Methodology
56
6.5.1.2.4. Evaluation of screen refusals (í µí±½ í µí².í µí²í µí²í µí² )
The volume of detritus retained by the screen depends on the quality of the water to be treated
and the spacing between the bars.
í µí± í µí±í µí±í µí±¡í µí±í µí±í µí±í µí±í µí± is expressed in liters per inhabitant equivalent per year, and is estimated at :
- 5 to 10 l/inhab/year for a fine screen.
- 2 to 5 l/inhab/year for a coarse screen.
It is calculated using the following formula:
í µí±½ í µí².í µí²í µí²í µí² =
í µí±µ í µí±°í µí±¬ × í µí±½ í µí²í µí²í µí² ×í µí¿í µí¿ −í µí¿
í µí¿í µí¿í µí¿
With :
NIE : Number of Inhabitant Equivalent for the horizon 2050.
Vret : The volume of detritus retained by the screen.
For the coarse screen
For urban wastewater, the quantity of waste recovered by the grates per inhabitant per
year is estimated at:
í µí± í µí±í µí±í µí±¡í µí±í µí±í µí±í µí±í µí± =
2+5
2
(l/inhab/year).
For the fine screen
í µí±¤í µí± ℎí µí±í µí±£í µí±: í µí± í µí±í µí±í µí±¡í µí±í µí±í µí±í µí±í µí± =
5+10
2
(l/inhab/year).
The screen sizing calculation results are summarized in the table 19 (Chapter 3).
6.5.1.3. Dessander and oil removal sizing
The dessander is a structure in which dense particles with low velocity can settle. These
are mainly sands. It is preferable to recover these particles upstream of the plant, rather than
allowing them to accumulate at certain points where they can cause a variety of problems. They
also limit the life of metal parts in pumps and other equipment. In the desander, 90% of particles
equal to or larger than 200 µm (sand) must be eliminated. (FNDAE, 2002).
The purpose of the oil removal is to retain oil by natural flotation or accelerated by injection of
fine bubbles. These matter are likely to interfere with the biological phase of treatment.
56
6.5.1.2.4. Evaluation of screen refusals (í µí±½ í µí².í µí²í µí²í µí² )
The volume of detritus retained by the screen depends on the quality of the water to be treated
and the spacing between the bars.
í µí± í µí±í µí±í µí±¡í µí±í µí±í µí±í µí±í µí± is expressed in liters per inhabitant equivalent per year, and is estimated at :
- 5 to 10 l/inhab/year for a fine screen.
- 2 to 5 l/inhab/year for a coarse screen.
It is calculated using the following formula:
í µí±½ í µí².í µí²í µí²í µí² =
í µí±µ í µí±°í µí±¬ × í µí±½ í µí²í µí²í µí² ×í µí¿í µí¿ −í µí¿
í µí¿í µí¿í µí¿
With :
NIE : Number of Inhabitant Equivalent for the horizon 2050.
Vret : The volume of detritus retained by the screen.
For the coarse screen
For urban wastewater, the quantity of waste recovered by the grates per inhabitant per
year is estimated at:
í µí± í µí±í µí±í µí±¡í µí±í µí±í µí±í µí±í µí± =
2+5
2
(l/inhab/year).
For the fine screen
í µí±¤í µí± ℎí µí±í µí±£í µí±: í µí± í µí±í µí±í µí±¡í µí±í µí±í µí±í µí±í µí± =
5+10
2
(l/inhab/year).
The screen sizing calculation results are summarized in the table 19 (Chapter 3).
6.5.1.3. Dessander and oil removal sizing
The dessander is a structure in which dense particles with low velocity can settle. These
are mainly sands. It is preferable to recover these particles upstream of the plant, rather than
allowing them to accumulate at certain points where they can cause a variety of problems. They
also limit the life of metal parts in pumps and other equipment. In the desander, 90% of particles
equal to or larger than 200 µm (sand) must be eliminated. (FNDAE, 2002).
The purpose of the oil removal is to retain oil by natural flotation or accelerated by injection of
fine bubbles. These matter are likely to interfere with the biological phase of treatment.