Velp Scientifica
– UDK139 Operating Manual
10004762/B5
118
41 - Method to control the efficiency of an anaerobic digester by
determination of volatile acids content in digesting sludge
The volatile acids content in a slurry submitted to anaerobic digestion is considered the
best index of process performance when compared to alkalinity level.
If the ratio by weight between volatile acids (C2 - C6) content, expressed as acetic acid,
and total alkalinity expressed as calcium carbonate is maintained lower than 0,2 (eg.
300/2000), the digestion proceedes regularly. Values of 0,3 - 0,4 show some malfunction
and the need of intervenction. At values higher than 0,8 methane production is inhibited.
A quick method for determining volatile acids (acetic, propionic, butirric, valerianic, etc.) in
digesting sludge is based on steam distillation and titration in the distillate.
Procedure
1) Reagents:
• sulphuric acid 1 : 1. Dilute 1 volume of concentrated sulphuric acid (H
2
SO
4
) slowly
added to the same volume of water, stirring at intervals.
Do not add water in the acid to avoid dangerous splashings.
• Sodium hydroxide (NaOH), titrated 0,04 N solution.
• Phenolphthalein indicator 0,5 g in 100 ml of distilled water
• Methyl orange indicator solution. 0,05 g in 100 ml of distilled water
• Acetic acid (CH
3
COOH), 2000 mg/l. Dilute 1,9 ml glacial acetic acid to 1000 ml by
deionized or distilled water. Titrate by sodium hydroxide 0,04 N. 10 ml of the solution
(0,33 meq) require 8,3 ml of sodium hydroxide 0,04 N (phenolphthalein indicator).
2) Sample:
150 : 200 ml of digesting slurry are centrifuged to remove solids and
avoid a possible production of volatile acids by hydrolysis. 5 min at low speed. The
sample is the supernatant liquid.
3) Distillation:
100 ml of sample in a 250 ml test tube are acidified by sulphuric acid
1 : 1, added shaking in 1 ml volumes up to the change of methyl orange indicator
from red to yellow (pH 2,7).
Place the tube with acidified sample in the unit UDK139.
Select the mode “analysis without titration”.
Place in position in the steam distillator UDK139 an Erlenmeyer flask empty; take off
the distillate collecting tube from the condenser and place a new silicone tube,
introduced in the flask.
Set the following parameters:
Reagents addition:
H
2
O=0.
H
3
BO
3
=0.
NaOH=0.
Distillation time = 4 min
steam=100%.
Push START to begin the distillation.
Avoid collection of hydrogen sulphide and carbon dioxide present in the sample
discarding the first 10 ml of condensate. Collect 100 ml of distillate.
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