Solids Handling - Wastewater Operators - page 378

10.6.1.1.4 ph adjustment

Occasionally, the pH of the aerobic digester will fall below the lev-
els required for good biological activity. When this occurs, the operator
must perform a laboratory test to determine the amount of alkalinity
required to raise the pH to the desired level. The results of the lab test
must then be converted to the actual quantity of chemical (usually lime)
required by the digester.

Chemical Used (mg) × gester Vol. (MG) Di × 3.785 L/gal

Chemical
Required (lb)

(10.20)

=

Sample Vol. (L ))454 g/lb × × 1000 mg/g

 Example 10.20

Problem: The lab reports that 225 mg of lime were required to increase
the pH of a 1-L sample of the aerobic digester contents to pH 7.2. The
digester volume is 240,000 gal. How many pounds of lime will be required
to increase the digester pH to 7.2?

Solution:

225 mg × 240,000 gal × 3.785 L/gal

Chemical Required = = 450lb

1L × 454 g/lb × 1000 mg/g

10.6.2 anaerobic digestion

Anaerobic digestion is the traditional method of sludge stabiliza-
tion that involves using bacteria that thrive in the absence of oxygen. It
is slower than aerobic digestion but has the advantage that only a small
percentage of the wastes is converted into new bacterial cells. Instead,
most of the organics are converted into carbon dioxide and methane
gas. In operation, process residual (thickened or unthickened sludge) is
pumped into the sealed digester. The organic matter digests anaerobi-
cally by a two-stage process. Sugars, starches, and carbohydrates are
converted to volatile acids, carbon dioxide, and hydrogen sulfide. The
volatile acids are then converted to methane gas. This operation can
occur in a single tank (single stage) or in two tanks (two stages). In a
single-stage system, supernatant and digested solids must be removed
whenever flow is added. In a two-stage operation, solids and liquids
from the first stage flow into the second stage each time fresh solids are
added. Supernatant is withdrawn from the second stage to provide addi-
tional treatment space. Periodically, solids are withdrawn for dewater-
ing or disposal. The methane gas produced in the process may be used
for many plant activities.

Equipment used in anaerobic digestion includes a sealed digestion
tank with either a fixed or a floating cover, heating and mixing equip-
ment, gas storage tanks, solids and supernatant withdrawal equipment,