quick method used to determine TAC and fatty volatile acid content (VFA) in liquid sludge

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These determinations are important to ensure that anaerobic digesters operate properly.

Measure 25 mL of sludge as precisely as possible. Centrifuge this sludge once at 5,000 rpm for 10 minutes. Collect the supernatant liquid in a 400 mL beaker. Resuspend the sludge pellet with 50 mL of distilled water, taking care not to lose any of the solids.

Centrifuge the sample again at 5,000 rpm for 10 minutes and collect the supernatant liquor in a beaker. Repeat the sludge pellet wash centrifugation procedure one more time.

The final liquid collected contains the soluble bicarbonates and soluble volatile acids initially present in the sludge.

M-alk. measurement

A magnetic agitator is used to stirr the liquid collected in the beaker. The electrodes of a pH-meter are plunged into the liquid and its initial pH recorded.

Using a 1/10 mL burette, pour H2SO4 0.1 N until pH = 4, or V mL:

Formula: M-alk. measurement

or:
M-alk. = V × 4 × 0.05 in g· L-1of CaCO3.

VFA measurement

Thereafter, pour H2SO4 0.1 N until pH = 3.5. Then boil the liquid at pH = 3.5 for 3 minutes precisely. Set aside to cool. Bring the cooled liquid beneath the electrodes of the pH-meter and, while agitating the liquid, use a burette to add 1/10 NaOH 0.1 N until pH = 4, V2 is the volume obtained.

Continue pouring until pH = 7 and let V3 be the new volume.

Formula: VFA measurement

or:
Volatile acidity = (V3 – V2) × 4 × 0.06 expressed in g· L-1 of acetic acid.

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