ANAEROBIC DIGESTION

What is AD?  Process: microbs attack OM + no oxygen = biogas + solid + liquid residue  Common: stabilisation of sewage sludge, digestive tract or ruminants, landfill, marshlands

Why AD?  Landfill  CH4: fire, greenhouse gas  Leachate: water pollution  Impermeable landfill caps: lateral movement  Remedy: make use of landfill CH4 (but)  Site operational problems (corrosive trace gas)  Unpredictable generation rates  Maintenance issues  Inadequate gas collection system

Why AD?  Promotion of controlled degradation  Strategic plant location  Gas: More consistent supply, recover all gas  Digestate: agricultural or horticultural application  Waste mgmt: reduce landfill space, reduce leachate and landfill gas

The AD Process  Essentially 4 steps  Hydrolysis  Acidogenesis  Acetogenesis  Methanogenesis

The AD Process  Hydrolysis  Hydrolytic bacteria produce extracellular enzyme  break down and liquefy complex insoluble organic polymers  Proteins – amino acids, fats – LCFA, Carbohydrate – simple sugars  Hydrolysis rate governed by substrate availability, bacterial population, pH and temp.

The AD Process  Acidogenesis  Make acetic acid and VFA from preceding monomers  CO2 and H2 from catabolism of carbohydrate  Also some simple alcohols  Proportion of different by-products depend on environmental condition, bacterial species

The AD Process  Acetogenesis  Degrade LCFA & VFA to acetate, CO2 and H2  Methanogenesis  Methane end-product  Acetoclastic: use acetic acid/acetate (75% CH4 produce)  Hydrogenothropic: use CO2 & H2  Decrease VFA, pH naturally regulated

 T_Fk T_Fk T_Fk  VVHI VVHI VVHI

Feedstock  Yes: Biodegradable materials  No: Non-biodegradable & inorganic material  Toxic to reactor contents  Reduce reactor space  Digestate heavy metal

Feedstock  Pre-treatment  Size reduction  Homogenous supply  Remove contaminants (source separation or mechanical)  Sewage sludge  Common  Suitable if heavy metal below digester toxic level or land application

Feedstock  Municipal Waste  70% organic, readily degradable ¼ of total  E.g. paper & card better recycle or incinerate

Feedstock  Garden waste  Shred for homogeneity  High lignin content  Organic industrial waste  Food/ drink processing, organic chemicals, pharmaceutical and fermentation industries  Suitable solid/liquid form, individually or mixed with other wastes

Feedstock  Manures  Good organic characteristics (solid or liquid)  Can mix with other waste to enhance process stability  Relatively low gas yield

Feedstock  Typical gas yields and solids content of different wastes

Feedstock  Typical Biogas Quality

Reactor  Feedstock preparation – reactor (digester)  Where optimize microbiological processes of AD, produce gas and digestate  Diverse reactor designs  great diversity of waste composition  choice of operational parameters

Reactor Type - One Stage ‘Wet’ system (<15% TS) ‘Dry’ system (>20% TS)

‘Wet’ system (<15% TS) Reactor Type- One Stage

Reactor Type – Two Stage  Separate phases  Control process, more methane yield

Reactor Type - Two Stage Without biomass retention With biomass retention

Reactor sizing  Effective tank volume affected by hydraulic retention time (HRT) and organic loading rate (OLR)  V = HRT. Qwhere Q = flow rate  OLR = S0/HRTS0 = feed conc  Sizing fix one criteria

Reactor Sizing  HRT  Affects OM removal and specific gas production  Calculate min. value  Below which substrate does not degrade and not produce gas  Avoid anaerobe washout  Min HRT 4-10 days for mesophilic AD

Reactor Sizing  OLR  Overload risk with highly digestible feedstock e.g alcohols  Substrate with small VS, can put large volumes  Thermophilic plant 2x load  E.g Mesophilic plants: 3-4 kg VS/m3 digester, thermophilic: 7-8 kg VS/m3

 Xccnw&NR=1

Operational Parameters  Temperature  Degradation rates, yields, increase with temp  Thermophilic: require increase heating  Thermophilic less stable, go two stage

Operational Parameters  Mixing  Eliminate scum  Uniform temperature  Better microbial and waste contact  Release methane to headspace  Eg. Internal impellers, biogas re-circulation, mix feedstock with recycle liquors

Operational Parameters  Nutrients  C/N ratio 20/1 – 30/1: optimal methane prod  Nitrogen  methane-forming bacteria growth  Phosphorous  Phosphorous requirement 1/7 or 1/5 of nitrogen  Others (decreasing order): iron, cobalt, nickel, molybdenum, selenium, riboflavin, vitamin B 12  Supplementation  Nitrogen – urea, aqueous ammonia, ammonium chloride  Phosphorous – phosphoric acid or phosphate salt

Process Monitoring  Stable process:  Low VFA <1000mg/l; CO %  Temporary imbalance because:  Temperature change  Organic loading  Substrate type  Prolonged imbalance because (start-up):  materials toxic to methane bacteria  extreme pH drop

Process Monitoring  pH, alkalinity and VFA – integral expression of reactor acid-base conditions  pH  Stable AD: pH  Control pH drops < 6.5:  Lime: insoluble calcium carbonate  Sodium bicarbonate: metal cation toxicity  Anhydrous ammonia: excess ammonia  Mixtures Ca(OH)2, NaOH, KOH

Process Monitoring  VFA  Depends on substrate  Normal mgAc/l  VFA increase due to loading increase  Unstable process; VFA increase, alkalinity drops  Normal VFA/Alk< 0.3

Process Monitoring  Alkalinity  Acid neutralising capacity of medium  From ammonia (protein degradation), bicarbonate (CO2 solubilisation in liquid phase)  Typical values mg/l CaCO3

Process Monitoring  Toxicity  Ammonia  High loading & pH, NH3 >1250 mg/l : AD failure  Low loading & neutral pH, NH3 > 5000 mg/l: still tolerated  Free ammonia high pH, toxic to AD system  Ammonia remedy: reactor dilution, C/N adjustment

Process Monitoring  Sulfides  Threshold value: mg/l  Introduced from waste, biological sulfate reduction, protein degradation containing sulfur  Heavy metals  Toxic at low conc: copper, zinc, nickel  Remedy: react with sulfides to precipitate as metal sulfides (insoluble)

Example AD Monitoring Meat waste, 13% TS, 180g/day Food Waste, 1% TS, 250g/day

AD Complete Picture