Co-digestion of biowaste on wastewater

treatment plant (WWTP)

• Why co-digestion on wastewater

treatment plants?

• Recycling of used coffee capsules

• Management of contraries (metal,

glass, sand) in AD

• Open questions, outlook

Overview

Principle of co-digestion on WWTP

Co-substrates

primary sludge

biogas

dewatered sludge

excess sludge

rejection water

Co-digestion of biowaste on WWTP

• ~ 10.000 digesters on WWTP in the EU

• Most of them have relevant capacities for

additional, external substrates

• Existing infrastructure, qualified personnel

• Investment costs for co-digestion low

(storage tank for substrates, pipes and

pumps, ~ 100.000 €).

• Collection and pre-treatment of co-

substrates by external companies

Co-digestion on WWTP

Energy balance on WWTP

• Germany, Austria: average of 43 % self-

production of electrical power from biogas

produced from sewage sludges

• co-digestion: energy self sufficient WWTP

can be realised

• Co-substrates: separatly collected

biowaste, kitchen waste, waste from dairy

industries….

Co-digestion on WWTP

Energy balance of WWTP Zirl, Tyrol, Austria

Co-digestion on WWTP

0

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2006 2007 2008 2009 2010 2011 2012 2013 2014

Bio

gas

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Biogas from sewage sludge Biogas from Co-substrates

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Production electricity Consumption electricity • Total consumption of

electricity ~ 1000 kWh/a

• Dosage of co-substrates

started in 2008.

• From 2009 to 2014:

surplus of electricity

• 2014: problems with

digester foaming,

reduction of substrate

dosage

Recycling of used coffee capsules

Co-digestion on WWTP

• Lifestyle products“ in

food industry e.g. coffee

capsules

• Special challenge for

waste recycling

• Used capsules: coffee

powder and a capsule

made of aluminum

• Used coffee powder biogas production

• Aluminum metal recycling

• Collection and pretreatment of the used

capsules required

Recycling of used coffee capsules

Co-digestion on WWTP

• Separate collection system

for the used capsules in

Austria (Nespresso

company)

• Pretreatment by the

company Höpperger

Recycling in Pfaffenhofen,

Tyrol, Austria

Pretreatment of used coffee capsules

Co-digestion on WWTP

• Collected capsules

transproted to the central

pretreatment facility in

Tyrol, Austria

• Pretreatment using a

hammer mill

(Wackerbauer, Germany)

• Addition of a liquid

required (washout of the

coffee powder). Liquid

biowaste like whey can be

used

Pretreatment of used coffee capsules

Co-digestion on WWTP

• Produced co-substrate (coffee powder-

whey) transported to the WWTP by a

tank truck (pumpable substrate)

• Storage at the WWTP in a storage tank

• Co-digestion->biogas->CHP->

electricity and heat production

• 300 Nm3 CH4/t dry coffee powder

Digester operation using co-substrates

Co-digestion on WWTP

• Biological stable process (no overloading problems)

• Synergistical enhancement in sewage sludge

methanization, no increase of digestate and disposal

costs up to 25 % co-substrates addition (volatile

solids)

• Most mechanical pretreated biowastes can be stored

without loss of the energy potential

• „New“, anknown digester foaming problems->

research needed! (Known problems with filamentous

bacteria deriving from activted sludge stage)

• Unwanted material (contraries) in the co-susbtrates

(aluminum, glass, sand, plastics etc)

Contraries (impurities) in co-substrates

Co-digestion on WWTP

• Aluminum (coffee capsules), glass, sand, plastics

(biowaste)

• Damage of pumps, clogging of pipes, reduction of the

active, mixed digester volume due to formation of

sediments

• Research topics:

• Quantification of contraries

• Removal of contraries

• Monitoring of the active, mixed digester volume

Contraries (impurities) in co-substrates

Co-digestion on WWTP

• Aluminum (coffee capsules),

glass, sand, plastics (biowaste)

• Damage of pumps, clogging of

pipes, reduction of the active,

mixed digester volume due to

formation of sediments

• Research topics:

• Quantification of contraries

• Monitoring of the active,

mixed digester volume

• Removal of contraries

Contraries (impurities) in co-substrates

Co-digestion on WWTP

• Quantification of contraries

• Sampling

• Extraction of contraries

• Classification, quantification,

size distribution etc.

• Samples taken from sewage

sludge, co-substrates and

• From the digester sediments

(by industry divers)

Contraries (impurities) in co-substrates

Co-digestion on WWTP

• Monitoring of the active, mixed

digester volume

• Important issue for WWTP

• Sensitive tracer method

developed (Volutrack)

• Change of the active, mixed

digester volume can be

monitored

• WWTP Zirl: 1350 m3 original

volume, linear decrease of the

active digester volume (25 m3/a)

• Operational safety, planning and

budgeting of cleaning actions

y = -0,0619x + 3818,6

-

200

400

600

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1.000

1.200

1.400

01.01.2008 31.12.2009 31.12.2011 30.12.2013 30.12.2015

acti

ve, m

ixe

d d

ige

ste

r vo

lum

e (

m³)

Contraries (impurities) in co-substrates

Co-digestion on WWTP

• Removal of contraries

• Removal from the mechanical pretreated

biowaste difficult due to the high viscosity

(~16 % TS content)

• Dilution of the substrate required

• New idea: Installation of a hydrocyclone in

the external heating circuit

• Internal dilution of the substrate with

digestate ~ 3% dry coffee powder

• Removal of glass, sand etc. in the cyclone

• Prototype installed, promising results

• New research project accepted for further

optimisation of the process

digester content ~ 3 % TS

Co-substrate ~ 16 % TS

heat exchanger

Outlook

Co-digestion on WWTP

• Management of contraries in AD on WWTP, ongoing

projects

• Synergistical effects, enhanced sludge methanisation by

the addition of co-substrates, reduction of disposal costs

for the dewatered sludge

• Serial or parallel operation of the digesters? (degradation

of organic matter, removal of contraries, foam etc.)

• Pre-treatment of substrates and sewage sludge for

enhanced degradation

• Energy storage in the substrate, demand driven

methane/energy production on WWTP

• WWTP as a component in a future, decentralized

energy system based on renewable sources

THANK YOU FOR

YOUR ATTENTION