4.8 Example: Intermittent Sand Filter as Tertiary Treatment Step in Ben Sergao, Morocco
In Ben Sergao, Morocco, a system was designed consisting of a settler for primary treatment, a trickling filter for the secondary step, and an intermittent sand filter for tertiary treatment.
Due to the low strength of wastewater after the pre-treatment by trickling filter, a higher hydraulic loading rate of 0.357 m3/m2/d (instead of 0.1 m3/m2/d) was possible. The resulting design saved space and cost whilst ensuring that the water is safe for reuse in terms of pathogens and helminth eggs.
During the operation, feeding was done 5 days per week, and the filter was left to rest for 2 days over the weekends.
The cylindrical sand filter had a diameter of 1.6 m and an area of 2 m2 a filter medium of 0.2 m pea gravel and 1.5 m sand.
The following performance parameters were recorded for the whole system.
| Parameter | Treatment Performance | |||
| Settler | Trickling Filter | Sand filter | Total | |
| TSS | 64.6% | 83.7% | 90.2% | 99.4% |
| COD | 52.6% | 76.8% | 64% | 96% |
| BOD5 | 56.4% | 79% | 87.7% | 98.9% |
| Helminths (eggs/100mL) | 47 | 0 | ||
| Coliforms (/100mL) | 5.16 | 170 | ||
Specifically for the intermittent sand filter, the following values for TSS, COD and BOD5 were measured:
| Parameter | In | Out |
|---|---|---|
| TSS (mg/L) | 23.5 | 2.3 |
| COD (mg/L) | 118 | 42.5 |
| BOD5 (mg/L) | 36.5 | 4.5 |
The objective was the removal of pathogens and helminth eggs. The results showed that the treatment realised a complete removal of helminth eggs, and the effluent was safe for discharge.
