4.4.2 Sizing of the digester

Biogas Sanitation 4 Biogas Installation Planning 4.4.2 Sizing of the digester

The purpose of this chapter is to practice how to size the digester and what information is required.

Parameter	Value	Unit	Source
Min Average Temperature during the coldest month	16.4	°C	Climate data
Retention time	45	days	Based on the BMP test at a temperature of 16°C
Total inflow rate	3.72	m³/day	Calculated in the previous exercise
Total organic loading rate	34	kg_VS/day	Characterisation study
Specific organic loading rate for fixed dome digester	0.75	kg_VS/m³/day	Literature values

Assumptions for Case Study

The first step is calculating the necessary hydraulic volume for digestion. This will be determined based on the retention time of 45 days from the BMP test.

Where,

Parameter	Description	Unit
V_DG	Digester Volume	m³
IN _TOTAL	Total Inflow per day	m³/day
HRT	Hydraulic Retention time	days

Before proceeding, we must verify that the digester volume is suitable for the organic loading rate (OLR). If the organic loading rate exceeds the recommended limit of 0.75kg_VS per m³ of the digester volume per day, it is advised to expand the volume.

To calculate the organic loading rate per m³ of digester volume per day, we need to divide the total organic loading rate by the digester volume.

Where,

Parameter	Description	Unit
OLR_S	Specific Organic Loading Rate	kg_VS/m³/day
OLR	Organic Loading Rate	kg_VS/day
V_DG	Digester Volume	m³

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