Digester Training slides v203 - 2010-05-19

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Percolation & Digestion

Introductory Training Module

19 May 2010

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Agenda

Percolation

Sand separation and dewatering

Anaerobic Digestion

Denitrification & biomass retention

Hydrogen Sulphide removal

Power generation

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Lancashire Waste PFI Contract

Under the Project Contract, GRLL’s key obligations are:

To divert at least 56% of residual waste arisings from landfill disposal

To divert at least 93% of green and kitchen waste from landfill disposal

To divert at least 90% of co-mingled dry recyclables from landfill disposal

To divert all separately collected dry recyclables from landfill disposal

To plant 100,000 trees per year beginning 2010/2011 to mitigate carbon dioxide emissions

To assist the Councils in limiting waste arisings growth to no more than 1% per year

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The UR-3R Process®

Municipal Solid Waste

Renewable Energy

Reduced Greenhouse Gas Emissions

Organic Growth Media

Recycled Products

Composting and Refining

Waste Stream Separation

ISKA®

Percolation

Energy Production

“Key technology to achieve Zero Waste Goals”

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Percolation

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ISKA® Percolation

Clean Percolated

Waste

High Organic Waste from

sorting

Sand, Glass & Grit Removal

Percolator

Biogas

Recirculated Hydrolysis Solution

Air

Digestion

Water

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ISKA® Percolation Washes out easily extractable

organics: Separates degradation from biogas

generation – reduced tankage for organic solids

odour reduction – aerobic process mass reduction SNAP readily amenable to composting

Homogenisation - stable feed to composting

Cleans organics – some glass and stones washed out

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Advantages of percolation Mass reduction

Recovery of renewable energy

Allows anaerobic digestion to be operated as a liquid digester

Homogenises organic fraction of waste This also allows the easily biodegradable organics to be removed The organics from percolators are mixed and are of similar biodegradable nature

Compost hall is smaller Less organics go to compost hall The organics from percolators tend to compact

Allows composting to be started quicker in the compost hall because waste removed from percolators is hydrolysed

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Solid Separation and Dewatering

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Dewatering

Dewatering is required to: Remove waste (SNAP) from percolators for composting Produce instantaneous percolate Composting is retarded when waste is too wet (minimum

pressure on dewatering/screw press be 2 bar)

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ISKA® Percolation – Back end Kufferath Screw press for solid / liquid separation

Sand Separators

Vibrating screens for fiber removal

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Anaerobic Digestion

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DigesterFeedTank

AnaerobicDigester

BiomassRetention

Denitrification(Ammonia Removal)

ProcessWaterTank

Heat Exchanger

Sludge (Biom

ass)

Cen

trat

e

Low N

H3 W

ater

Percolate

Bio

gas

Recirculation

Amm. Sulphate

Anaerobic Digestion Circuit Schematic

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BiomassRetention Sludge (B

iomass)

Cen

trat

e


ProcessWaterTank

Low N

H3 W

ater

Amm. Sulphate

Anaerobic Digestion

DigesterFeedTank

AnaerobicDigester

Heat Exchanger

Percolate

Bio

gas

Recirculation

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Slow step (3 days)

Rate limiting step (>4 days)

Also, pH sensitive

Slower step (3 - 4 days)

The rate and conditions for each anaerobic digestion step play an important role in determining digester performance

Stage 3 Methane formation

Stage 1 Hydrolysis, liquefaction and fermentation

Complex waste organics Carbohydrates Proteins Lipids

Simpler, soluble organics

Propionate, butyrate etc.

(long chain fatty acids)

H2, CO2 Acetate

CH4, CO2

1

1

1 1

2 2

3

4 5

Bacterial groups: 1. Fermentative bacteria 2. H2-producing acetogenic

bacteria 3. H2-consuming acetogenic

bacteria 4. CO2-reducing methanogens 5. Aceticlastic methanogens Stage 2

Hydrogen and acetic acid formation

VFA

VFA

Anaerobic Digestion Anaerobic digestion is a biological process in which biodegradable organic materials

are decomposed in the absence of oxygen to produce methane and carbon dioxide.

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An Anaerobic Digester is a cow, not a tractor!

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Due to practical / mechanical issues, hydrolysis is often rate limiting step in AD

Hydrolysis is mechanical / biological breakdown of complex organics to simpler organics

Percolation expedites hydrolysis by washing out the easily biodegradable organics for AD by the mechanism of washing

Anaerobic Digestion & Percolation in Combination

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Acceptable pH of 6.8 to 8 (Optimum 7.1 to 7.4)

35oC Optimum temperature 39oC Too low temperatures inhibit activity Too high temperatures kill bacteria

What is necessary for growth may become toxic COD / BOD Volatile Fatty Acids (VFAs)

By-products can become toxic if not treated or removed Ammonia Sulphur compounds

Extremely sensitive to environmental conditions, loading rate fluctuations and toxins

Anaerobic Digestion

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Too much of a good thing is bad for a digester!

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Effect of Temperature

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Also, pH sensitive

Slow step (3 days)








H2, CO2 Acetate

CH4, CO2

1

1

1 1

2 2

3

4 5





VFA

VFA

Anaerobic Digestion If the VFAs are not utilized at the rate they are produced, then

it can kill the methanogenic activity due to lower pH

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….and if it wasn’t already enough of a challenge

Digesters never stop running!Digester runs all day and night – 8,760 hours a year

= the life of 3 Mercedes Benz cars with 45 services every year

Digesters need to be robust and maintainable!

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iomass)

Cen

trat

e


ProcessWaterTank

Low N

H3 W

ater

Amm. Sulphate

Biomass Retention

DigesterFeedTank

AnaerobicDigester

Heat Exchanger

Percolate

Bio

gas

Recirculation

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In a CSTR:

The HRT in our digesters is 10 – 12 days

So, SRT is 10 – 12 days

Is this long enough…?

Biomass Retention

=(Solids Retention Time)

SRT(Hydraulic Retention Time)

HRT

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Also, pH sensitive

Slow step (3 days)








H2, CO2 Acetate

CH4, CO2

1

1

1 1

2 2

3

4 5





VFA

VFA

SRT = 12+ days

Anaerobic Digestion If the SRT is too short, bacteria will never reach maturity

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The centrifuge separates the solids (biomass) from the digester liquor

Flocculent added to aid the process, and solids returned to the digester

This breaks the SRT / HRT connection, so:

In a CSTR with biomass return:

Biomass Retention

≠(Solids Retention Time)

SRT(Hydraulic Retention Time)

HRT

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Biomass centrifuge is upstream of the Denitrification circuit.

Temperature and pH in the Denitrification cicuit would kill all bacteria.

Even if Denitrification is not run, centrifuge should be run periodically to ensure biomass is retained in the digesters.

Biomass Retention

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iomass)

Cen

trat

e


ProcessWaterTank

Low N

H3 W

ater

Amm. Sulphate

Ammonia Removal

DigesterFeedTank

AnaerobicDigester

Heat Exchanger

Percolate

Bio

gas

Recirculation

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Dentrification (Ammonia Removal)

Ammonium ion (NH4+) and

Ammonia (NH3) are toxins in the AD process

(NH4+) inhibits methane

formation @ 1500ppm

(NH3) inhibits methane formation @ 150ppm

(NH4+) and (NH3) exist in

equilibrium, based on pH, as follows:

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Denitrification

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The Denitrification process is simple (on paper…):

1. Heat the high-Ammonia liquor (approximately 75°C), which reduces the solubility of gases in the liquid phase.

2. Bubble air (as much as is practical) through the liquid, to strip out any entrained gases (especially CO2). This has the effect of increasing the pH of the liquid phase ( > 9 ).

3. Increase in pH shifts the (NH4+) / (NH3) equilibrium in favour of (NH3) – which is gaseous Ammonia

4. Strip the gaseous Ammonia from the liquid phase with more air

5. Bubble the Ammonia-rich air stream through Sulphuric Acid, to produce Ammonium Sulphate. Find a use for the Ammonium Sulphate…

6. Return the now low-Ammonia liquor to the process water tank.

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The Denitrification process is tricky (in practice…):

1. Requires a lot of energy to heat up the total volume of high-Ammonia liquor to 75°C.

2. Not always easy to achieve or maintain a pH > 9 in the aeration tanks.

3. pH < 9 means the (NH4+) / (NH3) equilibrium favours (NH4

+), which remains in the liquid phase.

4. Digesters require the most stable conditions possible, so shock changes in Ammonia levels (even if a reduction) can cause digester health problems.

5. As always – regular, small changes are preferred.

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Hydrogen Sulphide Removal and Power Generation

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Hydrogen Sulphide Removal and Power Generation

Biogas is: 60-65% Methane

35% Carbon dioxide

up to 30000ppm H2S

Gas engine requires less than 500ppm H2S

Operates down to 25% Methane

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Hydrogen Sulphide Removal

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Hydrogen Sulphide Removal

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Emission Limits

NOx 475 mg/Nm3

CO 1000mg/Nm3

SO2 350mg/Nm3

VOC 950mg/Nm3

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Biogas Storage

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Power Generation

Generate up to 1.8 MWe

Expected power production 1.4MWe

Must operate in a way that limits flaring to less than 10%/annum

Efficiency of Gas engines reduces below 75% throughput

Manage system so that we maintain high efficiency

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Two 1 MW Gas Powered Generators

Power Generation

Flare Backup

Documents

Digester Training slides v203 - 2010-05-19