KIT – University of the State of Baden-Württemberg andNational Large-scale Research Center of the Helmholtz Association

Institute for Water and River Basin ManagementDepartment of Aquatic Environmental Engineering

www.kit.edu

CAPACITY EVALUATION AND OPTIMISATION OF A CO-DIGESTER PLANTHoffmann, Erhard and Blank, Andreas

Institute for Water and River Basin Management,Department of Aquatic Environmental Engineering2

Capacity Evaluation and Optimisationof a CO-Digester Plant

Structure

Introduction: Background and Targets

Material and Methods

Results and Conclusions

Institute for Water and River Basin Management,Department of Aquatic Environmental Engineering3

Digesters have been built in 1983Start-up of Co-Digestion in 1994with an amount of 5000 t/a Bio-WasteToday´s throughput:30000 t/a of Bio-Waste and 3200 t/a ofKitchen-Waste

Introduction

Capacity Evaluation and Optimisationof a CO-Digester Plant

Fig.1: Schema of the WWTP Baden Baden/Sinzheim

Fig.2: WWTP Baden Baden/Sinzheim (200000 PE)

Institute for Water and River Basin Management,Department of Aquatic Environmental Engineering4

Introduction

Capacity Evaluation and Optimisationof a CO-Digester Plant

Fig.1: Schema of the WWTP Baden Baden/Sinzheim

Fig.3: Grinder/Pulper

Fig.4: Screw press

Institute for Water and River Basin Management,Department of Aquatic Environmental Engineering5

IntroductionToday's challenges

The digester volume is halved dueto the rehabilitation of onefermenterHow to cope with the unchangedamount of substrate?

Possibilities:Pre-treatment of the substratesand/or inclusion of an existing biomass reactor

Capacity Evaluation and Optimisationof a CO-Digester Plant

22.2013.10312.0Total2.131.9055.2Excess sludge9.295.25165.3Primary sludge3.381.288.8Kitchen waste7.404.6882.7Bio-waste suspension

[to CODtot/d][to DM/d][m³/d]

Fig.5: Biogas plant

Tab.1: Substrates

Institute for Water and River Basin Management,Department of Aquatic Environmental Engineering6

Introduction

Fig.8: Schematic representation of the Biogas plant

Capacity Evaluation and Optimisationof a CO-Digester Plant

Fig.6: CODsol-concentration of the substrates Fig.7: CODtot -concentration of the substrates

1

10

100

1000

PS ES KW BW Mix

COD t

ot [g

/L]

1

10

100

1000

PS ES*10-³ KW BW Mix

COD s

ol [g/

L]

Mean

Max

Min

84%

16%

Institute for Water and River Basin Management,Department of Aquatic Environmental Engineering7

Introduction

Fig.8: Schematic representation of the Biogas plant

Capacity Evaluation and Optimisationof a CO-Digester Plant

Fig.9: DM-concentration of the substrates Fig.10: oDM-concentration of the substrates

0

40

80

120

160

200

PS ES KW BW Mix

oDM

[g/L

]

0

40

80

120

160

200

PS ES KW BW Mix

DM [g

/L]

Mean

Max

Min

84%

16%

Institute for Water and River Basin Management,Department of Aquatic Environmental Engineering8

Material and MethodsSludge disintegration (batch experiments):

Ultra sonic generator(SONOTRONIC Nagel GmbH,1000 W, >6m³/d)Exposure time (max. 10 min)Energy input (0 to 17 kWh/kgDM)

Capacity Evaluation and Optimisationof a CO-Digester Plant

Fig.12: Functional diagram of the High-Output Ultrasonic Reactor:1. Feed pipe, 2.Ultrasonic transmitter, 3. Cavitation field,4. Flow, 5.Downpipe, 6. DrainpipeFig.11: Ultrasonic Reactor: 5 generators each with 1000W

Institute for Water and River Basin Management,Department of Aquatic Environmental Engineering9

Material and MethodsHydrolysis (batch experiments) :

Double-walled tempered hydrolysis reactors

Temperature range:36° - 52° C

Hydraulic retention time:24 h - 72 h

Laboratory analysesCODtot, CODsol, DM, oDM,organic acids, NH4-N

Capacity Evaluation and Optimisationof a CO-Digester Plant

Fig.13: Batch-scale hydrolysis reactors

Institute for Water and River Basin Management,Department of Aquatic Environmental Engineering10

Material and MethodsContinuous one-stage flow experiments

Digestion temperature: 36°C

HRTDigestion: 20 d

Volumetric loading:3.5 kg CODtot/(m³·d)

Laboratory analysesCODtot, CODsol, DM, oDM,organic acids, NH4-N

Capacity Evaluation and Optimisationof a CO-Digester Plant

Fig.14: Continuous flow pilot plant

Institute for Water and River Basin Management,Department of Aquatic Environmental Engineering11

Material and MethodsContinuous flow two-stage experiments

Hydrolysis (1st-stage)Hydrolysis temperature: 42°CHRTHydrolysis: 24 hVolumetric loading:68.8 kg CODtot/(m³·d)

Digestion (2nd-stage)Digestion temperature: 36°CHRTDigestion: 11 d - 16 dVolumetric loading:4.6 – 7.1 kg CODtot/(m³·d)

Capacity Evaluation and Optimisationof a CO-Digester Plant

Fig.14: Continuous flow pilot plant

Institute for Water and River Basin Management,Department of Aquatic Environmental Engineering12

ResultsSludge disintegration

Specific energy input decreases according to the dry matterconcentration increaseDisintegration efficiency achieves up to 50 % based on CODtot

Capacity Evaluation and Optimisationof a CO-Digester Plant

0

10

20

30

40

50

60

0 2 4 6 8 10 12 14 16 18 20specific Energy Input [kWh/kgDM]

Disin

tegra

tion [

%]BWS PSKW ESDS Nickel et al. (2003)

0

2

4

6

8

10

12

14

0 25 50 75 100 125 150 175 200 225DM [g/l]

[kWh/k

gDM]

LBW DS PS KW ES Mix

Fig.15: Specific energy input Fig.16: Disintegration efficiency

Institute for Water and River Basin Management,Department of Aquatic Environmental Engineering13

ResultsHydrolysis step

The optimal temperature for the hydrolysis process was about42 °C and the optimal HRT about 24 hDegree of disintegration was about 20 % based on CODtotIncrease of organic acid concentration from 1320 to 5200 mg/L

Capacity Evaluation and Optimisationof a CO-Digester Plant

0123456789

101112131415

0 24 48 72t[h]

sCOD

[g/L

]

0,000,250,500,751,001,251,501,75

sCOD

(t=x)

/sCOD

(t=0)36°C

42°C52°C

24 h 48 h 72 h36 °C

42 °C52 °C

02468

101214161820

DDCO

D [%]

Fig.18: Degree of disintegrationFig.17: Efficiency of hydrolysis process depending on time and temperature

Institute for Water and River Basin Management,Department of Aquatic Environmental Engineering14

ResultsContinuous flow experiments:

HRTHydrolysis: 23 h, HRTDigester:14.3 dEffluent process water quality:1300 mg CODsol /L, organic acids 350 mg/L, 1400 mg NH4-N/LSubstrate-mix as given in table 1

Capacity Evaluation and Optimisationof a CO-Digester Plant

5550oDM/DM [%]5765Methane [%]

260.0293.3[L Biogas/kg CODtot, in]425.8503.6[L Biogas/kg oDMred.]899.9970.6[L Biogas/kg CODred.]3.54.8368.6Load [kgCODtot/(m³·d)]1.882.2336.2Load [kgoDM/(m³·d)]2014.30.96HRT [d]

2. stage1. stage (hydrolysis)1-stage operationtwo-stage operationTab.2: Results of the flow pilot plant

‘ operation

Institute for Water and River Basin Management,Department of Aquatic Environmental Engineering15

ResultsContinuous flow experiments:

Transfer of the half-scale results to the full-scale operation ofthe Baden-Baden/Sinzheim WWTP

Capacity Evaluation and Optimisationof a CO-Digester Plant

Fig.19: Mass and volume flows of the Co-digestion of the WWTP Baden Baden/Sinzheim

Fig.20: Mass and volume flows of the Co-digestion during the rehabilitation of one digester

Institute for Water and River Basin Management,Department of Aquatic Environmental Engineering16

Conclusions I

An ultrasonic pre-treatment of the co-substrates bio-wastesuspension and excess sludge leads to disintegrationefficiencies up to 51 %. However treating kitchen-waste orprimary sludge in the same manner is not promising as thedisintegration yields are rather low.

Out of the hydrolysis temperatures of 37, 42 and 52 ˚C whichhave been investigated, a process temperature of 42 ˚C turnedout to be optimal. The results indicated that the hydrolysis step isalmost completed after 24 h (batch tests) and leads to anaverage disintegration degree of 18.5 %. Thus an only minorincrease of the solubilization/disintegration degree duringhydrolysis could be expected at prolonged HRTs.

Capacity Evaluation and Optimisationof a CO-Digester Plant

Institute for Water and River Basin Management,Department of Aquatic Environmental Engineering17

Conclusions II - Transfer to the future full scale operationCompared to the one-stage operation a two-step technology withan upstream, separated hydrolysis

allows to reduce the total HRT to about 25 %,leads to an increase of the specific biogas production[L Biogas/kg CODtot,in] by 12.8 % at the same timeand an increase of the daily COD input from 22.2 to 42.3 tonsleading to a biogas production of 12495 m³/d compared to aformer daily amount of 5772 m³/d.

Capacity Evaluation and Optimisationof a CO-Digester Plant

Fig.21: Mass flows and volumes of the co-digestion plant (after termination of the reconstruction works)– Scale-up of the half-scale investigation results to the full-scale treatment plant