anaerobic digestion of municipal solid waste residual waste

7/27/2019 Anaerobic Digestion of Municipal Solid Waste Residual Waste

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Anaerobic digestion of municipal solid waste: residual wasteL. De Baere, Managing Director OWS

The anaerobic digestion of the so-called grey or residual waste fraction coming from MSW isbecoming more and more important. Anaerobic digestion of municipal solid waste developedvery rapidly in Europe during the mid-nineties but was mainly geared to the treatment ofbiowaste, due to the fact that source separate collection of the biowaste fraction was beingimplemented in many countries. The purpose of the source separation at the household levelwas to be able to achieve a high-quality compost with a very low concentration in heavymetals.

In the last 5 years however, the application of anaerobic digestion on residual waste hasincreased dramatically. In 1998, only 13% of the digestion capacity in Europe was installedfor the treatment of residual waste but by 2006, more than 52% of the digestion capacity forMSW was designed to treat residual waste. Current construction of anaerobic digestion

plants for MSW is about 2/3 for residual waste and 1/3 for source separated biowaste.

The most difficult material to treat is the residual waste that is produced after the separatecollection of both recyclables as well as the organics. This means that the easily digestiblematerials are removed and residual waste with a large degree of impurities, such as glass,stones and plastics, is left over. The purpose of the anaerobic digestion of this residual wasteis then mostly to produce an inert stabilized material that can be landfilled in accordance tothe German regulations. The oldest digestion plant treating residual waste after separatecollection of the biowaste is located in Bassum, Germany, utilizing the Dranco technology.

Partial stream digestion plant in Pohlsche Heide, Hille, Germany

A more recent plant digesting the organic fraction derived from household residual waste, isthe plant in Pohlsche Heide, Hille, constructed in 2005. The plant is designed to handle100.000 t/year, of which 37.500 t/y are anaerobically digested. The plant treats bothindustrial waste and residual MSW, as well as dewatered sludge coming from wastewatertreatment plants in the area surrounding the plant. Only the organics from the residual MSWand the sludge are subjected to digestion (see Figure 1).

The residual MSW is first shredded and screened over a rotating sieve of 60 and 300 mm.After removal of the ferrous metals, the fraction less than 60 mm is sent to a ballisticseparator. The ballistic separator removes the heavies, such as glass, stones etc, present inthis fraction. The heavy components are sent directly to the composting phase. The refined

organic fraction after removal of the heavies is subsequently conveyed to a metering bin, inorder to meter out the material in the right proportion during feeding of the digester. A part ofthe organics is diverted without being digested and is mixed with the digestate coming fromthe digester. The other part of the organics is fed to the mixing-feeding unit of the digester, in


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which one part of fresh organics is intensively mixed with around six to eight parts ofdigestate. Iron chloride and process water are added as needed. A tonnage of 50 to 100 tonper week of sludge is added to the mixture as well, together with the steam necessary toraise the temperature to around 50C in order to ob tain thermophilic operating conditions.The mixture is pumped by means of a piston pump to the top of the digester, where themixture starts to ferment immediately.

Figure 1: Flow sheet partial stream Dranco digestion plant in Pohlsche Heide, Germany

The digester has a volume of 2260 m, with a total height of 25m. The residence time in the

digester is 21 days on average. The biogas produced is extracted from the top of thedigester, while the digesting material moves from top to bottom through the digester bymeans of gravity only. The digestion tank is not mixed and does not contain any movingdevices. The digested material is extracted from the conical bottom of the digester by meansof screws and is either recycled to the mixing-pumping unit for the addition of fresh feedstockinto the digester, or is removed from the process and sent to a second mixing unit prior to thecomposting phase.

Bypassed non-digested organics are mixed with digestate in a ratio of 1:2 and subjected toan intensive composting phase in heated tunnels. After the intensive composting for 2 to 3days, the material is moved to non-heated tunnels and is further stabilized and dried. Thefinal end product is sufficiently stabile to meet the standards for inert landfilling, as specifiedby German law.

FEEDINGPUMP

STEAMGENERATOR

MIXING UNIT

Biogas

DRANCODIGESTER

2.260 m

Steam

DOSING UNIT

Sewage sludge

GAS STORAGE GAS COOLING

GAS BLOWERS

GAS ENGINE

RTO

Existingflare

SILOXANEADSORBER

Process water

Municipal solid waste

INTENSIVECOMPOSTING

COMPOSTING Landfill

ROTATING SIEVE(60/300 mm)

MAGNET

Fraction< 60 mm

Fraction60-300 mm

Fraction> 300 mm

Landfill

AIR SEPARATOR

NON-FERROUSSEPARATOR

To intermediatestorage

Lightparts

Heavyparts

SHREDDER

MAGNET

BALLISTICSEPARATOR

Hardmaterials

Industrial waste

SHREDDER

To intermediatestorage

MAGNET

INCINERATION

PLANT

SHREDDER


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The biogas produced is collected at the top of the digester and stored for a brief time in a gasstorage. The biogas is further cooled down and pushed through an activated carbonscrubber, in order to remove siloxanes. The cleaned biogas is utilized either for theproduction of steam, for the production of electricity in gasengines or for the RTO-unit(Regenerative Thermal Oxidation) for the burning of the most polluted process air (seeFigure 2).

Figure 2: Air flow diagram

Digestion process and results

The feedstock after pretreatment of the residual household waste amounts to about 500 t perweek. The material has a high dry matter content (54% DM) and a volatile solids content of55% (Table 1). The sludge that is added contains about 21% of dry matter with a volatilesolids concentration of 61%. The digestion results in the production of 120Nm of biogas perton of organics derived from residual waste and 23,5 Nm per ton of sludge. The biogas hasa methane concentration of 56,2% on average.

DELIVERY

Hall + point source

PRETREATMENT

Hall + point source

DIGESTION

HallPoint source

COMPOSTING

Tunnel filling hallTunnel

DUST FILTER AIR HUMIDIFIER

ACID WASHER

RTO

BIOFILTER

CHIMNEY

Ammonium sulphate

Biogas

Fresh air

Fresh air

Fresh air

Fresh air

H2SO4


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Table 1: Composition of organics after pretreatmentOn fresh weight Dry matter on fresh weight Ash content of dry matter

Inerts 9,7 % 94 % 94,8 %

Plastics 4,6 % 74 % 26,4 %

Textiles 1 % 57 % 21,2 %Rest 84,7 % 54 % 44,6 %

Figures 3, 4 and 5 show an overview of the results obtained during the weeks 34 through 43.Figure 3 shows the tonnage fed during each week. During week 39, the digester was fedduring 3 days only, due to maintenance works in the pretreatment. The biogas yield per tonfluctuated only very slightly during this 10-week period. Figure 4 shows the weekly tonnageof organic waste and sludge. The sludge represented on average about 10% of the feed, butrose to 25% during the week that organics were only pretreated during 3 days. Figure 5shows the biogas production per week in Nm, which averaged around 55.000 Nm and aspecific productivity of 4,5 Nm per Nm of active reactor volume and per day.

Figure 3

INPUT IN REACTOR & BIOGAS PRODUCTION-DRANCO PLANT POHLSCHE HEIDE-

0

100

200

300

400

500

600

700

34 35 36 37 38 39 40 41 42 43

Week (2006)

To

ns

0

20

40

60

80

100

120

140

Nm/Mg

eq.waste

Total reactor input Biogas production


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Figure 4

Figure 5

INPUT IN THE REACTOR-DRANCO PLANT POHLSCHE HEIDE-

0

100

200

300

400

500

600

34 35 36 37 38 39 40 41 42 43Week (2006)

Tonsofmunicipalsolid

wastel

0

25

50

75

100

125

150

Tonsofsludge

Input municipal solid waste in reactor Input sludge in reactor

BIOGAS PRODUCTION PER WEEK & BIOGAS PRODUCTIVITY-DRANCO PLANT POHLSCHE HEIDE-

0

10.000

20.000

30.000

40.000

50.000

60.000

70.000

34 35 36 37 38 39 40 41 42 43

Week (2006)

Nm/Week

0

1

2

3

4

5

6

7

Nm/m

reactor.day

Biogas production Biogas productivity


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anaerobic digestion of municipal solid waste residual waste

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