Environmental Biotechnology

Dr. Benndorf

1 Introduction
2 Biological Fundamentals
3 Biological Waste Gas Treatment

4 Biological Treatment of Wastes

5 Prospects
(More sustainable industrial processes)

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4.1 Composting

Composting (Latin: compositum composition, compound ...)

link together with
Composition of wastes (mixture)
Diversity of microorganisms
Variety of composting processes.

Composting is performed from ancient times. If there is agriculture, there is composting!

Composting means the microbial degradation of organic substances under aerobic

conditions in order to produce a more stable matter which can be used as soil conditioner
or fertilizer.
In an industrial scale separately collected biowastes are treated by composting to close the
material cycle.

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Basic scheme:
raw organic material (fresh) + O2
stabilized organic material + CO2 + H2O + ∆Hm
The reaction enthalpy ∆Hm causes a temperature increase.

Composting includes

 degradation of substances
(products are CO2, H2O from org. matter → mineralization)
 modification and conversion of substances
(substances from degradation steps are changed to new substances)
 assembly/composition of new complex long–chain compounds (humous matter).
This humous matter causes grainy structure and earthy, dark colour of completed compost.

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 Important natural materials and their degradability by composting

Natural Material Degradation Degrading Organisms

starch very fast fungi and bacteria

proteins very easily numerous fungi and bacteria
pectins fast many fungi and bacteria
fructans, mannans, easily numerous microorganisms
xylans
fats and wax good many bacteria
chitins relatively easily many fungi and bacteria
hemicellulose easily (arabans) to poor bacteria and fungi
(galactans)
cellulose relatively good but difficult to many fungi
affect (unsoluble in water)
lignin very slow higher fungi (Basidiomyceten)

from: ATV-Handbuch: Mechanische und biologische Grundlagen der Abfallbehandlung, Verlag

Ernst & Sohn Berlin, 2002, S. 100 f.

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5
 °C thermophilic cooling completion
mesophilic
90
80

70

pH
60
temperature

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50
8
40
7
30 6

20 5

4
time

Temperature and pH-value versus time during composting

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 Optimal Enviromental Conditions for the Microbial Society

Substrate/Nutrient Supply Examples:

mainly C, N Kind of waste C:N

but also S, P, K, Ca, Mg, Fe and foliage (leaves) 30 ... 60
other trace elements cutted grass 12 ... 25
kitchen wastes 18 ... 25
Important is the relation C : N
• C : N = 20 :1 … 40 : 1 (availability for sludge (wastewater
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microorganisms!) treatment)
wheat straw 128
waste paper 200 ... 400
sawdust 500

The different raw materials have to be reduced to small pieces and mixed in order to
achieve a good C/N–ratio.

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 Optimal Enviromental Conditions for the Microbial Society

● moisture (water content)

• > 25 %
• If the water content in less than 25 % the composting will be stopped.

● oxygen supply
• Is necessary for biodegradation (aerobic process)!
• Rule of thumb for the oxygen need: 1 g O2/1 g organic matter

● pH
• A pH–value > 7 is beneficial for a fast start up of composting. So sometimes lime is
added to increase the pH–value.

● great surface area

• crushing is necessary!

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 Connection between water content and oxygen supply

Oxygen supply is performed by the air flow through the void

spaces of the material and by the diffusion into the aqueous phase.

The higher the water content the smaller the void space
(pore volume).
Bad solubility of O2 in water at high temperatures!

So called «structure stability» of the input material is needed in order to

maintain the void spaces (pores).

But with running composting process mass and volume are reduced (40 … 60
%).

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 Composting Processes - Basic requirements

The processes have to be configured so that

• reduction to small pieces and homogenization of raw material (input)

• regulation of water content in the material
• maintaining of a favourable structure by means of repeated mixing for sufficient
oxygen supply and to avoid anaerobic zones
are achieved.

The processes have to be directed so that a compost with defined properties and
constant quality can be produced in spite of different kind, mass and composition of raw
materials according to the seasons.

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acceptance balance biowaste
registration
waste air
intermediate bunker dust
storage

Block Diagram of Composting Processes

silo press water
waste air
Selection of disturbing matter dust
pretreatment separation of metals press water
grinding, sieving residues
homogenization
waste air
shifting dust
aeration press water
composting
irrigation condensation water
waste air
conditioning sieving dust
separation of disturbing matter residues
waste air
dust
storage
compost
Main Tasks of Mechanical Treatment of Biowastes
(Pretreatment of biological processing)

process step task equipment/apparatus

removal of • removal of matter which complicates the • loaders (wheel loader,
troublesome process run excavator, grab)
material • improvement of product quality • sorting station
• Fe- and Non-Fe-Separator
• sifting plants
• pulper
• float/sink-tank
material flow • separation of coarse components • sifting plants
separation • split up aerobic and anaerobic treatable • air separator
partial flows • pulper
• float/sink-tank
confectioning for • crushing of coarse components • desintegrator
the biological step • regulating of suitable particle size distributions • sieving
• adjustment of the water and nutrient content • pulper
as well as of the structure (addition of special • feeder and homogenizer
material, water and nutrients)
• homogenization

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Tense shaft screen

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Sieve drum,
Fa. Avermann
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Separation of iron metals by magnets

Positioned crosswise to the mass flow Positioned lengthwise at the discharge

point of two belt conveyors

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Cutting mill

Rotor diameter: 400 – 800 mm

mass: 2,8 … 10 t
power: 55 … 200 kW

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 Composting Processes

Industrial processes can be differentiated:

1• pile composting
• composting in silos, drums, reactors

2• static processes
• dynamic processes
(with material moving now and again or continuously)
• quasidynamic processes
(automated pile composting in closed halls)

3• discontinuous processes (batch)

• continuous processes

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Excavator wheel and conveyor belt for the pile turnover

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Composting plant Cascais
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Composting of Biowastes 1000 kg input of biowastes
Mass Balance 650 kg water (65 %)
227,5 kg odmbio (65 % odmbio in the d. m.)
(taking a standardized
biowaste as an example)
122,6 kg d. m. min

pretreatment
sieving disturbing matter
separation of disturbing matter 50 kg

950 kg input aerobic stage loss of composting

water, air aerobic stage 639 kg
degradation odmbio 60 % 131,1 kg odmbio
311 kg output aerobic stage 508,8 kg water

subsequent treatment disturbing matter

sieving 20 kg
separation of disturbing matter

291 kg compost
102, 5 kg water (35 %)
81,2 kg odmbio (65 % odmbio in the d. m.)
109,4 kg d. m. min

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 Properties of quality product compost

1 hygienic safety (for humans, animals, plants)

2 degree of composting
3 compatibility to plants
4 organic substance (35% in the dry matter)
5 water content
6 density (increases with composting period)
7 particle size: fine I < 5 mm
fine II < 12 mm
medium < 25 mm
coarse < 40 mm
8 salt
9 pH
10 plant nutrients

Compost contains normally all important plant nutrients.

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 Properties and Components of Compost
Parameter Unit Average value Range for ¾ of all samples1)
loss of ignition % per d. m. 35 25 … 45
(org. matter)
ignition residue % per d. m. 65 55 … 75
water content % 36 35 … 50
density kg/m3 680 550 … 850
soluble salt content g/l 4 2…8
pH-value 7.6 7.0 … 8.3
N total % per d. m. 1.1 0.8 … 1.5
N anorg. mg/l f. m. 150 100 … 400
P2O5 total % per d. m. 0.7 04 … 1.0
P2O5 soluble mg/l f. m. 1200 500 … 2000
K2O total % per d. m. 1.2 0.6 … 1.5
K2O soluble mg/l f. m. 2500 1000 … 5000
MgO total % per d. m. 0.4 0.2 … 0.7
MgO soluble mg/l f. m. 250 150 … 500
CaO total % per d. m. 3 2…6
1) several hundreds of samples
d. m. – dry matter; f. m. – fresh matter

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Quality management
Product and process of composting have always to be tested

Product test
 evaluation of quality (value forming components) and observing of allowable
concentration of undesired substances
 transfer of diseases is improbable (hygienic safety)

Process test
Samples with test organisms (test pathogens) are introduced into the composting process
→ into the plant/reactor. These samples are tested for survivors. Viable test organisms are
removed after the typical residence time (composting period).

Process tests can be classified in:

 Prototype tests: obligatory tests for new composting processes
 Starting tests: perfomed in the first 18 months of a plant operation
 Subsequent tests: in a defined period during 3 years

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Phytohygiene
Test organisms
(Organisms which are especially resistant against the environmental conditions during the
composting)

tobacco mosaic virus ≤ 8 lesions/plant

plasmodiophora brassicae
(cabbage disease) index of plants be seized ≤ 0.5
Tomato seeds ≤ 2% seeds per sample
capable of germination

Human and veterinary hygiene

Test organism
Salmonella senftenberg
no salmonella in the samples and in the product
(3 different zones, 4 different points of the pile, samples are duplicated)

Otto-von-Guericke-University Magdeburg, Institute of Process Equipment and Environmental Technology

Dr.-Ing. Hartmut Haida
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“Biological Drying” of wastes
(domestic wastes without paper and glass
but with organic material)
Process steps
• Pretreatment (separation of metals, shredder)
• Intensive rot → 7 … 10 days (closed box)
• Wrapping (shrink – wrapping in films/foils)
⇒”Dry Stabilat”

Task of intensive rot

• decrease of water content < 15 %
• reduction of mass and volume (25 … 30 %)
• increase of calorific value > 11000 kJ/kg
The degradation of organics is not the preferred goal because of the reduction of the
calorific value.

This material (”Dry Stabilat”) can be stored and incinerated

→ energetic utilization of wastes!

Residues from incineration can be deposited at landfills!

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Schematic of biodrying box with process air circulation and dehumidification based on a Herhof system:
(1) enclosed box
(2) air forced through the waste matrix, heated by the exothermic aerobic biodegradation of readily decomposable waste fragments
(3) leachate collection and circulation system
(4) forced aeration system with partial air recirculation, mixing ambient air and conditioned process air
(5) heat exchanger
(6) cooling tower
(7) water (vapour condensate
(8) exhaust air treatment through biofilter or regenerative thermal oxidation (RTO).

Appropriate conditions for microbial activity allow for the biodegradation of the waste placed within the bioreactor, providing the necessary heat to
evaporate moisture from the waste fragments. Evaporated moisture is removed by the air convection, achieved by forced aeration. The exhaust
air is going through various treatment stages that improve its drying capacity (ability to carry moisture) before it is partly re-circulated into the
reactor, after being mixed with ambient air.

C.A. Velis, P.J. Longhurst, G.H. Drew, R. Smith, S.J.T. Pollard. Biodrying for mechanical–biological treatment of wastes: A review of
process science and engineering. Bioresource Technology, Volume 100, Issue 11, 2009, 2747–2761

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