Environmental Engineering FOURTH EDITION

186 WIRONMENEL ENGINEERING

Influent *

*

Influent Aeration

Short

aeration

(sorption)

Aeration

d Influent Return sludge

Return sludge

Tapere Aeration Step Aeration

Figure 9-13. Diagrams of tapered aeration and step aeration.

Return Air

Sludge

+++++++

Long

aeration

(bacterial growth)

Figure 9-14. The biosorption modification of the activated sludge process.

lattice structure of their filaments that prevents settling5 A trend toward poor settling
may be the forerunner of a badly upset and ineffective system. The settleability of
activated sludge is most often described by the sludge volume index (SVI), which is
determined by measuring the volume occupied by a sludge after settling for 30 min in
a 1-L cylinder, and calculated as
(lOOO)(volume of sludge after 30 min, mL)
mgL of SS

SVI = (9.11)

EXAMPLE 9.5. A sample of sludge has an SS concentration of 4000 mgL After settling

for 30 min in a 1-L cylinder, the sludge occupies 400 mL. Calculate the SVI:

= 100.

(1 000) (400 mL)

4000mg/L

SVI =

SVI values below 100 are usually considered acceptable; SVI > 200 is a badly

bulking sludge. Some common loadings, as a function of the SVI, for final clarifiers are

5Y0u may picture this as a glass filled with cotton balls. When water is poured into the glass, the cotton

filaments are not dense enough to settle to the bottom of the glass.