Comparison of oxygen transfer parameters and oxygen demands in bioreactors operated at low and high dissolved oxygen levels

ABSTRACT

The proper design of aeration systems for bioreactors is critical since it can represent up to 50% of the operational and capital cost at water reclamation facilities. Transferring the actual amount of oxygen needed to meet the oxygen demand of the wastewater requires α- and β-factors, which are used for calculating the actual oxygen transfer rate (AOTR) under process conditions based on the standard oxygen transfer rate (SOTR). The SOTR is measured in tap water at 20°C, 1 atmospheric pressure, and 0 mg L⁻¹ of dissolved oxygen (DO). In this investigation, two 11.4-L bench-scale completely mixed activated process (CMAS) reactors were operated at various solid retention times (SRTs) to ascertain the relationship between the α-factor and SRT, and between the β-factor and SRT. The second goal was to determine if actual oxygen uptake rates (AOURs) are equal to calculated oxygen uptake rates (COURs) based on mass balances. Each reactor was supplied with 0.84 L m⁻¹ of air resulting in SOTRs of 14.3 and 11.5 g O₂ d⁻¹ for Reactor 1 (R-1) and Reactor 2 (R-2), respectively. The estimated theoretical oxygen demands of the synthetic feed to R-1 and R-2 were 6.3 and 21.9 g O₂ d⁻¹, respectively. R-2 was primarily operated under a dissolved oxygen (DO) limitation and high nitrogen loading to determine if nitrification would be inhibited from a nitrite buildup and if this would impact the α-factor. Nitrite accumulated in R-2 at DO concentrations ranging from 0.50 to 7.35 mg L⁻¹ and at free ammonia (FA) concentrations ranging from 1.34 to 7.19 mg L⁻¹. Nonsteady-state reaeration tests performed on the effluent from each reactor and on tap water indicated that the α-factor increased as SRT increased. A simple statistical analysis (paired t-test) between AOURs and COURs indicated that there was a statistically significant difference at 0.05 level of significance for both reactors. The ex situ BOD bottle method for estimating AOUR appears to be invalid in bioreactors operated at low DO concentrations (<1.0 mg L⁻¹).

KEYWORDS:

• α-factor
• β-factor, inhibition
• nitrite accumulation
• oxygen demand
• oxygen transfer rate

Funding

The authors would like to express their appreciation to the Vice Provost of Research at Mercer University for funding Seed Grant #227410.