Theory to Practice: Oxygen Transfer and the New ASCE Standard

Oxygen transfer is an important part of wastewater treatment and accounts for as much as 60% of the energy consumption for the activated sludge process. Prior to 1984, no standard method for quantifying oxygen transfer existed, which created problems in the design and warranties for treatment plants. The ASCE Standard for the Measurement of Oxygen Transfer in Clean Water and the ASCE Standard Guidelines for In-Process Oxygen Transfer Testing have found widespread application and have reduced the variability in new designs and allowed operators and engineers to access the process operation of existing treatment plants. A new clean water standard is in press as of this writing and the in-process Guidelines are undergoing updating for reissue. This paper illustrates the key concepts of both the Standard and the Guidelines and shows why they are important and reduced the variability of testing. The paper also highlights key new areas of the revised clean water Standard, which includes an optional correction for test water total dissolved solids concentration, and applications to loop (ditch) activated sludge process and the high purity oxygen activated sludge process.

In 1977, under the sponsorship of the US EPA, a Committee organized by ASCE began the study of methods to quantify oxygen transfer rates in wastewater treatment. The Committee met as a group in Asilomar, California in 1978 (US EPA, 1979) and proposed consensus methods for establishing uniform and repeatable test conditions, estimating clean water parameters (mass transfer coefficient or KLA, and equilibrium oxygen concentration *∞C ) from reaeration data, and translating clean water rates to process conditions. The resulting methods were evaluated over next several years by the committee members, consultants and manufactures and refined through the collective experience of the group. The final result was the 1984 version of the ASCE Standard for the Measurement of Oxygen Transfer in Clean Water. The Standard was subsequently improved, updated and republished in 1991 and will be published again in 2006.

Following the development of the Clean Water Standard, it was realized that the next most important gap in knowledge was the characterization of process water transfer rates. The Standard provided ways of calculating expected process water rates from clean water rates, by adjusting for standard conditions, such as barometric pressure, temperature and the effects of the contaminants in the process water (α and β factors for KLA and *∞C , respectively), but there were no consensus-based process water measurement procedures, and a lack of knowledge of the conditions that affect process water testing. The US EPA and ASCE funded a new effort to develop process water testing methods, which were published by the US EPA in 1989 and later adopted into a standard guideline (ASCE, 1997).

Several major changes and improvements were realized over the process of these projects. The first was in the methodology of estimating KLA from the reaeration data. Prior to the Standard, the log deficit method was used which required a priori knowledge of *∞ C . Several methods for specifying *∞ C existed, but all had pitfalls, allowing the introduction of errors which might bias the overall transfer rate by ± 15 to 20% (Boyle, et al. , 1974). The Standard uses a non-linear regression technique which avoids the use of a priori methods and eliminates the possibility of bias. A second major change was the realization that α factors, which prior to 1980 had almost always been routinely specified as 0.8, were dependent not only on just the wastewater type, but also on the aeration devices (i.e., fine pore diffusers have lower α factors than surface aerators, Stenstrom and Gilbert, 1981) and on the conditions of the activated sludge
mixed liquor (i.e., processes operating with longer mean cell retention times (MCRT) have higher α factors that processes operating at low MCRT, Rosso, et al, 2005). A series of other improvements were realized including the impact of cobalt as an interference in the Winkler dissolved oxygen (DO) measurement procedure, the impact of the lag in DO probes on estimates of KLA (Philichi and Stenstrom, 1989). Finally, a new testing methodology, developed as part of the second project, called off-gas analysis was developed and perfected (Redmon, et al., 1983), and this method, in the ten years following its development, has become the method of choice for measuring oxygen transfer in subsurface aeration systems for conventional processes as well as occasional use in novel process such as biological aerated filters (Newman, et al., 2005)

This paper describes some of the key concepts used in developing the Clean Water Standard and illustrates why they are still important. Several new aspects of the new Standard are described. Finally, in a similar fashion, several key concepts used in process water testing are also described.

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