Static Aeration Systems —
Problems and Performance
R.O. BALL, Waste Consultant
H.J. CAMPBELL, Jr., Waste Consultant
Engineering Service Division
E.I. Du Pont de Nemours & Company
Wilmington, Delaware 19898
INTRODUCTION
Since the discovery of the activated sludge process by Ardern and Lockett in 1914 (1), a
variety of aeration systems have been investigated. Initially, large bubble, diffused aeration
systems were used, mainly in municipal applications. For industrial applications,
mechanical forms of aeration, such as high and low speed surface mixing and submerged
turbine mixing, have been employed. In recent years, several types of high efficiency
aeration systems have been proposed.
It can be seen from the oxygen transfer equation that there are several ways that
aeration efficiency can be improved:
(1)
where.
Dt=
-Kl
Dt=
DO deficit at time t
Da =
DO deficit at time zero
K =
mass transfer coefficient
If the DO deficit is increased for example, then the aeration efficiency will be improved
proportionally. Another method is to increase the mass transfer rate. The static aeration
systems that have been developed thus far attempt to improve the mass transfer rate with
respect to conventional diffused and mechanical types of aeration.
DESCRIPTION OF STATIC AERATION SYSTEMS
Two types of static aerators which have been developed and marketed were studied.
The first type consists of a plastic tube 12 in in diameter, 5 ft in length with five helical
elements. The elements are turned alternately left hand and right hand in the tube to provide
greater turbulence. The second type of static aerator is 18 in in diameter, 6 ft in length, and
has a continuous spiral element in the tube. This element is at approximately the same pitch
as the individual elements in the other type of aerator. A cutaway view of the first type of
aerator is shown in Figure 1, and the second in Figure 2. For each system, air is introduced
through large diameter orifices beneath the tube. The large bubbles are sheared into very
fine bubbles in the tube, which also acts as an air lift pump, thereby mixing the air and
water. This provides for a very high surface area to volume ratio as well as providing
intimate mixing.
The benefits of using these types of aerators, exclusive of their efficiency, are
numerous. The aerator tubes are constructed of plastics, without moving parts, and possess
an advantage for use with industrial wastes where corrosion can be a problem. Also, as the
orifices which deliver the air are approximately jSj-$J in diameter, the possibility of plugging
in the air distribution system is minimized. Due to the relatively low surface turbulence
inherent in these systems, surface misting and heat losses from the aeration tanks are
reduced. The static aerators are also particularly suitable for deep aeration tanks where
328