Head-to-Head Continuous Aeration Performance Monitoring of Full-Scale Densified and Conventional Activated Systems

[b]BACKGROUND AND OBJECTIVES[/b] Increased loads and constrained facility footprints are driving water resource recovery facilities (WRRFs) to adopt intensification strategies that increase treatment capacity without expanding tankage. Among these, densified activated sludge (DAS) is a promising and cost-effective approach that selectively wastes poorly settling floc through hydrocyclones, reducing the sludge volume index (SVI), increasing biomass inventory, and enabling higher clarifier loading rates. The benefits of DAS are well-documented, but little information is available on its aeration performance under full-scale conditions, particularly for in-process inefficiencies quantified by the alpha factor (aF). Because aeration represents the dominant energy demand at WRRFs, reliable aF values are essential for both proper design and operation of aeration systems. Metro Water Recovery's Robert W. Hite Water Treatment Facility (RWHTF) in Denver, Colorado, treats approximately 130 mgd of municipal wastewater. To evaluate process intensification strategies, a full-scale 2-MG treatment train, with a dedicated secondary clarifier and return activated sludge (RAS) was operated as a densification pilot in parallel with 11 conventional activated sludge (CAS) treatment trains. To assess aeration performance and generate representative aF values suitable for scale-up design, a continuous, side-by-side off-gas testing campaign was conducted on the DAS and CAS systems. The DAS system was configured as an anaerobic-anoxic-oxic (A2O) process in the North Aeration Basin 2 (NAB2), while the CAS train operated as a MLE process in the NAB4. In the DAS system, the IMLR can be routed to multiple upstream zones, but usually A3 (A2, A3 and A4, see Fig. 1). A4, a swing zone, was operated as anoxic during the testing. [b]TESTING METHODOLOGY[/b] Continuous off-gas testing was performed using the I-FLOAT™ system (Inflatable Fast Lightweight Off-gas Analysis Technology), enabling rapid deployment and automated, continuous monitoring of aeration performance and process emissions (see Fig. 2). Three locations per basin were monitored over 24-hour periods. Concurrent side-by-side testing was conducted using two hoods connected to a single analyzer alternating every 10 minutes, yielding one data point per zone every 20 minutes (see Fig 1). Testing followed ASCE 18-18 guidelines (ASCE, 2018). [b]RESULTS AND FINDINGS[/b] During testing, the DAS system operated at ~25% granule mass fraction, MLSS of 5,000 mg/L, and SVI₃₀ of 68 mL/g, compared to <5% granulation, 3,900 mg/L MLSS, and SVI₃₀ of 89 mL/g for CAS. Aerobic/total SRTs were approximately 11/14 days (DAS) and 6/8 days (CAS). Despite these differences, in Zone A6, DAS and CAS exhibited similar aeration performance, with average OTEs of ~8.3–8.4% and aF values of 0.42. Diurnal aF ranged from 0.32 to 0.55, highlighting substantial diurnal variability and the importance of performing continuous monitoring over spot-check testing. This zone operated at relatively high air fluxes (2.2 scfm/diffuser for DAS; 2.7 for CAS). Greater differences were observed in Zone B4, where average OTEs were 14.4 (DAS) and 22.0 (CAS), corresponding to aF values of 0.72 and 0.91. Both systems operated at lower air fluxes, with CAS exhibiting both higher aF and higher OTE (see Table 1). The lower aF observed for DAS in Zone B4 may be influenced by differences in bubble coalescence associated with higher MLSS and potential effects of microbial by-products (e.g., SMPs or EPS, Germain et al., 2007). However, these mechanisms were not confirmed, and MLSS differences alone are unlikely to fully explain the differences in aF, based on previous aF-MLSS correlation studies (Krampe & Krauth, 2003).The absence of similar discrepancies in Zone A6 suggests convergence of bubble behavior at higher air fluxes, regardless of differences in MLSS or microbial by-products. In Zone C4, DAS results were considered representative (OTE 13.4, aF 0.83), although data loss occurred over night. CAS data were excluded due to an air leakage that compromised the data. Oxygen uptake rates (OURs) were estimated via a similar approach presented in Reifsnyder et al., 2018. OURs values for CAS were approximately 15–20% higher than DAS across Zones A6 and B4. [b]CONCLUSIONS AND RELEVANCE[/b] This study presents a first-of-its-kind, continuous, full-scale, side-by-side off-gas comparison of DAS and CAS systems. Results indicate that in initial aerobic zones operating at higher air fluxes, DAS and CAS exhibit comparable aF values, supporting similar aeration sizing assumptions. However, at lower air fluxes in downstream zones, DAS systems exhibited reduced aF relative to CAS. Results imply that under low-air-flux conditions, downstream air demands in DAS systems may be approximately 15–20% higher than CAS to achieve equivalent oxygen transfer rates, which translates to larger aeration equipment sizing, increased capital costs, and higher operating expenses. These findings provide early, design-relevant insights into mechanistic differences in aeration performance of DAS and CAS systems, and provide the basis for future studies. Results reflect performance at the tested granule mass fraction. Further evaluation at higher fractions (e.g., >40%) is needed. Ongoing testing at re-balanced air fluxes will refine aF guidance for DAS systems under typical operating conditions.