1. Anaerobic digestion of sewage and domestic wastes. How small can it be?
Ioannis Markidis
WISE CDT PhD Student, University of Bath
Supervisors: Dr Marta Coma & Dr Tom Arnot

2. Research Aim Substrate Biogas VOLUME ↓ Digestate MODELLING
- Influent COD
- Volatile solids
- Particle size
- Methane potential
Substrate
Biogas
Digestate
- Biogas yield
- Methane content
- Produced volume
Scale down AD
Optimize biogas production
Explore added value options
Decentralized approach
VOLUME ↓
- Nutrients (P, N)
- Heavy metals
- COD removal
- Quantity
- Organic loading rate
- HRT / Volume
- Sludge age
- Temperature & pH
MODELLING

3. Benefits & Restrictions
Why small scale?
Local treatment (reduced transportation and costs)
Local fertilizer & renewable energy production
More flexible & simpler infrastructure (low footprint)
Reduced load in existing sewers and WWTP
Local economic & social benefits
Challenges:
Technical limitations (operational conditions, gas leakage, feedstock availability)
Feedstock limitations (co-digestion unavailable)
Economies of scale
Policies, energy prices, gate fees

4. What was found in literature
Publications in Scopus, since 2002, with the keyword ‘’Anaerobic digestion’’

5. AD database construction
Parameters
Feedstock type
Food waste
Sewage
Biogas yield (m3 kg-1 VS)
Methane content (%)
Source separated black waste
Farm - animal
Organic loading rate (kg COD m-3 d-1)
COD removal efficiency (%)
Temperature (o C)
Hydraulic retention time (d)
Application
Full scale
Pilot
Reactor volume and type
Lab
Feasibility studies

6. Feedstock types and operational parameters

7. Feedstock types and operational parameters

8. Analysing the literature data

9. Developing a simple AD model
COD based kinetics (Monod equation)
Stoichiometry & Elemental balances (Buswell equation)
Mass balances
Reactor design (operational parameters)
To predict the influence of various parameters on the process
To correlate gas production and substrate composition
To simulate the outcomes of literature cases
To explore various additional potential scenarios

10. Factors affecting the AD process
Baseline composition: % carbohydrates, proteins, lipids, VFA and non biodegradable fraction
Fixed parameters: Volume, HRT, ODM, DM, Temperature, Kinetics

11. The influence of composition
‘’Small variations in composition can have strong impacts on the methane produced’’

12. Factorial analysis of variations in composition

13. Model simulations versus the literature data

14. Outcomes & future steps
68% of small scale AD applications are installed in rural areas
Gather more data from industrial collaborators and existing AD plants
Dilute influents require larger reactor volumes - Hard to maintain a low OLR
So pre-treatment / dewatering is needed – reduced household water use will be beneficial
The developed is model robust and can be widely used
Use advanced process modelling tools to simulate small scale AD scenarios
Small variations in composition have a big impact on the methane produced
Lab experiments to validate the modelling results and test feedstocks
SA: Early indications of the limits within which small AD might work
Life cycle assessment to evaluate the economic potential of small scale AD

15. Thank you for your attention!
Questions?