4.5 What is the space requirement of ISFs?

For an efficient treatment, an area between 7 – 10 m² per m³ of wastewater per day is recommended for ISF if used as a secondary treatment step. When used as a tertiary treatment step, the hydraulic loading rates can be higher, and an area between 0.7 – 2 m² per m³ of wastewater per day may be considered sufficient. The specific area requirement can be calculated in consideration of the hydraulic or organic loading rate.

The total required area is recommended to be divided into several, but at least two cells or beds in parallel. The cells are the same size, to ensure equal feeding rates. The feeding rate impacts the required area within the above-mentioned and recommended ranges. If the different cells are fed in rotation within the day, the cells work usually more efficiently, and slightly higher feeding rates are possible. If rotation only takes place weekly, then it is advised to consider lower feeding rates.

Method to calculate the size of a sand filter

We want to calculate the required area for the filter bed, to treat 10m³/d (Q_d) of effluent with the BOD₅ concentration of 150 mg/L (C_in).

Firstly, we calculate the required area based on the organic loading rate.

Where,

Parameter	Description	Unit
AOLR	Surface area of filter bed based on OLR	m2
Qd	Average daily flow rate of wastewater/sewage	m3/d
Cin	Influent BOD concentration	mg/L or g/m3
OLR	Organic loading rate	g/m2/d

The Organic Loading Rate for ISFs should range between 2.5 – 360 g/m²/d.

Secondly, we calculate the required area based on the hydraulic loading rate.

Where,

Parameter	Description	Unit
AHLR	Surface area of filter bed based on HLR	m2
Qd	Average daily flow rate of wastewater/sewage	m3/d
HLR	Hydraulic loading rate	m3/m2/d

The recommended Hydraulic Loading Rate is 0.1 m³/m²/d.