Phosphorus recovery and VFAs production from sewage sludge 

fermentation

D. Crutchik, N. Frison, A. Jelic and F. Fatone

Natalia Herrero García

• Municipal wastewater contains around 100‐120gCOD/(inhabitant per day);

• Up to now, the utilization of sewage sludge has been limited mainly to theproduction of biogas for co‐production of thermal and electric energy andcompostable material;

• The sewage sludge might be considered as a challenging feedstock to beprocessed for bio‐based applications (waste‐to‐chemicals and bio‐productvalue chain);

• Volatile Fatty Acids (VFAs) could be considered intermediates for a wide rangeof applications

BackgroundBackground

Production & Application of Waste‐derived VFAsProduction & Application of Waste‐derived VFAs

Lee et al., 2014. Chemical Engineering Journal.

The operating conditions for VFAs productionshould be addressed based on the finalapplication of the VFAs

Proper process control can manipulate the typeof VFA produced, which is critical to theperformances of the downstream applications.

Organic‐rich Wastes

Pretreatment of Waste

Anaerobic technologies for VFA production

Volatile fatty acids (VFA)

‐ph‐Temperature‐Hydraulic retention time‐Solids retention time‐Organic loading rate‐Additives

Applications:‐Polyhydroxyalkanoates‐Electricity‐Biogas‐Hydrogen‐Lipids for Biodiesel‐Biological nutrient removal

Ope

ratin

g cond

ition

s

Fermentation of  Sewage Sludge 

Struvite (MgNH4PO4∙6H2O)

Magnesium hydroxideMg(OH)2

To  anaerobic digester

PS&WASWAS

PS

‐Acetic Acid;‐Propionic Acid;‐Butyric Acid;‐Others

Scenario: Integration of VFA Production and  Phosphorus Recovery

Scenario: Integration of VFA Production and  Phosphorus Recovery

S/L

(After Dynamic Thickening)

SCFAs (SSFL)

T=37ºC

Role of SCFAs in Wastewater TreatmentRole of SCFAs in Wastewater Treatment

• SCFAs are rbCOD and help the denitrificationprocesses;

• Enhanced Bio‐P removal (4‐5 mgVFA are required foreach mg P removed);

• Hydrogen production;• Biological Nutrients Removal;• Lipids for biodiesel;• Polyhydroxyalkanoates (PHAs).

Lee et al., 2014. Chemical Engineering Journal.

Addition of Mg(OH)2 as magnesium ion source and struvite seed crystals (5 g/L) to promote reaction.

Mg2+ + NH4+ +  HnPO4

n‐3 + 6 H20   MgNH4PO4∙6H20  + nH+

Struvite: High comercial value as slow release fertilizer

Phosphorus Recovery Via Struvite CristallyzationPhosphorus Recovery Via Struvite Cristallyzation

Crutchik et al,. 2013

Initial pH fixed at pH 8.5 by addition NaOH

ObjectiveObjective

• To study the effect of the sludge type and the initialfermentation pH on the production and composition ofSCFAs;

• To evaluate the feasibility of phosphorus recovery (asstruvite, NH4MgPO4∙6H2O) from sewage sludge fermentationliquid (SSFL);

• To validate effect of initial pH and sludge type in propionateproduction and phosphorus recovery, best operatingconditions were evaluated in a bench‐scale sequencing batchfermentation reactor

Parameter Units PS PS&WAS WASTS g/L 29.7 ± 0.6 39.1 ± 0.8 58.1 ± 0.4

VS g/L 23.6 ± 0.5 33.5 ± 0.7 45.9 ± 0.5

Total COD mgCOD/gTVS 846.4 ± 4.0 914.8 ± 3.8 997.4 ± 6.1

Total Nitrogen (TN) mgN/gTVS 32.5 ± 0.8 41.9 ± 0.7 56.2 ± 0.5

Total Phosphorus (TP) mgN/gTVS 17.2 ± 0.4 17.5 ± 1.6 18.7 ± 1.1

COD/N ratio gCOD/gN 26.6 21.8 18.7

Origin: WWTP of Verona municipality (North of Italy);Type of Dynamic Thickening:• Gravity Belt thickening for Primary Sludge;• Screw Drum for Waste Activated Sludge (6‐8 g polyacrilamide /kgTS)

Characteristics of the PS, WAS and PS&WASCharacteristics of the PS, WAS and PS&WAS

N°Experiment

Sludge Type

COD/N (gCOD/gN) Initial fermentation pH

1‐5 PS 26.6 4.96 (Uncontrolled),8,9,10,11

6‐10 PS&WAS 21.8 5.76 (Uncontrolled),8,9,10,11

11‐15 WAS 18.7 6.19 (Uncontrolled),8,9,10,11

The response surface methodology (RSM) was applied 

y b b x b x b x x b x b x

Outline of the Batch Fermentation ExperimentsOutline of the Batch Fermentation Experiments

18.7523.75

0,010,020,030,040,0

5,57,0

8,510,0 Ratio COD/N

%HPr  on SCFAs

Initial pH

0,0‐10,0 10,0‐20,0 20,0‐30,0 30,0‐40,0

18.7523.75

04080

120160200240280

5,57,0

8,510,0 Ratio COD/N

SCFAs E

fficiency (m

gCOD/gVS

S)

120‐160 160‐200 200‐240 240‐280

3D Surface plots: Maximal SCFA Production Efficiency and Percentage of Propionic Acid3D Surface plots: Maximal SCFA Production Efficiency and Percentage of Propionic Acid

Initial pH

Initial fermentation pH<8.5 & higherfraction of PS promote highest  % HPr

Low initial pH & higher fraction of PS favoured the production of the SCFA

18.7523.75

0,02,0

4,0

6,0

8,0

10,0

5,57,0

8,510,0

Ratio COD/N

PO4‐P release (m

gP/gVS

S)

Initial pH

0,0‐2,0 2,0‐4,0 4,0‐6,0

18.7523.75

0

25

50

75

100

5,57,0

8,510,0 Ratio COD/N

%PO

4‐P recovered (m

gP/gVS

S)

Initial pH

0‐25 25‐50 50‐75 75‐100

3D Surface Plots: P Released and Percentage of P Recovered

3D Surface Plots: P Released and Percentage of P Recovered

Increase of PO4 ‐P release observed at higherinitial pH 

Higher PO4‐P recovery noted at low  initial pH & higher fraction of WAS

0,00

1,00

2,00

3,00

4,00

5,00

6,00

7,00

8,00

9,00

0,0

20,0

40,0

60,0

80,0

100,0

120,0

140,0

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75

pH

Prop

ionate produ

ction 

(mgCOD/gV

SS)

Time (d)

PERIOD I (WAS)                           PERIOD II  (WAS+PS)      PERIOD III (PS)

Yield of propionate pH effluent

Period Sludge Type

COD/N (gCOD/gN) Initial fermentation pH

I WAS 26.6 8.5II PS&WAS 21.8 5.76 (Uncontrolled)III PS 18.7 6.19 (Uncontrolled)

SBFR: SCFA ProductionYield of Propionic AcidSBFR: SCFA ProductionYield of Propionic Acid

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

1,8

0 10 20 30 40 50 60 70

HP

r/H

Ac

(gC

OD

/gC

OD

)

Time (d)

WAS WAS+PS PS

SBFR: SCFAs CompositionAcetic acid/ Propionic acid (HPr/HAc) 

SBFR: SCFAs CompositionAcetic acid/ Propionic acid (HPr/HAc) 

PERIOD I                               PERIOD II                            PERIOD III

0

0,2

0,4

0,6

0,8

1

1,2

1,4

(mgPO

43‐‐P/gVS

S)PS+WAS

0

0,4

0,8

1,2

1,6

2

2,4

(mgPO

43‐‐P/gVS

S)

WAS

0

0,2

0,4

0,6

0,8

1

(mgPO

43‐‐P/gVS

S)

PS96 % 

Recovery 55% 

Recovery 

PO43‐‐P released 

(mgP/gVSS)Final PO4

3‐‐P  recovered (mgP/gVSS)

% PO4‐P recovered

WAS 2.11 1.43 55.0PS+WAS 1.27 1.22 96.0

PS 0.96 0.85 89.0

SSFL: PO43‐ Release & Recovery by Struvite Crystallization. SSFL: PO43‐ Release & Recovery by Struvite Crystallization. 

Before                      After

89 % Recovery 

Before                      AfterBefore                      After

ConclusionsConclusions

• Fermentation of sewage sludge provide a suitable source of SCFAs andPO4

3—P for a wide range of applications;

• The sewage sludge type and the initial fermentation pH affect theproduction and composition of SCFA;

• Higher production of Propionic acid was observed at an initialfermentation pH in a range between 5.5‐8 and with higher fraction of PS(high COD/N ratio);

• The alkaline fermentation of WAS enhanced the release of nutrients (Nand P), which can be recovered by struvite crystallization up to 11 mgStruvite/gVSS.

Phosphorus recovery and VFAs production from sewage sludge fermentation

D. Crutchik, N. Frison, A. Jelic and F. Fatone

Thank you for your attention

Natalia Herrero García