Bag Biodigester: Conceptual Design Engineers Without Borders University of Minnesota Leo A. Kucek 13 Nov 2008

Bag Biodigester: Background Developed in 1960s, Taiwan Long PVC cylinder Tested in Nepal by GGC at Butwal, 1986 Reqirements: – PVC bag is easily available – Pressure inside the digester is increased and – Welding facilities are easily available

Bag Biodigester: Background Polyethylene (PE) tubular biodigester technology: – Cheap and simple – Small-scale farmers – Low cost of the installation – Robust: rural or urban areas, both in low and hilly lands

Bag Biodigester: Alternatives Plug flow digester: – trench lined with concrete or an impermeable membrane – Reactor covered with flexible cover gas holder anchored to the ground – South Africa, Anaerobic filter: – 1950's – dilute and soluble waste water, low solids – Simple, reduces the reactor volume – packing medium: stones, plastic, coral, mussel shells, reeds, etc. – Methane producing bacteria form film on the large surface of the packing medium – "fixed film“ Upflow anaerobic sludge blanket: – 1980, the Netherlands – Methane producing bacteria in granules of sludge blanket (floor) – Feed enters from bottom; biogas is produced during liquid upflow

Bag Biodigester: Reaction Stage 1: Hydrolysis – Waste: carbohydrates, lipids, proteins and inorganic materials – Solubilize large molecules into simpler ones (via extracellular bacterial enzyme) – Example: cellulose (polymer of glucose)  glucose (via cellulolytic bacteria) Stage 2: Acidification – Monomer (e.g. glucose) from Stage 1 fermented under anaerobic condition – Produce various acids (via acid-forming bacterial enzymes) – C6 broken down  more reduced state (acids) – Products: acetic acid, propionic acid, butyric acid and ethanol Stage 3: Methanization: – Acids produced in Stage 2 converted to methane by methanogenic bacteria – Several reactions, several by-products – Desired product: Methane (CH 4 )

Bag Biodigester: Biogas Biogas Composition: – % CH 4 – 30 to 40 percent CO 2 – Other gases: H 2 (5-10), N 2 (1-2), H 2 O (0.3), H 2 S (Trace) Ignition temperature: °C Odourless, colourless gas Clear blue flame (similar to that of LPG gas) 20 MJ/m 3 60 percent efficiency in a conventional biogas stove.

Bag Biodigester: Sizing m 3 biogas/ kg Human Waste Assuption: C/N ~ 20 (Sawdust?) Design Specification: Heat 5 L of water from 25°C to 100°C per home (30 homes) 5 L / home x 30 homes = 150 L H 100°C – H 25°C = kJ/kg = kJ/kg Necessary heat = 48,000 kJ / 0.6 = 80 MJ Average density: 0.98 kg/L

Bag Biodigester: Sizing Necessary heat: 80 MJ Biogas heat content: 20 MJ/ m 3 Biogas produced: 4 m 3 Biogas to human dung ratio: m 3 / kg Amount of human dung necessary: 160 kg

Bag Biodigester: Sizing Case Study #1: – 10 fattening pigs – Digester volume required: 4 m 3 (V Liq ) – PE Bags: 80, 125, 200 cm (D) – D = 0.80 m – Cross-Sectional Area = π(D/2) 2 = 0.5 m 2 – On average, total volume is 80% liquid V Total = V Liq + V Gas V Liq = 0.80 V total V Gas = (1-0.80)/0.80(V Liq ) = 1 m 3 V total = π(D/2) 2 L = 5m 3  L = 10 m L D

Bag Biodigester: Sizing Case Study #1, (cont.): – Ditch for biodigester: – W1 = 90 cm – W2 = 70 cm – H = 90 cm – L D = 10 m W1 W2 H LDLD

Bag Biodigester: Materials Case Study #1 (cont.): – Bag PE: 80 cm ID, 100 m roll, 50 kg / 100 m roll μm thickness (calibre) (UV) – Ceramic tubes (2) L tubes = 75 to 100 cm, D tubes = 15 cm (ID) – Plastic (PVC) Hosepipe D Hosepipe = 12.5mm (ID) (Length is site-specific (to cooking site)) – Others: 2 PVC adapters (male and female) of 12.5mm internal diameter. 2 rubber washers (from car inner tube) of 7cm diameter and 1mm thickness with a 12.5 mm diameter central hole. 2 rigid plastic (perspex) washers of 10 cm diameter and a central hole of 12.5mm. 2 m of PVC pipe of 12.5mm internal diameter. 4 used inner tubes (from bicycle, motor cycle or motor car) cut into bands 5 cm wide. 1 transparent plastic bottle. 1 PVC elbow of 12.5mm internal diameter. 3 PVC "T" pieces of 12.5mm internal diameter. 1 tube of PVC cement.

Bag Biodigester: Materials Case Study #2 – 5.5 m of a strong, flexible, plastic sheet (> 2.8 mm width) – 4 m x 3" PVC tubing (feed and exit tubes) – 2 m 3 sand, 1 m 3 rock (mix w/ cement for walls) – 9 50-kg sacks of cement (walls & floor) – 60 cement blocks (12 cm X 20 cm X 40 cm): three rows for pins and hangers – 1 x 1/2" PVC tubing (rectangular frame, circumference of 16.6 m) – PVC tubing to cooking site – Others (minor): Rebar Tubes, fittings Rope Gallon jugs Hardware

Bag Biodigester Materials Other costs to consider: – Fencing – Housing (roof) – Additional PVC piping – Maintenance: Joe the Plumber!

Bag Biodigester: Final Remarks Dry human waste: expect 1 month fermentation until any biogas produced Concerns about pressure? Case Study #1: Payback in approximately 1 year (price of liquid fuel, no odors from pigs) Case Study #2: ~$300 Materials Cost

Bag Biodigester: The Next Step Economic data: – Materials & labor – Financial sustainability ($/fuel) Effects of changing reactor size Feed & product composition Separations? Safety?