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
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
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
What was found in literature Publications in Scopus, since 2002, with the keyword ‘’Anaerobic digestion’’
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
Feedstock types and operational parameters
Feedstock types and operational parameters
Analysing the literature data
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
Factors affecting the AD process Baseline composition: % carbohydrates, proteins, lipids, VFA and non biodegradable fraction Fixed parameters: Volume, HRT, ODM, DM, Temperature, Kinetics
The influence of composition ‘’Small variations in composition can have strong impacts on the methane produced’’
Factorial analysis of variations in composition
Model simulations versus the literature data
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
Thank you for your attention! Questions?