Wastewater Calculations - Mathematics and Statistics for the Environment - page 725

Labtestchemical (m g) × Digester volume × 3.785

Chemical required(lb) = (24.136)

Sample volume(L) × 454 g/lb × 1000 mg/g

■ EXAMPLE 24.105
Problem: 240 mg of lime will increase the pH of a 1-L sample of the aerobic digester contents to
pH 7.1. The digester volume is 240,000 gal. How many pounds of lime will be required to increase
the digester pH to 7.3?

Solution:

240 mg × 240,000 gal × 3.785 L/ga l

Lime required = = 480 lb

1L × 454 g/lb × 1000 mg/g

24.13.3 a erobiC t anK v olume C alCulations
The aerobic tank volume can be computed, where no significant nitrification will occur, by the fol-
lowing equation (WPCF, 1985):

( )

QX YS
XKP

+

i i i

V = (24.137)

( )

+ 1 θ

dv c

where
V = Volume of aerobic digester (ft 3 ).
Q i = Influent average flow rate to digester (ft 3 /d).
X i = Influent suspended solids concentration (mg/L).
Y = Fraction of the influent BOD consisting of raw primary sludge (in decimals).
S i = Influent BOD (mg/L).
X = Digester suspended solids concentration (mg/L).
K d = Reaction-rate constant (per day).
P v = Volatile fraction of digester suspended solids (in decimals).
θ c = Solids retention time (days).

■ EXAMPLE 24.106
Problem: The pH of an aerobic digester is found to have declined to 6.1. How much sodium hydrox-
ide must be added to raise the pH to 7.0? The volume of the digester is 370 m 3 . Results from jar tests
show that 34 mg of caustic soda will raise the pH to 7.0 in a 2-L jar.

Solution:

NaOH required = (34 mg) ÷ (2 L) = 17 mg/L = 17 g/m 3

NaOH to be added = 17 g/m 3 × 370 m 3 = 6290 g = 6.3 kg = 13.9 lb

24.13.4 a naerobiC d igestion p roCess C ontrol C alCulations
The purpose of anaerobic digestion is the same as aerobic digestion: to stabilize organic matter,
to reduce volume, and to eliminate pathogenic organisms. Equipment used in anaerobic digestion
includes an anaerobic digester of either the floating or fixed cover type. These include biosolids
pumps for biosolids addition and withdrawal, as well as heating equipment such as heat exchangers,
heaters and pumps, and mixing equipment for recirculation. Typical ancillaries include gas storage,