If the septic tank is used as a pretreatment before a constructed wetland, it can receive the black water and grey
water. Wastewater from food and catering services and industries should be pretreated in a septic tank (black
water) and a grease trap (grey water).
Hydraulic Retention Time (HRT): (minimum) 4 days (< 50 p.e.) and 2 days for larger systems.
The total daily discharge is multiplied by the HRT. For example if 0.75 m³ is discharged, an effective volume of 3 m³
is needed for 4 days HRT.