6.3 How to design the BSFL process

How to estimate the larvae requirement for the treatment process:

Firstly, we need to understand the organic loading rate in kg/d, with the following formula:

Where,

Parameter	Description	Unit
OLR	Organic loading rate	kgVS/d
VS	Total sludge volume per day	m3
ρ	Density	kg/m3
CTS	Concentration of TS	%
CVS	Concentration of VS	% from TS

Secondly, we need to calculate the number of larvae required to consume the organic material provided per day, using the following formula:

Where,

Parameter	Description	Unit
NL	Number of larvae	–
OLR	Organic loading rate	mgVS/d
CR	Consumption rate	mg/larvae/d

How to calculate the area requirements:

After determining the number of required larvae, and based on recommended larvae density, we can estimate the area requirements by dividing the number of larvae by the larvae density.

Where,

Parameter	Description	Unit
A	Area ()	m2
NL	Number of larvae	–
ρL	Larvae density	larvae/m2

How to estimate the larvae production:

Knowing the bio-conversion rate, we can estimate the larvae production by applying the following formula:

Where,

Parameter	Description	Unit
ML	Mass of larvae	kg/d
OLR	Organic loading rate	kg/d
BCR	Conversion rate (%)	%

Practice:

Assuming we would like to treat 5 m³ of faecal sludge per day with a total solid content of 20% and VS is 80% from TS, with faecal sludge density of 1,050kg/m³, the consumption rate is identified to be 50 mg/larvae/day and larval density of 13,300 larvae/m², the bio-conversion rate is 15%.

Firstly, please calculate the expected organic loading rate:

Solution:

Secondly, please calculate the number of larvae required if the consumption rate is estimated to be 50 mg/larvae/day.

Solution:

21,100,000 larvae are required per cycle.

Thirdly, please calculate the required area, if the recommended density is 13,300 larvae/m².

Finally, please calculate the expected production of larvae, if the bio-conversion rate is assumed to be 15%