status and perspectives for digestate treatment - nitrogen...

BOKU, Dept. IFA-Tulln I Inst. f. Environmental Biotechnology I Werner FUCHS

Status and Perspectives for Digestate Treatment- Nitrogen Recovery as one Important Task forClosure of Nutrient Cycles

1

Werner Fuchs

University of Natural Resources and Life Sciences ViennaDept. IFA-TullnInst. f. Environmental BiotechnologyKonrad Lorenz Str. 20, A-3430 Tulln, Austria

<[email protected]>

BOKU, Dept. IFA-Tulln I Inst. f. Environmental Biotechnology I Werner FUCHS 2

Main drivers for application ofdigestate treatment

Legal restrictions regarding the amount of N applied per hectare via

livestock manure or agricultural wastes

European Nitrate Directive

National regulations

Landfarming is only applicable during the growing season

National regulations to providesufficient storage capacity for the winter period

Intensification of biogas production in regions with high

livestock densities

strong competition for land areawhere manure or fermentation residues can be applied


Expectations from digestate treatment

Cost savings

Reduction of transportations costs for application on farmland

Reduction of storage costs

Marketing of resulting products

Additional advantages

Decrease of N loss

Decrease of environmental burden


Overview ofpossible process combinations

Solid fractionSolid fraction

DryingDrying

Solid-liquidseparation


DryingDrying

CompostingComposting

Liquid phaseLiquid phase

DigestateDigestate

Dried solidsDried solids

CompostCompost

Dried digestateDried digestate

UltrafiltrationUltrafiltration

StrippingStripping

Purified waterPurified water

Enhancedsolids removal


Biologicaltreatment

Biologicaltreatment

PreconditioningPreconditioning

EvaporationEvaporation

Landapplication

Landapplication

Concentratedliquid

Concentratedliquid



Reverseosmosis

Reverseosmosis

N-reducedliquid

N-reducedliquid

Landapplication

Landapplication

Directdischarge

Directdischarge

Partially purifiedwater


Further treatment/Indirect discharge


Land application /Marketing






Indirect discharge /Land application


Solid fractionSolid fraction

DryingDrying



DryingDrying

CompostingComposting

Liquid phaseLiquid phase

DigestateDigestate

Dried solidsDried solids

CompostCompost

Dried digestateDried digestate

UltrafiltrationUltrafiltration

StrippingStripping

Purified waterPurified water



Biologicaltreatment

Biologicaltreatment

PreconditioningPreconditioning

EvaporationEvaporation

Landapplication

Landapplication

Concentratedliquid

Concentratedliquid



Reverseosmosis

Reverseosmosis

N-reducedliquid

N-reducedliquid

Landapplication

Landapplication

Directdischarge

Directdischarge














Advantages of N-recovery

Increased amount of digestate applicable per hectare

Recovery of a valuable product - substitution of artificial fertilizers

Less ammonia emissions during drying of residual digestate

Enhancement of digester performance

High NH3 concentrations are inhibitory to microflora

N removal increases process stability using N rich substrates (chickenmanure, slaughterhouse waste)

Lower ammonia emissions from fertilized soils


Options for NH4-Recovery

Ammonia retention in the concentrated liquid phase by means ofmembrane processes

Ammonia transfer to the gas phase and recovery

Stripping

Evaporation

Membrane contactors

Ammonia precipitation as struvite in the liquid phase

N-Recovery from liquid by means of algae photo-reactors


Typical scenario

No area for land application is available,export of nutrients is necessary

Aim

Recovery of organics and nutrients

Discharge of liquid phase

Products

UF concentrate

often recycled to the AD plant

RO concentrate

N-rich liquid fertilizer

Pure water of direct discharge quality

Membrane Purification –Ultrafiltration & Reverse Osmosis

Clean water (discharge, irrigation)

Micro-/Ultra-filtration

3-stepreverse osmosis

Solid/liquidseparation

Sieving/Flotation

Digestate


NH4-recovery with membranes

High practical experience available

RO membranes are very susceptible to particles→ complete suspend solids removal necessary

To high effort if only N-removal required

Un-dissociated NH3 easily passes the membrane Cooling necessary

Acidification necessary

Product: liquid fertilizer concentrate (RO concentrate) Contains NH4 + all organic/inorganic components (P, K, salts)

Not well defined


0%

20%

40%

60%

80%

100%

4,0 5,0 6,0 7,0 8,0 9,0 10,0 11,0 12,0S

ha

reof

am

mon

ia[%

]

pH [-]

120 C

100 C

40 C

60 C

20 C

80 C

NH3 transfer to the gas phase

Equilibrium between dissolved NH4+ (ammonium) and NH3 (ammonia)

NH4+ ↔ H+ + NH3

Enhanced transfer to gas phase at

high pH

high temperature

low pressure pressure

Problem:high buffer capacity of digestate

HCO3- + OH- ↔ CO3

2- + H2OHCO3

- + H+ ↔ H2CO3

pH increase without chemicals through CO2 strippingreduces also buffer capacity


Principle of the Stripping Processes

10

Stripping

Regenerationof Strip gas

Digestate N-depleted digestate

N-rich product

Additives (e.g. NaOH)

Additives (e.g. H2SO4)


Typical site scenario

Not enough area available to apply tothe nitrogen load

Surplus of thermal energy

Aims

Reduction of nitrogen load

Product

Recovered ammonia or ammonium-sulfate solution,well defined and high quality

Production of liquid N fertilizers

Flue gas cleaning

Bulk chemical for industry

Ammonia Stripping/Flash Evaporation


Stripping technologies

Air/gas stripping

Medium temperature level (excess thermal energy from CHP plant)

High air flow rate → increased electrical energy

Addition of NaOH to increase pH

Ammonia recovered in a scrubber as ammonium sulfate

Steam stripping

High temperature level for steam production(usually not available from CHP plant)

Low electrical energy demand

Ammonia recovered by condensation as ammonia solution


NH3-stripping in packed columns

Air stripping Steam strippung

Ammoniasolution

EffluentEffluent Ammoniumsulfate

Sulphuricacid

Influent Influent


Treateddigeste

Scrubbingcolumn

Stripping airrecirculation

NH -stripping

3

Acid dosage(H SO )2 4

Basedosage(NaOH) Collection tank

ammoniumsulfate

Digestatestorage tank

CO -stripping

2


Stripping in Stirred Tank Reactors

Packed columns

Only suitable after extensive solids removal

Clogging through precipitates (carbonates, struvite, organics)

High cleaning effort

Alternative:

Stripping in stirred tank reactors

No solids removal necessary

But less efficient ammonia transfer

Pressure reduction to increase transfer rate

Problems withprecipitation

Stripping columns


Anastrip-Process

Semi continuous processusing 2 stirred tank reactors

Stripping gas: released CO2

Process conditions:T ≤ 80 °C, P ~ 350 - 400 mbar

New ammonia capture technique

Released NH3 + CO2converted with FGD (flue-gas desulfurization)-Gypsum, CaSO4

Products: Ammonium sulfate + CaCO3

Advantage

No solid liquid separation required

Integrated into biogas process, reduction of N-inhibition

Disadvantage: high investment costs

Stripreactor I

Stripreactor II

Product

NH4recovery


Anastrip-Process

2 reference plants in Germany

Biogas plant Röblingen am See, near Halle/Saale(2012), equivalent 3 MWel

Benas Biogas plant Ottersberg, near Bremen(2007), 5 kWel

Technical data (according to technology provider)

Input material: energy corps, manure, food waste

Treated digestate amount: max. 600 m³/d, 3 - 4 g/L NH4-N

N removal efficiency: > 90 %

Gypsum consumption: max. 11 t/d

Products: (NH4)2SO4(40%): max. 19 m³/dCaCO3 (dry): max. 7 t/d

Energy demand: ca. 50 kWel

2 - 3 MWtherm (from CHP plant)


Vapour

Product

Feed

Heat exchanger

Flash vessel

Heating medium

18

Flash Evaporation

Overheated liquid streampassed through a throttling deviceand undergoes a pressure reduction→ partial evaporation occurs

Vapour (water/ammonia) is condensed

T = 90 – 110 °Clow under pressure

Thermal energy supply byCHP plant possible

Less extensive suspendedsolids removal necessary(particles > 5 mm,depending on throttle)


Flash evaporation

Well established technology fortreatment of N-rich effluents

Can be operated withhigher particle content

Little experience as technologyfor biogas plants

Proposed by acouple of companies

No detailedinformation available


Membrane Contactors

Hydrophobic hollow-fibre membraneswith gas filled pores

Submerged in the digestate

Sulphuric acid is circulated through the lumen of the fibres

Driving force for NH3 transfer:Difference in partial pressure between digestate and sulphuric acid.

Membrane

NH3

(NH ) SO4 2 4

H SO2 4

Digestate


Advantages of Membrane Contactors

Available surface remains undisturbed at high and low flow rates

Very high specific surface obtainable (5,000 - 30,000 m²/m³)

No foaming

No fluid/fluid dispersion (no emulsion)

Modular design

Straightforward scale-up


Membrane module development

1.2 m² 3 m²

0.009 m² 0.05 m²


Experimental set up – technical scale

500 L anaerobic reactor Substrate:

Residues from a bioethanolplant (maize, wheat)

BOKU, Dept. IFA-Tulln I Inst. f. Environmental Biotechnology I Werner FUCHS December13, 2011

24

Membrane contactor - process stability

Voltaile Fatty Acids (VFA) - Reference reactor

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2000

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26.0

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Date

VF

AC

on

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ntr

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[mg

/l]

Acetic acid Propionic acid iso-Butyric acid

Butyric acid iso Valerianic acid Valerianic acid

Volatile Fatty Acids (VFA) - membrane contactor

0

2000

4000

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26.0

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23.0

8.

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9.

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15.1

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07.0

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07.0

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04.0

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02.0

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30.0

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25.0

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Date

VF

Aco

nce

ntr

ati

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[mg

/l]

Acetic acid Propionic acid iso Butyric acid

Butyric acid iso Valerianic acid Valerianic acid

Membrane contactor Reference reactor


Gas Volume

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18.11.

07.01.

26.02.

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me

[ml]

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Reference Reactor Membrane Contactor

Membrane contactor – biogas production


Membrane contactors – open questions

Module design

Long term membrane stability,pore wetting

Cleaning procedure

outside

inside


Conclusions

N-recovery helps to overcome limited area availabilityand to recover N as a valuable product

A variety of strategies and technical solutions existbut limited number of full scale installations

Membrane treatment (UF+RO) is only meaningfulif complete digestate treatment is required

Stripping in stirred tank reactors has high potential,reference plants are available

Flash evaporation is very reasonable technology,applicability for digestate treatment needs to be demonstrated

Membrane contactors are a promising technologybut only in the beginning of its development


Thank you for your attention

status and perspectives for digestate treatment - nitrogen...

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