Environmental Engineering FOURTH EDITION

206 ENVIRONIvENTAL ENGINEElUNG

treatment is known as the yield, expressed as kilograms of suspended solids produced
per kilogram of BOD removed.
Phosphorus removal processes also invariably end up with excess solids. If lime
is used, calcium carbonates and calcium hydroxyapatites solids are formed. Similarly,
aluminm sulfate produces solids in the form of aluminum hydroxides and aluminum
phosphates. Even so-called ‘‘totally biological processes” for phosphorus removal
end up with solids. The use of an oxidation pond or marsh for phosphorus removal
is possible only if some organics (algae, water hyacinths, fish, etc.) are harvested
periodically.
The quantities of sludge obtained from various wastewater treatment processes
may be calculated as shown in Fig. 10-1. The symbols are defined as follows:

So = influent BOD (kgh, 5 days, 2OoC),

Xo = influent suspended solids (kgh),

h = fraction of BOD not removed in primary clarifier,

i = fraction of BOD not removed in aeration or trickling filter,

k = fraction of Xo removed in primary clarifier,

j = fraction of solids not destroyed in digestion,

Y = yield = AX/AS, where AS = hSo - ihSo.

Xe = plant effluent suspended solids (kgh),

AX = net solids produced by biological action (kgh), and

For domestic wastewater, typical values for these constants would be as follows:

So = (250 x x Q) kg/h if Q is m3/h,

So = (250 x 8.34 x Q) kg/h if Q is mgd,

Xo = (220 x x Q) kg/h if Q is m3/h,

Xo = (220 x 8.34 x Q) kg/h if Q is mgd,

k = 0.6,

h = 0.7,

Xe = (220 x

Xe = (220 x 8.34 x Q) mgd

x Q) m3g/h if Q is m3/h,

if Q is mgd,

j = 0.5 for anaerobic,

---_-_-___--_-_

(aerobic or

Primary only Primary & Secondary

Figure 10-1. Schematic diagram of a generalized secondary treatment plant.