Symptoms

• Sand presence in the collected screenings

• Water level increase in the screening channel

• Sand reduction collected form the grit chamber

Main causes

• Reduced approach velocity

• Obstruction occurrence in the screening channel

Investigations and analyses

• Confine the area where sand accumulates

• Measure the approach velocity corresponding to the different wastewater flow rate;

• Examine the as-built drawings to check the presence of some shape irregularity in the approaching channel

Remedial actions

• If the approaching velocity is less than 0.5 m s^-1 then an increase is needed:

a temporary solution could be the flow rate increase through a recycle flow, a reduction of the screening channels (if there are more than two working in parallel) a water level reduction modifying the out flow weir

• Empty the screen channel and remove all the bottom irregularities