Lecture 10 - Biological treatment (chap 9.1+9.2+9.4+9.5)

Question 1

How much of Danish household biowaste is reprocessed?

Answer 1

8%

Question 2

Mention types of agricultural waste with biogas-potential.

Answer 2

• Manure (37 mil. tonnesare generated per year)
• Straw ( 5+ mill. tonnes excl. deep litter and animal feed)
• Catch/cover crops
• Buffer zones
• Corn and beet root residues, grass and clover grass
• Aquatic biomass

Question 3

How is the trend in historical biogas production in DK?

Answer 3

Increasing (slow increase from 95-2013 then very high yearly increase with around 2000 TJ/year from 2014-2018)

Question 4

Wich 4 points was presented from the Danish resource strategy (2013)?

Answer 4

• Min. 50% of all houehold waste must be recycled by 2022 (2012-75% was compusted)
• Min. 60% of the organic part of waste from the service sector must be recycled by 2018 (2012 - 83% was combusted)
• Min. 25% of garden waste must be used for energy production by 2018
• Min. 80% of the phosphorous in sewage sludge must be reused by 2018.

Question 5

Which regulation defines regulations on spreading digested biomass on soil?

Answer 5

The statutory Order on Sludge (“Affald til jord-bekendtgørelsen” nr. 1650/2006).

Question 6

Which regulation specifie that food waste has to undergå a hygienisation/pasteurisation treatment?

Answer 6

Commission regulation (EU) No 142/2011

Question 7

Explain what a composting process is.

Answer 7

Compostingis a microbial aerobic transformation and stabilization of heterogeneous organic matters in aerobic conditions and in solid state.

• ‘Compost is the stabilized and sanitized product of composting which is beneficial to plant growth’

Question 8

What are the main microbial biomass responsible for the degradation of organic waste?

Answer 8

1. Bacteria-can degrade waste under anaerobic as well as aerobic conditions;
2. Actinomycetes-enzymatically better equipped to degrade more complex substrates (hemicellulose, cellulose and lignin)
3. Fungi -more complex organisms than actinomycetes; important in the degradation of hemicellulose, cellulose and lignin

Question 9

List types of organic material that are

1) Readily degradable
2) Slow degradable
3) Usually resistant

Answer 9

1) Sugars starches, glycogen, pectin, fatty acids and glycerol, lipids, fats and phospholipids, amino acids, nucleic acids, proteins
2) Hemicellulose, cellulose, chitin, low molecular weight aromatics and aliphatics
3) Lignocellulose and lignin

Question 10

Explain the technological process factors for composting (biodegradability, moisture content, oxygen content, temperature and hygiene, nutrients, pH)

Answer 10

• Biodegradability - depends on the composition of the feedstock, the efficiency of the technology, and the length of the composting period including curing
• Moisture content - optimum moisture content is feedstock-specific and varies between 40 and 70%.
• Oxygen content, material structure, particle size and aeration - a minimum free pore space of 20–30% is recommended for a sufficient supply of oxygen to the waste
• Temperature and hygiene - Most composting studies have concluded that the optimum temperature during the high-rate decomposition period is about 55 ◦C. During curing, the optimum temperature is around 40 ◦C
• Nutrients - With the exception of nitrogen, biodegradable wastes generally contain enough macronutrients (sulfur, phosphorus, potassium, magnesium, calcium) and micronutrients to sustain the composting process. The C/N ratio at the beginning of the composting process is between 20 and 30 for most wastes. Actual availability of the carbon and nitrogen is very important
• pH - optimum pH range is between 7 and 8

Question 11

Generally, the composting process is technically divided into three degradation phases. Explain these phases.

Answer 11

1. High-rate degradation
   a. High-rate degradation is the thermophilic part of the composting process where the feedstock is degraded to a ‘fresh compost’. During the high-rate degradation phase, volume and mass are reduced by degradation of the easily degradable organic matter that is usually responsible for both vector attraction (disease-spreading organisms like flies, rats, etc.) and the most intense odor emissions released from a composting facility. Additionally, pathogens are destroyed due to the thermophilic temperatures.
2. Stabilization
   a. During stabilization the ‘fresh compost’ is degraded to ‘stabilized compost’. As the temperature decreases, the degradation continues and the organic matter is further stabilized.
3. Curing.
   a. During curing, the compost matures to ‘cured compost’. Ambient temperatures are reached and the humification started earlier continues. However, it should be noted that even ‘cured compost’ is not completely stable; it still has an oxygen demand.

Question 12

Which types of composting systems are available?

Answer 12

Open technologies –> exhaust gas of the composting process escapes the environment.

Enclosed technologies –> buildings or reactors. Enable treatment of exhaust gas.

Static technologies

Dynamic technologies –> Rotating drums, turning and agitation based techniques.

Question 13

The mass balance for composting processes is dependent on many factors. Which ones?

Answer 13

• most important is the type and composition of waste
• the intended quality of the final product: the better the quality and the more stable the compost
• composting technology

Question 14

What are the potential emissions of composting?

Answer 14

Air emissions
o primarily related to CH4, NH3, N2O, odours, dust and bioaerosols
o 0.77–3.0% of the carbon degraded during the composting process is emitted as CH4
o emission of N2O may be in the order of 0.13–1.8% of the initial N content

Liquid emissions
o	leachate (water that percolates through the compost and exits at the bottom) and condensate (water that evaporates from the compost and condensates in condensers or on cooler building surfaces such as roofs and walls) are collected as excess water

Question 15

Explain anaerobic digestion and the products from this process.

Answer 15

Anaerobic digestion is typically a biological conversion process without an external electron acceptor (which in aerobic processes is O2) –i.e.CO2 from the degradation itself is electron acceptor. However, also conversion of externally added CO2and hydrogen is anaerobic digestion.

Products:
Biogas: is the gas mixture of mainly CH4and CO2deriving from anaerobic digestion. Also contains small amounts of water, NH3, H2S and volatile organic compounds.
- about 50-90% of the energy content of the waste fed to the digester is captured in biogas, depending on the type of waste
Digestate: is the liquid (or solid) residue remaining after anaerobic digestion

Question 16

Explain anaerobic digestion and the products from this process.

Answer 16

Anaerobic digestion is typically a biological conversion process without an external electron acceptor (which in aerobic processes is O2) –i.e.CO2 from the degradation itself is electron acceptor. However, also conversion of externally added CO2and hydrogen is anaerobic digestion.

Products:
Biogas: is the gas mixture of mainly CH4and CO2deriving from anaerobic digestion. Also contains small amounts of water, NH3, H2S and volatile organic compounds.
About 50-90% of the energy content of the waste fed to the digester is captured in biogas, depending on the type of waste

Digestate: is the liquid (or solid) residue remaining after anaerobic digestion

Question 17

What are the 4 steps of anaerobic degradation?

Answer 17

1. Hydrolysis of particulate matter and larger molecules
2. Acidogenesis (formation of acids) generating primarily acetate but also other Volatile Fatty Acids (VFA)
3. Acetogenesis (formation of acetate). Here, hydrogen is used as an electron acceptor
4. Methanogenesis
   
   1. Acetate => CO2 + CH4
   2. Hydrogen + CO2 => CH4 + water

Question 18

Why will the practical yield of methane obtained in a biogas reactor always be lower compared to the theoretical and lab-measured yield?

Answer 18

• At a finite retention time a fraction of the organic material will be lost in the effluent, typically 5–10 %.
• Lignin is not degraded anaerobically.
• Often a part of the organic material is inaccessible due to binding in particles or structural organic matter.
• Limitation of nutrients.

Question 19

Which types of processes and reactors are used for anaerobic digestion of municipal organic waste?

Answer 19

Moisture content of the substrate
- Wet and dry anaerobic digestion
o Question of the moisture content in the biological reactor. A dry process has moisture content of less than 75% and the biomass looks like a thick slurry. The wet process has moisture content above 90% and the biomass looks like a liquid.

According to the feeding mode

• Continuous mode: continuous stirred tank reactor(CSTR) or compartmentet plug flow reactor
• Batch mode

Digestion process temperature
- Mesophilic (between 35 °C and 40 °C)
- Thermophilic (between 50 °C and 55 °C)
Selection of process temperature is a balance between several factors. A higher temperature gives faster biological degradation and some kind of material is more easily treated at high temperature. However, the thermophilic process is more difficult to operate and the need for heating and insulation add an extra cost to the treatment.

Shape of the reactors
- Vertical and horizontal

Question 20

Explain the difference between wet and dry anaerobic digestion

Answer 20

Wet digestion: TS of the substrate is less than 15%

• The substrate needs to be conditioned
• Higher consumption of water and energy for heating
• Higher capital costs
• Higher biomass conversion efficiency

Dry digestion: TS of the substrate is 20%-40%

• Very robust: less conditioning
• The digesters are plug flow reactors -generally do not apply mechanical mixers and may use biogas injection
• Complete mixing may not happen -ideal contact of microorganisms and substrate cannot be guaranteed

Question 21

Explain the differences between a batch and continuous feeding system

Answer 21

Batch systems: digesters are filled once with fresh feedstock, with or without addition of innocula, and sealed for the complete retention time, after which they are opened and the effluent removed
o does not require fine shredding of waste, sophisticated mixing or agitation equipments, or expensive, high-pressure vessels

Continuous systems: fresh feedstock continuously enters the digester and an equal amount of digested material is removed
o One stage systems: all biochemical processes take place simultaneously in a single reactor
o Two/multi-stages systems: offers optimization of the digestion conditions by providing separate reactors for each step:
1. hydrolysis, acidification and liquefaction,
2. methanogenesis-acetate, hydrogen and carbon dioxide are converted into methane

Question 22

What are the potential emissions from anaerobic degradation?

Answer 22

The main emissions may be:
• Diffuse air emissions from leaks and solid handling
(0.1-3% of the biogas may be emitted)

• Air emissions from conversion of biogas
(e.g.combustion of the gas in a gas engine for heat and electricity)

• Emissions to air, water or soil from digestate (if not considered a product) or digester liquid after dewatering. (e.g.CO2, CH4, N2O)

Question 23

High quality digestate fit for use as fertilizer is defined by essential features such as…

Answer 23

• declared content of nutrients, pH, dry matter and organic dry matter content,
• homogeneity,
• purity (free of inorganic impurities such as plastic, stones, glass, etc.),
• sanitised and safe for living organisms and the environment with respect to its content of biological (pathogenic) material and of chemical pollutants (organic and inorganic)

Question 24

What is the rule thumb regarding the use of the digestate as fertilizer?

Answer 24

The rule of thumb is that if efficient pollutant removal cannot be guaranteed either by pre-treatment or through the AD process, the respective material must not be used as feedstock in biogas plants where digestate is used as fertilizer or for other agricultural purposes.

Question 25

The potential pollutants of anaerobic digestion can be divided in the 4 categories:
1) Physical impurities
2) Chemical impurities
3) Organic pollutants
4) Pathogens and other unwanted biological matter.
Give som examples for all the categories

Answer 25

1) Undigestible materials
2) Heavy metals
3) Persistent organic pollutants as PCB, dioxins, furans, PAHs plasticizers, flame retardants, medicine.
4) Bacteria viruses, intestinal parasites, weed and crop seeds and crop diseases.

Question 26

The potential pollutants of anaerobic digestion can be divided in the 4 categories:
1) Physical impurities
2) Chemical impurities
3) Organic pollutants
4) Pathogens and other unwanted biological matter.
Give som examples for all the categories

Answer 26

1) Undigestible materials
2) Heavy metals
3) Persistent organic pollutants as PCB, dioxins, furans, PAHs plasticizers, flame retardants, medicine.
4) Bacteria, viruses, intestinal parasites, weed and crop seeds and crop diseases.

Question 27

Mention examples of mechanical pre-treatment for biowaste wet AD

Answer 27

• Shredding and sieving
• Screw press
• Extrusion press
• Wet hammer mill
• Pulper and separation
• and many other variations

Question 28

List the major technological elements of an anaerobic digestion plant presented in the textbook. Describe their functions.

Answer 28

1. Reception of waste: Weighbridge, tipping place, bunker.
2. Pretreatment: Grinders/shredders, separators, hygienization and treatment of residues from pretreatment.
   o Function: Removal of unwanted items, removal of plastic bags used in collection or other items that can disturb the process or the quality of the residues, size reduction of the biomass prior to anaerobic treatment, and hygienization of the biomass, if needed.
3. Digestion: Feeding, mixing.
   o Function: The anaerobic digestion shall convert the waste into biogas and digest.
   The digestion system is comprised by the following functions:
   - Storage and feeding system.
   - Preheating of the biomass.
   - Mixing of new biomass and active microorganisms.
   - Gas collection system.
   - Separation of solid and liquid digest.
4. Gas handling: Collection, treatment, storage and utilization.
   o Function: The gas storage and treatment system collects and prepares the produced biogas for the final utilization of the gas.
   o The need for gas treatment depends to a great extent on the final utilization of the gas. The gas coming from the digestion process normally is saturated with water and has a methane content of about 64 %. Further, it contains carbon dioxide and has a minor content of hydrogen sulfide and ammonium.
5. Management of digest from digestion.
   o Function: Brings the digestate from the digestion process into proper conditions for the final utilization or disposal.
   o The management of the digestate may (depending on the final utilization or disposal of the digestate) require posttreatment in terms of dewatering, wastewater treatment, composting, and storage facilities for digestate.
6. Odor control.
   o Function: Controls and minimizes odor from the raw waste and from the anaerobic digestion and other biological processes at the plant.
   o Five main systems for odor treatment are in use:
   • Dilution of the ventilation air.
   • Biological filters.
   • Chemical scrubbers.
   • Incineration.
   • Catalytic oxidation/regenerative thermal oxidation.