X= Digester total suspended solids, mg/L
Kj= Reaction rate constant, d~
l. May range from 0.05 d~
l
at 15
0
C (59
0
F) to 0.14
d~
l
at 25
0
C (77
0
F) (assume 0.06 d~
l
at 15
0
C)
Pv = Volatile fraction of digester suspended solids (expressed as a decimal) = 0.8
(80%) as stated in the initial assumptions.
Qc = Solids retention time (sludge age), d
Using values obtained above with winter conditions governing, the aerobic digester vol-
ume is:
V= (1,224 rV/d)(50,OOOmg/L) 982ft
3
f6222m
3
^
(50,000 mg/L)(0.7)[(0.06 0(0.8)+1/31.7 d] ' l ' 'The air requirement per 1,000 ft
3
(2.8 m
3
) of digester volume with summer conditions
governing is:
Volume of Air =
1328 ft /inm
= 59.41 ft3
/min/10
3
ft
3
(0.97 m
3
/min/Mm
3
)
21.7o2 10 itThe mixing requirements for diffused aeration range from 20 to 40 ft^3 /min/10^3 ft^3 (0.32 to
0.64 m^3 /min/Mm^3 ). Therefore, adequate mixing will prevail.
DESIGN OF AN AERATED GRIT CHAMBER
Domestic wastewater enters a wastewater treatment facility with an average daily flow
rate of 4.0 Mgd (15,140 L/d). Assuming a peaking factor of 2.5, size an aerated grit cham-
ber for this facility including chamber volume, chamber dimensions, air requirement, and
grit quantity.
Calculation Procedure:- Determine the aerated grit chamber volume
Grit removal in a wastewater treatment facility prevents unnecessary abrasion and wear
of mechanical equipment such as pumps and scrappers, and grit deposition in pipelines
and channels. Grit chambers are designed to remove grit (generally characterized as non-
putrescible solids) consisting of sand, gravel, or other heavy solid materials that have set-
tling velocities greater than those of the organic putrescible solids in the wastewater.
In aerated grit chamber systems, air introduced along one side near the bottom causes
a spiral roll velocity pattern perpendicular to the flow through the tank. The heavier parti-
cles with their correspondingly higher settling velocities drop to the bottom, while the
rolling action suspends the lighter organic particles, which are carried out of the tank. The
rolling action induced by the air diffusers is independent of the flow through the tank.
Then non flow dependent rolling action allows the aerated grit chamber to operate effec-
tively over a wide range of flows. The heavier particles that settle on the bottom of the
tank are moved by the spiral flow of the water across the tank bottom and into a grit hop-
per. Screw augers or air lift pumps are generally utilized to remove the grit from the hop-
per.
The velocity of roll governs the size of the particles of a given specific gravity that