Water Quality Management - River Dynamics and Integrated River Management - page 581

The tool for implementation of the pollutant clean up programs is the National Pollution Discharge
Elimination System (NPDES) permit system, which requires industrial and municipal point source
dischargers to obtain wastewater discharge permits that limit the quantity of pollution that can be discharged
into the receiving water (Novotny, 2003, p. 12). The NPDES permit limit, in most cases, is based on the
technological limitations of the available treatment technologies, the so-called “Best Available Technology
Economically Achievable,” and is unrelated to the waste assimilation capacity of the receiving water body.
Failure to meet permit limits can result in high fines and even closure of a chronically offending facility.
Instream aeration— In the U.S., despite the fact that all industrial wastewater and nearly all domestic
wastewater receives biological (secondary) treatment and a growing portion of the wastewater receives
tertiary treatment. Many rivers still suffer from low DO concentrations. For example, the 2008 report to
the USEPA on water body impairment in Illinois (the 303d report named after Section 303d of the CWA)
indicated that 282 river reaches spanning more than 3000 river miles were impaired because of low DO.
In these cases, it may be necessary to introduce additional DO to the river. Davis and Masten (2004,
p. 293) note that often a relatively inexpensive method for improving stream quality is to add oxygen to
the wastewater to bring it close to saturation prior to discharge. The Fox River Water Pollution Control
Center in Brookfield, Wis., U.S., does this, and its NPDES permit limits require it to discharge effluent
with DO concentrations at or above the following values throughout the year: 9.5 mg/L (January-
February), 9.0 mg/L (March-May and November-December), 8.5 mg/L (June and October), and 8.0 mg/L
(July-September).
Consideration of the addition of instream aeration stations to improve DO concentrations has been
done since at least the late 1960s. The presumption, in the early studies, was strong that on any river
where secondary treatment of effluents is insufficient to achieve desired DO concentrations, instream
aeration will be a more economical means than advanced waste treatment to achieve desired dissolved
oxygen levels (Whipple et al., 1970). A study done by the Metropolitan Water Reclamation District of
Greater Chicago (MWRDGC) confirmed this as they found instream aeration was 7.5 times cheaper than
advanced treatment for meeting DO standards in the Calumet-Sag Channel (Robison, 1994).Whipple et
al. (1970) studied the possibility of using mechanical surface aerators and submerged diffuser aerators.
They found the oxygen transfer rates of the diffuser aerators tested were only about two-thirds of those of
the mechanical aerators, for comparable conditions. The costs of operation per horsepower-hour were
almost the same for the two types of aerators. Thus, they concluded that, based upon the much lower
transfer rate of diffusers, the mechanical aerator would be more economical by a wide margin. However,
they also concluded there are certain other factors in favor of the diffuser aerators, particularly the lesser
interference with boating, and the fact that the mechanical aerators must be serviced to remove heavy
drift, and to adjust cables or remove the aerators in time of flood. Hence, mechanical surface aerators
generally are not considered for practical applications of instream aeration.
A wide variety of devices have been used for aeration in wastewater treatment facilities, but a only a
few different types of devices have been installed in rivers. The following subsections summarize devices
that have been installed and/or are under consideration for use in the Chicago Waterway System.
Porous Ceramic Diffusers inject atmospheric oxygen supplied by blowers located on shore that travels
trough a manifold system to the instream diffusers located near the bottom of the river. Figures 9.13 and
9.14 show a schematic diagram and a photograph of the operation of the Devon Avenue Instream Aeration
Station in Chicago, which is one of two ceramic diffuser aeration stations operated by the MWRDGC
since the early 1980s. The Oxygen Transfer Efficiency (OTE) of ceramic diffusers increases with depth
and spacing of the diffusers and particularly with fineness of bubble. Fine bubble diffusers are subject
to clogging, and, thus, coarse bubble diffusers are used in practice, such as at Devon Avenue. The North
Branch of the Chicago at Devon Avenue is at least 3 m deep, which is a sufficient for a reasonable OTE.