Water tools

Biogas and energy from COD removed anaerobically

Every kilogram of COD destroyed anaerobically ends up largely as methane. For high-strength streams the energy recovered can exceed the plant's own demand.

Flowm³/d

Influent CODmg/L

Effluent CODmg/L

Methane yieldm³ CH₄/kg COD removedTheoretical 0.35 at 0 °C and 1 atm; 0.30 to 0.35 in practice.

Methane content of biogas%

Methane lower heating valuekWh/m³

CHP electrical efficiency

COD removed

7,500kg/d

Methane

2,475m³/d

Biogas

3,808m³/d

Thermal energy in methane

24,676kWh/d

Electricity from CHP

8,637kWh/d

Electricity per m³ treated

4.32kWh/m³

Continuous electrical output

360kW

How it works

Methane from the COD destroyed, biogas from its methane content, and energy from the lower heating value and the CHP efficiency:

V_{C H_{4}} = Q (C O D_{i n} - C O D_{o u t}) Y_{C H_{4}} V_{b i o g a s} = \frac{V_{C H_{4}}}{x_{C H_{4}}}

(1)

E_{t h} = V_{C H_{4}} L H V E_{e l} = η_{C H P} E_{t h}

(2)

where

$V_{C H_{4}}$: methane produced, m³/d
$Q$: flow, m³/d
$C O D_{i n}, C O D_{o u t}$: influent and effluent COD, mg/L
$Y_{C H_{4}}$: methane yield, m³ per kg COD removed
$x_{C H_{4}}$: methane volume fraction of the biogas
$E_{t h}$: thermal energy, kWh/d
$L H V$: lower heating value of methane, kWh/m³
$E_{e l}$: electricity from the CHP unit, kWh/d
$η_{C H P}$: electrical efficiency of the CHP unit

COD used for cell synthesis and sulfate reduction does not become methane, which is why the practical yield sits below the theoretical 0.35 m³ per kg COD at standard conditions. Gas dissolved in the effluent is also lost at low strength and low temperature.

Biogas and energy from COD removed anaerobically

How it works

Related reading