Contact  |  Sitemap  |  Downloads 
 

    
     Print this page   

   

 

:: Lime in sludge treatment
 Why use lime in sludge treatment ?
 Lime as a flocculation agent
 Lime for hygienisation
 Lime to decrease phosphorous
 Lime for odours reduction

 

 

Lime in sludge treatment > Lime for odours reduction

 

Lime to reduce bad odours

 

The purpose for using lime for sludge tretment is to ionise sulphur or ammonia based volatile odorous compounds into their ionic form.  Only the volatile form is odorous.

 

Organoleptic properties of sludge :

 

Odorous compounds

Odour quality

Detection threshold

(µg/m3)

Low

High

Sulphides

H2S

Rotten eggs

0.7

14

C2H6SH

Rotten cabbage

2.5

50.8

(CH3)2S2

Sulphur

0.1

346

(CH3)3S3

Sulphur

6.2

6.2

N-based

compounds

NH3

High

26.6

39600

(CH3)2NH

Fishy

84.6

84.6

(CH3)3N

Fishy

0.8

0.8

Fatty acids

CH2O2

Acrid

45

37800

CH3COOH

Vinegar

2.5

250000

C2H5COOH

Rancid

84

60000

C3H7COOH

Rancid

1

9000

 

  • Liming avoids volatile sulphide (H2S)

     

 

At pH>12, only non-volatile ionic species (HS-/S--) coexist

 

pKa(H2S/HS-)=7 << pKa(HS-/S--)=13 < pKa(H2O/OH-)=14

H2S + OH-  D  HS- + H2O     HS- + OH-  D  S-- + H2O

  • Liming avoids volatile mercaptans (RSH)

 

At pH>12, only non-volatile ionic species (RS-) coexist

 

pKa(C2H5SH/C2H5S-)=13 < pKa(H2O/OH-)=14

C2H5SH + OH-  D  C2H5S- + H2O

  • But liming enhances NH3 release 

At pH>12, only volatile NH3 is formed

 

pKa(NH4+/NH3)=9.25 << pKa(H2O/OH-)=14

NH4+ + OH-  D  NH3 + H2O

NH3 formation is promoted by temperature rise

 

                                                               K=exp(-ΔG°/RT)