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5,12 CHAPTER FIVE

Air pressure requirements depend on submerging diffusers and friction loss through
piping. Power requirements vary from 0.5 to 2.0 kW/mgd (0.00013 to 0.00053 kW/m 2
per day), with the average about 1.0 kW/mgd (0.00026 kW/m 2 per day). When porous
plates or tubes are used, air should be filtered to avoid clogging of the diffusers. "Oil-
free" compressors should be used to avoid contamination of the air.
Diffuser-type aerators require less space than spray aerators and generally more than
tray aerators. They have practically no head loss through diffusion units, and this is usu-
ally an important aspect in overall plant design. Aeration units have few cold weather op-
erating problems, and in moderate climates there is no need to house them. In some in-
stances, diffusion aeration basins are used to provide chemical mixing.

Design of Spray Aerators
Exposure time for each drop from a spray aerator depends on its initial velocity and tra-
jectory. Drop size, and the resulting area-volume ratio, is a function of the dispersing ac-
tion of the nozzle. The initial velocity V of a drop emerging from an orifice or nozzle ap-
pears in the formula
V = Cv ~/2gh
and the discharge by the equation
O = " gg
where h = total head on nozzles, ft
g = acceleration from gravity, ft/s 2
A = area of opening, ft 2
Cv = coefficient of velocity
Cd = coefficient of discharge (Ca = CvCc, where Cc is coefficient of contraction)

Coefficients of velocity, contraction, and discharge vary with the shape and other char-
acteristics of the orifice or nozzle.
The trajectory of the spray used in an aerator may be vertical or inclined. If the angle
between the initial velocity vector and horizontal is zero, theoretical exposure time t of
the water drops is given by the formula

t = 2Cv sin 0 k/2 h

The sine of an angle of less than 90 ° is less than 1.0, so a vertical jet gives the longest
exposure time for a given value of h. But an inclined jet has the advantage of a longer
path and less interference between falling drops. Wind also influences the path of the ris-
ing and falling drops, so an allowance must be made for its action.
Nozzle design is important in achieving optimum dispersion of water. Among special
designs used are rifled nozzles, centrifugal (West Palm Beach) nozzles, Sacramento float-
ing cones, impinging devices, and rotating reaction nozzles.
The size, number, and spacing of spray nozzles depend on the head of water being
used, space available for aeration facilities, and interference between adjacent sprays. The-
oretically, numerous small nozzles capable of producing atomized water would be the
most efficient design. However, from a practical standpoint, very small nozzles should be
avoided because of clogging and high maintenance requirements. Nozzles used in most
spray aerators are 1.0 to 1.5 in. (2.5 to 3.8 cm) in diameter and have discharge ratings of