AD Workshop - Optimising Processes for the Stable Digestion of Food Waste

Ammonia removal from food wastedigestate using gas stripping

Dr Sonia Heavens.heaven@soton.ac.uk

Dr Mark Walkerm.walker@soton.ac.uk

Contents• Introduction

• Ammonia removal scenarios

• Batch ammonia stripping from food waste digestate

• Integrating ammonia stripping with anaerobic digestion

• Part 1 - Ammonia release during hydrolysis

• Part 2 - Side-stream ammonia removal

• Modelling ammonia removal during anaerobic digestion

• Conclusions

Ammonia

Proteins

Digestate

Headspace

NH3(l)

NH4+

(aq)

NH3(g)

Gas bubbling

Gas removal

Introduction• Why remove ammonia?

• Reduced toxicity on acetoclastic methanogens• Valuable product

• Why gas stripping?• Easy to integrate with AD plants, proven chemical technology

• How can gas stripping be used?• 4 scenarios identified: side-stream, post-hydrolysis, in situ and post-

digestion

• Results presented in 3 sections1. Batch ammonia stripping from digestate2. Integration of ammonia stripping with anaerobic digestion3. Modelling the performance of a combined ammonia and anaerobic

digestion process

Ammonia removal scenario 1‘Post-digestion’

mesophilic digester (35°C)

mixing tank

pasteuriser (70

°C)

ammonia capture

food waste

recycleddigestate

digestate

removedammonia

biogas

Ammonia removal scenario 2‘in situ’

gas-mixed mesophilic

digester (35°C)

mixing tank

pasteuriser (70

°C)ammonia capture

foodwaste

digestate

biogas

removedammonia

Ammonia removal scenario 3‘Side-stream’ biogas

gas-mixed mesophilic digester (35°C)

mixing tank

pasteuriser (70

°C)

ammonia capture

foodwaste

digestate

removedammonia

Stripping reactor

Ammonia removal scenario 4‘Post-hydrolysis’

gas-mixed mesophilic digester (35°C)mixing tank

(hydrolysis)

pasteuriser (70

°C)

ammonia capture

foodwaste

digestate

biogas

removedammonia

Stripping reactor

biogas

biogas

Batch ammonia stripping from food waste digestate

Objectives• To understand kinetics of ammonia removal with regard to

important process characteristics

• Temperature (35-70°C)

• pH (unmodified or with NaOH to increase to 10-14.5)

• Gas flow rate (0.125-0.70 l l-1 min-1)

• To collect data on removal of ammonia from ‘real’ food waste digestate which is applicable to all scenarios

• To gain some qualitative understanding of how (removed) ammonia could be captured

• 2 digestate samples used from AD plants feeding both commercial and domestic source-separated food waste

Experimental setup

Initial findings• Confirmation of theoretical framework

• Nitrogen strips carbon dioxide and causes pH decrease• With biogas stripping, temperature and flow rate both increase

ammonia removal rate

• A complex system• Ammonia removal causes pH to decrease• VFA concentration changes pH behaviour as ammonia is

removed• pH changes ammonia removal behaviour• No ammonia removal below pH of ~7.5• If ammonia removal ceases, water removal through evaporation

can cause an increase in ammonia concentration

Results – ammonia removal kinetics

y = 7797e‐0.031xR² = 0.9642

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

0 20 40 60 80 100 120 140

Ammon

ia co

nc. (mg/l)

Time (hrs)

y = 7797e‐0.031xR² = 0.9642

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

0 20 40 60 80 100 120 140

Ammon

ia co

nc. (mg/l)

Time (hrs)

Curve fitted in the form C=C0et/τ where τis the decay constant

τ = 1/0.031 = 32.3 hours

Run # Reactor # Temperature (°C)

Flow Rate ( l l-1 min-1)

Alkali addition

Length of Run (hrs)

Initial Final Time constant

(hrs)

r2 % Ammonia Removed

Digestate 1 pH Ammonia (mg/l)

pH Ammonia (mg/l)

0.1 2 35 0.375 N 306.25 8.43 7189 8.87 4318 575.4 0.98 45%1.1 1 55 0.125 N 120.07 8.46 7479 8.89 6155 699.6 0.91 21%2.1 1 70 0.125 N 97.50 8.70 7743 8.49 446 30.7 0.96 96%3.1 1 70 0.125 N 118.50 8.49 7798 9.36 285 31.3 0.96 96%4.1 1 70 0.375 N 30.00 8.50 7764 8.86 1090 15.1 0.99 89%5.1 1 70 0.25 N 63.48 8.65 7810 8.89 497 22.1 0.99 95%6.1 1 70 0.375 N 28.83 8.50 7847 8.99 1585 17.7 1.00 83%7.2 2 70 0.5 N 24.40 8.59 8076 9.04 1963 17.5 1.00 80%8.2 2 70 0.625 N 44.68 8.62 8245 7.98 773 17.7 0.97 95%9.2 2 70 0.375 Y 6.25 12.18 6783 10.99 1539 4.2 1.00 78%

10.1 1 70 0.75 N 22.72 8.45 8821 9.03 6925 17.8 0.97 -10.2 2 70 0.75 N 52.00 8.45 8821 8.01 470 17.1 0.97 96%11.1 1 70 0.125 Y 64.88 10.19 7815 10.36 1706 45.2 0.93 78%11.2 2 70 0.125 Y 89.80 10.19 7815 10.12 551 36.0 0.91 93%12.1 1 70 0.25 Y 55.10 10.25 8134 10.30 235 15.3 0.96 97%12.2 2 70 0.25 Y 55.30 10.25 8134 10.18 202 15.0 1.00 98%13.1 1 70 0.25 Y 71.78 11.43 5816 11.18 357 27.8 0.73 95%13.2 2 70 0.375 Y 71.85 10.32 6232 10.41 191 8.2 0.74 98%14.1 1 70 0.375 Y 22.78 13.86 5404 11.28 281 7.8 0.96 95%14.2 2 70 0.375 Y 22.88 13.86 5404 11.36 391 8.9 0.95 94%

Digestate 215.1 1 70 0.125 N 98.48 8.20 5978 8.57 1389 64.2 0.99 81%15.2 2 70 0.25 N 98.48 8.20 5978 7.63 1302 66.9 0.98 88%16.1 1 70 0.375 N 52.87 8.13 5834 8.82 1701 41.8 1.00 82%16.2 2 70 0.5 N 52.87 8.13 5834 8.13 1664 36.0 0.95 82%17.1 1 70 0.625 N 48.50 8.24 5849 7.86 1395 33.4 0.98 85%17.2 2 70 0.75 N 45.33 8.24 5948 7.85 1937 37.8 0.80 90%18.1 1 70 0.375 Y 18.75 12.24 4328 11.18 537 9.2 0.99 91%18.2 2 70 0.5 Y 18.75 12.24 4328 11.28 458 8.6 0.98 93%19.1 1 70 0.375 Y 11.17 9.15 5258 9.96 2587 15.7 0.99 49%19.2 2 70 0.5 Y 11.17 9.06 5045 9.93 2214 13.0 0.98 64%20.1 1 70 0.5 Y 6.33 14.56 4764 12.08 1044 4.1 1.00 78%20.2 2 70 0.625 Y 6.33 14.61 4976 11.84 940 3.7 0.99 83%

35/55°C gives decay constant 500-700 hours

70°C gives decay constant 17-31 hours

70°C + pH modification gives decay constant ~4 hours

Results – effect of gas flow rate (no pH modification)

0.125-0.375 l l-1 min-1 results in an increase in ammonia removal rate

Further increase in gas flow rate does not deliver any process advantage

Results – effect of gas flow rate (with pH modification)

Without pH modification kinetics digestate dependent

The effect of pH is to increase ammonia

removal rate and less digestate specific

Results – effect of VFA concentration

VFA spiked sample had reduced ammonia removal

Lower initial pH with VFA

pH downswing seen in initial experiments

Results – ammonia recovery

12.1_70_0.25_10 and 12.2_70_0.25_10

Ammonia concentration (mg/l)

R1 R2

Initial Digestate 8877 8877

Final Digestate 256 221

Acid trap 7681 7430Water trap 19301 20996

Condensate trap 29227 34978

Crystals (with water)

330 240

Lower flow rate results in higher strength ammonia

solutions due to decreased evaporation

Results – ammonia recovery

11.1_70_0.125_10

Outcomes – batch ammonia removal• Increasing pH, temperature and gas flow rate increase the rate of

removal of ammonia

• ...but above 0.375 l l-1 min-1 further increase in flow rate realises no benefits (and causes more evaporation)

• Moving from ‘AD’ temperatures (35/55°C) to 70°C greatly decreases time needed for ammonia removal, which with pH modification can be reduced to around 4 hours

• Ammonia removal from two digestates shows different behaviour except at high pH

• VFA can reduce the effectiveness of ammonia stripping

• Ammonia readily ‘trapped’ using condensation or water bubbling

Integrating ammonia stripping with AD

Integrating ammonia stripping with AD

Part 1Ammonia released during

semi-continuous anaerobic hydrolysis

Ammonia removal scenario 4‘Post-hydrolysis’

gas-mixed mesophilic digester (35°C)mixing tank

(hydrolysis)

pasteuriser (70

°C)

ammonia capture

foodwaste

digestate

biogas

removedammonia

Stripping reactor

biogas

biogas

Aim – ‘post hydrolysis’ removal scenario

mixing tank (hydrolysis)

ammonia capture

foodwaste

biogasStripping reactor

biogas

batch ammonia removal process

Need to understand ammonia release kinetics during hydrolysis step to design an appropriate ammonia

removal system

Experimental setup• 8 x 600 ml working volume, stirred mesophilic digesters fed on

homogenised food waste

• Retention times from 2-10 daysRetention time (d)

# digesters Digestate removed daily (g)

Food waste added daily (g)

10 2 60 608 2 75 757 2 85 856 2 100 1005 2 120 1204 2 150 1503 2 200 2002 2 300 300

Results – pH

Initial pH drop caused by acid production and

washout of methanogenic biomass

Lower retention times resulted in lower steady

state pH

pH inhibition of biological reactions expected

Results – gas production

biogas was found to be CO2 and increased with shorter retention time

Specific gas production is similar suggesting an

inhibition of the hydrolysis process

Results – ammoniaInitially higher

ammonia production (higher pH)

As pH decreases ammonia production

decreases

Similar ammonia concentration at all

retention times which supports that biological

hydrolysis/fermentation is inhibited

Only ~ 15% of the total ammonia in the food waste is

being released

Outcomes – ammonia released during hydrolysis

• Ammonia and biogas (carbon dioxide) data show that the hydrolysis and fermentation process is being inhibited in its early stages

• Inhibition is likely due to pH

• Increasing the retention time has no effect on the ‘degree’ of hydrolysis

• Only effective at releasing ~15% of the bioavailable ammonia

• Not a useful pre-treatment to ammonia removal

Integrating ammonia stripping with AD

Part 2AD of food waste with side-stream

ammonia removal

Experimental setup

3.6-litre digester (mesophilic)

0.36-litre stripping reactor

continuous circulation of biogas

pump

water trap (100ml DI water, thermophilic only)

acid trap (100ml 40% H2SO4)

daily transfer of digestate

excess digestate

2gVS/ld food waste biogas

stripping system digester system

Experimental setup – ammonia stripping system

Mesophilic 0.36-litre stripping reactors

Thermophilic 0.36-litre stripping reactors

Pump

SM1SM2

ST1ST2

Water traps

Acid traps

Results – methane production

Transient behaviour throughout experiment

Initially thermophilic stripping caused instability

Later thermophilic stripping shows process advantage (hydrolysis?

population selection?)

Mesophilic stripping destabilises process

Results – ammonia concentration

Initial ammonia reduction causes a

reduction in pH of all reactors from 8.2-7.8

No further ammonia removal caused by pH

decrease and VFA increase

pH decreased and VFA increased through experiment

Outcomes – side-stream ammonia removal

• Using side-stream ammonia stripping initial rapid reductions in ammonia were achieved (~5500 → 4500 mg l-1) after which no ammonia removal was observed, probably due to the interaction of ammonia stripping with pH and VFA throughout the experiment

• Under the conditions investigated, ammonia removal was unable to prevent the accumulation of VFA and subsequent destabilisation of the anaerobic digestion process and cessation of ammonia removal

• Ammonia stripping could have potential for the intermittent removal of ammonia since it is most effective when the concentration of ammonia is high and at the corresponding high pH

Modelling the integration of ammonia removal and food waste AD

Objectives

• Use experimental data from batch ammonia removal work to simulate removal of ammonia from a full anaerobic digestion process

• Model two of the most promising ammonia removal scenarios

• Post digestion – 70°C, 0.375 l l-1 min-1 and with pH modification

• In situ ammonia removal during mesophilic anaerobic digestion

• Both scenarios modelled under a modest and high rate loading condition (2 and 5 g VS l-1 day-1) and for a freshly inoculated and mature digester (initial ammonia conc 500 mg l-1 and 6000 mg l-1)

• Requires some simplifying assumptions regarding the way ammonia is released during digestion and to what concentration (see report).

Scenario 1 – post digestion ammonia removal

mesophilic digester (35°C)

mixing tank past

euris

er

(70°

C)

ammonia capture

food waste

recycled digestate

digestate

removed ammonia

biogas

• Digestate recycle ratio can be used to set the digester retention time, which was limited to 30 days for biological stability. High recycle ratio leads to greater ammonia removal

• Ammonia stripped digestate is used to ‘dilute’ the incoming food waste leading to a reduced in-digester ammonia concentration

• Ammonia removal time constant = 4 hours as per run 20.1/20.2

Scenario 1 – modelling results

31% removal (5 g VS l-1 day-1 )

72% removal (2 g VS l-1 day-1)

Final conditions independent of initial conditions

Scenario 2 – in situ ammonia removal

gas-mixed mesophilic

digester (35°C)

mixing tank past

euris

er

(70°

C)

ammonia capture

food waste

digestate

biogas

removed ammonia

• No digestate recycle therefore organic loading rate determines retention time

• Ammonia removal time constant = 575 hours as per run 0.1

Scenario 2 – modelling results

81% removal (2 g VS l-1 day-1)

63% removal (5 g VS l-1 day-1 )

Outcomes – ammonia removal modelling

• In situ ammonia stripping at mesophilic conditions results in a lower in-digester ammonia concentration despite the much lower rate of ammonia removal

• Post digestion ammonia removal not effective at removing ammonia from the digester in a high rate system since there is a limitation on the ammonia of digestate that can be recycled

• An energetic analysis is needed:

• Post digestion is ‘heat hungry’ - excess digestate is pasteurised

• In situ is ‘pumping hungry’ - 30 x the gas pumping cost

Conclusions - Ammonia removal from food wastedigestate using gas stripping

Main conclusions• Ammonia removal by gas stripping took place with decay constants

of 4-700 hours in the envelope of conditions investigated in this project

• Low pH (<7.5) and high VFA can both severely limit the ammonia stripping performance

• Preliminary laboratory investigation into the integration of ammonia stripping as a post-hydrolysis or side-stream process showed insufficient ammonia removal for stable anaerobic digestion

• Modelling the ammonia removal process during anaerobic digestion showed in situ ammonia stripping to be a promising treatment option in decreasing in-digester ammonia concentrations, especially at a high organic loading rate

Thank you for your attention!