Velp Scientifica
– UDK139 Operating Manual
10004762/B5
100
30 - Kjeldahl method to determine total nitrogen on sludges from
wastewater treatment plants
The final use of sludges from municipal wastewater treatment plants should be on
agricultural land for the recovery of useful nutrients. This is suggested also by the
Council of European Economic Community (EEC Directive 86/278) related to the
agricultural use of sludges from wastewater treatment if the level of contained toxic
elements (Cd, Cu, Ni, Pb, Zn, Cr, Hg) is not too high.
The total nitrogen content ranges from 2,2 to 4,2%, dry matter, and is represented by
ammonia, nitrite, nitrate, urea and organic nitrogen.
The method operates first a reduction of nitrite and nitrate to ammonia with metallic
chromium in acid solution and then the digestion of organic nitrogen to ammonia with
concentrated sulphuric acid and catalysts.
Finally ammonia is steam distilled and titrated.
Procedure
1) Sample:
about 0,5 g of sludge dried at 105 °C up to constant weight.
(10 - 20 mg of N) or a corresponding amount of wet sludge, accurately weighed and
quantitatively transferred in a test tube.
2) Reagents for the reduction:
for each sample add in the test tube:
0,5 g chromium powder (Cr)
20 ml hydrochloric acid 7% about (HCl) (200 ml of concentrated acid, 36% , to 1 liter
by ammonia free distilled water).
3) Riduction:
let reaction run at room temperature for 5 min, shaking at intervals.
Heat to starting boiling for 4 minutes and let cool.
4) Reagents for digestion:
for each sample add in the test tube:
2 catalyst tablets ST (code CT0006609)
10 ml concentrated sulphuric acid (96-98%)
5) Digestion:
heat for 60 min at 420 °C.
6) Cooling:
let cool the tubes to 50-60 °C.
7) Distillation-titration:
place in position the test tube with digested sample in the
unit UDK 139.
factor: 0.00
Addition of reagents:
dilution water=50ml
H
3
BO
3
=30ml
NaOH=50 ml
Titrant: HCl 0.2 N
Distillation time= 4 min
Steam power= 100%
Press the button starting distillation cycle.
References:
method CNR
– IRSA, Rome, Italy
287
