biocarrier on oxygen transfer efficiency
The existence of biocarrier certainly affects OTE, but it is not completely intuitive. In one hand, the biocarrier may increase the contact time of bubble with water by interfering the bubble rise. On the other hand, biocarriers may act as a catalyst of bubble coalescence and reduce the total surface area. There are not many publications based on scientific methodologies exist on this topic, but following are the some of the outcomes of recent studies.
A full scale integrated fixed film sludge (IFAS) process with AnoxKaldnesTM biocarrier and coarse bubble aeration was tested along with activated sludge process (ASP). The process diagrams are shown in Fig. 1, where biocarriers are added only the first six aeration tanks of IFAS process. Process conditions are summarized in Table 1. Oxygen transfer efficiency (OTE) was measured using the American Society of Civil Engineer’s (ASCE) standard method based on steady-state method described here. Since oxygen must penetrate through the biofilm in IFAS process, dissolved oxygen (DO) was elevated to above 3 mg/L in IFAS versus 1-2 mg/L in ASP (Rosso, 2011).
Table 1. Plant operating condition during the study (Rosso, 2011)
Fig. 2 shows OTE comparison between IFAS and ASP. In spite of the much lower MLSS of IFAS, it is clear that OTE in the first six tanks that contain biocarriers were much lower than their counterpart in ASP. The air consumption of IFAS was consistently higher than that of ASP during the course of the study (Rosso, 2011). The large difference can be attributed to the lower α-factor and lower driving force for oxygen dissolution at higher DO.
If the effect of DO on OTE is eliminated by using αSOTE as shown in Fig. 3, the difference between IFAS and ASP became much smaller, but αSOTE of IFAS still seemed slightly higher. Here, αSOTE is the OTE in mixed liquor at zero DO, 20 oC, and zero salinity (see here for more detail).
Fig. 2. Comparative OTE profiles for ASP and IFAS during the January and June tests. IFAS cells are installed only at testing positions 1-6, and positions 7-9 are ASP for both tanks (Rosso, 2011).
Fig. 3. Comparative αSOTE profiles for ASP and IFAS during the January and June tests (Rosso, 2011).
Very similar results were obtained in other studies (Viswanathan, 2008), where OTE and αSOTE (or SOTEPW in the paper) were measured by off-gas analysis method in ASP and IFAS. It was found that ASP had slightly higher αSOTE (12.6% vs 11.7%). The α-value, which indicates the relative OTE in process water to that in clean water as describedhere, was calculated at 0.646 and 0.6 for ASP and IFAS, respectively. Though dissolved oxygen (DO) are not indicated in the paper, assuming higher DO is necessary for IFAS, the actual OTE of ASP can be considered 10-30% higher than the α-factor suggests.
It is also noticeable that ASP proceeds to IFAS in the serially connected plug flow reactors used in the experiment. Since OTE tends to be lower in upstream in plug flow reactors as discussed here, the ASP might have some extent of disadvantages in oxygen transfer. Therefore, if the αSOTE in those two processes were compared in side-by-side tests as the other experiment discussed above (Rosso, 2011), bigger differences of αSOTE are expected.