Environmental Engineering Reference
In-Depth Information
Sludge Retention Time
Sludge retention time (SRT) is the length of time the sludge remains in the digester:
Digester volume(gal)
Sludge volu
SRT(days)
=
(5.3)
me addedper day(gpd)
■
Example 5.3
Problem:
Sludge is added to a 525,000-gal digester at the rate of 12,250 gal per day.
What is the sludge retention time?
Solution:
Digester volume
Sludge volumeaddedper
525,000 gal
12,250 gpd
SRT
=
=
=
42.9 days
day
Estimated Gas Production in Cubic Feet/Day
The rate of gas production is normally expressed as the volume of gas (ft
3
) produced
per pound of volatile matter destroyed. The total cubic feet of gas that a digester will
produce per day can be calculated by
3
Gasproduction
=
VM
(lb/day)
×
%VMreduction
× Production rate (ft /lb)
(5.4)
in
■
Example 5.4
Problem:
The digester receives 11,450 lb of volatile matter per day. Currently, the
volatile matter reduction achieved by the digester is 52%. The rate of gas production
is 11.2 ft
3
of gas per pound of volatile matter destroyed.
Solution:
Gas production = 11,450 lb/day × 0.52 × 11.2 f t
3
/lb = 66,685 ft
3
/day
Percent Volatile Matter Reduction
Because of the changes occurring during sludge digestion, the calculation used to
determine percent volatile matter reduction is more complicated:
(
)
×
%VM%VM
%VM
−
100
in
out
%VMreduction
=
(5.5)
[
]
(
)
−
VM
×
%VM
out
in
in
■
Example 5.5
Problem:
Using the data provided below, determine the percent volatile matter
reduction for the digester:
Raw sludge volatile matter = 74%
Digested sludge volatile matter = 55%
