Question

A restaurant (Restaurant A) uses an anaerobic digester to treat the refuse oil after cooking, with the goal to use the CH4 generated as fuel source for cooking. It typically uses canola oil for cooking (C18H34O2). If the restaurant produces 5 Gallons of refuse oil every day, estimate the CH4 generated per day (Use the COD/TOC approach).

Answer 1

A restaurant (Restaurant A) uses an anaerobic digester to treat the refuse oil after cooking, with the goal to use the CH₄generated as fuel source for cooking. It typically uses canola oil for cooking (C₁₈H₃₄O₂). If the restaurant produces 5 Gallons of refuse oil every day, estimate the CH4 generated per day :

TOC is the carbon content of an organic compound/substrate, whereas COD is the oxygen equivalent of the same compound/substrate, serving as electron donor during full oxidation. Therefore TOC/COD ratio is a function of the oxidation state of carbon in the compound.

Complete oxidation of C₁₈H₃₄O₂ :

C₁₈H₃₄O₂ + 51/2 O₂ ==> 18 CO₂ + 17 H₂O

1 mol of C₁₈H₃₄O₂ will required 25.5 moles of O₂ .

TOC : 12*18 g = 216 g /mol of C₁₈H₃₄O₂ .

5 gal ~ 18.93 L canola oil

COD indirectly measures the amount of organic matter, and for that reason, it can be applied to estimate the CH₄ yield of biomass substrate. This method is based on the assumption that 1 mole of methane requires 2 moles of oxygen to oxidise carbon to carbon-dioxide and water .

Every g of methane is thus equivalent to 4 g of COD.

Average density of canola oil : 0.92 g/cm3

mass of used oil : 17.42 Kg

Moles of C₁₈H₃₄O₂ : 17.42 Kg / 282 g/mol = 61.77 moles

moles of O₂ (COD) needed = 61.77 mol *25.5 moles/ mol = 1575.2 moles

so, amount of O₂ needed or (COD) = 50.4 kg

4 g COD = 1 g methane

So, Methane generation potential : 12.6 kg/day.